U.S. patent application number 11/712777 was filed with the patent office on 2007-06-28 for apparatus and method for projecting a variable pattern of electromagnetic energy.
This patent application is currently assigned to Microvision, Inc.. Invention is credited to Clarence T. Tegreene, Christopher A. Wiklof.
Application Number | 20070145136 11/712777 |
Document ID | / |
Family ID | 34103754 |
Filed Date | 2007-06-28 |
United States Patent
Application |
20070145136 |
Kind Code |
A1 |
Wiklof; Christopher A. ; et
al. |
June 28, 2007 |
Apparatus and method for projecting a variable pattern of
electromagnetic energy
Abstract
According to an embodiment, an interrogator includes a beam
generator operable to scan a variable-power beam across a field of
view, a detector aligned to receive an electromagnetic signal from
the field of view and generate a corresponding detection signal,
and a controller operatively coupled to the detector and the beam
generator and operable to vary the power of the beam as it scans
across the field of view responsive to the detection signal.
According to an embodiment, an illumination system includes an
illumination source operable to provide spatially-varying
illumination, a detector configured to receive scattered energy
from the spatially-varying illumination, and an electronic
controller operable to vary the spatial variation of the
illumination responsive to the scattered energy received by the
detector. According to an embodiment, a method includes
illuminating a field of view with a variable power illumination
pattern, receiving scattered light from the field of view, and
modifying the pattern of the variable power illumination responsive
to the scattered light
Inventors: |
Wiklof; Christopher A.;
(Everett, WA) ; Tegreene; Clarence T.; (Bellevue,
WA) |
Correspondence
Address: |
Christopher A. Wiklof;GRAYBEAL JACKSON HALEY LLP
Suite 350
155 - 108th Avenue NE
Bellevue
WA
98004-5973
US
|
Assignee: |
Microvision, Inc.
|
Family ID: |
34103754 |
Appl. No.: |
11/712777 |
Filed: |
February 27, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10630062 |
Jul 29, 2003 |
|
|
|
11712777 |
Feb 27, 2007 |
|
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Current U.S.
Class: |
235/454 ;
235/462.32; 235/462.33 |
Current CPC
Class: |
G06K 7/10851 20130101;
G06K 7/10564 20130101 |
Class at
Publication: |
235/454 ;
235/462.32; 235/462.33 |
International
Class: |
G06K 7/10 20060101
G06K007/10 |
Claims
1-38. (canceled)
39. An interrogator comprising: a beam generator operable to scan a
variable-power beam across a field of view; a detector aligned to
receive an electromagnetic signal from the field of view and
generate a corresponding detection signal; and a controller
operatively coupled to the detector and the beam generator and
operable to vary the power of the beam as it scans across the field
of view responsive to the detection signal.
40. The interrogator of claim 39 wherein the controller comprises a
frame buffer operable to hold values corresponding to the variable
power of the variable-power beam.
41. The interrogator of claim 39 wherein the controller comprises:
a frame buffer operable to hold variable values corresponding to
the variable power of the variable power beam; and a processor
operable to receive the detection signal and responsively vary the
values held by the frame buffer.
42. The interrogator of claim 39 wherein the controller comprises:
a processor operable to receive the detection signal and
responsively vary the power of the variable power beam.
43. The interrogator of claim 39 wherein the controller is operable
to receive the detection signal and responsively decode an image
corresponding to the electromagnetic signal from the field of
view.
44. The interrogator of claim 43 wherein the detection signal
corresponds to a machine-readable symbol in the field of view, and
decoding the image corresponds to converting the detection signal
into a decoded signal corresponding to data coded in the
symbol.
45. The interrogator of claim 39 wherein the controller is operable
to vary the power of the beam proportionally to the value of the
detection signal.
46. The interrogator of claim 45 where proportionally comprises
inversely proportionally.
47. An illumination system comprising: an illumination source
operable to provide spatially-varying illumination; a detector
configured to receive scattered energy from the spatially-varying
illumination; and an electronic controller operable to vary the
spatial variation of the illumination responsive to the scattered
energy received by the detector.
48. The illumination system of claim 47 wherein the detector
comprises a detector array.
49. The illumination system of claim 47 wherein the detector
comprises an imaging detector array comprising at least one
selected from the group consisting of a CCD array and CMOS
array.
50. The illumination system of claim 47 wherein the detector
comprises a non-imaging detector.
51. The illumination system of claim 47 wherein the detector
comprises a non-imaging detector comprising at least one selected
from the group consisting of a photo-diode, a photo-transistor, and
a photo resistor.
52. The illumination system of claim 47 wherein the illumination
source comprises a scanned illumination beam source.
53. The illumination system of claim 47 wherein the illumination
source comprises at least one selected from the group consisting of
a plurality of light emitters, a light emitter with a patterned
attenuator, and a beam scanner.
54. The illumination system of claim 47 wherein the
spatially-varying illumination comprises illumination varying along
an axis.
55. The illumination system of claim 47 wherein the spatially
varying illumination comprises illumination varying along at least
two axes.
56. The illumination system of claim 47 wherein the electronic
controller is further operable to capture an image of an area
illuminated by the spatially-varying illumination.
57. The illumination system of claim 47 wherein the electronic
controller is further operable to decode a symbol illuminated by
the spatially-varying illumination.
58. A method comprising: illuminating a field of view with a
variable power illumination pattern; receiving scattered light from
the field of view; and modifying the pattern of the variable power
illumination responsive to the scattered light.
59. The method of claim 58 wherein modifying the pattern of the
variable power illumination comprises at least one selected from
the group consisting of reducing the illumination power to a
portion of the field of view corresponding to a large amount of
scattered light and increasing the illumination power to a portion
of the field of view corresponding to a small amount of scattered
light.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to scanned beam systems and
variable illumination systems, and more particularly to control
systems for scanned beam imagers and variable illuminators.
BACKGROUND OF THE INVENTION
[0002] The field of imaging can be divided into one-dimensional
(1D) or linear imaging, two-dimensional (2D) imaging, and
three-dimensional imaging.
[0003] Examples of 1D imagers include bar code devices using linear
pixelated detectors, including for instance charge-coupled-device
(CCD) and complementary-metal-oxide-silicon (CMOS) detectors.
Linear pixelated imagers typically include illumination to reduce
exposure time and ambient lighting requirements, often in the form
of a sheet of light formed by a light emitting diode (LED) or LED
array. Line-Scan cameras are a specific type of 1D pixelated
imager. In line-scan cameras, a second dimension, usually
perpendicular to the detector array, is provided by movement of the
object or phenomenon being photographed.
[0004] 2D imagers are used for myriad applications, including for
instance, automated inspection, automated vehicle guidance, 2D
symbol reading, omni-directional linear symbol reading, document
capture, signature capture, endoscopic and laparoscopic
visualization, and many other important applications. The most
familiar form of 2D imaging is perhaps the digital camera,
available in both still and video variants. Generally, 2D imagers
use 2D pixelated arrays such as 2D CCD and 2D CMOS detectors.
Often, 2D imagers include illuminators to improve low-light
performance, depth-of-field, and motion blur immunity. 2D imagers
are characterized by field-of-view (FOV) having both height and
width.
[0005] Both linear and 2D imagers commonly trade off several
performance characteristics including depth-of-field, resolution,
and motion blur immunity. Frequently, designers choose to add
illumination to the system. In such situations, peak power
consumption and illuminator cost vs. DOF may be regarded as design
trade-offs.
[0006] In typical pixelated imagers, each pixel in an array
receives light energy from a conjugate point in the field-of-view
for a selected sampling interval. Each pixel converts light to
electrical charge that accumulates proportionally to the brightness
of its conjugate point.
[0007] Another familiar field of imaging is that of film
photography. Film photography is widely practiced and widely
understood in terms of the trade-offs between depth-of-field
(determined by lens f-stop), motion blur immunity (determined by
shutter speed), resolution, and dynamic range (both of the latter
determined largely by film chemistry and film speed). Flash
photography adds illumination to the field-of-view to enable
operation in dark or fast-moving environments. Unfortunately, flash
photography suffers from dynamic range issues, having a tendency to
overexpose nearby objects while underexposing distant parts of the
FOV.
[0008] Another field that relates to the present invention is that
of laser scanning. Laser scanning systems scan a beam, typically a
laser beam, over a surface and measure the light reflected from the
beam. Generally, laser scanners detect light instantaneously
scattered by the moving spot without imaging it onto a detector
array. Rather, the size of the projected spot itself determines
resolution and the detector may be of a non-imaging type such as a
PIN photo-diode or the like. Some laser scanners sequentially form
a non-coincident pattern of scan lines such as a raster pattern or
a "starburst" pattern. These can be especially useful for multiple
symbol and stacked 2D symbol reading, or omni-directional bar code
reading, respectively.
[0009] Scanned beam systems, in the form of conventional linear
(1D) bar code scanners have been in use since the mid-1970s as
fixed mount devices such as those used in grocery stores. By the
early 1980s, scanned beam systems had been adapted to hand held
form as several bar code companies introduced helium-neon based
hand held scanners, most commonly called laser scanners. In such
systems, the pattern of received light is referred to as the scan
reflectance profile and may be processed to decode bar code symbols
through which the beam is scanned.
[0010] Scanned beam bar code systems have not generally heretofore
been referred to as "imagers", because they generally do not
capture an image per se. Instead, they continuously scan a beam and
simply monitor the reflected light for a scan reflectance profile
that is indicative of a bar code symbol.
[0011] In more recent times, scanning laser systems have found use
in other image capture applications, such as scanning laser
ophthalmoscopes and scanning microscopes.
[0012] Another related field is that of illumination systems for
image capture. Used in still photography as well as cinematic and
video photography, such illumination systems are used to achieve
effects not possible with ambient light. Consumer systems typically
include a simple strobe (for still photography) or incandescent
light (for video photography) that emits more-or-less uniform light
across the entire field-of-view. Often, the resulting images have
too much contrast, foreground objects being over-exposed and the
background underexposed. Commercial and industrial photographers
and cinematographers frequently add multiple illuminators to a
scene to try to recapture some of the ambiance that is lost in
simple consumer systems. These attempts to manually manipulate the
illumination intensity across the field of view are frequently
laborious and require a high degree of artistry.
OVERVIEW OF THE INVENTION
[0013] In its various aspects, the present invention relates to
imaging systems, to laser scanners, and to illumination systems
that automatically create variable intensity illumination across
the field-of-view, responsive to scene characteristics. It also
relates to systems that capture and/or reproduce images with
enhanced dynamic range and systems that accelerate the
determination of optimal illumination.
[0014] In one aspect according to the invention a method and
apparatus illuminates a field-of-view with a variable-intensity
source; specifically, a source (or sources) that casts variable
illumination energy across the field-of-view. The variable
intensity source may be formed, for instance, by scanning one or
more beams of light across at least portions of the field-of-view
while modulating the intensity of the beam or beams. In this way,
darker and/or more distant regions may be illuminated more while
lighter and/or closer regions are illuminated less.
[0015] This may be especially advantageous for use with film
cameras or digital cameras where it is desirable to compress the
dynamic range of the scene enough to fit substantially within the
dynamic range of the film or imaging array, respectively. Such
dynamic range compression may also be desirable for pixelated
imagers where data from the illuminator frame buffer is not
available during the image recording process.
[0016] In another aspect, the color balance of a field-of-view or
portions of a field-of-view may be modified by differentially
illuminating the scene with illuminators of differing color.
[0017] In another aspect, a method and apparatus for variable
intensity illumination may include or augment a substantially
uniform illuminator. In this case, lighter and/or closer regions
may be illuminated entirely or to a relatively greater degree by
the quasi-uniform illuminator while darker and/or more distant
regions receive some "boost" in their illumination levels by the
variable intensity illuminator.
[0018] In other embodiments, variable illumination may be produced
by an array of static illuminators, by a variable attenuation
array, or by an array of scanned beams.
[0019] Variable illumination may operate on a pixel-by-pixel basis
or on a more generalized basis. In the latter case, illumination
spots may correspond to zones or regions in the field-of-view. Such
zones or regions may correspond to a few specific pixels or may
correspond to many pixels. For example, a 9.times.16 landscape
aspect ratio field-of-view may be divided into 12 spots arranged as
three zones vertically by four zones horizontally. Alternatively,
illumination regions may correspond to small groups of pixels such
as 2.times.2, 3.times.3 or larger arrays. The specific arrangement
of illumination zones may be selected to correspond to the
application requirements. For 1D imagers, it may be appropriate for
the variable illumination to be operative within moderately sized
groups of pixels comprising 25% or less of the linear FOV.
[0020] In "large spot" applications such as those that operate on a
more generalized basis, it may be advantageous to select
illumination patterns within the zones in such a way as to "smooth"
the edges of each spot. A smooth roll-off of illumination at the
edges of such zones may tend to reduce the abruptness of
differential illumination between zones, yielding a more attractive
or accurate rendition of the image.
[0021] When illumination is operative at a pixel-by-pixel basis, an
individual spot being illuminated corresponds to only a single
pixel of the image. In this case, it is possible to drive the
illuminator in such a way that a portion, up to substantially all,
of scene information is exhibited as the inverse of the data used
to drive the variable illumination. At one limit, the field-of-view
may be differentially illuminated to produce substantially uniform
light scatter at a detector. In this case, image information may be
retrieved wholly or substantially by a frame buffer used to drive
the differential illuminator. This mode may be especially
advantageous for non-imaging detectors such as PIN photodiodes,
avalanche photodiodes, photomultiplier tubes, and the like.
[0022] Another aspect according to the invention is related to
laser scanners, scanned beam imaging engines, and devices that use
scanned beam systems. Bar code laser scanners of the prior art
sometimes suffer from relatively poor immunity to variations in
ambient light across the field-of-view, specular reflections from
scanned surfaces, scanner orientation relative to the symbol
surface, symbol environmental degradation, and other effects that
hinder performance. In various aspects according to the present
invention, these and other artifacts may be eliminated or minimized
by modulating the laser intensity according to image
attributes.
[0023] In another aspect, detector sensitivity modulation or
diversification may augment or substitute for illuminator
modulation.
[0024] In another aspect, a scanned beam imager may operate with
enhanced dynamic range, even when using a detector with only modest
dynamic range. In such cases, the dynamic range of the light source
may substitute for extra dynamic range of the detector.
[0025] Another aspect of the invention relates to modulating output
of the illuminator synchronously with modulating responsiveness of
the detection system. The sinusoidal velocity profile of MEMS and
other scanners, the high data rate of imaging systems, and/or the
use of variable illuminator power can interact with how synchronous
detection may be implemented.
[0026] It may be important, especially with fast moving scenes and
when the implementation more closely resembles the leveled
illumination mode, to minimize the amount of time it takes for the
illumination system to converge, or arrive at the optimum
illumination energy for each spot. Some aspects according to the
present invention focus on methods and apparatus for speeding up
convergence.
[0027] Another aspect relates to methods and apparatus for
"probing" the field-of-view for image data. In this case,
especially bright illumination may be switched on for an instant to
determine the optical characteristics of one or a few dark or
distant spots, and then switched off for a time sufficient to meet
safety, covertness, or other requirements. During subsequent
frames, other spots may be similarly probed.
[0028] Another aspect according to the invention relates to
illuminator modulation being controlled by brightness
variation.
[0029] Another aspect according to the invention relates to
controlling the illuminator-to-detector coupling by pulse-width
variation or phase synchronization. In some situations, it may be
advantageous to modulate the illuminator(s) in a predictable way,
thereby creating a synchronous signal for the detector(s) to
monitor. In some cases, this may be a constant pulse frequency. In
other cases, it may comprise a predictably varying pulse frequency.
In either case, illuminator modulation may be achieved by varying
the duty cycle on a spot-by-spot basis.
[0030] Still another aspect according to the invention relates to a
combination of illuminator brightness variation and pulse-width
variation.
[0031] In another aspect, illuminator pulsing may be used alone,
without illumination intensity modulation, to create a synchronous
signal for detection.
[0032] Other aspects relate to various detector optical
configurations including staring, retro-collective, and
confocal.
[0033] Another aspect, according to the invention, relates to a
scanned beam imager module that may be used in a 2D bar code
reading system. By emulating the output of a CCD or CMOS imaging
engine, a scanned beam imaging engine may be conveniently
substituted into close derivatives of existing designs.
[0034] Another aspect relates to using synchronous modulation and
detection in adjoining systems to reduce or eliminate crosstalk
between such systems.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a block diagram of a simple scanned beam imager
and/or variable illuminator.
[0036] FIG. 2 is a block diagram of an apparatus and method for
modifying illuminator power.
[0037] FIG. 3 is a conceptual diagram showing an initial state for
an illuminator. In this case, the illumination energy is held
constant and the amount of scattered energy received at the
detector varies proportionally to the apparent brightness of the
spot.
[0038] FIG. 4a is a conceptual diagram showing a converged state
for an illuminator that has been programmed to provide a flat-field
or leveled scatter. In this case, the illumination energy is
modified in a manner inversely proportional to the apparent
brightness of each spot to result in substantially the same amount
of received energy at the detector.
[0039] FIG. 4b is a conceptual diagram showing a converged state
for an illuminator that has been programmed to compress FOV dynamic
range somewhat but still maintain differences in apparent
brightness.
[0040] FIG. 4c is a conceptual diagram showing a converged state
for a differential illuminator that has been programmed to augment
a conventional illuminator to reduce FOV dynamic range
[0041] FIG. 5a is an exemplary image showing regions differentially
illuminated to reduce FOV dynamic range per the techniques of
diagrams 4a, 4b and 4c.
[0042] FIG. 5b is a chart showing the apparent brightness ranges of
objects shown in FIG. 5a.
[0043] FIG. 6a is a diagram showing waveforms for converging
illuminator power per the method of FIG. 4a over several frames for
an exemplary 1D FOV.
[0044] FIG. 6b is a flow chart showing another explanation of how a
pixel value is converged.
[0045] FIG. 7 is a diagram indicating a non-converged state for two
exemplary beam scans across a 2D FOV.
[0046] FIG. 8 is a diagram indicating partial intra-frame
convergence for the two beam scans of FIG. 7 achieved by using
image processing.
[0047] FIG. 9 is a diagram indicating a pseudo-converged state for
the two beam scans of FIGS. 7 and 8 achieved intra-frame using
further image processing.
[0048] FIG. 10 is a diagram illustrating a post-processing method
of combining several complete images, each with a limited dynamic
range, into a final image.
[0049] FIG. 11 is an isometric view of a scanned beam imaging
engine or variable illuminator.
[0050] FIG. 12 is an isometric view of a scanned beam imaging
engine or variable illuminator having multiple detectors.
[0051] FIG. 13 is an isometric view of a scanned beam imaging
engine or variable illuminator having a pixelated detector.
[0052] FIG. 14 is an isometric view of an imaging engine or
variable illuminator having a non-scanning illuminator array and a
pixelated detector.
[0053] FIG. 15 is an isometric view of a scanned beam variable
illuminator.
[0054] FIG. 16 is an isometric view of an imaging engine or
variable illuminator having a static illuminator with variable
attenuator and a pixelated detector.
[0055] FIG. 17 is a side sectional view of a combination static and
variable illuminator where variable illumination is produced by a
variable attenuator.
[0056] FIG. 18 is a diagram showing a variable attenuator casting
shadows on a field of view.
[0057] FIG. 19 is an isometric view of a scanned beam imager having
a scanned beam engine.
[0058] FIG. 20 is a block diagram of a variable illuminator module
connected to an imager module.
[0059] FIG. 21 is an isometric view of a camera with a variable
illuminator attached.
[0060] FIG. 22 is a block diagram of a variable illuminator or
scanned beam imager.
[0061] FIG. 23 is a block diagram of a synchronous scanned beam
imager where the illuminator and detector are pulsed
synchronously.
[0062] FIG. 24 is a diagram showing pulsed illuminator and detector
waveforms of a synchronous scanned beam imager.
[0063] FIG. 25 is a side view of a compact three color light source
where the output beams are combined by an X-cube.
[0064] FIG. 26 is a diagram of a bi-sinusoidal scanning pattern
compared to a rectilinear pixel coordinate grid.
DETAILED DESCRIPTION OF THE INVENTION
[0065] FIG. 1 shows a block diagram of a scanned beam imager 102
that comprises one form of a variable illumination system. An
illuminator 104 creates a first beam of light 106. A scanner 108
deflects the first beam of light across a field-of-view (FOV) to
produce a second scanned beam of light 110. Taken together, the
illuminator 104 and scanner 108 comprise a variable illuminator
109. Instantaneous positions of scanned beam of light 110 may be
designated as 110a, 110b, etc. The scanned beam of light 110
sequentially illuminates spots 112 in the FOV. Spots 112a and 112b
in the FOV are illuminated by the scanned beam 110 at positions
110a and 110b, respectively. While the beam 100 illuminates the
spots, a portion of the illuminating light beam 100 is reflected
according to the properties of the object or material at the spots
to produce scattering or reflecting the light energy. A portion of
the scattered light energy travels to one or more detectors 116
that receive the light and produce electrical signals corresponding
to the amount of light energy received. The electrical signals
drive a controller 118 that builds up a digital representation and
transmits it for further processing, decoding, archiving, printing,
display, or other treatment or use via interface 120.
[0066] The light source 104 may include multiple emitters such as,
for instance, light emitting diodes (LEDs), lasers, thermal
sources, arc sources, fluorescent sources, gas discharge sources,
or other types of illuminators. In a preferred embodiment,
illuminator 104 comprises a red laser diode having a wavelength of
approximately 635 to 670 nanometers (nm). In another preferred
embodiment, illuminator 104 comprises three lasers; a red diode
laser, a green diode-pumped solid state (DPSS) laser, and a blue
DPSS laser at approximately 635 nm, 532 nm, and 473 nm,
respectively. While laser diodes may be directly modulated, DPSS
lasers generally require external modulation such as an
acousto-optic modulator (AOM) for instance. In the case where an
external modulator is used, it is typically considered part of
light source 104. Light source 104 may include, in the case of
multiple emitters, beam combining optics to combine some or all of
the emitters into a single beam. Light source 104 may also include
beam-shaping optics such as one or more collimating lenses and/or
apertures. Additionally, while the wavelengths descried in the
previous embodiments have been in the optically visible range,
other wavelengths may be within the scope of the invention.
[0067] Light beam 106, while illustrated as a single beam, may
comprise a plurality of beams converging on a single scanner 108 or
onto separate scanners 108.
[0068] Scanner 108 may be formed using many known technologies such
as, for instance, a rotating mirrored polygon, a mirror on a
voice-coil as is used in miniature bar code scanners such as used
in the Symbol Technologies SE 900 scan engine, a mirror affixed to
a high speed motor or a mirror on a bimorph beam as described in
U.S. Pat. No. 4,387,297 entitled PORTABLE LASER SCANNING SYSTEM AND
SCANNING METHODS, an in-line or "axial" gyrating, or "axial" scan
element such as is described by U.S. Pat. No. 6,390,370 entitled
LIGHT BEAM SCANNING PEN, SCAN MODULE FOR THE DEVICE AND METHOD OF
UTILIZATION, a non-powered scanning assembly such as is described
in U.S. patent application Ser. No. 10/007,784, SCANNER AND METHOD
FOR SWEEPING A BEAM ACROSS A TARGET, commonly assigned herewith, a
MEMS scanner, or other type. All of the patents and applications
referenced in this paragraph are hereby incorporated by
reference
[0069] A MEMS scanner may be of a type described in U.S. Pat. No.
6,140,979, entitled SCANNED DISPLAY WITH PINCH, TIMING, AND
DISTORTION CORRECTION; U.S. Pat. No. 6,245,590, entitled FREQUENCY
TUNABLE RESONANT SCANNER AND METHOD OF MAKING; U.S. Pat. No.
6,285,489, entitled FREQUENCY TUNABLE RESONANT SCANNER WITH
AUXILIARY ARMS; U.S. Pat. No. 6,331,909, entitled FREQUENCY TUNABLE
RESONANT SCANNER; U.S. Pat. No. 6,362,912, entitled SCANNED IMAGING
APPARATUS WITH SWITCHED FEEDS; U.S. Pat. No. 6,384,406, entitled
ACTIVE TUNING OF A TORSIONAL RESONANT STRUCTURE; U.S. Pat. No.
6,433,907, entitled SCANNED DISPLAY WITH PLURALITY OF SCANNING
ASSEMBLIES; U.S. Pat. No. 6,512,622, entitled ACTIVE TUNING OF A
TORSIONAL RESONANT STRUCTURE; U.S. Pat. No. 6,515,278, entitled
FREQUENCY TUNABLE RESONANT SCANNER AND METHOD OF MAKING; U.S. Pat.
No. 6,515,781, entitled SCANNED IMAGING APPARATUS WITH SWITCHED
FEEDS; and/or U.S. Pat. No. 6,525,310, entitled FREQUENCY TUNABLE
RESONANT SCANNER; for example; all commonly assigned herewith and
all hereby incorporated by reference.
[0070] Alternatively, illuminator 104, scanner 108, and/or detector
116 may comprise an integrated beam scanning assembly as is
described in U.S. Pat. No. 5,714,750, BAR CODE SCANNING AND READING
APPARATUS AND DIFFRACTIVE LIGHT COLLECTION DEVICE SUITABLE FOR USE
THEREIN which is incorporated herein by reference.
[0071] In the case of a 1D scanner, the scanner is driven to scan
output beams 110 along a single axis. In the case of a 2D
scanned-beam imager, scanner 108 is driven to scan output beams 110
along a plurality of axes so as to sequentially illuminate a 2D FOV
111.
[0072] For the case of 2D imaging, a MEMS scanner is often
preferred, owing to the high frequency, durability, repeatability,
and/or energy efficiency of such devices. A bulk micro-machined or
surface micro-machined silicon MEMS scanner may be preferred for
some applications depending upon the particular performance,
environment or configuration. Other embodiments may be preferred
for other applications.
[0073] A 2D MEMS scanner 108 scans one or more light beams at high
speed in a pattern that covers an entire 2D FOV or a selected
region of a 2D FOV within a frame period. A typical frame rate may
be 60 Hz, for example. Often, it is advantageous to run one or both
scan axes resonantly. In one embodiment, one axis is run resonantly
at about 19 KHz while the other axis is run non-resonantly in a
sawtooth pattern to create a progressive scan pattern. A
progressively scanned bi-directional approach with a single beam,
scanning horizontally at scan frequency of approximately 19 KHz and
scanning vertically in sawtooth pattern at 60 Hz can approximate an
SVGA resolution. In one such system, the horizontal scan motion is
driven electrostatically and the vertical scan motion is driven
magnetically. Alternatively, both the horizontal scan may be driven
magnetically or capacitively. Electrostatic driving may include
electrostatic plates, comb drives or similar approaches. In various
embodiments, both axes may be driven sinusoidally or
resonantly.
[0074] Several types of detectors may be appropriate, depending
upon the application or configuration. For example, in one
embodiment, the detector may include a PIN photodiode connected to
an amplifier and digitizer. In this configuration, beam position
information is retrieved from the scanner or, alternatively, from
optical mechanisms, and image resolution is determined by the size
and shape of scanning spot 112. In the case of multi-color imaging,
the detector 116 may comprise more sophisticated splitting and
filtering to separate the scattered light into its component parts
prior to detection. As alternatives to PIN photo diodes, avalanche
photodiodes (APDs) or photomultiplier tubes (PMTs) may be preferred
for certain applications, particularly low light applications.
[0075] In various approaches, photodetectors such as PIN
photodiodes, APDs, and PMTs may be arranged to stare at the entire
FOV, stare at a portion of the FOV, collect light
retro-collectively, or collect light confocally, depending upon the
application. In some embodiments, the photodetector 116 collects
light through filters to eliminate much of the ambient light.
[0076] The present device may be embodied as monochrome, as
full-color, and even as a hyper-spectral. In some embodiments, it
may also be desirable to add color channels between the
conventional RGB channels used for many color cameras. Herein, the
term grayscale and related discussion shall be understood to refer
to each of these embodiments as well as other methods or
applications within the scope of the invention. In the control
apparatus and methods described below, pixel gray levels may
comprise a single value in the case of a monochrome system, or may
comprise an RGB triad or greater in the case of color or
hyperspectral systems. Control may be applied individually to the
output power of particular channels (for instance red, green, and
blue channels) or may be applied universally to all channels, for
instance as luminance modulation.
[0077] FIG. 2 is a block diagram that illustrates one control
approach for adjusting variable illuminator intensity. Initially, a
drive circuit drives the light source based upon a pattern, which
may be embodied as digital data values in a frame buffer 202. The
frame buffer 202 drives variable illuminator 109, which may, for
instance comprise an illuminator and scanner as in FIG. 1. For each
spot or region, the amount of scattered light is detected and
converted into an electrical signal by detector 116. Detector 116
may include an A/D converter that outputs the electrical signal as
a binary value, for instance. One may refer to this detected value
as a residual. The residual is inverted by inverter 208, and is
optionally processed by optional intra-frame image processor 210.
The inverted residual or processed value is then added to the
corresponding value in the frame buffer 202 by adder 212. This
proceeds through the entire frame or FOV until all spots have been
scanned and their corresponding frame buffer values modified. The
process is then repeated for a second frame, a third frame, etc.
until all spot residuals have converged. In some embodiments and
particularly those represented by FIG. 4a, the pattern in the frame
buffer represents the inverse of the real-world image in the FOV at
this point, akin to the way a photographic negative represents the
inverse of its corresponding real-world image.
[0078] Inverter 208, optional intra frame processor 210, and adder
212 comprise leveling circuit 213.
[0079] The pattern in the frame buffer 202 is read out and
inverted, by inverter 214. The inverted pattern may be subjected to
optional inter-frame image processing by optional inter-frame image
processor 216 and then output to a display, to storage, to
additional processing, etc. by input/output 120.
[0080] Optional intra-frame image processor 210 includes line and
frame-based processing functions to manipulate and override imager
control. For instance, the processor 210 can set feedback gain and
offset to adapt numerically dissimilar illuminator controls and
detector outputs, can set gain to eliminate or limit diverging
tendencies of the system, and can also act to accelerate
convergence and extend system sensitivity. These latter aspects
will be discussed in more detail elsewhere. To ease understanding,
it will be assumed herein that detector and illuminator control
values are numerically similar, that is one level of detector
grayscale difference is equal to one level of illuminator output
difference.
[0081] As a result of the convergence of the apparatus of FIG. 2,
spots that scatter a small amount of signal back to the detector
become illuminated by a relatively high beam power while spots that
scatter a large amount of signal back to the detector become
illuminated with relatively low beam power. Upon convergence, the
overall light energy received at from each spot may be
substantially equal.
[0082] One cause of differences in apparent brightness is the light
absorbance properties of the material being illuminated. Another
cause of such differences is variation in distance from the
detector. Because of the inherently adaptive nature of the
illumination in the present system, greater depth-of-field often
results as a natural byproduct. Furthermore, such increased
depth-of-field may be realized with systems having lower
illuminator output power and lower power consumption than would be
possible otherwise. Because the amount of optical power
illuminating any one spot or region can be established at a
"correct" level, spots are not typically substantially
over-illuminated. Compared to other systems that must illuminate
all spots sufficiently to capture determinate energy from the
darkest spots of interest in the FOV, the present system may output
that relatively high amount of illumination energy only to those
specific darkest spots of interest, other spots with higher
apparent brightness receiving lower illumination energy.
Furthermore, illumination output energy is frequently limited by
comfort and/or safety requirements. In the case of bar code
scanners for instance, laser safety requirements frequently limit
output to Class II or lower for general-purpose scanners and Class
IIIa or lower for specialized, long-range scanners. Because such
safety regulations typically rely on measurements of incident
energy integrated over a relatively large spot corresponding to the
pupil size of the human eye and over a relatively long period of
time, a system that limits illumination energy both spatially and
temporally stands to have an advantage in achieving a numerically
lower, nominally safer classification. Therefore, in some
applications, the present system may achieve greater scan range at
a more restrictive safety classification than prior art
scanners.
[0083] Optional intra-frame image processor 210 and/or optional
inter-frame image processor 216 may cooperate to ensure compliance
with a desired safety classification or other brightness limits.
This may be implemented for instance by system logic or hardware
that limits the sum total energy value for any localized group of
spots corresponding to a range of pixel illumination values in the
frame buffer. Further logic may enable greater illumination power
of previously power-limited pixels during subsequent frames. In
fact, the system may selectively enable certain pixels to
illuminate with greater power (for a limited period of time) than
would otherwise be allowable given the safety classification of a
device. In this way, the system can probe distant and/or dark
regions of the FOV over multiple frames, acquiring grayscale values
for such spots without exceeding desired power limits.
[0084] While the components of the apparatus of FIG. 2 are shown as
discrete objects, their functions may be split or combined as
appropriate for the application. In particular, inverters 208 and
214, intraframe processor 210, adder 212, and interframe processor
216 may be integrated in a number of appropriate configurations
[0085] The effect of the apparatus of FIG. 2 may be more
effectively visualized by referring to FIGS. 3 and 4a-4c. FIG. 3
illustrates a state corresponding to an exemplary initial state of
frame buffer 202. A beam of light 110 produced by a variable
illuminator 109 is shown in three positions 110a, 110b, and 110c,
each illuminating three corresponding spots 112a, 112b, and 112c,
respectively. Spot 112a is shown having a relatively low apparent
brightness, spot 112b has a medium apparent brightness, and spot
112c has a relatively high apparent brightness, as indicated by the
dark gray, medium gray and light gray shading, respectively.
[0086] In an initial state corresponding to FIG. 3, the
illuminating beam 110 may, for example, be powered at a medium
energy at all locations, illustrated by the medium dashed lines
impinging upon spots 112a, 112b, and 112c. In this case, dark spot
112a, medium spot 112b, and light spot 112c return low strength
scattered signal 114a, medium strength scattered signal 114b, and
high strength scattered signal 114c, respectively to detector 116.
Low strength scattered signal 114a is indicated by the small dashed
line, medium strength scattered signal 14b is indicated by the
medium dashed line, and high strength scattered signal 114c is
indicated by the solid line.
[0087] FIG. 4a illustrates a case where the frame buffer 202 has
been converged to a flat-field response. After such convergence,
light beam 110 produced by variable illuminator 109 is powered at
level inverse to the apparent brightness of each spot 112 it
impinges upon. In particular, dark spot 112a is illuminated with a
relatively powerful illuminating beam 110a, resulting in medium
strength scattered signal 114a being returned to detector 116.
Medium spot 112b is illuminated with medium power illuminating beam
110b, resulting in medium strength scattered signal 114b being
returned to detector 116. Light spot 112c is illuminated with
relatively low power illuminating beam 110c, resulting in medium
strength scattered signal 114c being returned to detector 116. In
the case of FIG. 4a, image information is no longer determined
primarily by the strength of the signals being returned to the
detector, but rather by the power of the beams used to illuminate
the FOV.
[0088] It is possible and in some cases preferable not to
illuminate the FOV such that all spots return substantially the
same energy to the detector. For example, it may be preferable to
compress the returned signals somewhat to preserve the relative
strengths of the scattered signals, but move them up or down as
needed to fall within the dynamic range of detector 116. FIG. 4b
illustrates this variant of operation. In this case, the
illumination beam 110 is modulated in intensity by variable
illuminator 109. Beam position 110a is increased in power somewhat
in order to raise the power of scattered signal 114a to fall above
the detection floor of detector 116 but still result in scattered
signal 114a remaining below the strength of other signals 114b
scattered by spots 112b having higher apparent brightness. The
detection floor may correspond for example to quantum efficiency
limits, photon shot noise limits, electrical noise limits, or other
selected limits. Conversely, apparently bright spot 112c is
illuminated with the beam at position 110c, decreased in power
somewhat in order to lower the power of scattered signal 114c to
fall below the detection ceiling of detector 116, but still remain
higher in strength than other scattered signals 114b returned from
other spots 112b with lower apparent brightness. The detection
ceiling of detector 116 may be related for instance to full well
capacity for integrating detectors such as CCD or CMOS arrays,
non-linear portions of A/D converters associated with non-pixelated
detectors such as PIN diodes, or other selected limits set by the
designer. Of course, illuminating beam powers corresponding to
other spots having scattered signals that do fall within detector
limits may be similarly modified in linear or non-linear manners
depending upon the requirements of the application. For instance,
in some applications, the apparent brightness range of spots may be
compressed to fit the dynamic range of the detector, spots far from
a mean level receiving a lot of compensation and spots near the
mean receiving only a little compensation, thus producing an image
with lots of grayscale information. Conversely, in applications
where it is desirable to maximize gamma and maximize contrast such
as bar code or text reading, it may be desirable to choose
illumination energy based on a global or local threshold algorithm
that tends to force scattered signal strengths one way or the other
toward the low or high limits of the detector.
[0089] FIG. 4c is a diagram showing a variant of FIG. 4b where the
dynamic illumination created by variable illuminator 109 is used to
augment static illumination produced by static illuminator 402. In
FIG. 4c, static illuminator 402 illuminates spots 112a, 112b, and
112c with some power via light rays 404a, 404b, and 404c,
respectively. The power of rays 404 may be a static value,
user-adjustable, or may be determined using known exposure
techniques for artificially lit or flash photography. While
nominally of equal power in this example, rays 404a, 404b, and 404c
may be of non-equal power due to system limitations or for purpose.
The particular powers of rays 404a, 404b, and 404c are generally
not addressable in linearly independent ways. In the present
example, it is assumed that the power of static illumination rays
404 has been set to optimize the illumination of the spot with
highest apparent brightness, 112c. Accordingly, dark spot 112a
returns a low strength scattered signal 406a to detector 116,
medium spot 112b returns a medium strength scattered signal 406b to
detector 116, and light spot 112c returns a high strength scattered
signal 406c to detector 116. In the present example, it is assumed
that scattered signal 406a and 406b are too weak to readily
distinguish from one another and preserve image detail. That is
they fall below or too close to the sensitivity floor of detector
116.
[0090] FOV 111 is also illuminated by dynamic illumination from
variable illuminator 109. After convergence, beam 110 is powered at
a level inverse to the apparent brightness of each spot 112 it
strikes, taking into account the illumination power provided by
static illuminator 402. In particular, dark spot 112a is
illuminated with a relatively powerful illuminating beam 110a,
resulting in medium strength scattered signal 114a augmenting low
strength signal 406a from static illuminator 402 being returned to
detector 116. Medium spot 112b is illuminated with a relatively
weak illuminating beam 110b, resulting in medium strength scattered
signal 114b being returned to detector 116, augmenting low strength
signal 406b. Light spot 112c is not illuminated because signal 406c
is of sufficient strength to place the apparent brightness of spot
112c near the upper limit of detector 116 dynamic range.
[0091] FIGS. 5a and 5b illustrate how scenarios 3 and 4a through 4c
might work in a typical real-world scene. Referring to FIG. 5a, an
FOV 111 includes a foreground object 112c, a middle ground object
112b, and a background object 112c. Object 112c has a high apparent
brightness corresponding roughly to spots 112c in FIGS. 3-4c. This
high apparent brightness may be the result of lighter coloration
than other objects in the FOV or closer distance than other
objects, or both, as in this example case. Similarly, the middle
ground object 112b has a somewhat lower apparent brightness while
the background object 112a has an apparent brightness that is lower
yet. In addition, a far background field 502, has a very low
apparent brightness. One noticeable difference between FIGS. 3-4b
and FIG. 5a is that while regions or spots 112 in FIGS. 3-4b have
little or no difference in apparent brightness within them while
spots 112 in FIG. 5a have discernable value differences within
them.
[0092] Referring to FIG. 5b, brightness ranges for the various
spots or regions of FIG. 5a are indicated. Scale 504 indicates
apparent brightness of regions 112a-112b and 502, which may be
expressed as irradiance (Watts/cm**2), illuminance (lumens) or
other selected units. Lower dynamic range limit 506 and upper
dynamic range limit 508 define the dynamic range of an arbitrary
detector.
[0093] Range brackets 510c, 510b, 510a and 510d indicate the
apparent brightness of regions 112c, 112b, 112a, and 502,
respectively, for FOV 111 with no illumination. As shown in FIG.
5b, while highlights of region 112c are detectable, shadows are
lost in darkness. All but the brightest highlights of the middle
ground object 112b are undetectable and the background object 112a
is lost entirely. Far background 502 is undetectable.
[0094] Range brackets 512c, 512b, 512a, and 512d indicate the
apparent brightness of regions 112c, 112b, 112a, and 502,
respectively, under static illumination. This may correspond, for
instance, to prior art flash photography. Note the typical feature
of many flash photographs where the dynamic range 512c of the
foreground object 112c is "washed out" with its highlights
exceeding the dynamic range maximum of the detector. The middle
ground object 112b is illuminated with substantially the same power
as the foreground object but because of its greater distance does
not increase in apparent brightness as much as the foreground
object and thus remains largely in the shadows. The background
object 112a receives even less illumination because of its still
greater distance and remains undetectable. Far background 502
remains undetectable.
[0095] Bar 514 indicates an idealized depiction of a scene
illuminated as in FIG. 4a. Upon convergence, all parts of the FOV,
with the exception of far background 502, are illuminated inversely
to their apparent brightness, resulting in all spots scattering the
same light back to the detector. Because of the very great range
and lack of backscatter from far background 502, it does not rise
in brightness in any substantial Way. In this case, image
information is retained by the illuminator frame buffer, consistent
with the resolution of the scanned spot.
[0096] It may be noted that the vertical position of bar 514 is
coincident with or slightly above the upper limit of the brightest
spot in the FOV to be imaged, in this case the brightest highlight
of the foreground object 112c corresponding to the upper end of
range 510c. This ensures all spots in the FOV have determinate
values and thus may be assigned grayscale values. In other
applications, it may be preferable to leave certain highlights
indeterminate. This may be especially true in the case of a
substantially bimodal image such as a bar code symbol. In this
case, it may be desirable to set the maximum illuminator power at
or near the edge threshold of the bars for instance, leaving the
white background indeterminate and effectively filtering out any
noise therein. Other examples of when it may be desirable to leave
certain bright spots indeterminate include when illuminator power
is simply not sufficient to overcome their ambient brightness.
[0097] Ranges 516c, 516b, 516a, and 516d correspond to regions
112c, 112b, 112a, and 502, respectively for cases corresponding to
FIGS. 4b and 4c. In the case of 4b, the foreground object region
112c receives a small amount of illumination, enough to raise its
dynamic range 516c to a level appropriate for it and other scene
content. This may be chosen by moving the brightest pixel close to
the ceiling of the photodetector, which may for instance correspond
to the full well capacity of a CCD detector array, or it may be
chosen according to other programmed modes akin to programmed modes
used in digital and film cameras. The middle ground object 112b is
illuminated moderately to raise its dynamic range from range 510b
to 516b and the background object 112a is illuminated significantly
to raise its dynamic range from 510a to 516a. In this example, all
regions that can be illuminated, i.e. all regions except region
502, have been dynamically illuminated sufficiently to bring them
into the dynamic range of the detector while maintaining
significant contrast within that dynamic range.
[0098] For the case corresponding to FIG. 4c, the entire FOV has
been illuminated by a static illuminator in an amount appropriate
to brighten the foreground object 112c to a desired level. A
dynamic illuminator may optionally illuminate darker regions of the
foreground object to compress its darker regions toward its
highlights. The dynamic illuminator has added moderate illumination
power to the middle ground object 112b and rather significant
illumination power to the background region 112a to raise the
dynamic ranges of those regions to correspond to ranges 516b and
516a, respectively.
[0099] For this example, the mode logic may have been used to
determined it optimum to compress all three regions to fit within
the dynamic range of the detector. For this example, logic has
selected lower static illumination power to keep the foreground
object highlights in dynamic range. This was partially enabled by
the fact that the differential illuminator would sufficiently
illuminate the middle ground and background objects, thus reducing
the need to over expose the foreground object in the interest of
retaining some background detail.
[0100] For the cases corresponding to both FIG. 4b and 4c, the far
background 502 has remained substantially unchanged, having dynamic
range 516d. In some embodiments, these pixels may be synthetically
loaded according to scene content, user preference, prior or later
image capture, etc.
[0101] FIG. 6a is an example of how reflectance values for several
spots along a linear scan path might be converged to a
substantially constant reflectance value with grayscale values
being retained as the inverse of the illumination beam power
profile for the scan path. A FOV 111 comprises a scan path 112
having a plurality of spots. The plurality of spots in scan path
112 correspond to several apparent brightness levels including
white spots 602, light gray spots 604, medium gray spots 606 and
black spots 608. Shown below FOV 111 are several vertically aligned
waveforms. Waveform 610 illustrates the illuminator power
corresponding to scan path 112. In this example, the illuminator
power is held constant for the first scan at a level of 00 out of a
possible 7 binary values ranging from -11 to +11.
[0102] Waveform 611 is an idealized response from a detector having
dynamic range limited to three states: 00 (nominal), .gtoreq.+01,
and .ltoreq.-01. It ignores optical effects such as Gaussian
distortion and assumes gain equivalent to illuminator gain --i.e.
.+-.01 detector units correspond to .+-.01 illuminator units. In
waveform 611, a 00 strength beam swamps the detector when scattered
from white spots 602. This is seen by detector values 612 at the
high rail (.gtoreq.+01) in locations corresponding to white spots
602. Conversely, a 00 strength beam reflected from medium gray
spots 606 and from black spots 608 results in an undetectable
response of .ltoreq.-01 in waveform locations 616 corresponding to
spots 606 and 608. Light gray spots 604 scatter a medium energy
signal corresponding to 00 detector response levels 614.
[0103] In accordance with the process of FIG. 2, detector waveform
611 is inverted and added to illuminator waveform 609 to produce
new illuminator waveform 621. Because initial illuminator waveform
609 was constant, illuminator waveform 621 is simply the inverse of
detector waveform 611, with low -01 power regions 622 corresponding
to high detected energy regions 612, medium 00 power regions 624
corresponding to medium detected energy regions 614, and high +01
power regions 626 corresponding to low detected energy regions
616.
[0104] Beam 112 is then scanned across FOV 111 again using
illuminator power waveform 621 which may, for instance, be
implemented in the form of a frame buffer. Detector waveform 631
results from the second pass of beam 112. This time, medium gray
spots 606 have joined light gray spots 604 in falling within the
dynamic range of the detector, but there are still spots that fall
outside the range of the detector. Detector waveform 631 is
inverted and added to previous illuminator waveform 621 to produce
third pass illuminator waveform 641 comprising power levels 642 of
-10 corresponding to white spots 112, levels 644 of 00
corresponding to light gray spots 604, levels 646 of +01
corresponding to medium gray spots 606, and levels 648 of +11
corresponding to black spots 608. Beam 112 is finally scanned
across FOV 111 using illuminator power waveform 641. Resulting
detector power waveform 651 is constant and within the dynamic
range 613 of the detector, indicating complete convergence. Thus
the inverse of illuminator power waveform 641 has become an image
of linear scan path 112 across FOV 111. Consequently, by comparing
spots against scale 610, we can see that white spots 602 have a
grayscale value of +10, light gray spots 604 have a grayscale value
of 00, medium gray spots 606 have a grayscale value of -01, and
black spots 608 have a grayscale value of -10.
[0105] As can be seen, the system can record an image having a
dynamic range greater than that of the detector. In the example of
FIG. 6a, the image was determined to have a grayscale range of 5
levels (-10 to +10) whereas the detector had only one determinate
grayscale level.
[0106] The flowchart of FIG. 6b shows logic for an embodiment of
illuminator power adjustment. In step 662, the frame buffer is
initialized. In some embodiments, the buffer values may be set to
fixed initial values near the middle, lower end, or upper end of
the power range. Alternatively, the buffer may be set to a
quasi-random pattern designed to test a range of values. In yet
other embodiments, the buffer values may be informed by previous
pixels in the current frame, some approaches being described in
FIGS. 8 and 9. In still other embodiments, the buffer values may be
informed by previous frames or previous images.
[0107] Using the initial frame buffer value, a spot is illuminated
and its scattered light detected as per steps 664 and 666,
respectively. If the detected signal is too strong per decision
step 668, illumination power is reduced per step 670 and the
process repeated starting with steps 664 and 666. If the detected
signal is not too strong, it is tested to see if it is too low per
step 672. If it is too low, illuminator power is adjusted upward
per step 674 and the process repeated starting with steps 664 and
666.
[0108] Thresholds for steps 668 and 672 may be set in many ways.
For detectors that are integrating, such as a CCD detector for
instance, the lower threshold may be set at noise equivalent power
(NEP) (corresponding to photon shot noise or electronic shot noise,
for example) and the upper threshold set at full well capacity.
Instantaneous detectors such as photodiodes may be limited by
non-linear response at the upper end and limited by NEP at the
lower end. Thus these points may be used as thresholds for step 668
and step 672, respectively. Alternatively, upper and lower
thresholds may be programmable depending upon image attributes,
application, user preferences, illumination power range, electrical
power saving mode, etc.
[0109] Additionally, upper and lower thresholds used by steps 668
and 672 may be variable across the FOV. For instance, when the
apparatus is used as a dynamic range compressor as illustrated by
FIGS. 4b, 4c, 5a, and 5b, illuminator energy for a given spot may
be selected according to the range of illumination energies and/or
detected scatter from the range of relevant spots across the FOV.
For instance, whereas a medium gray spot 112b may require only a
little illumination power to raise its scatter or reflectance up
above the minimum level required for detection in the absence of
additional, darker spots; the presence of additional darker spots
112a may dictate a somewhat higher step 672 minimum threshold for
that spot in order to raise its apparent brightness high enough in
the detector dynamic range to make room for additional, darker
spots to also fall within that dynamic range. This is illustrated
in the relative positions of ranges 516b and 516a, respectively in
FIG. 5b.
[0110] As described above, the process of FIG. 6b may be used to
achieve determinateness, that is, illuminate many or all pixels at
levels sufficient to fall within the range of the detector. In
other embodiments, the levels used in steps 668 and 672 may be set
at a single value, such as the midpoint of detector sensitivity for
example. This could be used to achieve complete convergence.
[0111] After a scattered signal has been received that falls into
the allowable detector range, the detector value may be inverted
per optional step 676 and transmitted for further processing,
storage, or display in optional step 678. Steps 676 and 678 are
identified as generally optional depending upon the
application.
[0112] For applications involving scanned beam imaging and when the
illuminator power itself contains a significant portion of pixel
information, pixel illuminator power (or the inverted value of
pixel illuminator power) may be transmitted. On the other hand,
when the range between upper and lower thresholds is large (for
steps 668 and 672, respectively), illuminator power may be used
essentially to compensate for relatively large-scale differences
across the FOV, with most pixel information being retained in the
detector value. This may be used, for instance, when illuminator
power modulation is used to compensate for overall FOV
reflectivity, range, transmissivity, or other effect that modifies
the signal in a gross sense. For some applications, most or all of
the useful image information may then be determined by the detector
and illuminator power omitted from further processing.
[0113] In yet another type of application, illuminator power may be
used to define other programmable parameters during further
processing. In some applications, high illuminator power may
indicate greater imaging range. For the case of a bar code or other
decodable indicia imager for instance, illuminator power may thus
be used to set a band pass frequency for image decoding. In this
case, higher power implies longer range, longer range implies a
smaller apparent mean feature size (owing to the multiplicative
effect of scan angle with distance), and a smaller apparent mean
feature size means the decoder should be looking for smaller,
higher frequency features when performing a decode. When the
illuminator power is used in this way, signal inversion step 676 is
optional.
[0114] In addition to or, as illustrated above, instead of
transmitting illuminator power for further operations, the detector
value may be transmitted as in optional step 680. For the case of a
bar code or other decodable indicia imager as described immediately
above, the detector may contain most or all of the important image
information for decode. In other applications and particularly
those where the detector dynamic range is very limited, there may
be very little effective image information in the detector value
resulting from the selected illuminator power and transmission of
the detector value may be omitted.
[0115] In still other applications significant useful portions of
the image data may be present in both the illuminator power and the
detector value. An example of this type of application is an
endoscope where illuminator power is used to extend the working
range of the device and most of the image information is present in
the detector value, but a few bits of apparent pixel brightness
information retained by illuminator power act as the most
significant bits of the pixel value.
[0116] In other applications, and particularly those corresponding
to FIGS. 4b and 4c, illumination power and resultant detected light
may not be transmitted at all, the apparatus having achieved its
goal simply by appropriately illuminating the FOV. This type of
operation may be especially useful when the illuminator is used as
a "flash apparatus" or to augment a flash apparatus for array
imaging technologies such as digital or film photography.
[0117] As will be explained later, several aspects of the methods
illustrated by foregoing FIGS. 2 through 6b do not necessarily
require a scanned beam to selectively illuminate, but rather may
use other forms of selective illuminators such as an illuminator
array or illumination attenuator array, for example.
[0118] Two possible side effects of the system described herein are
losses in temporal or spatial resolution. That is, during the time
spent converging the image, any movement in the image relative to
the scanner can necessitate the need to re-converge (increasing
latency) and/or can result in indeterminate spot values
(effectively decreasing spatial resolution) corresponding to edges
having high contrast relative to detector dynamic range. One
approach to overcome- this issue is to increase frame rate and/or
spatial resolution sufficiently to make any indeterminate spots so
fleeting or small as to render them insignificant. This approach
may be used to special advantage in some cases with linear CCD/CMOS
imagers or linear laser scanners, either of which can deliver
refresh rates up to several hundred or even several thousand frames
per second at high effective resolution. Another technique may be
understood by referring back to FIG. 2, where optional intra-frame
image processor 210 and optional inter-frame image processor 216
may cooperate to speed convergence.
[0119] As indicated above, optional intra-frame image processor 210
includes line and frame-based processing functions to manipulate
and override imager control and can accelerate convergence and
extend system sensitivity. Specifically, to control source power
levels, optional intra-frame image processor 210 may load grayscale
values into the frame buffer to override values that would normally
be loaded by inverted residual addition. The intra-frame image
processor 210 can also load values to other pixels in the frame
buffer beyond the currently processed pixel.
[0120] FIGS. 7, 8, and 9 illustrate methods used by optional
intra-frame image processor 210 and optional inter-frame image
processor 216 to increase the rate or decrease the time for
convergence with FIG. 7 showing operation corresponding to one
frame of the process of FIG. 6a. FIG. 7 shows two neighboring scan
lines 112a and 112b across 2D FOV 111. In this example, scan line
112a is a left-to-right scan line while scan line 112b is a
right-to-left scan line. FOV 111 comprises three regions; a medium
gray region 606 abutted on each edge by light gray regions 604a and
604b across which scan lines 112a and 112b pass. Superimposed over
the scan lines are individual pixels 702 and 704. Only a few of the
pixels are shown for clarity. The areas of interest for this
discussion are the few pixels in each scan line corresponding to
the transitions from light gray to medium gray and back again. The
shading of the pixels indicates the calculated or indeterminate
gray values determined by the scanned beam imager. For this
discussion, it is assumed that illuminator power for scan lines
112a and 112b is initially set at constant value 609.
[0121] Comparing FIG. 7 to FIG. 6a, pixels 702a, 702b, 702c, and
702d corresponding to light gray regions 604 are determinate on the
first pass as were gray level 00 regions in FIG. 6a. Thus, pixels
702 are illustrated as light gray equal to the actual gray level of
the corresponding FOV spots. As scan line 112a proceeds from left
to right across the transition from region 604a to region 606,
pixels 704a corresponding to the right side of the edge are
illustrated as black. This indicates their value to be
indeterminate. That is, the detector receives a signal below its
minimum sensitivity or floor so it is indeterminate if the actual
gray level of region 606 is a little darker than the minimum level
the detector will detect or much darker. Proceeding farther along
the scan line, all pixels corresponding to spots in region 606 are
indeterminate during the current frame (although, per FIG. 6a, the
illumination power would be reduced for those spots on the
subsequent frame and pixels 704a would then become determinate). As
the scan line 112a crosses the edge from region 606 to region 604b,
it again receives enough optical energy for the signal to be within
the range of the detector, and thus pixels 702b are determinate and
are shown shaded light gray in correspondence with the shading of
spots within region 604b. The situation is repeated on subsequent
right-to-left scan line 112b, with pixels corresponding to regions
604a and 604b being determinate and pixels corresponding to region
606 indeterminate (dark).
[0122] FIG. 8 illustrates a technique for achieving faster
convergence for some spots. The technique of FIG. 8 results in some
indeterminate (dark) pixels becoming determinate prior to the
subsequent frame. A side effect is that it may create some other
indeterminate (light) pixels. The particular signs, light vs. dark,
of the additional indeterminate pixels are not significant for this
example as they are functions of the particular example of FIG. 8.
As in FIG. 7, scan line 112a produces determinate light gray pixels
702a corresponding to spots in region 604a. As before, pixel values
become indeterminate pixels 704a after crossing the edge from
region 604a to region 606. This time, however, an adaptive
illuminator power is used to regain determinism while the scan path
is still within region 606. After one or more pixel values become
indeterminate (dark), illuminator power is increased until detected
energy again rises above the lower limit of the detector, thus
producing determinate medium gray pixels 802a. As the scan line
crosses the edge from region 606 to 604b, subsequent pixels 804a
are indeterminate (light). This may be caused by the illuminator
power being set at a level appropriate to darker region 606,
resulting in excess signal from lighter region 604b swamping the
detector. In a manner analogous to what happened after the scan
path crossed the edge from region 604a to 606, illuminator power is
decreased until reflected energy is again within the dynamic range
of the detector, resulting in determinate light gray pixels 702b.
This process is repeated during subsequent scan 112b.
[0123] From inspection of FIG. 8, it can be seen that three
indeterminate pixels were produced after an edge of excessive
dynamic range was crossed. Thus, in this example, the logic of
optional intra-frame image processor 210 required three successive
indeterminate (dark) or indeterminate (light) pixels be acquired
before resetting the illumination power higher or lower,
respectively. Setting a relatively large number of indeterminate
pixel acquisitions of the same sign prior to illuminator power
adjustment may be useful when detector dynamic range is small
relative to FOV dynamic range and/or when relatively high
frequency, small features relative to imager addressability are
present in the FOV, for example. This can reduce any tendency for
the acceleration process to induce instability. A smaller number of
indeterminate pixel acquisitions may be more appropriate when
features are larger or when the dynamic range of the detector is
greater. A further refinement automatically sets the gain of the
convergence accelerator based upon observed and/or historical FOV
attributes such as apparent feature size distribution and apparent
dynamic range.
[0124] The illuminator power adjustment step size may be a function
of detector dynamic range and the convergence algorithm. For
instance, it may be preferable for the initial illuminator
adjustment to be no greater than the dynamic range of the detector.
Alternatively, it may be advantageous to take larger steps to speed
intra-frame convergence. Numerous search algorithms are known and
may be applied.
[0125] For the case where detector dynamic range is relatively
large compared to the apparent dynamic range of the FOV, it may be
advantageous to dynamically adjust the illuminator power to keep
the scattered signal centered within the dynamic range of the
detector. This can increase the system's immunity to loss of
convergence when crossing edges.
[0126] As an alternative to selecting an initial illuminator power
to a constant value, an initial power pattern, for instance
embodied as a bitmap in a frame buffer, having variable output may
be employed. Especially when detector dynamic range is very limited
this may help to speed convergence in scenes having generally large
features. This may be thought of as a pre-loaded search algorithm
comprising illuminator power diversification.
[0127] FIG. 9 illustrates a method for accelerating convergence
that overcomes the side effect of the additional indeterminate
(light) pixels 804a and 804b of FIG. 8. The technique of FIG. 9
makes use of a characteristic of many images that neighboring spots
within given regions tend to have similar grayscale values. In
particular, spots along one side of an edge tend to have grayscale
values similar to neighboring spots along the same side of the
edge. Along the opposite side of the edge, the converse is true.
Therefore, it is reasonable to use the determinate light gray value
of pixels 702a as reasonable guesses of the indeterminate values of
pixels 804b. Similarly, grayscale values of pixels 802b may be
substituted for indeterminate values of pixels 704a, determinate
values of pixels 802a for indeterminate pixels 704b, and
determinate values of pixels 702c for indeterminate pixels 804a.
FIG. 9 illustrates this approach as arrows pointing from
determinate pixels to their associated indeterminate pixels. This
procedure may be carried out after scans 112a and 112b to fill in
unknown values and create a pseudo-converged image to be verified
during the subsequent frame. A similar procedure may also be
carried out a priori, using the illumination map of one scan line
as the starting point for the illumination map of the subsequent
line. Over a period of lines, edges begin to emerge, further
informing the image processor(s) of likely values for
yet-to-be-scanned pixels in the frame. Edge finding and other
applicable algorithms are known to those having skill in the art of
image processing and may be applied as is advisable for the
application.
[0128] Referring back to the FIG. 2 discussion of probing dark
and/or distant spots in conjunction with the foregoing discussion
of FIG. 9, a way to improve convergence time of such distant spots
may be seen. Because surrounding pixels have a reasonable
probability of similar gray values, the system can determine a
reasonable initial set of pixel values for rapid convergence by
applying probe bursts sparsely across a region, and selecting
intervening pixel values by interpolation between determinate
values. Over a period of several frames, the system may eventually
probe all pixels in dark regions to provide complete FOV grayscale
information not otherwise obtainable. To prevent overexposure to
laser light, the rule set and burst approach is defined with
care.
[0129] Optional inter-frame image processor 216 performs
frame-based image processing and may be used to inform the system
of edge tracking and probing functions, as well as converting the
frame buffer values to values appropriate for display or further
processing. Optional inter-frame image processor 216 may include
image de-skewing to compensate for a moving FOV, white balance
compensation, gamma correction including grayscale expansion,
compression, or shifting, gamut correction including gamut
expansion, compression, or shifting), pixel interpolation,
suppression of non-valid pixel values, noise reduction, and
combining frame buffer and detector data.
[0130] Some of the optional inter-frame image processor 216
functions are based upon edge finding and tracking techniques such
as gradient or Sobel operators for edge finding and local
maximum/minimum feature extraction for tracking. These and other
techniques for edge finding and local maximum/minimum feature
extraction are known to those having skill in the art of image
processing. Also, as optional intra-frame image processor 210
operates, it may leave indeterminate values in the frame buffer.
Optional inter-frame image processor 216 can "scrub" these from the
output by tracking which pixels are indeterminate and optionally
combining this data with other FOV information.
[0131] When several edges have identical movement vectors, optional
inter-frame image processor 216 can infer overall FOV movement
relative to the system and calculate resulting skew and perform
de-skewing algorithms. This can be especially useful in automated
systems such as bar code readers and machine vision systems.
[0132] White balance processing can compensate for differences in
source efficiency or power as well as differences in detector
efficiency. Stored calibration values make this process fairly
straightforward. To simulate ambient illumination effects, optional
inter-frame image processor 216 may shift values to an effective
illumination color temperature.
[0133] Optional inter-frame image processor 216 may reduce noise
using noise correlation principles to distinguish between
variations in frame buffer data related to structure in the scene
and noise artifacts, and can apply a smoothing function to "clean
up" the image. Techniques for doing this are known to the art.
[0134] The foregoing has illustrated methods and apparatus for
variable FOV illumination using inter- and intra-frame convergence.
Another alternative exists in the form of post-processing pixel
selection. By this alternative means, a digital imager may capture
2 or more frames of a FOV, each at a different static illumination
energy and/or different detector sensitivity. Subsequently, pixels
are selected and extracted from the frames to create a resultant
combined frame having extended effective dynamic range. To do this,
brighter pixel values may be extracted from lower power
illumination and darker pixel values extracted from higher power
illumination, for instance. For cases where illumination powers or
detector sensitivities are known, relative illumination power or
relative sensitivity, respectively, may be used to determine
combined pixel values. In other cases, differences in the apparent
brightness from frame to frame of given pixels may be s used to
determine combined pixel values, using the assumption that a
particular pixel or the mean of a group of pixels has a constant
absolute reflectance or scattering coefficient, apparent
differences then being used to calculate relative illumination
powers or detector sensitivities.
[0135] FIG. 10 is a diagram illustrating a post-processing method
of combining several complete images 1002, 1004, 1006, and 1008,
each with a limited dynamic range, into a final image 1010 having
greater dynamic range. Image 1004 is captured using a high static
illumination energy and thus captures relatively dark region (or
spot) 1012, which may for instance comprise a house in the
background. Around the house is dark region 1014, which does not
have a high enough scattering coefficient to return a usable signal
to the detector. Also within image 1004 are three overexposed
regions 1016, 1018, and 1020, each of which returned a signal above
the upper limit of the detector dynamic range.
[0136] Image 1002 is captured using very high static illumination
energy that reveals regions 1014a and 1014b within dark region
1014. In this image, dark region 1014b returns sufficient signal to
reveal details that may for instance be surrounding landscape while
very dark region 1014a remains below the detection threshold. The
very high static illumination used to produce image 1002 has now
overexposed region 1012 to produce combined overexposed region 1022
(which consists of regions 1012, 1016, and 1020).
[0137] Image 1006 is captured using medium static illumination
energy and results in proper exposure of medium region 1016. Dark
region 1014 now encompasses region 1012 as well. Depending upon the
method of varying the apparent static illumination energy, image
1006 may also capture region 1018 with proper exposure. In addition
to or as an alternative to changing the actual amount of
illumination energy, the apparent illumination energy may also be
varied by changing the light sensitivity of the detector, for
instance by changing the effective shutter speed, changing the
effective aperture, adding a filter over the detector, or changing
the intrinsic response for instance by varying electrical bias or
amplification. The methods described herein are also applicable to
that technique.
[0138] Finally, image 1008 is captured using low static
illumination energy, thus capturing region 1020 with proper
exposure. All other features have faded into dark region 1014.
[0139] After, or in parallel to, capture of images 1002, 1004,
1006, and 1008, appropriately exposed regions from each may be
combined to produce resultant image 1010. In this image, regions
1020, 1016 (and 1018), 1012, 1014b, and 1014a are shown as
progressively darker. That is, the apparent brightness of each has
been shifted to account for the differences in static illumination
energy between the frames in which they were captured, thus
producing a resultant image 1010 with greater dynamic range than
any of the source images 1002-1008. Such a large dynamic range
image may be compressed to meet the dynamic range limitations of an
output device such as a display, printer, or image processor using
known techniques.
[0140] Other applications may work better if the relative
brightness of each of the frames is not shifted when combining, but
rather the resultant image is comprised of regions each having
substantially the same dynamic range. Alternative resultant image
1022 illustrates this mode where each of the regions 1014b, 1012,
1016 (and 1018), and 1020, captured in frames 1002, 1004, 1006, and
1008, respectively, is combined without shifting. Thus, the
apparent dynamic range of image 1022 is no greater than any of its
constituents, but regions are included that have different actual
apparent brightness that exceed the dynamic range of the image. One
example where this mode may be preferable is in indicia imaging
such as bar code scanning.
[0141] Frames 1002 through 1008 may be captured sequentially or in
parallel. When capturing sequentially, a relatively high frame rate
is preferable to eliminate relative movement of regions between
frames. Additionally or alternatively, image processing software
may track the movement of regions between frames to s build a
resultant frame free of undesirable artifacts.
[0142] FIGS. 11-16 are isometric views of alternative imaging
engines or variable illuminators. FIG. 11 shows a scanned beam
imager or variable illuminator having a single non-pixelated
detector 1101. Chassis 1102 carries illuminator assembly 104.
Illuminator assembly 104 comprises an emitter 1103 that emits raw
beam 1104. Emitter 1103 may be a laser diode and in a preferred
embodiment, a monochrome laser diode with peak emission at
approximately 635 to 650 nm. Raw beam 1104 is shaped by beam optics
1106, which may for instance comprise a collimating lens and an
aperture, to produce first beam 106. First beam 106 is reflected by
scanning mirror 108, here shown as deflecting beam 106 in two axes,
as indicated by rotation angles 1108, to produce two dimensionally
scanned beam 110. Two positions of scanned beam 110 are shown as
beam 110a and 110b. Reflected or scattered beam 114 is collected by
optional collection optic 1110, which focuses scattered beam 114
onto detector 116, here shown as a photodiode.
[0143] Return beam 114 is shown as having width. This is indicative
of the gathering effect of optional collection optic 1110, which
serves to increase the subtended angle over which the scattered
light may be collected, increasing the numerical aperture and
therefore the intensity of the collected signal. It may thus be
seen that detector 116 is non-imaging in this embodiment. That is,
detector 116 is a staring detector that simply collects all light
scattered from the FOV. To improve the signal-to-noise ratio (SNR),
it may be advantageous for collection optic 1110 to include a
filter to exclude wavelengths not scanned, fluoresced, or otherwise
indicative of FOV response to scanned beam 110.
[0144] As an alternative or in addition to a staring detector 116,
scanned beam imager or variable illuminator 1101 may use confocal
or retro-collective collection of return beam 114. Confocal and
retro-collective collection schemas de-scan the return signal with
the scanning mirror or a synchronized scanning mirror, thus using
spatial filtering to ensure the return signal is dominated as much
as possible by the scanning spot. Confocal systems detect through
an aperture that is arranged confocally to the beam source, thus
detecting over a reduced DOF for maximum resolution.
Retro-collective systems collect light from around and on the
scanning spot, resulting in a balance between maximum signal
isolation and maximum DOF. Scanner 108 may alternatively be driven
in a single axis for 1D imaging such as a linear bar code scanner
for instance. Scanner 108 may alternatively be driven in three axes
or more for 3D imaging etc.
[0145] FIG. 12 shows a scanned beam imager or variable illuminator
having multiple non-pixelated detectors 116 and 116' with
associated optional collection optics 1110 and 1110', respectively.
Optional collection optic 1110 instantaneously gathers reflected
beam 114 and focuses it onto detector 116 while optional collection
optic 1110' instantaneously gathers reflected beam 114' and focuses
it onto detector 116'. FIG. 12 is one example of a scanned beam
imager or variable illuminator having multiple detectors wherein
the embodiment includes two detectors 116 and 116'. Other
embodiments may include three, four, or more detectors 116, 116',
116'', 116''', etc.
[0146] The scanned beam imager or variable illuminator of FIG. 12
may have one or more of several operative modes. In one mode,
detector 116' may be selected or driven to have a different gain
than detector 116, making detector 116' less sensitive than
detector 116. In this case, detector 116 may have sufficient
dynamic range to capture an entire signal for an FOV with high
apparent brightness, for instance a bar code symbol at a relatively
close distance such as 2 inches to 5 inches. Conversely, detector
116' may have sufficient dynamic range to capture an entire signal
for an FOV with low apparent brightness, such as a bar code symbol
at a relatively long distance such as 5 inches to 12 inches for
instance. In other applications where the dynamic range of each
detector is less than that necessary to capture the entire dynamic
range of an image, the detectors 116 and 116' may be used in
tandem, each capturing a portion of the dynamic range, wherein
their respective captured pixels may be combined as per the method
of FIG. 10 to produce an entire output image 1010 or 1022. Thus the
apparatus pictured in FIG. 12 may collect frames in parallel
without any modulation of variable illuminator output, relying
instead on differing gains to produce a multiplicity of images for
selection of appropriate dynamic range.
[0147] In other applications, detectors 116 and 116' may have
overlapping dynamic ranges and may be used in tandem to drive
illuminator modulation to an optimum output level. In this
approach, the two detectors five ranges of detection; a first no
return signal range (indeterminate low) not detectable by either
detector, a second low return signal range detectable only by
sensitive detector 116, a third medium return signal range
detectable by both detectors 116 and 116', a fourth high return
signal range detectable by low sensitivity detector 116' and
swamping high sensitivity detector 116, and a fifth very high
return signal range that is indeterminate high because it swamps
both detector 116 and detector 116'. Spots scattering or reflecting
an illumination beam in the first or second range would be
increased in illumination power according to which range they fell
in, spots e returning a signal in the fourth or fifth range would
be decreased in illumination power according to which range they
fell in, and spots returning a signal in the third, medium range,
would be considered substantially converged and would retain their
illumination power.
[0148] Taken to one limit, the second, third, and fourth dynamic
ranges could represent very small differences corresponding to the
least significant bit, and thus illuminator power selection would
represent almost all of the image information.
[0149] In another embodiment, the magnitudes of the second, third,
and fourth ranges could be varied, for instance with the third
range smaller than the other two to help better drive the
illuminator to convergence while maintaining reasonable overall
dynamic range for rapid convergence. In this case a relatively
significant portion of overall dynamic range could be comprehended
by ranges two, three, and four.
[0150] In another embodiment, the range sizes could be quite large,
allowing operation with little or no beam power modulation. In this
mode, most of the image information would be determined by detector
responses.
[0151] In another mode, the apparatus of FIG. 12 could have
substantially identical detectors 116 and 116', but have
differences in collection optics 1110 and 1110'. For instance,
collection optic 1110' could include a filter to reduce signal
strength, thus attenuating the response of detector 116'.
Alternatively, collection optics 1110 and 1110' could have
different collecting powers, for instance by being different sizes,
collection optic 1110 being formed to have a higher numerical
aperture than collection optic 1110'. This approach would have a
similar effect to filtering collection optic 1110', resulting in
detector 116 having higher effective sensitivity than detector
116'. This mode could be operated according to the alternatives
described for modifying detector sensitivities.
[0152] Additionally, detectors 116 and 116' may be selectively
additive. This may be done electrically according to known methods.
Overall detection sensitivity may thus be varied linearly according
to the number of detectors in the array. For the case where
effective detector sensitivities are weighted, detectors may be
combined in selected combinations to create a broader number of
sensitivity ranges. For instance, if the relative sensitivities of
four detectors 116''', 116'', 116', and 116 is 1,2,4, and 7,
respectively; any relative sensitivity ranging from 1 to 14 may be
selected. In this case, selection of detector 116''' alone would
result in a relative sensitivity of 1, selection of detector 116''
would result in a relative sensitivity of 2, selection of detectors
116''' and 116'' would result in an additive relative sensitivity
of 3, etc. up to selection of all four detectors resulting in an
additive sensitivity of 14. Thus, many sensitivity ranges may be
synthesized from a relatively small number of detectors. Such
sensitivity ranges may be used to positive effect using methods and
apparatuses described elsewhere.
[0153] In yet another mode, collection optic 1110' may include a
filter making it insensitive to the emission wavelength or
wavelengths of illuminator 104. In this case, detector 116' may be
used to sense ambient light. Knowledge of the ambient light level
may be used for instance to select the power output range of
illuminator 104 and/or the sensitivity range of detector (or
detector array) 116. Thus, the optimal convergence power may be
selected to achieve a desired SNR for instance.
[0154] In another mode, collection optics 1110, 1110', 1110'',
etc.; detectors 116, 116', 116'', etc., or both may be filtered to
select response to a narrowed wavelength range. For instance, three
detectors may each be filtered to detect red, green, and blue,
respectively, for the purpose of imaging or variably illuminating a
FOV in RGB color. To enable this case, illuminator 104 may be
selected to create broad band substantially white illumination or a
tri-chromic (or higher) narrow band spectrum such as combined laser
emission spectra corresponding to the filter functions of each of
the RGB detectors 116, 116', and 116''. Additionally or
alternatively, detectors may be tuned to fluorescence or other
shifted spectral response created for instance by fluorescing down
converting or quantum well up-converting materials in the FOV.
[0155] In another mode, collection optics 1110, 1110', etc. may be
aimed to enhance collective sensitivity across the FOV. For
instance with a linear bar code scanner, collection optic 1110 may
be aimed to primarily collect light returned from the right side of
the FOV and collection optic 1110' aimed to primarily collect light
returned from the left side of the FOV. Frames may then be
combined, converged, etc. to effectively extend the FOV beyond that
normally achievable by normal collection optics.
[0156] FIG. 13 shows a beam scanner having a pixelated detector 116
to collect returned signals 114 through optional collection optic
1110. Reflected signals 114a and 114b are shown having limited
width to draw attention to the imaging nature of pixelated detector
116. Compared to the non-pixelated staring detectors of FIGS. 11
and 12, the pixels in pixelated detector 116 each detect a
conjugate location or spot in the FOV.
[0157] Pixelated detector 116 may comprise a CCD, CMOS, or other
array device. Pixelated detector 116 and optional collection optic
1110 are shown integrated into scan engine 101. Alternatively,
pixelated detector 116 and optional collection optic 1110 may
comprise a second body 1302, for instance a digital camera that is
logically coupled to variable illuminator 109. In this case the
image detected by the digital camera is fed back in its native or
in an inverted form to a controller controlling variable
illuminator 109 to update the illumination pattern as shown by the
apparatus of FIG. 2 and elsewhere in this document. This
arrangement of separate bodies is also shown in FIGS. 20 and
21.
[0158] FIG. 14 shows an imaging engine having a non-scanning
multiple beam source and a pixelated detector. Light source 1402
and optional lens 1404 collectively form variable illuminator 109.
Light source 1402 emits a plurality of beams 110, here shown as to
exemplary beams 110a and 110b, onto the FOV. Scattered rays 114a
and 114b are returned from the spots illuminated by beams 110a and
110b, respectively. The scattered rays are collected by collection
optic 1110 and focused onto detector array 116. The apparatus is
operated as indicated elsewhere.
[0159] Variable light source 1402 may comprise for instance an
array of individual emitters such as an LED array, a VCSEL array,
an array of incandescent sources, an array of gas discharge
sources, an array of fluorescent sources, or other emitter array
technology as may be appropriate. Illumination beams 110 may have a
one-to-one correspondence with scattered beams 114. Alternatively,
each illumination beam 110 may correspond to several scattered
beams 114. The former being operative to select the apparent
brightness of individual pixels and the latter being operative to
select regional illumination as may be determined appropriate by a
logical combination of the apparent brightness detected by the
individual cells in the detector array. In still another
embodiment, illumination beams 110 may be overlapping such that at
least some of scattered beams 114 are responsive to the power of
two or more neighboring illumination beams 110. As in FIG. 13,
pixelated detector 116 and optional collection optic 1110 may be
formed in a body separate from variable illuminator 109.
[0160] FIG. 15 shows a scanned beam variable illuminator 109. This
would correspond to the case described for FIG. 13 wherein the
detection system 1302 is removed to a separate body.
[0161] FIG. 16 shows an imaging engine having a variable
illuminator 109 comprising non-scanning light source with a
variable attenuator. In this case, variable illuminator 109 is
analogous to variable illuminator 109 shown in FIG. 14. Rather than
independently generating illumination beams 110 as in FIG. 14, a
non-variable light source such as a flood or spot illuminator
creates relatively broad area of illumination that is selectively
attenuated by variable attenuator 1606. In the example of FIG. 16,
non-variable light source 1605 is comprised of one or more emitters
1602 and optional reflector 1604 for directing the illumination in
the direction of the FOV. One or more emitters 1602 may comprise
for instance an incandescent bulb, a non-modulated scanning beam
such as a scanning laser beam, an LED array, a xenon or other
gas-discharge flash system, a gas discharge constant output source,
a fluorescent source, or other illumination source as may be
convenient and appropriate. The illumination created by
non-variable light source 1605 is selectively passed by the
attenuation cells 1608 of attenuator array 1606. Attenuator array
1606 may comprise for instance a transmissive liquid crystal array,
a thermo-optic or electro-optic array, a mechanical shutter array,
or other means for selectively transmitting light. As an
alternative to transmissive attenuation, variable illuminator 109
may be constructed to provide variable reflected illumination. In
this case attenuator array 1606 may for instance comprise a liquid
crystal on silicon (LCOS) array, a deformable mirror array, or
other means for variably reflecting light. As in FIG. 14, variable
illuminator 109 may emit illumination beams 110 that have
one-to-one or many-to-one correspondence to scattered beams
114.
[0162] FIG. 17 is a cross-sectional view of an illuminator that
combines variable illumination with non-variable illumination. The
example of FIG. 17 uses a transmissive attenuator to control the
variable power portion of the illuminator. Emitter 1602 emits first
non-variable rays 1702. Non-variable rays 1702 are non-variable in
the sense that they are not directly controlled to control
spot-by-spot, spatially variable illumination. Non-variable rays
1702 are, of course, controllable in terms of illumination duration
and/or overall power output.
[0163] The example of FIG. 17 includes a transmissive liquid
crystal-based variable attenuator 1606. First rays 1702 impinge
upon polarizer 1704 to produce polarized rays that pass through
liquid crystal layer 1706. Polarizer 1704 may for instance be a
reflective-type polarizer. Reflective polarizers often include
sub-wavelength spaced parallel conductors that allow the
polarization parallel to their orientation to pass while reflecting
the polarization component perpendicular to their orientation. An
example of such a polarizer is disclosed in U.S. Pat. No.
6,449,092, entitled REFLECTIVE POLARIZERS HAVING EXTENDED RED BAND
EDGE FOR REDUCED OFF AXIS COLOR, hereby incorporated by reference.
In some embodiments, reflected beams 1704 are reflected forward by
reflector 1604 to pass through diffuser 1716 to form non-variable
illumination 404. In other embodiments, polarizer layer 1704 or
reflector 1604 may include a polarization rotation medium such as a
quarter-wave plate to rotate an additional portion of light 1702
into the correct polarization for passage through polarizer 1704.
This arrangement effectively recycles a portion of rejected light
energy and increases the proportion of light 1702 that passes
through variable attenuator 1606 and decreasing the portion of
light emitted as non-variable illumination 404.
[0164] Liquid crystal layer 1706 includes individually controllable
cells 1608 that for instance affect the polarization direction of
light passed therethrough. After passing through liquid crystal
layer 1706, light impinges upon analyzer 1708. Depending upon the
control signals applied to individual cells 1608, light maybe
attenuated a little or may be attenuated significantly. For
example, non-variable light ray 1702a passes through cell 1608a,
which controls the light's polarization direction to pass a
significant proportion through analyzer 1708 to produce bright
variable illumination beam 1710a. In contrast, non-variable light
ray 1702b passes through cell 1608b. Cell 1608b selects the
polarization direction of the light to pass a small proportion
through analyzer 1708 to produce dim variable illumination beam
1710b.
[0165] Several variants of liquid crystal shutters are known to the
art including nematic, smectic, and ferromagnetic. In principal,
many types of liquid crystal devices may be used in the present
embodiment or in the embodiment of FIG. 16, the choice affecting
only details of illuminator fabrication rather than any basic
principles.
[0166] Optional output optic 1712 may alter the paths of variable
illumination beams 1710a and 1710b to produce illumination beams
110a and 110b, respectively. In the example of FIG. 17, variable
illumination beam 1710a and corresponding illumination beam 110a
are shown with long dashes, indicating relatively high power
arising from the minimal attenuation imparted by cell 1608a.
Correspondingly, variable illumination beam 1710b and associated
illumination beam 110b are shown with short dashes, indicating
relatively low power arising from the significant attenuation
imparted by cell 1608b.
[0167] FIG. 18 is a diagram shown in cross-section that illustrates
the converged illumination pattern created by a variable
illuminator 109 onto a FOV comprising alternating light and dark
regions. Light source 1602, here illustrated as an LED, shines
illumination through optional optical element 1802 through a
variable attenuator 1606 onto a FOV 111. FIG. 18 is shown in a
state it might reach after convergence. In this state, variable
attenuator 1606 comprises several non-attenuated portions 1608a,
1608a', 1608a'', and 1608a''' which pass relatively bright rays of
light 110a, 110a', 110a'', and 110a''', respectively on to dark
regions 112a, 112a', 112a'', and 112a''', respectively.
Inter-digitated between non-attenuated portions 1608a (here
intended to refer to the collective of non-attenuated portions) of
variable attenuator 1606 are attenuated portions 1608c, 1608c',
1608c'', and 1608c''' which form shadows (or pass attenuated rays
of light) 110c, 110c', 110c'', and 110c''', respectively on to
light regions 112c, 112c', 112c'', and 112c''', respectively. This
pattern of bright light and shadows results in relatively uniform
signal return to a detector.
[0168] It can be seen in FIG. 18 that the pattern of transmissive
and obscure portions 1608 in variable attenuator 1606 forms what is
essentially an inverse image of the FOV. By extension then, the
electrical or digital pattern in the electronics that drive regions
1608 also forms and inverse image of the FOV. For the case of a bar
code scanner, this electrical or digital pattern in the electronics
may be conveniently used as the basis for decode algorithms.
[0169] FIG. 19 is an isometric view of a scanned-beam imaging
engine 1101 in a hand-held imager 1902 such as one used for reading
1D and 2D bar code symbols, reading OCR, capturing signatures, and
other enterprise activities. In one mode, the scanned-beam imaging
engine 1101 includes a microcontroller that has an interface that
emulates a CCD or CMOS scan engine. This mode may be especially
useful to reduce time-to-market for applications involving
replacement of CCD or CCD image capture sections in existing
products. Another mode includes a native interface that may offer
higher performance features unique to a scanned beam imaging
engine. For instance, the first mode may enable constant
illumination while the second mode enables variable illumination
for extended depth-of-field and/or dynamic range.
[0170] FIG. 20 is a block diagram showing an embodiment where
illumination and detection are performed by separate modules or
separate bodies with limited or no intra-frame communication
therebetween. First module 2002 comprises frame buffer 202, which
drives variable illuminator 109 as per FIG. 2. Light scattered or
reflected by the FOV is detected by detector 116 in second module
2004. Detector 116 is coupled to communications interface 2006,
which returns detected pixel values from second module 2004 to
inter-frame processor 216 through inverter 214 in first module
2002. Interframe processor 216 combines the previous frame buffer
values with returned inverted detected values, for instance adding,
and reloads frame buffer 202 with updated values. The process of
illumination, detection, communication, inversion, and combining is
then repeated until the image is converged according to appropriate
rules. Appropriate rules for full convergence, dynamic range
compression, etc. are discussed elsewhere in this document. Either
first module 2002 or second module 2004 may include optional static
illuminator 402. First module 2002 and second module 2004 may be
separate physical bodies, separate electrical modules, separate
logical modules, or may be integrated.
[0171] FIG. 21 is an isometric view of an embodiment of FIG. 20
where second body 2004 is a camera: and first body 2002 is a
separable illuminator or "flash" unit. In this example, flash unit
2004 includes a first illuminator region 2102 for emitting variable
illumination and a second illuminator region 2104 for emitting
static illumination. In embodiments corresponding to the block
diagram of FIG. 20, the user depresses a shutter button 2101 on the
camera 2002. In accordance with exposure logic in the camera it may
choose to use flash. In this case flash unit 2004 illuminates the
scene and a frame is captured by the detector in camera 2002. The
frame is sent to the flash unit 2004, and the flash unit performs
logic illustrated by the block diagram of FIG. 20. If "modified
fill flash" or leveling is required, the camera captures a second
frame while static illumination and variable illumination are
operative. Optionally, the amount of static illumination may be
decreased from the first frame to allow for the additional
illumination and greater dynamic range of the variable illuminator.
The second frame is then sent to flash unit 2004 and the process
repeated until proper illumination balance is achieved.
[0172] Camera 2002 may be a still or a video camera. Additionally,
flash unit 2004 may be integrated into the camera body. Finally, it
may be desirable for the illuminator 2004 to have its own detection
system. This may be especially valuable for cameras used for sports
or other action photography where it is s desirable to have
intra-frame processing and convergence acceleration for faster
response.
[0173] FIG. 22 is an electrical block diagram showing one
embodiment according to the present invention, drawing particular
attention to an embodiment where the controller 118 is distributed
into microprocessor 118a and memory 118b blocks connected to one
another and to emitter 104, scanner 108, and detector 116 by buss
2202. Interface block 120 may for instance include wired and
wireless data interfaces; visible indicators, audio indicators,
tactile indicators, and/or displays; input means such as
temperature sensors, ambient light sensors, trigger, orientation
and/or location sensors, remote memory or storage, a keyboard,
mouse, microphone, and/or other devices for communicating
operatively relevant information.
[0174] Optional remote unit 2204 may be connected to interface 120
by wired or wireless means including a networked or Internet
connection. The nature of remote unit 2204 may be determined based
on application requirement. For example, a remote expert comprising
artificial intelligence and/or humans may be useful when the
embodiment involves image analysis such as medical imaging or
machine vision. A remote decoder may be used to advantage in a bar
code or OCR reading application. Multiple imagers may be networked
to one or a plurality of remote units.
[0175] FIG. 23 is a block diagram of an embodiment that uses a
synchronous illuminator and detector. Timer-controller 2301
controls the synchronization of the illuminator(s) 104 and
detector(s) 116. Functional implications of this and alternative
synchronous embodiments are illustrated in FIG. 24.
[0176] Embodiments related to FIG. 23 pulse the illuminator. The
detector is then "tuned" to the pulse rate of the illuminator. When
a detector 116 coupled with a circuit that makes it sensitive to a
signal modulated at the same frequency as the illuminator, the
system may be referred to as a synchronous detector 2305.
[0177] Timer-controller 2301 comprises an RF source 2302 that may
be controlled by controller 118. RF source 2302 modulates the
illuminator 104, which outputs a modulated beam 106 that is
deflected by scanner 108 to produce scanned beam 110. In some
embodiments, illuminator 104 is a red laser diode of the type
typically used in bar code scanners. Such bar code scanners may,
for example, use a red laser diode having a wavelength between 635
and 670 nanometers with a rated power output of 10 to 30
milliwatts.
[0178] Scanner 108 may be one or a combination of several types of
scanners capable of producing an appropriate scan rate. In some
embodiments, scanner 108 is a MEMS mirror.
[0179] Scanned beam 110 scans FOV 111 and is reflected or scattered
back as reflected beam 114 to synchronous detector 2305.
Synchronous detector 2305 is tuned to detect the pulse modulation
frequency of illuminator 104. Light may be collected by collection
optics (not shown). Collection may be made retro-collectively,
confocally, or may be made via staring optics. The staring optics
may use reflectors, lenses, filters, vignetting structures, or
combinations thereof. Non-imaging collection optics are described
for example in the book entitled "High Collection Nonimaging
Optics" by W. T. Welford and R. Winston, 1989, Academic Press.
[0180] One way to tune a detector 116 to a pulse modulation
frequency is to use lock-in amplifier 2306, which amplifies a
signal at one or more particular frequencies. Lock-in amplifier
2306 may include circuitry to convert the detected modulated signal
to base band or, alternatively, may pass a modulated signal to the
controller. The controller converts the signal into an image and
performs other necessary functions appropriate for the
application.
* * * * *