U.S. patent application number 10/914732 was filed with the patent office on 2005-01-13 for light source detection and categorization system for automatic vehicle exterior light control and method of manufacturing.
Invention is credited to Bechtel, Jon H., Bush, Gregory S., Ockerse, Harold C., Reese, Spencer D., Stam, Joseph S., Tuttle, Darin D..
Application Number | 20050007579 10/914732 |
Document ID | / |
Family ID | 31186769 |
Filed Date | 2005-01-13 |
United States Patent
Application |
20050007579 |
Kind Code |
A1 |
Stam, Joseph S. ; et
al. |
January 13, 2005 |
Light source detection and categorization system for automatic
vehicle exterior light control and method of manufacturing
Abstract
In at least one embodiment, optical systems include a lens
system assembly, a spectral filter material and a pixel array
configured such that small, distant light sources can be reliably
detected. In at least one embodiment, the optical systems provide
accurate measurement of the brightness of the detected light
sources and identification of the peak wavelength and dominant
wavelength of the detected light sources. In at least one
embodiment, the optical systems provide improved ability to
distinguish headlights of oncoming vehicles and taillights of
leading vehicles from one another, as well as from other light
sources.
Inventors: |
Stam, Joseph S.; (Holland,
MI) ; Bechtel, Jon H.; (Holland, MI) ; Reese,
Spencer D.; (Fort Wayne, IN) ; Tuttle, Darin D.;
(Byron Center, MI) ; Bush, Gregory S.; (Grand
Rapids, MI) ; Ockerse, Harold C.; (Holland,
MI) |
Correspondence
Address: |
BRIAN J. REES
GENTEX CORPORATION
600 NORTH CENTENNIAL STREET
ZEELAND
MI
49464
US
|
Family ID: |
31186769 |
Appl. No.: |
10/914732 |
Filed: |
August 9, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10914732 |
Aug 9, 2004 |
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10208142 |
Jul 30, 2002 |
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6774988 |
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Current U.S.
Class: |
356/218 |
Current CPC
Class: |
G01J 1/04 20130101; G01J
1/0403 20130101; G06K 9/00825 20130101; B60Q 2300/112 20130101;
B60Q 2300/45 20130101; B60Q 2300/42 20130101; G01J 1/32 20130101;
G06K 9/2018 20130101; G01J 1/42 20130101; B60Q 2300/314 20130101;
B60Q 1/1423 20130101; B60Q 2300/41 20130101 |
Class at
Publication: |
356/218 |
International
Class: |
G01J 001/42 |
Claims
The invention claimed is:
1. A light source detection and classification system, for use in
an automatic vehicle exterior light control system, comprising: at
least one pixel array; and at least one pigment spectral filter
material disposed between light sources to be detected and said
pixel array.
2. A light source detection and classification system as in claim
1, wherein said at least one pigment spectral filter material is
red.
3. A light source detection and classification system as in claim
2, wherein said pigment spectral filter material has a
fifty-percent transmission point between 580 and 620
nanometers.
4. A light source detection and classification system as in claim
3, wherein said pigment spectral filter material has a
fifty-percent transmission point at 605 nanometers.
5. A light source detection and classification system as in claim
1, wherein said at least one pigment spectral filter material is
infrared.
6. A light source detection and classification system as in claim
2, wherein said pigment spectral filter material has a
fifty-percent transmission point between 630 and 780
nanometers.
7. A light source detection and classification system as in claim
3, wherein said pigment spectral filter material has a
fifty-percent transmission point between 640 and 700
nanometers.
8. A light source detection and classification system as in claim
3, wherein said pigment spectral filter material has a
fifty-percent transmission point at 680 nanometers.
9. A light source detection and classification system as in claim
3, wherein said pigment spectral filter material has a
fifty-percent transmission point at 660 nanometers.
10. A light source detection and classification system as in claim
1, wherein pixels of one contiguous half of said pixel array are
filtered with said spectral filter material.
11. A light source detection and classification system as in claim
1, wherein individual pixels of said pixel array are filtered with
said spectral filter material such that alternating stripes of
filtered and unfiltered pixels are defined.
12. A light source detection and classification system as in claim
1, wherein individual pixels of said pixel array are filtered with
said spectral filter material such that a checkerboard pattern of
filtered and unfiltered pixels is defined.
13. A light source detection and classification system as in claim
1, wherein a red spectral filter material, a green spectral filter
material and a blue spectral filter material are disposed between
said light sources to be detected and said pixel array.
14. A light source detection and classification system as in claim
13, wherein twice as many pixels of said pixel array are red
filtered as compared to the number of pixels either green or blue
filtered.
15. A light source detection and classification system as in claim
13, wherein individual pixels of said pixel array are filtered such
that a Bayer pattern is defined.
16. A reflected light ray detection and classification system for
use in an automatic vehicle exterior light control system,
comprising: at least one pixel array; and a red spectral filter
material disposed between reflected light rays to be detected and
said pixel array.
17. A reflected light ray detection and classification system as in
claim 16, further comprising an infrared filter material disposed
between reflected light rays to be detected and the light ray
detection and classification system.
18. A reflected light ray detection and classification system as in
claim 16, further comprising headlights having high color
temperature.
19. A reflected light ray detection and classification system as in
claim 16, wherein said color temperature is greater than 3500
Kelvin.
20. A reflected light ray detection and classification system as in
claim 16, wherein said color temperature is 3700 Kelvin.
21. A reflected light ray detection and classification system as in
claim 19, wherein said color temperature is greater than 4500
Kelvin.
22. A reflected light ray detection and classification system as in
claim 16, wherein said at least one headlight is high intensity
discharge.
23. A reflected light ray detection and classification system as in
claim 16, wherein said at least one headlight is at least one light
emitting diode.
24. A reflected light ray detection and classification system as in
claim 16, wherein said at least one headlight is
halogen-infrared.
25. A reflected light ray detection and classification system as in
claim 16, wherein said at least one headlight is halogen.
26. A reflected light ray detection and classification system as in
claim 16, wherein said at least one headlight is blue enhanced
halogen.
27. A reflected light ray detection and classification system as in
claim 16, wherein pixels of one contiguous half of said pixel array
are filtered with said spectral filter material.
28. A reflected light ray detection and classification system as in
claim 16, wherein individual pixels of said pixel array are
filtered with said spectral filter material such that alternating
stripes of filtered and unfiltered pixels are defined.
29. A reflected light ray detection and classification system as in
claim 16, wherein individual pixels of said pixel array are
filtered with said spectral filter material such that a
checkerboard pattern of filtered and unfiltered pixels is
defined.
30. A reflected light ray detection and classification system as in
claim 16, further comprising a green spectral filter material and a
blue spectral filter material disposed between said light sources
to be detected and said pixel array.
31. A reflected light ray detection and classification system as in
claim 30, wherein twice as many pixels of said pixel array are red
filtered as compared to the number of pixels either green or blue
filtered.
32. A reflected light ray detection and classification system as in
claim 30, wherein individual pixels of said pixel array are
filtered such that a Bayer pattern is defined.
33. An optical system, for use in an automatic vehicle exterior
light control system, comprising: a dual lens system assembly for
projecting a field of view onto a pixel array such that a first
image of said field of view is formed on a first portion of said
pixel array and a second image of said field of view is formed on a
second portion of said pixel array; and at least one pigment
spectral filter material disposed between said field of view and
said pixel array.
34. An optical system as in claim 33, wherein said spectral filter
material is disposed such that said first image is filtered and
said second image is unfiltered.
35. An optical system as in claim 33, further comprising an
infrared spectral filter material disposed between said field of
view and said pixel array.
36. An optical system as in claim 33, wherein each lens system of
said dual lens system assembly is symmetrically truncated.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/208,142, filed on Jul. 30, 2002, the entire
disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to automatic vehicle exterior
light control systems. More specifically, the present invention
relates to light source detection and categorization systems for
use with automatic vehicle exterior light control.
[0003] Automatic exterior light control systems for vehicles have
been developed that utilize various light sensors and, or, an array
of sensors (commonly referred to as an array of "pixels" or "pixel
arrays") to control the state of external lights of a controlled
vehicle. These systems may, for example, be employed to detect the
headlights of oncoming vehicles and the taillights of leading
vehicles and to dim, or switch, the high beam headlights of a
controlled vehicle off when headlights or taillights are detected.
An automatic exterior light control system is described in detail
in commonly assigned U.S. Pat. No. 6,587,753 to Stam et al., the
disclosure of which is incorporated herein by reference. Automatic
exterior light control systems employing a pixel array are capable
of capturing images of the scene forward of the controlled vehicle
and base the exterior light control decisions upon various aspects
of the captured images.
[0004] Automatic exterior light control systems are required to
accurately distinguish various features of a given scene to
separately categorize scenarios which require control activity from
scenarios which do not. For example, taillights of leading vehicles
have to be distinguished from red traffic lights and red roadside
reflectors. When the high beams of the controlled vehicle are on,
it is desirous to automatically switch to low beams, or dim the
high beams, when taillights of leading vehicles are detected.
Conversely, detection of red traffic lights and red roadside
reflectors should not invoke switching, or dimming, of the
controlled vehicle's high beams.
[0005] A significant feature of sensors and optical systems for
exterior light control relates to their ability to detect and
accurately measure the brightness and color of small, distant,
light sources. In order to perform satisfactorily, an optical
system for such an application is preferably capable of identifying
taillights of leading vehicles at distances of at least 100 meters
and most preferably over 200 meters.
[0006] The resolution of an optical system is an important feature
in accurately distinguishing between various objects within a given
scene. The center-to-center distance between adjacent pixels of a
pixel array is commonly referred to as the "pixel pitch." The given
pixel pitch and the associated lens system design are two
significant components in deriving the corresponding optical system
resolution.
[0007] A typical taillight is about 10 cm in diameter, or less. The
angle subtended by such a taillight, at 200 meters, when projected
upon a sensor or pixel array of a sharply focused optical system,
is approximately 0.03.degree.. Typical pixel arrays, with
associated lens systems for use in exterior vehicle light control
applications, are designed to image 0.1.degree.-0.5.degree.
horizontally and vertically per pixel. Consequently, with known
optical systems, a projection of a taillight onto a typical pixel
array may be of a size substantially less than that of one pixel
pitch.
[0008] In known automatic vehicle exterior light control systems,
the associated small size of projected, distant, light sources
severely impairs the ability of the system to accurately determine
the color and brightness of the corresponding light source. This is
due largely to the fact that known sensors, and individual sensors
of known pixel arrays, have a non-uniform response; depending upon
the region of the pixel where light is incident, the sensitivity of
the pixel will vary. For example, light falling on a transistor, or
other structure that does not absorb photons, within the pixel will
not be absorbed at all and thus will not be detected. Additionally,
the given pixel may be more or less sensitive to light falling on
various regions depending on the distance between the point where a
photon is absorbed and the corresponding collection node to which
the generated electrons must diffuse.
[0009] U.S. Pat. No. 6,005,619 to Fossum and incorporated in its
entirety herein by reference, describes the pixel structure of a
photogate active pixel for use in a CMOS pixel array suitable for
use in a vehicle exterior light control system. FIG. 26 depicts a
group of four pixels from the Fossum device. As is readily seen
from FIG. 26, there are several different materials and structures
that form each pixel represented by 2600, 2602, 2604 and 2606.
Thus, the sensitivity of the pixel to incident light rays varies
depending upon the particular region of the pixel onto which
photons are incident. As discussed above, micro-lenses may be
employed to focus light rays on the most sensitive area of the
pixel.
[0010] Pixel non-uniformity becomes especially problematic when
attempting to determine the color and brightness of small, distant
light sources. For example, the optical system described in U.S.
Pat. No. 6,587,753 utilizes a lens system assembly with first and
second lens systems. The first lens system projects the associated
field of view onto one half of the associated pixel array and the
second lens system projects substantially the same field of view
onto the other half of the pixel array. A red spectral filter is
placed in the optical system such that the light rays projected by
the first lens system are red filtered. By comparing light rays
projected through the red spectral filter with the unfiltered, or
complementary filtered, light rays, a relative red color of each
object within the field of view is determined. When the light
sources are small, distant sources, minor misalignment of the first
lens system relative to the second lens system can cause
significant error in the relative color measurement. The distant
light source projected by the red spectral filtering lens system
may be incident onto a sensitive region of a pixel, while the light
source may be projected onto an insensitive region by the second
lens system, causing an erroneously high redness value. In a
subsequent image, the above scenario may be reversed causing an
erroneously low redness value to be attributed to the light source.
Consequently, distant taillight detection of known systems is less
than satisfactory in certain scenarios.
[0011] Therefore, there remains a need in the art of automatic
vehicle exterior light control systems for an optical system
capable of more accurately detecting the brightness and color of
small, distant light sources.
SUMMARY OF THE INVENTION
[0012] According to a first embodiment of the present invention, a
lens system is provided that projects light rays emitted by a
small, distant light source onto a sensor, or pixel array, such
that the projected light rays substantially cover at least one
entire sensor, or at least one pixel pitch, respectively. The
optical system in accordance with this first embodiment comprises a
pixel array with an associated Nyquist frequency limit. The optical
system has an associated spatial frequency cutoff. In the preferred
optical system, the spatial frequency cutoff is less than, or equal
to, the Nyquist frequency limit of the pixel array.
[0013] In another embodiment of the present invention, micro-lenses
are placed proximate each pixel. Each micro-lens projects the
corresponding incident light rays such that they are focused upon
the most sensitive portion of the given pixel. In optical systems
that employ a single sensor, or individual sensors not grouped into
an array, it is preferred to use micro-lenses on each sensor.
[0014] As another alternative to the preceding embodiments, a light
ray scattering film material is superimposed between the lens
system assembly and the pixel array, or is integrated into the lens
system assembly. Light rays projected by the lens system assembly
toward the sensor, or pixel array, are scattered such that the
projected light rays are incident more uniformly across the
individual pixels.
[0015] In yet another embodiment of the present invention, spectral
filtering material is incorporated into the optical system such
that light rays in various spectral bands can be accurately
categorized by color. Thereby, for example, red light sources can
be distinguished from white light sources. Particular embodiments
of the present invention are capable of discerning amongst light
sources throughout the visible spectrum. In yet other embodiments,
ultra-violet and infrared light rays are filtered and prevented
from being projected onto the sensor or pixel array. Preferably,
the spectral filter material is a pigment, as opposed to a dye.
[0016] In yet another embodiment of the present invention, an
optical system is provided with a pixel array having various
spectral band filter material on individual pixels. In this
embodiment, an optical system having a polychromatic modulation
transfer function that first drops to e.sup.-1 at a spatial
frequency less than, or equal to, the reciprocal of the minimum
center-to-center distance between any two similarly filtered pixels
is preferred.
[0017] In one embodiment of the present invention, various external
vehicle lights are used, such as high intensity discharge (HID),
tungsten-halogen and blue enhanced halogen, to provide greater
ability to distinguish reflections from various roadside reflectors
and signs from headlights of oncoming vehicles and taillights of
leading vehicles. Particular spectral filter material is employed
in combination with the external vehicle lights to produce desired
results.
[0018] In a further embodiment of the present invention, a method
of manufacturing an optical system is provided which insures that
the lens system assembly is fixed proximate the sensor, or pixel
array, in a precise position. An encapsulate block is transfer
molded over the pixel array such that the encapsulate block is
fixed to the associated circuit board. The encapsulate block
provides an attachment surface for placement of the lens system
assembly. A precision lens system placement machine is employed to
move the pixel array relative to the lens system assembly; it is
preferred to have the lens system assembly fixed in space with
respect to a target and move the pixel array relative thereto,
however, the pixel array may be fixed in space relative the target
and the lens system assembly would be moved relative thereto. At
the point the target is projected upon the pixel array as desired,
the lens system assembly is fixed relative the pixel array
preferably utilizing an ultra-violet light curable epoxy, or other
adhesive.
[0019] In another embodiment of the manufacturing method of the
present invention, a precision transfer molding apparatus is
employed to transfer mold a lens system assembly proximate the
pixel array. Molding the lens system assembly and the encapsulate
block in one step simplifies the manufacturing method and results
in lower cost.
[0020] In yet another embodiment of the manufacturing method of the
present invention, a precision transfer molding apparatus is
employed to transfer mold an encapsulate block with a precision
lens system assembly mounting structure. A lens system assembly is
provided with a mating mounting structure such that the lens system
assembly can be "snap" fit into place. This manufacturing method
provides for ease in lens system assembly replacement.
[0021] The preferred lens system is molded of polycarbonate to
withstand the potentially high temperatures to which the optical
system may be exposed in the automotive environment. The preferred
transfer molded encapsulate block, or encapsulate block and lens
when molded in combination, is formed from an epoxy material with
low thermal expansion and high temperature stability to ensure that
the optical system can withstand the automotive environment.
[0022] The features and advantages of the present invention will
become readily apparent from the following detailed description of
the best mode for carrying out the invention when taken in
connection with the accompanying figures and appended claims.
BRIEF DESCRIPTION OF THE FIGURES
[0023] FIG. 1 depicts an optical system in accordance with the
present invention, within a controlled vehicle, relative to the
taillights of a leading vehicle;
[0024] FIG. 2 depicts a block diagram of an automatic exterior
light control system for a vehicle with an optical system in
accordance with the present invention;
[0025] FIG. 3 depicts a preferred embodiment of the optical system
in accordance with the present invention;
[0026] FIG. 4 depicts the modulation transfer function plot of the
"sharply focused" optical system of a known optical system as
described in example 1a herein;
[0027] FIG. 5 is an encircled energy plot that shows the integral
of total energy of a projected spot as a function of the distance
from the center of the spot;
[0028] FIG. 6 depicts a point-spread-function plot of a
monochromatic projected spot for the optical system as described in
example 1a herein;
[0029] FIG. 7 depicts a modulation transfer function plot of the
optical system of example 1b in accordance with the present
invention;
[0030] FIG. 8 depicts an alternative embodiment of the optical
system in accordance with the present invention;
[0031] FIG. 9 depicts an encapsulated pixel array in accordance
with the prior art with the lead frame extending to the exterior of
the encapsulate block;
[0032] FIG. 10 depicts an alternate embodiment of the optical
system in accordance with the present invention;
[0033] FIG. 11a depicts a pixel array with superimposed spectral
filter material comprising a "Bayer pattern" in accordance with the
present invention;
[0034] FIG. 11b depicts an alternative embodiment of a pixel array
with superimposed spectral filter material in accordance with the
present invention;
[0035] FIG. 12 depicts an alternative embodiment of a pixel array
with a secondary substrate superimposed over the pixel array having
a spectral filter material on the secondary substrate in accordance
with the present invention;
[0036] FIG. 13 depicts an alternate embodiment of a lens system for
use with the optical system in accordance with the present
invention that provides for high resolution;
[0037] FIG. 14 depicts an alternate embodiment of a lens system for
use with the optical system in accordance with the present
invention which provides for chromatic aberration correction;
[0038] FIG. 15 depicts a perspective view of an optical system in
accordance with the present invention having a dual lens system
assembly with symmetrically truncated individual lens systems;
[0039] FIG. 16 depicts a perspective view of an optical system in
accordance with the present invention having a dual lens system
assembly with asymmetrically truncated individual lens systems;
1
[0040] FIG. 17 depicts a perspective view of the optical system of
FIG. 8;
[0041] FIG. 18 depicts a perspective view of an optical system in
accordance with the present invention having a cascaded lens system
for high resolution applications;
[0042] FIG. 19 depicts an optical system in accordance with the
present invention having a variable focus lens system;
[0043] FIG. 20 depicts an alternate embodiment of an optical system
in accordance with the present invention having a variable focus
lens system;
[0044] FIG. 21 depicts plots of the spectral distributions of
various vehicle exterior lights;
[0045] FIG. 22 depicts plots of the spectral reflectance ratios of
various colored road signs;
[0046] FIG. 23 depicts plots of transmission factors of red and
infrared filter material in accordance with the present
invention;
[0047] FIG. 24 depicts plots of the quantum efficiency versus
wavelength for an optical system in accordance with the present
invention;
[0048] FIG. 25 depicts a graph of red-to-clear ratios for various
light sources as detected by an optical system in accordance with
the present invention;
[0049] FIG. 26 depicts an exploded view of a group of four pixels
of a known pixel array;
[0050] FIG. 27 depicts an array of 3.times.3 dots produced by a
target and as projected by an optical system, in accordance with
the present invention having a dual lens system assembly, onto a
pixel array as a beginning point for the optical system
manufacturing method in accordance with the present invention;
[0051] FIG. 28 depicts the dots as described with regard to FIG. 22
once the lens system and pixel array have been aligned with regard
to the x and y coordinates;
[0052] FIG. 29 depicts the target as described with regard to FIG.
23 when lens system is "tilted" in relation to the pixel array with
regard to either the theta ("pitch") or alpha ("yaw")
coordinates;
[0053] FIG. 30a depicts an image of a target with vertical lines as
captured using an optical system with low resolution as
manufactured in accordance with the method of the present
invention;
[0054] FIG. 30b depicts an image of a target pattern with vertical
lines as captured using an optical system with high resolution as
manufactured in accordance with the method of the present
invention;
[0055] FIG. 31a depicts a low-frequency target pattern for use with
the manufacturing method in accordance with the present
invention;
[0056] FIG. 31b depicts an image of the target pattern of FIG. 26a
utilizing an optical system in accordance with the present
invention having a low frequency cutoff;
[0057] FIG. 32a depicts a mid-frequency target pattern for use with
the manufacturing method in accordance with the present
invention;
[0058] FIG. 32b depicts an image of the target pattern of FIG. 27a
utilizing an optical system in accordance with the present
invention having a low frequency cutoff;
[0059] FIG. 33a depicts a high-frequency target pattern for use
with the manufacturing method in accordance with the present
invention;
[0060] FIG. 33b depicts an image of the target pattern of FIG. 28a
utilizing an optical system in accordance with the present
invention having a low frequency cutoff;
[0061] FIG. 34 depicts a plot of the resolution metrics for various
Z-axis heights with superimposed linear representations;
[0062] FIG. 35 depicts a plot of the resolution result for various
Z-axis heights where zero on the x and y axis equals sharp
focus;
[0063] FIG. 36 depicts a modulation transfer function plot for the
optical system of example 2 described herein in accordance with the
present invention; and
[0064] FIG. 37 depicts a modulation transfer function plot for the
optical system of example 3 described herein in accordance with the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0065] Automatic vehicle exterior light control systems that employ
optical systems to detect objects in a scene forward of the
controlled vehicle are known. The detected objects are examined and
categorized such that the exterior lights of the controlled vehicle
can be controlled dependent upon the detected objects. One known
automatic vehicle exterior light control system is described in
detail in commonly assigned U.S. Pat. No. 6,587,753 to Stam et al.,
the disclosure of which is incorporated in its entirety herein by
reference. As used herein, the term "controlled vehicle" refers to
the vehicle that has the automatic exterior light control
system.
[0066] The optical systems in accordance with the present invention
are capable of detecting small, distant light sources and provide
for more accurate brightness and color discrimination when compared
to known systems. Thereby, the ability of the associated automatic
vehicle exterior light control system to categorize the objects in
the scene forward of the controlled vehicle is dramatically
improved. As used herein, the term "light source" includes devices
that emit light rays (i.e., headlights, taillights, streetlights,
traffic signals, etc.), as well as devices that reflect light rays
(i.e., roadside reflectors, roadside signs, reflectors on vehicles,
etc.). As used herein, the term "accurate" refers to the optical
system's ability to predictably and reliably measure brightness and
color of small, distant light sources that, in turn, facilitates
predictable and reliable distinguishing of the headlights of
oncoming vehicles and taillights of leading vehicles from one
another and from other light sources.
[0067] Referring initially to FIG. 1, an optical system 105 in
accordance with the present invention is shown within a controlled
vehicle 110. The controlled vehicle 10 is shown in relation to a
first vehicle 115, having taillights 116, leading the controlled
vehicle at a distance of approximately 200 meters and a second
vehicle 120, having headlights 121, approaching the controlled
vehicle 110 from the opposite direction. The overall field of view
of the optical system 105 is depicted in FIG. 1 to be within the
dashed lines 125 extending from the front of the controlled vehicle
110.
[0068] A block diagram of an automatic vehicle exterior light
control system 250 is depicted in FIG. 2 including an optical
system 205 in accordance with the present invention. As shown in
FIG. 2, a processing and control system 255 is employed to control
various parameters of the associated pixel array and to acquire and
process digitized images obtained by the optical system 205. The
processing and control system 255 ultimately generates control
signals 265 which are transmitted to the associated controlled
vehicle exterior light controller 270.
[0069] In general, optical systems in accordance with the present
invention incorporate a lens system assembly, a spectral filter
material system, and a pixel array mounted on a printed circuit
board. The optical systems are preferably manufactured in
accordance with methods that provide precise control of the
placement of the lens system assembly relative the pixel array. The
upper surface of the pixel array defines an "image plane."
[0070] The lens system assembly may include a single lens system or
a plurality of lens systems. Each lens system may include a single
optical surface or a plurality of cascaded lenses, each lens having
associated optical surfaces. The lens system(s) may be permanently
attached to the pixel array or may be detachable and replaceable.
Various lens system assemblies in accordance with various
embodiments of the present invention are depicted in FIGS. 3, 8,
10, and 13-20. Additional lens system assemblies will be apparent
to those skilled in the art and are contemplated for use with the
present invention. The lens system assemblies may include far field
baffles, shutter stops, aperture stops and other structures that
serve to affect the imaging performance of the system.
[0071] The spectral filter material system may comprise a single
spectral band rejection characteristic or may comprise a plurality
of spectral band rejection characteristics with individual pixels
or groups of pixels within the pixel array being uniquely filtered.
FIGS. 3, 8, 11a, 11b and 12 depict spectral filter material systems
in accordance with various embodiments of the present invention.
The optical systems in accordance with the present invention may
have individual sensors, pixels, or groups of pixels filtered
differently from others. The spectral filter materials may be of
either the absorption or reflective type.
[0072] The pixel array may be a low resolution device with few,
large pixels or may be a high resolution device with many small
pixels as described in more detail within the optical system
examples contained herein. Multiple pixel arrays or image sensors
manufactured either separately or in combination, each with
potentially unique lens systems and spectral filter
characteristics, may be employed for automatic vehicle exterior
light control in accordance with the present invention.
[0073] The pixel array may comprise individual sensors, or pixels,
with varying sizes from one area of the array to other areas. The
manufacturing methods in accordance with the present invention, as
described in detail herein, provide for precisely focusing the
optical system to achieve desired resolution. The resulting optical
system may be sharply focused, defocused, nearsighted or
farsighted.
[0074] Optical systems in accordance with the present invention
that employ a plurality of lens systems may have all lens systems
focused and filtered identically or may have each lens system
uniquely focused and, or, uniquely filtered. The filtering may be
provided on the individual pixels of a portion of the overall pixel
array associated with a given lens system, may be interposed
between the lens system and the pixel array or may be integrated
into the lens system assembly.
[0075] In operation, an optical system comprising a pixel array
produces a "digitized image" of a corresponding scene within the
field of view of the optical system. Light rays emanating from
objects within the scene are projected by the lens system assembly
onto the pixel array such that each pixel within the array is
exposed to light rays corresponding to a segment of the overall
scene. As will become apparent from the description of the optical
systems contained herein, the digitized image is merely an
approximation of the actual scene. The output of each pixel, which
is proportional to the light rays incident thereon during a given
period of time, is utilized by the corresponding digital image
processing system to reconstruct the actual scene in the form of a
digitized image. The size of the area, or segment, of the overall
scene projected onto a given pixel determines the resulting
resolution of the digitized image. As can be appreciated, if a
large field of view is projected onto a small number of large
pixels such that the output of each pixel represents a relatively
big area, or segment, of the overall scene, the resulting digitized
image will appear "grainy" and individual objects within the actual
scene are likely to appear as one collective object in the
digitized image. This is especially true with regard to small,
distant objects contained in the scene. Conversely, if a small
field of view is projected onto a large number of small pixels such
that the output of each pixel represents a relatively small area,
or segment, of the overall scene, the resulting digitized image
will appear "smooth" and individual objects within the actual scene
will likely remain distinct within the digitized image.
[0076] A number of factors, in addition to the size of the field of
view and the size of the individual pixels, influence an optical
system's ability to detect small, distant light sources. In an
effort to provide a more clear understanding of the present
invention, definitions of a number of terms as used herein are
provided at this point.
[0077] The term "spatial frequency" is used herein to quantify the
level of detail contained in a scene, measured herein in cycles per
millimeter (cycles/mm) on the image plane. As an example, a scene
containing a small number of alternating colored, wide lines would
be characterized as encompassing a low spatial frequency, while a
scene containing a large number of alternating colored, narrow
lines would be characterized as encompassing a high spatial
frequency. As a second example, a scene containing a small number
of alternating colored, large squares in a checkerboard pattern
would be characterized as encompassing a low spatial frequency,
while a scene containing a large number of alternating colored,
small squares in a checkerboard pattern would be characterized as
encompassing a high spatial frequency. The example with lines would
be further characterized to be a one-dimensional spatial frequency,
while the example with squares would be further characterized as
having a two-dimensional spatial frequency.
[0078] The term "spatial frequency response" is used herein to
quantify the ability of a given optical system to resolve detail in
a scene when projecting an image of the scene onto the surface of a
pixel array. As an example, a given optical system may have a high
spatial frequency response for low spatial frequency scenes, such
as a scene containing a low number of wide lines, which when
projected onto a pixel array remain clear and distinct. The same
optical system may have a low spatial frequency response for high
spatial frequency scenes, such as a scene containing a high number
of narrow lines, which when projected onto a pixel array may blur
together. Spatial frequency response is typically measured, or
computed, with respect to the image plane. The image plane, as
stated above, is defined by the upper surface of the pixel
array.
[0079] The term "modulation transfer function" (MTF) is used to
quantify a function of the spatial frequency response of a given
optical system for a given range of spatial frequencies. The MTF is
further defined to be "monochromatic" when applied to optical
systems imaging scenes that contain a single wavelength of light
and to be "polychromatic" when applied to optical systems imaging
scenes that contain a plurality of wavelengths of light, in which
case analysis is performed by considering the entire spectral band
imaged by the optical system. The scale of the vertical axis of the
MTF plots for the examples given herein is unity at the spatial
frequency of zero on the horizontal axis.
[0080] The term "spatial frequency cutoff" refers to the lowest
spatial frequency at which the spatial frequency response is first
less than, or equal to, a desired threshold. Generally, except
where defined otherwise herein, the threshold will be e.sup.-1.
[0081] The term "absolute spatial frequency cutoff" refers to the
lowest spatial frequency at which the spatial frequency response is
first equal to zero.
[0082] Generally, the term "Nyquist frequency limit" refers to the
maximum spatial frequency that can be sampled without error by a
given pixel array, measured herein in cycles/millimeter
(cycles/mm). In the general case, the Nyquist frequency limit of a
given pixel array is equal to 1/(2d), where d is the
center-to-center distance between adjacent pixels. However, there
are specific instances described herein where the Nyquist frequency
limit is redefined for a specific pixel array. The center-to-center
distance between adjacent pixels is commonly referred to as the
"pixel pitch."
[0083] When the spatial frequency response of the optical system
exceeds the Nyquist frequency limit of the pixel array, the
resulting digitized image is not likely to accurately depict the
actual scene. Therefore, the preferred optical systems in
accordance with the present invention have a spatial frequency
response that is less than, or equal to, the Nyquist frequency
limit of the associated pixel array.
[0084] As mentioned above, and as described in detail herein,
objects within the digitized image are categorized and provide the
basis for automatic vehicle exterior light control. The optical
systems in accordance with the present invention may perform
functions in addition to automatic vehicle exterior lighting
control such as rain sensing, adaptive cruise control, lane
departure warning, general artificial vision functions, etc.
Therefore, the chosen optical system configuration will be
dependent, at least in part, upon the given application.
[0085] Turning now to FIG. 3, a preferred embodiment of the present
invention is shown as optical system 305. A pixel array 310 is
mounted onto a printed circuit board 315 with a die attach adhesive
(not shown) and electrically connected with wire bonds (not shown).
While the present invention is not limited to any number of pixels,
size of pixels, type of pixels, or arrangement of pixels, it is
advantageous to use a reasonably low number of pixels to minimize
associated data processing, data transmission and data storage
requirements. Lens system assembly 330 contains a first lens system
331 and a second lens system 332; each lens system projects light
rays 340 onto the pixel array 310 such that two individual images
of the scene forward of the controlled vehicle are formed on
separate halves of the pixel array. Lens system assembly 330 is
preferably formed of molded polycarbonate to withstand the
potential high temperatures possible when the optical system 305 is
mounted in the region of a vehicle rear view mirror. An encapsulate
block 325 is preferably formed by transfer molding a clear epoxy,
such as type NT-300H available from Nitto Denko, Shimohozumi,
Ibaraki, Osaka, Japan, over the pixel array 310 and onto the
printed circuit board 315 such that the encapsulate block is
attached to the printed circuit board. Commonly assigned U.S. Pat.
No. 6,130,421 to Stam et. al., which is incorporated herein in its
entirety by reference, describes another structure that is
contemplated for use with the present invention.
[0086] An aperture stop 345 prevents light rays from entering the
optical system 305 other than through the two lenses' optical
surfaces. A far-field baffle (not shown) excludes light rays from
objects beyond the desired field of view from being projected onto
the pixel array.
[0087] Lens system assembly 330 is preferably attached to the
encapsulate block 325 by UV curable adhesive 335, which may be of
type Norland-68 available from Norland Products, Cranbury, N.J.
Preferably, proper alignment and focus of the lens system assembly
is achieved by operating the optical system 305 with the lens
system assembly placed over the encapsulate block with uncured
adhesive 335 therebetween. The position of the lens system assembly
330 is adjusted, as described in detail below with regard to the
preferred method of manufacture, until an image of an associated
target is of the desired resolution. At this point, UV light is
introduced to cure the adhesive and fix the lens system assembly in
place relative the pixel array. The preferred manufacturing methods
in accordance with the present invention are described in detail
below. Alternatively, the two lens surfaces may be formed integral
with the encapsulate block 325 as described below with respect to
an alternative manufacturing method.
[0088] Preferably, the centers of the first and second lens systems
331, 332 of the optical system 305, as shown in FIG. 3, are spaced
closer than the diameter of the individual lens systems resulting
in some truncation of each lens system. Truncation on only the
adjoining edges, as shown in FIGS. 3 and 16, may cause a lack of
symmetry that, in turn, causes non-uniformity in the aberrations
associated with the overall optical system. This non-uniformity may
be problematic for off axis imaging where a brightness comparison
is made between an object projected by the individual lens systems.
To ameliorate aberrations caused by non-symmetry, each lens system
may be truncated at the outside edge as well as the adjoining edge.
FIG. 15 depicts a symmetrically truncated lens system as part of an
optical system in accordance with the present invention.
[0089] Returning to FIG. 3, preferably red spectral filter material
320 is deposited over one half of the pixel array 310. The red
spectral filter material 320 is preferably chosen to withstand the
high radiant loading that may occasionally occur when the optical
system 305 is aimed directly at the sun. When the optical system is
aimed directly at the sun, the light rays of the sun are projected
onto the spectral filter material 320 and the pixel array 310. Many
known spectral filter materials contain light absorbing dyes that
are likely to degrade under these conditions. To overcome the
filter material degradation problem, spectral filter material 320
is preferably chosen to contain a pigment rather than a dye.
Pigments are typically inorganic and can be made to be much more
resistant to high radiant loading than dyes. Spectral filter
materials containing pigments and which are suitable for patterning
onto pixel arrays are available from Brewer Science, Inc. of Rolla,
Mo. Spectral filter material of type PSCRed-101 is preferred. As an
alternative to the use of a pigmented spectral filter material,
multi-layer, thin-film interference filters may also be used. A
process for depositing interference filters for use on pixel arrays
is described in U.S. Pat. No. 5,711,889 to Buchsbaum, the
disclosure of which is incorporated in its entirety herein by
reference.
[0090] To further protect the optical system, a photochromic, or
electrochromic, spectral filter (not shown) is preferably placed in
front of the optical system 305. This is especially preferred if
less stable filter materials are used. Photochromic spectral
material darkens when subject to high levels of light. Therefore,
during sunny days, the photochromic spectral filter materials will
darken and thus attenuate much of the light that could potentially
damage the optical system. During the evening, when the automatic
exterior light control feature is desired, the photochromic
spectral filter material would be clear. Electrochromic spectral
filters provide the same benefits as photochromic spectral filters;
however, electrochromic spectral filters require external apparatus
to control the associated darkening. Photochromic and
electrochromic spectral filters are envisioned for use with the
present invention.
[0091] Preferably, an infrared blocking spectral filter material
(not shown) is positioned in front of the optical system, or
elsewhere within the optical system, to prevent light rays of
wavelengths greater than about 680 nm from being projected onto the
pixel array. Optionally, the parameters of each lens system may be
further optimized utilizing additional spectral filter material for
the particular spectral bands imaged by the given optical system.
It is foreseeable that the vehicle containing the optical system of
the present invention will be parked for extended periods of time
with the sun being directly incident upon the optical system.
Therefore, it is beneficial to provide spectral filtering material
that blocks the harmful rays of the sun from entering the optical
system. Similarly, any light rays that do not contribute to the
ability of the optical system to function as desired should be
inhibited from entering the optical system.
[0092] The spatial frequency response of a pixel array is limited
to its Nyquist frequency limit. The Nyquist frequency limit is
equal to 1/(2d), where d is the pixel pitch. In the general case,
the pixel pitch is measured from the center of one pixel to the
center of the adjacent pixel. In the case where pixels are not
square, the maximum horizontal and vertical spatial frequency may
be different and the Nyquist frequency limit should be considered
with regard to the largest dimension.
[0093] When considering the Nyquist frequency limit of a pixel
array with a patterned color spectral filter material array, the
pixel pitch should be considered as the distance from the center of
one pixel of a particular color to the center of the closest pixel
of that same color. In the case of the Bayer pattern of FIG. 11a,
any pixel of a particular color is two pixels away from another
pixel of that same color; an exception is the case of the green
filtered pixels that are located diagonal to each other, in which
case the blue and red pixels should be considered with regard to
the pixel array Nyquist frequency limit.
[0094] When the spatial frequency response of the optical system is
reduced, distant light sources tend to blur together in the image.
This blurring may be disadvantageous when trying to discriminate
two objects from one another. For example, a taillight and an
overhead streetlight may blur into one object. To avoid this, it
may be necessary to compromise the accuracy of the digital sampling
in order to discriminate distinct objects from one another. The
strict Nyquist frequency limit criteria implies that the spatial
frequency cutoff of the optical system be less than, or equal to,
1/(2d), where d is the pixel pitch. The Nyquist frequency limit
criteria may be violated for the purpose of increased
discrimination without completely impairing the optical system
functionality and without deviating from the spirit of the present
invention.
[0095] From the plots depicted in FIGS. 4, 7, 36 and 37, it can be
seen that the modulation transfer function (MTF) is significantly
reduced at higher frequencies near the cutoff. Since the majority
of vehicular light sources are broadband emitters, the
polychromatic MTF over the spectral bandwidth used may be
considered. Optical systems wherein the polychromatic MTF first
drops to e.sup.-1 at a spatial frequency that is less than, or
equal to, the reciprocal of the pixel pitch are in accordance with
the present invention.
[0096] The strict Nyquist frequency limit may be violated and
acceptable performance still achieved. Optical systems in
accordance with the present invention are capable of imaging light
rays of all visible wavelengths, thus reducing the spatial
frequency response of the optical system for polychromatic sources.
Better polychromatic spatial frequency response may be achieved
through the use of an achromatic lens system as depicted in FIG.
14. Optical systems wherein the polychromatic MTF first drops to
e.sup.-1 at a spatial frequency less than, or equal to, the
reciprocal of the minimum distance between any two pixels
containing the same color spectral filter for a spectral band
imaged by the optical system, are also in accordance with the
present invention.
[0097] The following examples are intended as representations of
the present invention for illustrative purposes and in no way
should be interpreted as limiting the scope of the present
invention. As will be appreciated by the skilled artisan, the
present invention can embody a host of optical system
characteristics to satisfy particular operational desires.
EXAMPLE 1a
[0098] The optical system of this example is constructed as
depicted and described with regard to FIG. 3 and results in a
sharply focused optical system. The pixel array is a complementary
metal-oxide silicon (CMOS) active pixel array comprising a 64
pixels horizontal.times.80 pixels vertical array of 30 .mu.m,
photogate pixels. The thickness of the encapsulate block (dimension
C as shown in FIG. 3), measured from the top surface of the pixel
array to the top surface of the encapsulate block, is 3.75 mm. Each
lens system has a diameter of 4.4 mm, a radius of curvature of 2.6
mm and a conic constant of -0.4. The thickness of the lens system
assembly (dimension A in FIG. 3) is 3.0 mm. The centers of the
individual lens systems are spaced by 1.44 mm; each lens system is
truncated on one side to place the center-to-center distance of the
lens systems at the desired distance to form the lens system
assembly. Sub windows of 60 pixels horizontally.times.26 pixels
vertically are chosen for image acquisition in each half of the
pixel array providing a field of view of approximately 24.degree.
horizontal by 10.degree. vertical, or approximately 0.4.degree. per
pixel. A first analysis is performed on the optical system with the
lens system assembly positioned such that sharpest focus is
achieved. This occurs when the UV cured adhesive is 0.26 mm
thick.
[0099] FIG. 4 depicts the Modulation Transfer Function (MTF) plot
of the optical system of this example. Zemax lens design software,
as discussed in detail below, was utilized to analyze the optical
systems described herein and to generate the corresponding MTF
plots. Although the actual optical systems of examples 1 a and 1b
comprise truncated lens systems, the analysis was approximated with
data representative of non-truncated lens systems. An MTF plot
shows the spatial frequency response of an optical system and thus
describes the ability of a lens system to project objects as a
function of the resolution of an imaged scene. The results of the
optical system of this example have an absolute spatial frequency
cutoff of approximately 260 cycles/mm, shown at reference 405 in
FIG. 4, when imaging on axis at the single wavelength of 620 nm.
With 30 .mu.m pixels, the Nyquist frequency limit of the pixel
array is 16.67 cycles/mm.
[0100] The spatial frequency response of the optical system greatly
exceeds the sampling capability of the pixel array, resulting in
the potential for significant error in digital representation of
the imaged scene. The degree of error is less when considering the
application of imaging actual taillights. For this purpose, distant
taillights are typically located within 1.degree. of the optical
axis. Therefore, the on-axis MTF should be considered. However, a
typical taillight is a broadband emitter of light of a wide range
of wavelengths and a multi-wavelength MTF can be considered.
Combination of a red spectral filter material and an infrared
spectral filter material provides a spectral band of about 600-680
nm for imaging of taillights that reduces the spatial frequency
response of the lens system assembly somewhat.
[0101] The above analysis is further clarified with reference to
FIG. 5. FIG. 5 depicts an encircled energy plot for the optical
system of this example that shows the integral of total energy of
an imaged spot as a function of the distance from the center of the
spot. As shown, for on-axis imaging of light of wavelength 620
nanometers, 90% of the total energy is contained in a spot of
radius 3.0 .mu.m. This spot is significantly smaller than the pixel
size of 30 .mu.m and may lead to the errors previously described.
As described above, the magnitude of this problem is reduced when
considering multi-wavelength imaging. However, narrow band light
sources, such as LED taillights, must be considered when designing
an optical system for automatic vehicle exterior light control. For
a final illustration of the optical system of this example, FIG. 6
depicts a point-spread-function plot of a monochromatic spot. By
viewing FIG. 6, it is possible to see the amplitude of the imaged
spot relative to an area of the size of a pixel.
EXAMPLE 1b
[0102] In order to achieve accurate digital sampling of the imaged
scene in accordance with the present invention, the optical system
of example 1a should be modified to have a spatial frequency cutoff
that is less than, or equal to, the Nyquist frequency limit of the
pixel array. This is achieved in accordance with the present
invention by modifying the thickness of the encapsulate block to
produce a more "blurred" image with a lower spatial frequency
cutoff. FIG. 7 depicts the MTF plot of the optical system described
in example 1a modified with the thickness of the encapsulate block
(dimension C as shown in FIG. 3) reduced to 3.65 mm. This reduction
in block height limits the absolute spatial frequency cutoff of the
optical system to approximately 17 cycles/mm, shown at reference
705 in FIG. 7, roughly the same as the limit of the pixel array of
16.67 cycles/mm.
[0103] FIG. 7 also depicts plots of modulation transfer functions,
in dashed and dotted lines, of alternated embodiments of optical
systems in accordance with the present invention. As shown with
regard to reference number 710, the MTF depicted with a dashed line
represents an optical system with an absolute spatial frequency
cutoff that is equal to the reciprocal of the pixel pitch of the
optical system of examples 1a and 1b. As shown with regard to
reference number 715, the MTF depicted with a dotted line
represents an optical system with a modulation transfer function
that first drops to e.sup.-1 at a spatial frequency equal to the
reciprocal of the pixel pitch of the optical system of examples 1a
and 1b. As discussed above, optical systems that deviate from the
strict Nyquist frequency limit criteria, such as those shown by the
dashed and dotted lines in FIG. 7, will contain some sampling
error. However, in an effort to improve discrimination between
distant light sources that are in close proximity to one another,
sacrificing a small amount of sampling accuracy may improve overall
system performance.
[0104] An alternate embodiment in accordance with the present
invention is depicted in FIG. 8 as optical system 805. A single
lens system 825 is shown which is precisely transfer molded
directly over the pixel array 810 with no interposing encapsulate
block. A separate lens system may also be used which may be
directly attached to the pixel array or may be a separate component
containing one, or more, lens surfaces. The lens system 825
projects light rays 840 onto the pixel array 810 which is
preferably covered with a color spectral filter material array 820,
most preferably as shown in either FIG. 11a or 11b. Aperture stop
845 blocks light rays 840 from entering the optical system 805
through paths other than the lens system. An infrared spectral
filter (not shown) and, or, an ultra-violet spectral filter (not
shown) may be used to block infrared and, or, ultra-violet
radiation from entering the optical system. A far-field baffle (not
shown) may be used to prevent objects beyond the desired field of
view from being imaged by the optical system. The pixel array 810
is preferably mounted onto a printed circuit board 815 using
chip-on-board mounting as known in the art. Other methods of
constructing optical system 805 will be appreciated by those
skilled in the art and are in accordance with the present
invention. For example, the pixel array may be contained in a
package and soldered onto the circuit board as shown in FIG. 9. The
lens system may attach to a pixel array package or may attach to
the printed circuit board through the use of a lens system
mount.
[0105] In this embodiment of the present invention, the lens system
and the encapsulate block are preferably formed as a single
transfer molded lens system 825. Molding of lens system 825 in this
fashion is accomplished by providing a negative of the lens system
features in a transfer mold tool. The pixel array 810 is precisely
aligned and wire bonded (not shown) onto the printed circuit board
815. The entire lens system 825 (lens system and encapsulate block)
is then transfer molded in one piece over the pixel array. Stresses
in the encapsulate block along the edges of the pixel array 810 may
be reduced by beveling the edges of the pixel array chip. This is
accomplished by dicing the pixel array chips 810 from the
associated silicon wafer using a beveled dicing blade.
Alternatively, a packaged or chip-scale packaged pixel array, as
depicted in FIG. 10, may be placed onto a circuit board and
subsequently encapsulated by lens system 1030. The embodiment of
the present invention as depicted in FIG. 10 has a pixel array 1005
attached to a carrier 1010 via wire bonds 1025. The carrier 1010 is
preferably solder bonded 1020 to printed circuit board 1015. The
lens system 1030 is precisely transfer molded over the packaged
pixel array such that it attaches to circuit board 1015. The lens
system transfer molding method in accordance with the present
invention has the advantage of using fewer manufacturing steps and
eliminating a separate lens system. However, this method requires
all components to be very precisely aligned since subsequent
adjustment of the lens system relative to the pixel array is not
possible.
[0106] The optical system 805, of FIG. 8, incorporates a color
spectral filter material that is patterned onto the pixel array,
preferably as shown in either FIG. 11a or 11b. FIG. 11a depicts a
typical Red/Green/Blue (RGB) filter patterned over the pixel array
in a checkerboard fashion (this pattern is commonly referred to as
a "Bayer pattern"). For conventional optical systems, it is common
to cover twice the number of pixels with a green filter as are
covered by a red or blue filter, to mimic the human eye's higher
sensitivity to green light. For the present invention, it is
advantageous to cover twice as many pixels by a red filter compared
to the number of pixels covered by a blue or green filter in order
to maximize detection of distant taillights. Actual color of the
detected light source is determined by interpolation of neighboring
pixels of different colors by techniques well known in the art. As
an alternative to a checkerboard or mosaic pattern as depicted in
FIGS. 11a and 11b, patterning spectral filters with stripes is in
accordance with the present invention. The use of complementary
Cyan/Yellow/Magenta filter arrays is also in accordance with the
present invention. As previously discussed, use of a pigmented
filter material is preferred because it provides much greater
stability in high radiant loading conditions.
[0107] While use of a RGB spectral filter array provides a high
level of color information, use of a red filter alone is in
accordance with the present invention and is preferred because it
reduces the computational requirements of the associated processing
system. FIG. 11b depicts a checkerboard pattern using only a red
spectral filter on a portion of the pixels and no spectral filter
on the remainder. A cyan, or complementary, spectral filter may
alternatively be used on the non-red spectral filtered pixels. A
stripe pattern is also contemplated.
[0108] In an alternate embodiment of the present invention, as
shown in FIG. 12, a secondary substrate 1205, such as a piece of
glass, with a controlled spectral filter material 1210 pattern, is
directly attached to the surface of the pixel array 1215. Standard
microelectronic assembly equipment as known in the art is used to
place the spectral filtering substrate on the pixel array. A
mechanical bond is created between the two components through the
use of a UV cured optical grade epoxy 1220. By aligning the filter
pattern formed on the secondary substrate 1205 in a predetermined
position on the pixel array 1215, the filter-on-die solution, as
described above, is closely duplicated. It is within the scope of
the present invention to incorporate infrared and, or, ultra-violet
spectral filter material into the optical system utilizing this
method, in which case substantially the entire secondary substrate
would be coated.
[0109] For applications where higher resolution is desired,
multiple lenses, each lens having associated optical surfaces, are
commonly used. An embodiment facilitating simple manufacture of a
multi-element lens system is shown in FIG. 13. A transfer molded
encapsulate block 1325 is formed on PCB 1315 covering pixel array
1310. The transfer molded encapsulate block may be formed with an
integral optical surface 1326. Registration features 1356 are
provided in the transfer molded encapsulate block for proper
registration of a second lens 1330. Registration features 1356 may
be cutouts, pegs, posts, slots, grooves, snaps or any other feature
that facilitates proper alignment of lens 1330 with encapsulate
block 1325. Registration features 1356 may also serve to retain
lens 1330 to encapsulate block 1325. An adhesive may also be used
to retain lens 1330 to encapsulate block 1325. Finally, lens 1330
may be actively aligned and adhered to the encapsulate block by a
positioning system described in detail below.
[0110] Lens 1330 contains mating features 1355 to align with
registration features 1356. The lens 1330 has first and second
optical surfaces 1331, 1332. These optical surfaces, along with
optical surface 1326, may be any combination of optical surface
types including concave, convex, flat, aspheric, spherical,
refractive, diffractive, binary, or any other type of optical
surface. Lens 1330 may be made of any suitable material and is
preferably molded from a transparent plastic to facilitate
incorporation of mating features 1355. The encapsulate block 1325,
with optical surface 1326, and lens 1330, with optical surfaces
1331, 1332, combine to form a lens system. An aperture stop 1345
may be provided which mates or snaps to registration features 1356
in lens 1330.
[0111] Additional lens elements similar to lens 1330 may be further
stacked on top of lens 1330 and mated with registration features to
ensure proper alignment, as shown in FIG. 18. In this way, any
number of lens elements with a variety of prescriptions along with
stops, shutters, or other optical elements may be conveniently
stacked to achieve the desired design. Other optical elements may
be provided in the optical system which are not stacked from
encapsulate block 1325 but rather secured by another means, for
example, to provide a movable lens element.
[0112] For some applications, use of an achromatic lens is desired.
An achromatic lens corrects for, or reduces, chromatic aberration
caused by the variance in index of refraction of an optical
material with wavelength. An embodiment of the present invention
useful for forming an achromatic lens is shown in FIG. 14. The
encapsulate block 1425 is preferably formed from a material such as
Nitto 300H available from Nitto Dengo, Shimohozumi, Ibaraki, Osaka,
Japan, which has a relatively high, 1.57, index of refraction and
functions similar to a flint glass. The lens 1430 is preferably
formed from acrylic, or other lower index of refraction (n=1.49)
material, that functions similar to a crown glass. The lens 1430
and encapsulate block 1425 have mating lens surfaces 1431, 1426
which are preferably cemented together using a transparent
adhesive. The two pieces may be aligned by registration features
1455, 1456 that are pegs, snaps, or the like.
[0113] FIG. 15 depicts a perspective view of an optical system in
accordance with the present invention having a symmetrically
truncated lens system assembly 1530. As can be seen, each lens has
the adjoining edge 1509 and the outside edge 1507 truncated. Lens
system assembly 1530 is attached to the encapsulate block 1525 with
adhesive 1035. Aperture stop 1545, with opening 1550, is receivable
over the lens system assembly 1030 and encapsulate block 1525.
[0114] FIG. 16 depicts a perspective view of an optical system
constructed in accordance with the present invention having an
asymmetrically truncated lens system assembly 1630. An aperture
stop 1645, with opening 1650, snaps over encapsulate block 1625 to
prevent light from entering the optical system other than through
the lens surfaces. The aperture stop 1645 is secured to the printed
circuit board 1615.
[0115] FIG. 17 shows a perspective view of an optical system
constructed in accordance with the manufacturing method of the
present invention. Lens 1730 is attached to encapsulate block 1725
through posts 1755 molded integral with lens 1730. While two posts
1755 are shown, more posts may be used, or, registration may be
achieved by a "skirt" around the entire lens system similar to
those shown in FIGS. 15 and 16. Encapsulate block 1725, with
optical surface 1726, combines with lens 1730, with optical
surfaces 1731, 1732, to form a lens system. Encapsulate block 1725
is cast over the pixel array (not shown) and adheres to the printed
circuit board 1715.
[0116] As mentioned above, FIG. 18 depicts an optical system in
accordance with the present invention having a high-resolution lens
system. Encapsulate block 1825 is molded over the pixel array (not
shown) and adheres to the printed circuit board 1815. Encapsulate
block 1825 has an associated optical surface 1826. Lens 1835 is
cascaded with lens 1830 on top of encapsulate block 1825 with
registration features 1855, 1856, 1857, 1858 providing the
interconnections. Encapsulate block 1825, with optical surface
1826, combined with lens 1830, with optical surfaces 1831, 1832 and
lens 1835, with optical surfaces 1833, 1834 forms a lens system
providing high resolution.
[0117] Conventional lens systems which allow for variable focus,
variable focal length (zoom) and various apertures employ motorized
actuators which move the lenses relative to each other and the
image plane. Such systems are complex and costly and may not be
suitable for all environments, particularly the high-vibration and
extreme temperature in the automotive environment. As an
alternative to the use of motorized components, piezoelectric
materials may be used to move corresponding lens elements. Of
particular interest is the use of a piezoelectric polymer, such as
polyvinylidene fluoride (PVDF). A detailed description of the use
of electro-active polymers for use in mechanical actuators is
included in U.S. Published Patent Application No. 20010026265,
hereby incorporated in its entirety by reference thereto.
[0118] FIG. 19 illustrates an embodiment of the present invention
that facilitates movement of optical elements. A bottom lens 1925
is attached rigidly to a support structure 1903 that suspends lens
1925 above a pixel array 1910 that is mounted onto a circuit board
1915. A top lens 1930 is attached to the bottom lens 1925 with an
electro-active (piezoelectric) polymer 1906 therebetween. Another
polymer seal 1907 provides for additional stiffness and,
optionally, for pre-loading of polymer 1906. Seal 1907 may be
implemented as a spring or other means of providing for stiffness
and pre-loading, or may be omitted in accordance with the present
invention. Electrodes on surfaces 1908 and 1909 of lenses 1925 and
1930 provide for an electric field to be applied to polymer 1906
causing it to expand. Electrodes are preferably formed of an
indium-tin-oxide (ITO) conductive transparent coating or other
transparent conductor. Clips (not shown) allow for electrical
contact with the ITO coating.
[0119] In some cases, it may not be desirable to position the
electro-active polymer in the optical path of the optical system.
This may be due to limits in the transparency or other properties
of the polymer or may be due to manufacturing issues. Another
embodiment of an optical system in accordance with the present
invention that employs an electro-active (piezoelectric) polymer,
or non-polymer piezoelectric device, is shown in FIG. 20. In this
embodiment, lens 2025 is fixed rigidly to support structure 2003
with a lens holder 2008, which may be a ring, surrounding lens
2025. Lens 2030 is held by lens holder 2009 which is connected to
lens holder 2008 through piezoelectric device 2006 and material
2007. Material 2007 provides stiffness and preloading of
piezoelectric device 2006, if necessary. Electrodes (not shown) may
be contained in lens holders 2008, 2009. These lens holders may be
separate components or may be molded, or integrated, with lenses
2025, 2026.
[0120] In yet another embodiment, a lens may be formed from a
slightly pliable material, such as silicone. Lens holders may be
formed of a piezoelectric device and designed to expand laterally
when an electric field is applied, thus pinching the lens and
causing its thickness and curvature to increase, thus changing the
focus, or focal length, of the associated optical system.
[0121] The embodiments of the present invention depicted in FIGS.
19 and 20 are merely exemplary and are not intended to limit the
present invention to any particular structure. Any number of
variable focus lens systems may be used and may contain flat,
convex or concave surfaces. Associated optical surfaces may be
spherical, aspheric, diffractive, or may contain any other
optically active, or passive, surface. Individual lens systems may
be molded from a polymer, may be glass or any other suitable
material for visible or non-visible imaging. Any number of
actuators may be used to vary the distance between any number of
lenses or vary the distance between lenses and the sensor. Lenses
may be moved for the purpose of establishing a desired focus,
adjusting focal length or any other purpose. Focus, or focal
length, may be adjusted by analyzing the image acquired by pixel
array 1910 and setting the voltage to the actuators accordingly.
The adjustment may also be set in manufacturing, under user
control, by a secondary auto-focus sensor, or by a predetermined
routine for a given application.
[0122] In operation, an automatic vehicle exterior light control
system should preferably be able to distinguish headlights of
oncoming vehicles and taillights of leading vehicles from
non-vehicular light sources or reflections off of signs and
roadside reflectors. The ability to distinguish these various
objects may be enhanced with optimal combination of various color,
ultra-violet and infrared spectral filters. FIG. 21 depicts plots
of the spectral content of different types of vehicular related
light sources. FIG. 22 depicts plots of the spectral reflectance of
various colored signs. FIG. 23 depicts plots of the percent
transmission of red and infrared spectral filters used in a
preferred embodiment of the present invention. FIG. 24 depicts a
plot of the quantum effeciency of an optical system in accordance
with the present invention. Numerical data depicted by the plots of
FIGS. 21-24 is utilized, as described in detail below, to
categorize various light sources.
[0123] The brightness of a given detected light source can be
estimated by multiplying the spectral output of the source, as
shown in FIG. 21, by the infrared spectral filter transmission
factor, as shown in FIG. 23, multiplied by the spectral response of
the pixel array, as shown in FIG. 24. For the red filtered pixels,
this value is further multiplied by the transmission factor of the
red spectral filter. The brightness of detected reflections from
road signs can be estimated by multiplying the controlled vehicle's
headlight spectral output, as shown in FIG. 21, by the spectral
reflectance factor of the sign, as shown in FIG. 22, the infrared
spectral filter transmission factor, as shown in FIG. 23, and the
spectral response of the optical system, as shown in FIG. 24. For
red spectral filtered pixels, the preceding result is then
multiplied by the red spectral filter transmission factor, as shown
in FIG. 23.
[0124] The ratio in brightness between the object projected onto
the red filtered pixels in relation to the object projected onto
the non-red filtered pixels can be used to determine the relative
redness of an object. This ratio is then utilized to determine if
the object is a taillight or a headlight. FIG. 25 depicts the
computed ratios of the brightness of objects projected onto red
filtered pixels relative to those same objects projected onto the
non-filtered pixels. As shown in FIG. 25, taillights have a much
higher red-to-clear ratio than headlights, or most other
objects.
[0125] Discrimination amongst light sources can be further improved
with the use of blue-enhanced headlights. Such headlight bulbs are
commercially available and produce a bluer, or cooler, color light
that more closely approximates natural daylight. These headlight
bulbs are sometimes used in combination with high-intensity
discharge (HID), low beam, lights to more closely match the color.
Finally, Halogen-Infrared (HIR) bulbs, which contain a coating to
reflect infrared light back into the bulb, have a cooler light
output and may be used. HIR bulbs have the advantage of emitting
less red light as a percentage of their total output, as shown in
FIG. 21. As a result, the image of signs reflecting light will have
a lower brightness on red filtered pixels than on non-red filtered
pixels. Other light sources which emit less red light in proportion
to the total amount of light may be advantageously used to minimize
the false detection of road signs and reflections off of other
objects. HID high beam lights and LED headlights are examples of
such sources.
[0126] It is common to classify the color of white light sources
(such as headlights) by their color temperature or correlated color
temperature. Light sources with a high color have a more bluish hue
and, misleadingly, are typically called "cool-white light" sources.
Light sources with a more yellow or orangish hue have a lower color
temperature and, also misleadingly, are called "warm white light"
sources. Higher color temperature light sources have a relatively
higher proportion of short wavelength visible light to long
wavelength visible light. The present invention can benefit from
the use of higher color temperature headlights due to the reduced
proportion of red light that will be reflected by signs or other
objects that could potentially be detected.
[0127] Correlated color temperature for non-perfect Planckian
sources can be estimated by computing the color coordinates of the
light source and finding the nearest temperature value on the
Planckian locus. The color coordinates are calculated as well known
in the art. The text entitled MEASURING COLOUR, second edition, by
R. W. G. Hunt, incorporated in its entirety herein by reference, is
one source for known teachings in the calculation of color
coordinates. Using the CIE 1976 USC (u', v') color space, a
standard halogen headlight was measured to have color coordinates
of u'=0.25 & v'=0.52. From these coordinates, a correlated
color temperature of 3100K is estimated. The blue-enhanced
headlight of FIG. 21 has color coordinates of u'=0.24 and v'=0.51
and thus a correlated color temperature of approximately 3700K. A
measured high intensity discharge (HID) headlight has color
coordinates of u'=0.21 and v'=0.50 and thus a correlated color
temperature of 4500K. The present invention can benefit when the
controlled vehicle is equipped with headlights having a correlated
color temperature above about 3500K.
[0128] The computations described above were utilized to select the
optimum combined red and infrared spectral filters to maximize the
discrimination of taillights from other objects while still
allowing for sufficient detection sensitivity. Preferably, a long
wavelength pass, red, spectral filter having a 50% transmission
point between 580 and 620 nanometers, most preferably at about 605
nanometers, is employed. Preferably, the infrared spectral filter
is selected as a short wavelength pass filter having a 50% cutoff
between 630 and 780 nanometers, more preferably between 640 and 700
nanometers and most preferably at 680 nanometers. Optical systems
with greater sensitivity may benefit from IR filter cutoff
wavelengths slightly less than 680 nanometers, most preferably at
660 nanometers.
[0129] In the examples presented below, the optical systems are
designed and evaluated for performance for on-axis imaging to
facilitate detection of small, distant, light sources. Typically,
the resolution and sensitivity requirements with regard to off-axis
light sources are significantly less since off-axis objects are
only of interest for objects that are closer to the controlled
vehicle. However, the teachings of the present invention are
applicable to off-axis imaging in applications where more
resolution and sensitivity are required. Use of lens systems
corrected for off-axis aberrations are preferable for these
applications.
[0130] Once the desired filter material is incorporated into, or
onto, either the lens system assembly or the pixel array and an
encapsulate block is transfer molded over the pixel array, the lens
system assembly is preferably placed relative the pixel array as
follows. As described elsewhere herein, the lens system assembly
and the encapsulate block may, alternatively, be transfer molded as
one precision piece. Therefore, the following lens system assembly
placement method is not utilized in all embodiments of the present
invention.
[0131] The optical system in accordance with the present invention
is preferably manufactured using a precision placement machine to
accurately place the lens system proximate the pixel array. The
manufacturing method in accordance with the present invention
allows for providing a sharply focused optical system or a
"blurred" optical system. The "blurred" optical systems in
accordance with the present invention are either nearsighted, or
farsighted, depending on the specific control requirements.
Suitable multi-axis precision optical positioning systems are
available from Physik Instrumente of Waldbronn, Germany, and
Burleigh Instruments of Victor, N.Y. Preferably, the positioning
system moves the pixel array under the lens; the lens is fixed in
space with respect to a target (preferably, the target is generated
utilizing a backlit liquid crystal display (LCD)). The positioning
system can locate the optimal position in each axis by moving the
pixel array around underneath the lens system. It is also in
accordance with the present invention to move the lens system while
keeping the pixel array fixed in space. Preferably, the LCD
generates an array of predetermined sized dots as depicted in FIGS.
27-29. Preferably an array of nine dots is generated for lens
system placement.
[0132] The lens system and pixel array alignment process is
preferably initiated by manually selecting a starting point; the
positioning system is manipulated manually to a position such that
at least one distinct dot is projected by the lens system onto the
pixel array and detectable thereby. It is also in accordance with
the present invention to program the positioning system to perform
this initial lens system and pixel array placement. Most
preferably, the mechanical tolerance achieved when loading the
pixel array and lens system into the positioning system is such
that at least one distinct dot will be imaged by the optical
system.
[0133] A standard seed fill algorithm, as described in U.S. Pat.
No. 6,587,753, which is of common assignment herewith and is
incorporated herein in its entirety by reference, is used with a
high threshold to determine "objects" in the image. The seed fill
is a recursive algorithm that rasters through an image until it
finds a pixel whose grayscale value is above the given threshold.
When such a pixel is found, it adds the given pixel to an object
record. The identified pixel is zeroed in the image to prevent it
from being counted twice. Each of its neighbors is examined to see
if one or more of them exceed the threshold. If any neighboring
pixels are bright enough, the seed fill algorithm runs on that
pixel. The seed fill algorithm continues in successive fashion
through all pixels of the pixel array.
[0134] In order to provide the highest flexibility possible when
placing the lens system relative the pixel array, the only criteria
for a starting point are that the focus is good enough to
distinguish individual dots, and at least one dot is visible in the
starting image. If these criteria are met, the preferred
positioning system iteratively discovers the correct placement. If
less than 18 dots (3.times.3 in red and 3.times.3 in white for the
optical system of FIG. 3) are visible, a coarse correction
algorithm is used. The preferred coarse correction algorithm is
based on the bounding box of the dot, or dots, that are currently
visible. By determining the minimum and maximum with regard to the
x- and y-axis, the algorithm determines which direction to correct
the placement. The following is a simplification of the preferred
coarse correction algorithm:
If ((rightX-maxX)>(minX-leftX))
[0135] Move +X
[0136] Else
[0137] Move -X
[0138] End
[0139] Where:
[0140] rightX is the value of the right border coordinate
[0141] maxX is the value of the right border of the bounding
box
[0142] leftX is the value of the left border coordinate
[0143] minx is the value of the left border of the bounding box
[0144] Once all 18 dots are visible (with regard to a dual lens
system assembly as in FIG. 3; 9 dots with regard to a single lens
system as in FIG. 8), a fine X/Y correction method is preferably
employed as follows. Mathematically it is known where the optical
center of the optical system is. An attempt is made to move the
group of 9 dots (3 rows by 3 columns) into the center of the image.
A weighted average is used to determine the center of the dots; the
"center of brightness" of the center dot in the group is then
determined. The preferred fine correction formula for x-axis
position placement is: 1 x COM = ( x i * G v i ) x i
[0145] where:
[0146] x.sub.com is the x coordinate at the center of mass,
[0147] x.sub.i is the individual x coordinate of the pixel in
question
[0148] GV.sub.i is the individual grayscale value of the pixel in
question
[0149] The y-axis fine position correction is calculated in a
similar fashion. With regard to the optical system depicted in FIG.
3, the center dot in the red image will always fall in the 5.sup.th
spot and the center dot in the white image will always be in the
14.sup.th spot, regardless of tilt, because a raster scan through
all of the columns in each row is performed to complete the seed
fill algorithm and a high threshold (typically a grayscale value
at, or around, 80 LSBs) is used to filter out fringe pixels.
[0150] To most accurately calculate position, the above fine x/y
correction calculations are preferably performed for each dot
separately (assuming target position/spacing is known).
Alternatively, a "center of brightness" calculation is performed on
all of the dots in aggregate. However, it is difficult to line up
all of the dots precisely on their respective target coordinates
before rotation of the pixel array about z-axis (theta direction)
is corrected.
[0151] Another option for X/Y placement in accordance with the
present invention is to continue to use the bounding box (maximum
and minimum in both x-axis and y-axis), assuming that the array of
dots is symmetrical. However, this encounters the same problem as
using all of the dots if rotation is not corrected first; the
problem arises in situations where some of the dots are off the
pixel array.
[0152] In theory, neither of these optional algorithms would be too
far off from optimal in a high-resolution pixel array system
because the errors in each axis would show up exactly the same in
both the plus and minus directions. The center points of both boxes
(angled and straight) are in the same point in space. However, with
a low-resolution optical system, the error may be too great to
produce a reliable, repeatable, metric.
[0153] In order to correct for angular error, the same seed fill
algorithm is used to locate objects. The center locations of the
dots in the middle column (3 red, 3 white) are extracted to
determine an equation of a line that joins their centers of mass
(same calculation of x/y correction). Preferably, the line is
calculated through simple linear regression (least squares method).
The error in theta is the tangent of the slope of the regressed
line. By minimizing that angle, the proper theta position is
located. Alternatively, the average of the lines through all
columns of dots can be used to achieve a more accurate measure of
rotational error.
[0154] Once the x-axis, y-axis and angular positions are
established, the optical system is focused as follows. In order to
find the sharpest focus point for the optical system, the target
pattern is preferably changed from dots to a set of narrow vertical
lines as depicted in FIG. 31a; this is one advantage of using an
LCD to generate the target. In theory, the focus is sharpest where
the lines are most highly resolved (assuming the individual lines
are not wider than one pixel).
[0155] The sharpest point of focus is found using a simple
averaging of the grayscale values (grayscale values above the seed
fill's threshold) to find the point at which the lines of the
target are resolved the highest. Sharp focus is detected by
searching, first in coarse steps, subsequently in fine steps, until
a point two steps past the peak resolution is found. Once a down
slope is detected in line resolution, the pixel array is backed off
to the highest measured point, and that position is denoted as
providing sharpest focus. FIG. 30a depicts a low resolution image
of the line pattern 30b of a corresponding target.
[0156] An alternative method is to use a number of small dots, in
lieu of lines, to determine sharpest focus. In this scenario, the
precision placement machine moves the pixel array in the z-axis
direction until the dots resolve to their highest resolution and
smallest values. A large number of dots provide some averaging, and
better noise immunity than using a single dot. This method works
similar to the line method; simply locating the maximum of
resolution and the minimum of average size. Once those peaks are
passed, the z-axis height can be moved back into the ideal
position.
[0157] Another method of focusing the optical system in accordance
with the present invention is to use one large dot. Similar to the
use of several small dots, preferably, the single large dot
provides a large number of pixels over which to average the
results. The downside of this method is that saturation of the
central pixels can be problematic, saturation would create
misleading results for determining sharpest focus.
[0158] In another lens system placement method in accordance with
the present invention, the centroids of an array of 3.times.3 dots
is used to measure the focal length (and thus the focus) of the
optical system. The further the lens is located from the pixel
array, the further apart the objects will be projected by the lens
system onto the array. Since the target size is predetermined, the
lens system can be moved until the images of the dots appear at a
predetermined spacing on the pixel array. Tilt in both the x and y
dimensions can be corrected in a similar fashion. Tilt in either
direction will make the dot pattern appear trapezoidal as shown in
FIG. 29. The difference in object spacing between the top and
bottom rows, or the left and right rows, can be used to measure,
and thus correct, for tilt.
[0159] A final method to focus the optical system in accordance
with the present invention is to use a "resolution test" as
detailed below. By directly measuring the resolving capability of
the optical system, the desired focus position can be determined
consistently.
[0160] As discussed above, it has been discovered that, in certain
scenarios such as for detection of distant taillights, defocusing
the optical system provides better results than placing the lens
system at the sharpest point of focus. This "blurring" has several
desirable effects, including acting as an optical low pass filter
to prevent image aliasing and reducing the pixel-to-pixel variance
when imaging that which would otherwise be "sub-pixel sources."
[0161] To provide part-to-part defocusing uniformity, a test was
developed that directly measures the minimum angle that the pixel
array can resolve, this translates into a measure of focus. The
resolution test preferably uses equal-sized, alternating bars of
light and dark in the target as depicted in FIGS. 31a, 32a, and
33a. For each image (or average of several images to eliminate
noise), the preferred software calculates the Fourier transform of
each row. The Fourier transform translates the spatial domain
signal (similar to a sine wave in grayscale values) to the spatial
frequency domain. The fundamental frequency of the sine wave will
show up as an obvious peak in the frequency domain. However, as the
lines get closer together and the magnitude of the waveform
captured gets closer to DC (i.e., approximately a flat line), the
peak is reduced as shown in FIGS. 31b, 32b, and 33b. Due to the
fact that a discrete Fourier transform is employed, ripples are
observed in the results as depicted in FIG. 34. By taking a linear
regression of the peak metric, the point where the regressed line
crosses a chosen level on the y axis is located. The corresponding
point in the x-axis direction is the minimum angle that can be
resolved.
[0162] The graphs as shown in FIG. 34 depict both the measured
resolution metric (the peak of the DFT along with its nearest
neighbors) and the regression line that best fits it. As the line
moves away from sharpest focus, the point at which the regression
line crosses 1.0, the chosen resolution point moves consistently
and predictably to the right, indicating a larger minimum angle.
For clarity, the point at which the lines cross 1.0 in the y-axis
direction is denoted to be the resolution result, and that result
is plotted as a function of the z-axis offset from sharp focus (at
x=0). The result is a well-behaved metric that is very valuable as
depicted in FIG. 35.
[0163] Once an angular specification for the target to be resolved
is established and a level at which the resolution metric moves
from resolved to unresolved is determined, a repeatable, directly
measurable, level of defocus is set that is consistent across
individual optical systems. When the parameter for fuzziness
(blurriness)/sharpness is selected, the rows of dots are normalized
(i.e., each pixel in a given row is divided by the maximum
grayscale value for the associated pixel in that row, making the
high point of each row exactly 1.0) prior to running the Fourier
transform. This allows the sharpness metric to be set to 1.0. Above
1.0, the image is considered to be acceptably sharp and resolved.
Below 1.0, the image is fuzzy (blurry) and unresolved. In a
preferred embodiment, the standard for defocusing lenses is 0.75
degrees. This provides optimum system performance for the detection
of small, distant, taillights. Other values, 0.75 plus or minus
0.1, will be more appropriate in different applications and are in
accordance with the present invention.
[0164] Other methods of constructing an optical system in
accordance with the present invention will be appreciated by those
skilled in the art. For example, the pixel array may be contained
in a package and soldered onto the circuit board as shown in FIG.
9. The lens system may attach to a pixel array package or may
attach to the printed circuit board through the use of a lens
system mount. Examples of various pixel array packaging methods are
contained in U.S. Pat. No. 6,130,448 to Bauer et al., and hereby
incorporated by reference thereto.
[0165] Analysis of the optical systems in accordance with the
present invention can be performed using Zemax lens design software
available from Focus Software Inc., of Tucson, Ariz. The analysis
of the optical systems of the examples contained herein was
performed utilizing this software. Detailed explanations of the
analysis methods used in the following discussions can be found in
the Zemax 10.0 Users Guide and the book entitled "Practical
Computer-Aided Lens Design" by Gregory Hallock Smith, both of which
are incorporated herein by reference thereto. Also, for the
purposes of this discussion, the term "sharpest focus" will refer
to the optical parameters that achieve the minimum theoretical
geometric root-mean-square (RMS) spot size on the image plane for
parallel light rays of the primary design wavelength entering the
lens system normal to the entrance aperture of the first lens.
Since a primary consideration of the optical system in accordance
with the present invention is detection of distant taillights, a
primary design wavelength of 620 nanometers is selected for the
following examples. A summary of some key optical properties used
for analysis in the examples is given in Table 1. Actual properties
may vary from those specified and other materials may be used,
varying the results of the analysis accordingly. All simulations in
the examples contained herein are performed for an ambient
temperature of 23.degree. C.
1 Index of Refraction (n) at Material 587 nm Abbe Number (v) Nitto
300H 1.569 31.14 Norland-68 1.492 55.31 Polycarbonate 1.585
29.91
[0166] The following examples are intended as further
representations of the present invention for illustrative purposes
and in no way should be interpreted as limiting the scope of the
present invention. As will be appreciated by the skilled artisan,
the present invention can embody a host of optical system
characteristics to satisfy particular desires.
EXAMPLE 2
[0167] The optical system of this example of the present invention
employs a high resolution pixel array to achieve a wider field of
view and greater discrimination of distinct light sources. A 176
pixels horizontal.times.144 pixels vertical pixel array (commonly
referred to as quarter-common image format (QCIF)) containing 15
.mu.m pixels is used. The configuration of the optical system is as
shown in FIG. 3. Lens systems with a diameter of 3.5 mm, a radius
of curvature of 2.15 mm and a conic constant of -0.4 are used. The
lens system assembly is 3.0 mm thick (dimension A as shown in FIG.
3), the adhesive layer is 0.25 mm thick (dimension B as shown in
FIG. 3) and the encapsulate block is 2.5 mm thick (dimension C as
shown in FIG. 3). Sub-windows of 120 pixels horizontal by 50 pixels
vertical on each half of the pixel array are used for a field of
view of approximately 29.degree. horizontal.times.12.degre- e.
vertical (approx. 0.241/pixel). FIG. 36 depicts a MTF plot of the
optical system of this example using a wavelength of 620
nanometers. The absolute spatial frequency cutoff of the optical
system is approximately 32 cycles/mm, just less than the pixel
array's Nyquist frequency limit of 33.3 cycles/mm.
[0168] As an alternate construction of the optical system of this
example, the lens system assembly and the encapsulate block are
transfer molded as one piece. The radius of curvature of each lens
system is adjusted to compensate for the change in total index of
refraction. The radius of curvature is set to 2.10 mm and the total
thickness of material from the tip of the lens system to the
surface of the pixel array chip is 5.75 mm.
EXAMPLE 3
[0169] A 352 pixels horizontal.times.288 pixels vertical pixel
array (commonly referred to as a common image format (CIF)),
containing 7.9 .mu.m pixels, is used in this example. The optical
system of this example is constructed as shown in FIG. 8. The lens
system has a diameter of 4.0 mm, a radius of curvature of 2.375 mm,
and a conic constant of -0.4. The distance from the top of the lens
to the surface of the pixel array is 6.5 mm (dimension A as shown
in FIG. 8). A window of 280 pixels horizontal.times.120 pixels
vertical is used providing a field of view of 32.degree. horizontal
by 14.degree. vertical. The MTF plot of the optical system of this
example is shown in FIG. 37. The absolute spatial frequency cutoff
of this optical system is approximately 31 cycles/mm. Since the
spectral filter material is preferably as shown in either FIG. 11a
or 11b, the pixel pitch should be considered to be twice the pixel
dimension, or 15.8 .mu.m. Therefore, the associated pixel array
Nyquist frequency limit is 1/2d=31.6 cycles/mm.
[0170] In the preferred embodiment of the present invention, color
measurement is achieved by comparing the image formed through the
red filter on one half of the array with the corresponding image
formed on the other half without the filter. Color measurement, as
described above, is applicable to optical systems constructed in
accordance with the embodiment as depicted in FIG. 3.
[0171] Another method of determining the color of light sources,
applicable to optical systems constructed as depicted in FIG. 8,
involves the patterning of a checkerboard filter pattern on the
pixel array such that adjacent pixels are exposed to different
spectral bands of light. This spectral filter pattern can be a
simple checkerboard pattern of alternating red filtered and
non-filtered pixels. Alternatively, this pattern could be a
checkerboard of alternating red and cyan filters. A conventional
color filter mosaic or stripe pattern consisting of alternating
primary red/green/blue filters or complementary cyan/yellow/magenta
filters is in accordance with the present invention. In these
alternate embodiments of the present invention, color is determined
by interpolating the values of the adjacent different color pixels
to determine the actual color of the object imaged. While this
technique is very suitable for conventional imaging of large
objects, which subtend solid angles greater than the solid angle
imaged by each pixel, this technique will not adequately suffice
for color discrimination of small, distant, light sources which
subtend angles less than that imaged by a single pixel. The
limitation introduced in the case of the tiled filter approach is
overcome by limiting the spatial frequency response of the optical
system, as described above, such that accurate color measurement of
small, distant, point sources is possible.
[0172] By providing for more accurate intensity (brightness)
measurement and color measurement of point light sources, the
present invention enables the use of more sophisticated software
techniques for distinguishing between the headlights and taillights
of other vehicles and non-vehicular light sources. For example,
reflections from a controlled vehicle's headlights off of signs may
be distinguished by monitoring a change in brightness of an object
with a corresponding change in the controlled vehicle's high beam
brightness. If such a correlation is detected, the object is likely
a sign, or other object, reflecting light from the controlled
vehicle's headlights. Reflective objects will increase in
brightness proportional to the square of the distance traveled by
the controlled vehicle between consecutive frames, which can be
determined by the controlled vehicle's speed. By accurately
detecting the change in the brightness of the reflection,
reflective objects can be ignored.
[0173] The red color of distant taillights in an image is often
diluted by surrounding ambient light or reflections of the
controlled vehicle's high beams off of the back or bumper of a
leading vehicle. This problem is especially pronounced when the
controlled vehicle is following a semi truck with a reflective, or
partially reflective, back. However, distant taillights of a
leading vehicle will have relatively constant intensity and remain
in approximately the same position in the image for a period of
time. Detection of distant taillights can be improved by monitoring
the relative consistency of brightness and position. For objects
that exhibit consistent brightness and color over several frames,
the thresholds for brightness and redness can be reduced, thereby
improving detection of taillights. False detecting of stationary
nuisance light sources is prevented by waiting a sufficient number
of frames for such objects to have passed out of the image before
reducing the thresholds. Additional object recognition techniques
are described in U.S. Pat. No. 6,631,316, the disclosure of which
is incorporated herein by reference thereto.
[0174] As with the human eye, an optimum optical system in
accordance with the present invention may incorporate a variable
focus lens system. In operation, the vehicle exterior light control
system, analogous to the human eye, varies the focus of the
associated lens system and, or, the corresponding focal length
between being focused on objects far in front of the controlled
vehicle to being focused on objects near the front of the
controlled vehicle. Images of the scene forward of the controlled
vehicle are acquired while the optical system is focused at various
distances. Also in accordance with the present invention, a
three-dimensional representation is obtained when two or more
spatially separated optical systems are employed. Thereby, the
advantages of stereographic vision are exploitable. As will be
recognized by the skilled artisan, a vision system with multiple
optical systems, variable focal length and variable focus lens
systems is relatively costly and complex. However, multiple,
discretely focused, optical systems in accordance with the present
invention are combined to emulate a vision system that provides an
equivalent series of images. Lens systems with varying focal length
(e.g., a zoom lens) or multiple lens systems with different focal
lengths, may be provided to provide different fields of view and
thus vary the ability to resolve distant objects or image a very
wide area. Optical systems that accommodate a variety of focus
points and, or, provide a variety of focal lengths, either by
incorporating multiple lenses and multiple image sensors or by
providing variable optics, are contemplated and within the scope of
the present invention. Such a vision system may facilitate features
in addition to lighting control such as rain sensing, pre-crash
sensing, adaptive cruise control, lane-departure warning, night
vision, and other vehicle control and vision applications.
Additionally, optical systems with automatic, or manual, vertical
tilt and, or, horizontal pan are within the scope of the present
invention. Such flexibility may also compensate for variability in
the optical system with temperature or manufacturing variances.
[0175] Recognizing the desire to obtain images of the associated
scene at various focal points, and ignoring economics, multiple
spatially separated optical systems with infinitely variable lens
systems, large pixel arrays with small individual pixel size, and
R/G/B filter material as in FIG. 11a would be integrated with a
processor with infinite speed and memory. It should be appreciated
that optical systems in accordance with the present invention will
evolve with improvements and reduced cost in these related
components. A single sensor, a one-dimensional array of sensors or
a two-dimensional array of sensors may be utilized to facilitate
automatic vehicle exterior light control in accordance with the
present invention.
[0176] While the many aspects of the present invention are
particularly suited for the development of an automatic vehicle
exterior lighting control system, many additional applications of
the invention are contemplated and thus the invention should not be
perceived as limited to vehicle lighting control or any other
specific purpose. Since many of the aspects of the present
invention contribute to the production of a low-cost, robust,
optical system, many other imaging applications for use in vehicles
would readily benefit from these aspects. Non-vehicular optical
systems would also benefit. For example, a low cost machine vision
system, which tracks the position or orientation of objects, may be
constructed according to the embodiments and methods presented
herein. A vision system that tracks the location of a point light
source (i.e., a LED contained on an object) may be useful for a
variety of purposes. Aspects of the present invention may even be
useful for film cameras or other applications wherein the
electronic image sensor is replaced with another means of acquiring
an image.
[0177] The above description is considered that of the preferred
embodiments only. Modifications of the invention will occur to
those skilled in the art and to those who make or use the
invention. Therefore, it is understood that the embodiments shown
in the drawings and described above are merely for illustrative
purposes and not intended to limit the scope of the invention,
which is defined by the following claims as interpreted according
to the principles of patent law, including the doctrine of
equivalents.
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