U.S. patent application number 14/037961 was filed with the patent office on 2014-01-23 for optical system for direct imaging of light markable material.
This patent application is currently assigned to Sinclair Systems International LLC. The applicant listed for this patent is Sinclair Systems International LLC. Invention is credited to Richard Evans, Richard Hirst, Matthew Scott Howarth, John Michael Rodgers, John Tamkin.
Application Number | 20140022786 14/037961 |
Document ID | / |
Family ID | 43300455 |
Filed Date | 2014-01-23 |
United States Patent
Application |
20140022786 |
Kind Code |
A1 |
Tamkin; John ; et
al. |
January 23, 2014 |
Optical System for Direct Imaging of Light Markable Material
Abstract
An imaging system. An array of light sources and an array of
lenses corresponding to the light sources and having optical axes
substantially parallel to one another are provided. The lenses
produce collimated output beams. An afocal optical relay having an
optical axis substantially parallel to the optical axes of the
lenses is also included, the array of lenses being positioned
relative to the afocal optical relay so as to form an optical
system that produces an image of each collimated output beam on an
image plane, each image having a prescribed depth of focus and spot
size. The light sources preferably are lasers producing an array of
respective laser beams having high intensity and a long waist. A
system for writing information on a light-sensitive label includes
the imaging system. Methods of imaging and of writing information
on a light-sensitive label are also provided.
Inventors: |
Tamkin; John; (San Marino,
CA) ; Rodgers; John Michael; (Pasadena, CA) ;
Howarth; Matthew Scott; (Clovis, CA) ; Evans;
Richard; (Los Gatos, CA) ; Hirst; Richard;
(Fakenham, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sinclair Systems International LLC |
|
|
|
|
|
Assignee: |
Sinclair Systems International
LLC
|
Family ID: |
43300455 |
Appl. No.: |
14/037961 |
Filed: |
September 26, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12477874 |
Jun 3, 2009 |
8570356 |
|
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14037961 |
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Current U.S.
Class: |
362/237 ;
362/235 |
Current CPC
Class: |
F21V 13/04 20130101;
B41J 2/45 20130101; F21V 5/007 20130101; G02B 17/0828 20130101 |
Class at
Publication: |
362/237 ;
362/235 |
International
Class: |
F21V 13/04 20060101
F21V013/04; F21V 5/00 20060101 F21V005/00 |
Claims
1. An imaging system, comprising: a source array of light sources;
an array of lenses corresponding to the light sources having
optical axes substantially parallel to one another, the lenses
producing collimated output beams; and an afocal optical relay
having an optical axis substantially parallel to the optical axes
of the lenses; wherein the array of lenses is positioned relative
to the afocal optical relay so as to form an optical system that
produces an image of each collimated output beam on an image plane,
each image having a prescribed depth of focus and minimum spot
size.
2. The imaging system of claim 1, wherein each of the light sources
has a separately variable output power, and the light sources are
modulated so as to selectively vary their respective output
powers.
3. The imaging system of claim 2, wherein the light sources are
programmable laser diodes that may be individually modulated by
varying a current supplied to the diodes.
4. The imaging system of claim 3, wherein the light sources are
arranged in a linear array.
5. The imaging system of claim 4, wherein the afocal optical relay
comprises a series of powered mirrors.
6. The imaging system of claim 5, wherein the powered mirrors
comprise a first, concave mirror, a second, convex mirror, and a
third, concave mirror.
7. The imaging system of claim 6, wherein the light sources have a
substantially punctile structure in comparison to the
center-to-center spacing between the lenses, the collimated beams
produced therefrom being magnified by the optical relay such that
the images overlap by a selected amount.
8. The imaging system of claim 1, wherein the light sources are
laser diodes and the afocal optical relay comprises a series of
powered mirrors.
9. The imaging system of claim 1, wherein the light sources have a
substantially punctile structure in comparison to the
center-to-center spacing between the lenses, the collimated beams
produced therefrom being magnified by the afocal optical relay such
that the images overlap by a selected amount.
10. An imaging system, comprising: a source array of light sources;
an array of lenses corresponding to the light sources, the lenses
having optical axes substantially parallel to one another and being
positioned relative to their respective light sources so as to
produce first images of the light sources; and an optical relay
comprising at least one powered reflective surface, having an
optical axis substantially parallel to the optical axes of the
lenses, and being positioned relative to the first images of the
light sources to produce at an image plane magnified second images
of the light sources, whereby the powered reflective surface serves
to minimize power loss in the optical relay.
11. The imaging system of claim 10, wherein each of the light
sources has a separately variable output power, and the light
sources are modulated so as to selectively vary their separate
output powers.
12. The imaging system of claim 11, wherein the light sources are
programmable laser diodes that may be individually modulated by
varying a current supplied to the diodes.
13. The imaging system of claim 12, wherein the light sources are
arranged in a linear array.
14. The imaging system of claim 10, wherein the afocal optical
relay comprises a series of powered mirrors that form an afocal
system.
15. The imaging system of claim 10, wherein the light sources are
lasers.
16. The imaging system of claim 15, wherein the powered mirrors
comprise a first, concave mirror, a second, convex mirror, and a
third, concave mirror.
17. The imaging system of claim 10, wherein the light sources have
a substantially punctile structure in comparison to the
center-to-center spacing between the lenses, the first images
produced thereby being magnified by the optical relay such that the
second images overlap by a selected amount.
18. The imaging system of claim 1, wherein the light sources are
lasers.
19. An imaging system, comprising: a source array of lasers, the
source array of lasers producing an array of respective laser
beams; an array of lenses corresponding to, and disposed at a
selected location relative to, the source array of lasers so as to
produce magnified images of the respective laser beams; and an
afocal optical relay, disposed at a selected location relative to
the array of lenses, so as to produce, at an image plane, images of
the respective laser beams, wherein the images meet a selected blur
criterion.
20. The imaging system of claim 19, wherein each lens within the
array of lenses has a front focal plane and a back focal plane, and
wherein the lasers are disposed at a selected object plane relative
to the front focal planes of the respective lenses.
21. The imaging system of claim 20, wherein each laser beam has a
waist and wherein the optical relay has an object plane located at
a distance with respect to the waist such that adjacent images
formed by the optical relay overlap by a selected amount.
22. The imaging system of claim 19, wherein each laser beam has a
waist and wherein the optical relay has an object plane located at
a distance with respect to the waist such that adjacent images
formed by the optical relay overlap by a selected amount.
23. The imaging system of claim 22, wherein the optical relay is an
afocal system wherein aberrations in the image are minimized for an
objected located at the object plane.
24. The imaging system of claim 23, wherein the lasers are
multi-mode lasers.
25. The imaging system of claim 22, wherein each of the lasers has
a separately variable output power, and the lasers are modulated so
as to selectively vary their separate output powers.
26. The imaging system of claim 25, wherein the lasers are
programmable laser diodes that may be individually modulated by
varying a current supplied to the diodes.
27. A method of imaging, comprising: providing a plurality of light
sources; collimating light from the light sources so as to produce
a corresponding plurality of collimated light beams; and afocally
producing images of the plurality of light beams at an image plane,
each image having a prescribed depth of focus and minimum spot
size.
28. The method of claim 27, wherein the light sources have a
substantially punctile structure, and the collimated light beams
therefrom are caused to overlap by a selected amount at the image
plane.
29. The method of claim 28, further comprising magnifying the image
a fractional amount.
30. The method of claim 27, further comprising modulating the light
sources so as to selectively vary their individual power
outputs.
31. The method of claim 28, further comprising arranging the light
source in a linear array so that a two-dimensional pattern can be
produced by modulating the light sources while a target is moved
through the light beams in a direction perpendicular to the axis of
the linear array.
32. A method of imaging, comprising: providing a plurality of light
sources; collimating light from the light sources so as to produce
a corresponding plurality of collimated light beams; and
reflectively producing images of the plurality of light beams at an
image plane.
33. The method of claim 32, wherein reflectively producing images
comprises causing the plurality of light beams to be reflected
sequentially from a plurality of light powered surfaces.
34. The method of claim 33, further comprising magnifying the
image.
35. The method of claim 32, further comprising modulating the light
sources so as to selectively vary their individual power
outputs.
36. The method of claim 33, further comprising arranging the light
sources in a linear array so that a two-dimensional pattern can be
produced by modulating the light sources while a target is moved
through the light beams in a direction perpendicular to the axis of
the linear array.
37. The method of claim 32, wherein the light sources are laser
light sources.
38. A method of imaging, comprising: providing a plurality of laser
light sources having respective outputs; separately producing a
plurality of respective images of the outputs at a selected
location; and afocally producing a single image of the outputs at
an image plane, wherein the plurality of outputs at the image plane
meet a selected blur criteria.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/477,874 filed Jun. 3, 2009 which is a
continuation-in-part of U.S. patent application Ser. No.
11/511,103, filed Aug. 28, 2006, and published as U.S. Patent
Publication No. 2007/0068630 on Mar. 29, 2007, which claimed
priority to Provisional Patent Application No. 60/789,505, filed
Apr. 4, 2006, and to Provisional Patent Application No. 60/712,640,
filed Aug. 29, 2005, and was a continuation-in-part of U.S. patent
application Ser. No. 11/069,330, filed on Mar. 1, 2005, now U.S.
Pat. No. 7,168,472, which claimed priority to Provisional Patent
Application No. 60/549,778, filed Mar. 3, 2004, all of which are
incorporated herein by reference.
TECHNICAL FIELD
[0002] The embodiments of the present invention disclosed herein
relate generally to the field of precision laser direct imaging of
light markable media used in a printing application, and
particularly to writing produce labels "on the fly," with variable,
item-specific information, as the labels are about to be applied
thereto.
BACKGROUND
[0003] Automatic labeling is of interest to the produce industry,
in which it has become a common practice to label each item of
produce with some item-specific information, printed in the form
of, for example, text or a bar code. The information about the
produce may include, for example, its type, size, date harvested,
geographic origin, and whether or not the produce is organic. In
particular, it has become desirable to label each item with a Price
Look. Up ("PLU") number, which enables retailers to facilitate
quick handling and accurate pricing of produce at checkout.
However, in the past, labeling items with different PLU numbers,
for example, denoting "small," "medium," or "large" size
designations for apples, has required three separate labeling
machines, three separate label designs, and three label
inventories. Consequently, it has become desirable to be able to
apply variable, programmable, information "on the fly" to a produce
label tailored to an individual item, thereby requiring only a
single labeling machine and only a single, at least partially
blank, label design. More background regarding this approach can be
found at col. 1 line 11 through col. 2 line 45 of Hirst et al.,
U.S. Pat. No. 7,168,472, entitled Method and Apparatus for Applying
Variable Coded Labels to Items of Produce, which issued Jan. 30,
2007 (hereinafter "Hirst"), the entire disclosure of which is
hereby incorporated by reference herein, and at paragraphs 2-21 of
Griffen et al., U.S. Patent Application Publication No.
2007/0068630, entitled Multi-Layer Markable Media and Method and
Apparatus for Using Same, which was published Mar. 29, 2007
(hereinafter "Griffen"), the entire disclosure of which is also
hereby incorporated by reference herein.
[0004] As disclosed in both Hirst and Griffen, it is desirable to
write variable information directly onto a label using a light
beam. To do this in a rapid, consistent, and cost effective manner
presents challenges arising from the relationships between the
labeling machine, label material, and light beam optics. In
particular, it is desirable to provide a high power light beam so
as to reduce the required label exposure time. It is also desirable
to provide a light beam that has a long depth of focus at the label
so as to ensure that a focused image will be written on the label
despite potentially significant variations in the label position,
relative to the nominal image surface of the light beam optics. It
is further desirable to minimize aberrations in the light beam to
provide, as nearly as practical, a diffraction limited light beam
image at that image surface.
[0005] One method and apparatus for direct writing of a pattern
with a laser beam is described in Tamkin, U.S. Pat. No. 6,084,706
(hereinafter "Tamkin"). Tamkin discloses a three-mirror afocal
optical system in which the mirrors may have aspheric (e.g.,
parabolic, hyperbolic, or elliptical) or spherical surfaces. Such
an all-reflective architecture, which uses mirrors instead of
lenses throughout, achieves a high level of transmission efficiency
compared to a lens-based system, in which the lens medium
inevitably absorbs significant light energy at certain
wavelengths.
[0006] In general, an afocal optical system is an optical system in
which both the object and the image are assumed to be located at
infinity. Light rays entering and leaving an afocal optical system
are parallel. Examples include binoculars and telescopes, in which
the image, although magnified by the optical system, is focused by
the eye. Magnification may increase or decrease (i.e., fractionally
magnify) the size of the image, depending on whether a
magnification factor is greater than or less than one,
respectively. An afocal optical system may be formed by combining
two focal optical systems so that the rear focal point of the first
system coincides with the front focal point of the second system,
yielding an overall system that has no effective focal length.
Several embodiments of a three-mirror afocal system are described
in Tamkin, each having different magnifications.
[0007] In Tamkin, a single laser source and a beam splitter are
used to produce up to eight separate beams, which are then passed
through an optical system to produce a 15,000-pixel image, having
pixel sizes in the range of about 1-10 microns. The three-mirror
afocal system is then used to relay the scan beams with a desired
magnification and minimal loss of power. However, splitting the
power of a single laser into multiple scan beams greatly reduces
the power that can be delivered per unit time to a given spot on an
object, such as a label, thereby affecting the throughput of a
direct scan system. In addition, Tamkin does not address the
challenges of achieving the long depth of focus required in an
automatic "on-the-fly" labeling system.
[0008] A multiple laser diode array may be used in a direct write
application, rather than splitting a single laser into multiple
beams, as disclosed in Landsman, U.S. Pat. No. 6,640,713. However,
unless the laser diode array can be placed immediately adjacent the
light markable medium, as is the case in writing produce labels on
the fly, effective delivery of the laser light to the medium
remains a challenge.
[0009] Johnson, U.S. Pat. No. 6,177,980 (hereinafter "Johnson"),
discloses an optical system that couples an array of miniature lens
elements, or lenslets, with an image projection system in a low
resolution, large field microlithography application. Johnson
modulates the expanded beam of a single diode laser source using a
grating light valve or an array of micromirrors. The modulated
light is then focused by an array of lenslets into widely spaced
point images. The beam separation between the lenslets in Johnson
is substantially wider than the focused spot, which requires a
writing strategy that is not suitable for high-speed, in-line,
web-fed processes. While Johnson discloses the use of an afocal
system with an array of lenslets in a direct writing application,
it does not address the aforementioned challenges that exist in the
design of a direct write imaging system in which the position of
the image plane may change significantly with time, the initial
quality of the beam is poor, as in the output of a multi-mode diode
laser, the illumination power of the beam must be high, and a
physically compact, cost effective optical package is
desirable.
[0010] Accordingly, there is a need for an improved optical system
for photosensitive printing by direct writing with a laser beam on
a light markable medium, wherein the position of that medium may
vary significantly, the illumination power is high, and the optical
system should be compact and cost effective.
SUMMARY
[0011] An imaging system is disclosed.
[0012] In a first respect the imaging system includes an array of
light sources, an array of lenses corresponding to the light
sources and having optical axes substantially parallel to one
another. The lenses produce collimated output beams. An afocal
optical relay having an optical axis substantially parallel to the
optical axes of the lenses is also included, the array of lenses
being positioned relative to the afocal optical relay so as to
foiin an optical system that produces an image of each collimated
output beam on an image plane, each image having a prescribed depth
of focus and spot size.
[0013] In a second respect the imaging system includes an array of
lasers, the array of lasers producing an array of respective laser
beams. It further includes an array of lenses corresponding to, and
disposed at a selected location relative to, the array of lasers so
as to produce magnified images of the respective laser beams. An
optical relay is disposed at a selected location relative to the
array of lenses, so as to produce, at an image plane, images of the
respective laser beams, wherein the images meet a selected blur
criterion.
[0014] A system for writing information on a light-sensitive label
is also disclosed. The system includes an array of light sources
that produces an array of light beams, and an array of lenses
corresponding to the light sources for directing the light beams
toward an image plane. A labeling apparatus is provided for
positioning the light-sensitive label at the image plane. An
optical relay disposed between the source array and the labeling
apparatus produces a magnified image of the light beams on the
light-sensitive label so as to expose the label and thereby write a
pattern thereon.
[0015] Methods of imaging and of writing information on a
light-sensitive label are also disclosed.
[0016] It is to be understood that this summary is provided as a
means for generally determining what follows in the drawings and
detailed description, and is not intended to limit the scope of the
invention. Objects, features and advantages of the invention will
be readily understood upon consideration of the following detailed
description taken in conjunction with the accompanying drawings
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] Embodiments of the present invention will be readily
understood from the following detailed description in conjunction
with the accompanying drawings. To facilitate this description,
like reference numerals designate like structural elements.
Embodiments of the invention are illustrated by way of example and
not by way of limitation in the figures of the accompanying
drawings.
[0018] FIG. 1 is a perspective view of an automatic produce
labeling apparatus in which a laser beam is used to write coded
information on a multi-layer thermally-sensitive label.
[0019] FIG. 2 is a side view of a portion of a bellows with a label
attached thereto and aligned with the optical axis of a preferred
embodiment of the optical system disclosed herein.
[0020] FIG. 3 is a schematic side view of an array of laser beams
produced by a source array of laser diodes and collimated by an
array of microlenses.
[0021] FIG. 4 is a schematic end view showing the geometry of
custom-fabricated microlens array of FIG. 3.
[0022] FIG. 5 is a detailed end view of a lens portion of a single
lenslet within the microlens array shown in FIG. 3
[0023] FIG. 6 is a cross-sectional side view of a single lenslet
within the microlens array shown in FIG. 3.
[0024] FIG. 7 is a layout diagram for a preferred embodiment of an
optical system disclosed herein, showing example marginal rays from
a single laser diode as they propagate through the system.
[0025] FIG. 8 is an unfolded side view of the optical system of
FIG. 7, showing only the three powered mirrors, in which concave
surfaces of the first and third mirrors, and a convex surface of
the second mirror, are visible.
[0026] FIG. 9 is a thin lens schematic for the three-mirror afocal
portion of the optical system of FIG. 7, showing chief and marginal
rays emanating from a single, representative, laser diode at the
center of the diode array, and propagating through the entire
optical system.
[0027] FIG. 10 is a side view of a Gaussian laser beam profile
showing the change in beam width with propagation.
[0028] FIG. 11 is a wave optics illustration of the effect of
multi-mode operation of a diode laser on the waist of a Gaussian
beam produced by the laser when collimated by a lenslet.
[0029] FIG. 12 is a plot of the width of a laser beam as a function
of distance from a laser source.
[0030] FIG. 13 is a plot of the location of an output waist of a
laser beam as a function of the location of its input waist.
[0031] FIG. 14 is a pictorial view illustrating misalignment of a
laser beam spot with respect to a target label.
[0032] FIG. 15 is a reproduction of the optical layout diagram of
FIG. 7, further showing placement of a label edge sensor at the
input to an afocal optical relay.
[0033] FIG. 16 is a schematic of a dichroic beamsplitter within the
label edge sensor of FIG. 15.
[0034] FIG. 17 is a reproduction of the optical layout diagram of
FIG. 7, further showing placement of a detector for monitoring
laser power during calibration of an afocal optical relay.
DETAILED DESCRIPTION
[0035] As mentioned above, an advantage in using a direct-write
laser system for creating product labels is that the label
information may be changed "on-the-fly" according to variations in
the product, such as size. For example, instead of sorting a batch
of fruit by size prior to labeling, individual fruits may be
labeled immediately after measuring. In an embodiment of the
produce labeling method and apparatus of Hirst, referred to and
incorporated herein by reference in its entirety, a label is
acquired by a bellows from a strip of removable labels, exposed to
a light beam that causes a pattern of light to be written through
the label and onto the front surface of the label, and then applied
by the bellows to an individual item of produce. (Hirst, FIGS. 1A
and 1B; Hirst, col. 3, lines 45-59).
[0036] Such a method and apparatus, and the labels used therewith,
present several challenges in the design of an optical system for
writing on the label in the most effective way. One challenge
arises because the longitudinal position of the label may vary
significantly as the bellows rotates into position to apply the
individual label onto the produce. Consequently, the consistency of
the spot size written on the label depends, in part, on the depth
of focus of the light beam and, in part, on the quality of the
light beam. Another challenge arises because the beam of light is
generally required to be of an intensity sufficient to expose the
photosensitive media adequately. A further challenge is to produce
an intense, high quality beam with a relatively long depth of focus
in a physically convenient, cost-effective package.
[0037] Turning to FIG. 1, a labeling apparatus 40 is used to
measure, and to immediately apply a label 41 to a product 42 being
processed in a production line 44. In this example, size or other
data about product 42 is gathered by a sensor 46, and transmitted
to a laser coding device 48 that emits a laser beam 50 having an
optical axis 52. Labeling apparatus 40 transfers an adhesive-backed
blank label 41 from a roll 54 of blank labels 41 onto a bellows tip
56 of a rotary-mounted bellows 58. Upon acquiring label 41, bellows
58 holds the label 41 in place by maintaining low pressure at the
interface between label 41 and bellows tip 56. As bellows 58
rotates along a curved path 59 toward production line 44, label 41,
preferably of a multi-layer thermochromic type, passes through the
optical axis 52 of laser coding device 48 and is exposed to laser
beam 50. Laser beam 50 is directed to propagate through an optical
conditioning device 60, such as a lens system shown schematically
in FIG. 7. Optical conditioning device 60 conditions laser beam 50
so that it is suitable to accurately write coded label information
directly onto label 41. As the rotary-mounted bellows 58 continues
through its rotation, bellows 58 applies label 41 onto product 42,
and repeats the cycle just described.
[0038] In a commercial application of such a produce-labeling
system, a significant challenge is posed by the need for accurate
timing, processing speed, and the need to focus an image accurately
onto a moving target. For example, the labeling apparatus described
in Griffen at paragraphs 114-120, the disclosures of which have
been incorporated by reference above, is able to sustain a product
throughput of 720 items of produce per minute. It is therefore
desirable for the laser beam image projected onto label 41 to have
a large depth of focus so that the image will remain in focus and
retain its magnification throughout as much of the bellows' motion
as possible, as indicated in FIGS. 6-8 and in paragraphs 63-64 of
Griffen, the disclosures of which have been incorporated by
reference above. However, some depth of focus may be sacrificed in
favor of high power to expose the relatively large area of the
label 41, which is about 20 mm wide. Characteristics of single
laser diode sources or laser diode arrays suitable for use in such
a produce labeling system are given, for example, in Griffen at
paragraphs 0119 and 0120. They include wavelengths between 800 and
1600 nm and power levels of about 500 mW per laser diode.
[0039] As shown in FIG. 2, the bellows 58 in an automated labeling
system as described above, and in Hirst, FIGS. 1A-1B, and in
Griffen, FIGS. 6-8, has a bellows tip 56 that pneumatically
attaches to a light-sensitive label 41, acquires it from a backing
material (not shown), moves it through the optical axis 52 of the
optical beam conditioner 60, and applies it to an item of produce,
as previously explained. Griffen describes, in FIGS. 9A-9B, and in
paragraphs 65-66, an example of a particularly advantageous
embodiment of a light-sensitive label 41 that comprises a
three-layer structure shown in FIG. 2, the disclosures of which
have been incorporated by reference above. The label 41 preferably
has a translucent adhesive coating 62, a back, translucent
substrate layer 64, a middle, light absorbent layer 66, and a
front, thermochromic layer 68, arranged in that order, such that
when a beam of light illuminates the back of label 41 it passes
through adhesive coating 62 and substrate layer 64 to absorbent
layer 66, in which the radiant energy is transformed into heat,
which then causes thermochromic layer 68 to change color wherever
it is exposed to light from the back of the label. Such a complex,
multi-layer label comprising different materials may itself be
treated as an optical system characterized by a point spread
function, separate from, and in addition to, a point spread
function characterizing optical conditioning device 60. As the
bellows 58 travels through the optical axis 52, the position of the
label 41 along the optical axis 52 varies over time, primarily due
to inconsistency in the radial extension of bellows 58, but also
due to rotation of the bellows 58 along curved path 59, variation
in the surface shape of label 41, and other factors. This variation
in the position of label 41 is represented by .DELTA.z in FIG. 2.
Consequently, to write a sharp image on label 41 consistently, the
depth of focus of the optical system should be at least as long as
.DELTA.z.
[0040] In addition to the disclosures of Hirst and Griffen,
incorporated by reference in their entirety, including those
particular sections cited above as, the present disclosure
comprises a novel optical system design that performs the functions
of laser coding device 48 and optical conditioning device 60 and
the combination of that optical system with automatic produce
labeling apparatus 40. The optical system comprises a laser diode
source array that generates an array of laser beams, a microlens
array that individually collimates the laser beams, and an afocal
optical relay that conditions the laser beams and produces laser
spots that meet the requirements of a particular application such
as the produce labeling application.
[0041] FIG. 3 shows a close-up side view of a laser diode and
microlens source array assembly 90. Source array assembly 90
comprises a light source array component 100 and a lens array
component 105 spaced apart at a selected distance. Light source
array component 100 includes a power supply (not shown), and,
according to a preferred embodiment, an array of laser diodes 102
that produce an array of laser beams 104 propagating along
substantially parallel axes. Laser diodes 102 are preferably
addressable, programmable light sources, having output powers that
may be individually modulated by varying the current supplied to
each diode in the array. Laser light produced by source array 100
preferably has a laser wavelength of about 980 nm, the nominal
output power level of each of about 300 laser diodes 102 is about
500 mW, and laser diodes 102 are spaced apart by about 125 microns.
Laser diode arrays of the type described herein can be obtained,
for example, from OSRAM Opto Semiconductors, Inc. of Sunnyvale,
Calif. and Laser 2000 GmbH in Munich, Germany. Lens array component
105 preferably comprises a collimating microlens array 106, the
elements of which are individual lenslets 107 having substantially
parallel optical axes, the lenslets 107 thus producing a collimated
array of laser beams 104.
[0042] FIG. 4 shows an end view of a preferred embodiment of a
customized microlens array 106, having an array length 202 of about
35 mm and an array width 204 of about 5 mm. Microlens array 106 is
a custom-fabricated device manufactured by companies such as
Rochester Photonics Corporation of Corning, N.Y. Microlens array
106 is fabricated by construction of a repeating linear pattern of
microlens array elements, or lenslets 107. Lenslets 107 are
disposed adjacent to one another, with a center-to-center spacing
distance 208 of about 125 microns, forming a one-dimensional,
vertical row 210, of about 280 lenses. As shown in FIG. 5, which is
an enlarged end view of a single microlenslet 107, centered within
each lenslet 107 is an individual transparent microlens 212, having
a lens diameter 214 of about 500 microns. Microlenses 212 may be
replicated in polymer, solgel, or etched into the glass substrate.
In a preferred embodiment, a pair of clear aperture (i.e.,
transparent), aspherical, convex, conic section polymer microlenses
212 are used to control aberrations, instead of using a unitary
cylindrical lens design followed by a single-surface array, as is
common in existing laser array systems.
[0043] FIG. 6 shows a single lenslet 107 in cross section. Lenslet
107 is fabricated on a fused silica (glass) substrate 216 about 1
mm thick, with an index of refraction of about 1.45. Flanking
substrate 216 are two parallel photopolymer base layers 218, about
50 mm thick, with an index of refraction of about 1.54. Each
polymer microlens 212 preferably has an aspheric, hyperboloid
shape, and is formed so as to protrude laterally by a height 219 of
about 40 microns from either a front surface 220, or a rear surface
222 of polymer base layer 218. At the center of each lenslet 107 in
the vertical row 210, one bi-hyperboloid polymer lens 212 protrudes
from front surface 220, and another lens 212 protrudes from rear
surface 222. Thus, an individual laser beam 224 propagating from
left to right in FIG. 6 at perpendicular incidence to the plane of
the microlens array 106 passes through a pair of hyperboloid lenses
212, as well as through polymer base layers 218 and glass substrate
216 sandwiched between the lens pair.
[0044] In FIG. 7, an all-reflective afocal optical relay system 300
is shown positioned between an object plane 301, at which the
source array assembly 90 is positioned, and an image plane 302,
which is co-located with a target label 303. Alternatively, while
an all-reflective system is preferred to minimize power losses in
transmission, it is to be understood that optical relay system 300
may be refractive, comprising lenses instead of mirrors, without
departing from the broadest principles of the invention.
[0045] An output image 304, of the array of laser beams 104, is
formed at the image plane 302, the image 304 comprising individual
laser beam spots, each having a spot size 308. From a geometrical
optics point of view, rays of light comprising each laser beam 224
produced by a given laser diode 102 of the source array 100, are
collimated by a given lenslet array 107, and then the collimated
beams propagate through a series of polished mirrors 310-320, some
of which are powered, to produce a fractionally magnified output
image 304 of the laser beam spot at image plane 302. Because the
chief rays enter and leave the afocal optical relay system 300
parallel to the optical axis, the magnification does not change
with defocus. The depth of focus is strictly determined by the wave
optics characteristics of the focused laser spot at the final image
plane. This is one advantage of the preferred system design
shown.
[0046] As the rays comprising laser beam 224 propagate through
optical relay system 300, they are deflected by each of mirrors
310-320 along a folded optical path, according to the law of
reflection, which dictates that the angle of reflection equals the
angle of incidence with respect to a normal to the surface of the
mirror at the point of reflection. The first two mirrors shown, 310
and 312, are preferably flat mirrors, neither concave nor convex.
Therefore they do not alter the profile of beam 224; rather, they
direct the beam into the tilted mirror system. Mirrors 314, 316,
and 318 are preferably spherical powered mirrors comprising a
three-mirror afocal system 319. A three-element afocal system is
used instead of a two-element system to further control
aberrations. An output mirror 320 is preferably a flat mirror,
angled so as to direct conditioned laser beam 224 toward target
label 303 at the image plane 302. Mirrors 314-318 may be aspheric
when the reduction ratio becomes large, causing the NA to exceed
0.05. The three-mirror system 319 serves to minimize aberrations so
that the system performance remains diffraction-limited, rather
than aberration-limited.
[0047] Referring to FIG. 8, which shows an unfolded, front view of
the three-mirror afocal system 319, in a preferred embodiment, a
first mirror 314 and a third mirror 318 are preferably positive
powered mirrors which increase the size of output image 304; mirror
316 is preferably a convex, negative powered mirror which decreases
the size of output image 304. Mirrors 314-318 thus cooperate to
condition the laser beam 224 to produce the desired output image
304, having a desired spot size 308. A principal characteristic of
the optical system design that includes microlens array 106 is to
remove the punctile nature of laser diode emission, relative to the
array element spacing, thereby causing each of the laser beams 224
to diverge enough that overlapping spots are formed on the image
plane 302. Microlens array 106 effectively reduces the numerical
aperture (NA) of the output of each laser diode 102 by at least
about a factor of 10, thereby relaxing constraints on the design of
afocal optical relay system 300.
[0048] It is important to note that an afocal system maintains the
magnification of the output image 304 even if the object plane 301
or the image plane 302 is shifted. This is important because, as
the position of the bellows tip 56 shifts through the depth of
focus, due to rotation, vibration, and other mechanical errors, the
lateral position of the image will not change or become distorted
during the direct write operation.
[0049] The final magnification of the output image 304 may be tuned
by varying the relative positions of the mirrors within afocal
optical relay system 300. A prescription for a suitable afocal
optical relay system 300 is detailed in Table 1, and illustrated by
FIG. 9.
TABLE-US-00001 TABLE 1 Prescription for afocal optical relay system
300. Radius of Curvature, Optical Element Position, mm mm Object
infinity N/A 1.sup.st powered mirror -190.97 -80.78 (concave)
2.sup.nd powered mirror -86.54 21.19 (convex) 3.sup.rd powered
mirror -159.42 -156.67 (concave) Image infinity N/A
[0050] In FIG. 9, an unfolded, reduced linear ray trace diagram of
a laser beam 224 illustrates optical properties of the preferred
embodiment in greater detail. FIG. 9 shows a thin lens
representation of a lenslet 107 located at object plane 301, the
first powered mirror 314 having focal length f1, the second powered
mirror 316 having focal length f2, and the third powered mirror 318
having focal length f3, distributed in that order to output image
304, along the optical axis 52 of the optical conditioning device
60. Mirrors 314-318 form an afocal system; however, the object is
not actually located at infinity.
[0051] FIG. 9 shows a chief ray 321 representing laser beam 224
that enters mirror 314 from the left, parallel to the optical axis
52, and exits from mirror 318 to the right, again parallel to the
optical axis 52. Each such chief ray generated by each of laser
diodes 102 then follows a path through the optical relay afocal
system 300 like that of the representative chief ray 321 to the
final image 304. A marginal ray 322 represents half of the extent
of the width of laser beam 224. In a preferred embodiment,
fractional magnification occurs, so the laser beam width at the
entrance to the afocal system is greater than the laser beam spot
size 308 at the output of the afocal system.
[0052] An important feature of the preferred embodiments disclosed
herein is the positioning of the laser diode source array 100 with
respect to the microlens array 106, so as to provide both the
desired the depth of focus and beam width at image plane 302, while
also providing the maximum optical power. Lenslets 107 limit the
amount of light collected from each laser diode 102, thus limiting
the size of the laser beam 224 that exits each lenslet 107. At the
same time, for use in on-the-fly label writing as described herein,
and for other high speed imaging applications, it is important that
the images of the laser beam spots corresponding to adjacent laser
diodes 102 overlap at the image plane 302. This is to be able to
produce continuously written areas on label 41, whereby any spaces
between the written areas are the result of turning off one or more
laser diodes 102. Without a microlens 212, the spot sizes on the
facets of the laser diode source 102 are re-imaged onto the image
plane 302. These spots, about several microns in diameter, are thus
very small compared to the center-to-center lenslet spacing
distance 208. Use of a micro lens 212 "collimates" the beam from
each laser, yielding larger spots, about the same size as the 125
micron center-to-center spacing distance 208. Since, as a practical
matter, light from one laser diode source 102 should be captured by
only one lenslet 107, the actual image spot size 308 is slightly
smaller than the spacing distance 208, and the laser beams 224
exiting two adjacent lenslets 107 will not immediately overlap.
However, adjacent laser beams 224 can be caused to overlap at some
distance away from lenslet 107, because the laser beams 224 spread
out as a function of distance (d) according to equation (1).
Therefore, the image to be placed on image plane 302 is not that of
the plurality of laser beams 224 directly exiting lenslets 107;
rather, it is an image located at some distance away from the
microlens array, at which adjacent laser beams 224 overlap
sufficiently.
[0053] Turning to wave optics, FIG. 10 shows a more realistic
representation of the shape of a beam of light that propagates
through the optical system. It will be recognized by a person
having skill in the art that the output of laser diode 102
ordinarily is a Gaussian laser beam 330, and that a lens or powered
mirror of focal length f configured to collimate or focus Gaussian
laser beam 330 produces a waist, or minimum width, .omega..sub.om,
at waist plane 332 in the image space of that lens or mirror. The
laser beam width .omega..sub.m expands along the optical
propagation axis 52 of laser beam 330 as a hyperbolic function of
distance z from the waist .omega..sub.om, such that the width is a
function of the waist .omega..sub.om, distance z, focal length f,
wavelength .lamda., and a mode parameter M given by:
.omega. m ( .lamda. , .omega. om , z , M ) : = .omega. om [ 1 + (
.lamda. z M 2 .pi. .omega. om 2 ) 2 ] .5 ( 1 ) ##EQU00001##
[0054] The depth of focus is, then, a distance b in front of and in
back of the waist .omega..sub.om within which an acceptable blur
criterion is satisfied, as shown in FIG. 10. For a given laser beam
waist .omega..sub.om, the depth of focus b of the optical system is
determined by the width of the laser beam .omega..sub.m and the
mode content of the laser source, as described by its M.sup.2
value. The depth of focus is thus independent of the position of
the afocal relay system 300 relative to the diode lenslet source
array assembly 90. In a preferred embodiment of the optical system
disclosed herein, as used as used in the produce labeling
application, a distance 2b, equal to twice the depth of focus,
should exceed the variation in the label position, .DELTA.z, shown
in FIG. 2.
[0055] FIG. 11 shows the effect of multi-mode operation of laser
diode 102 on the laser beam width and consequently on the depth of
focus, b, as Gaussian laser beam 330 passes through lenslet 107.
With the use of beam optics, a laser beam 338 representing the
central mode of Gaussian laser beam 330, and a laser beam 340
representing an edge mode of Gaussian laser beam 330, it can be
seen that, as compared to single mode operation, in multi-mode
operation (M.sup.2>1), the expansion of the combined laser beam
366 occurs much more rapidly with distance z from the waist than
does the expansion of a single laser beam 368, represented by
arrows 342. This rapid expansion is accompanied by a corresponding
shrinkage of the depth of focus b at the image plane 302.
Preferably, the focal length of the lens is chosen to collect the
most light from each diode 102. Since the center-to-center spacing
distance 208 between array elements is fixed, a lens having a short
focal length will collect more of the diverging light. However, a
lens having a short focal length will also yield a narrow
collimated output beam. Larger focal lengths produce larger spots,
but if the focal length becomes too large, light spills over into
adjacent array elements, causing too much overlap. Typically, the
desired focal length would be that which matches the NA of the
lenslet 107 to the divergence angle of the laser beam 224. However,
these two competing goals are balanced to obtain the optimum focal
length.
[0056] According to Equation 2, the largest spot waist for an
optimum focal length occurs when the laser source 100 is located at
the front focus of microlens 312. If the focal length of microlens
312 is chosen so that the NA of the lenslet 107 matches the
divergence angle of laser diodes 102, then the laser beam width
.omega..sub.m (which, at image plane 302 is effectively the image
spot size 208) as a function of laser source position z is shown in
the plot in FIG. 12. The laser diode source 102 need not be located
at the front focus of the microlens 212, but according to Equation
2, the largest spot size 308 corresponding to the smallest focal
length occurs when the laser diode source 102 is located at the
front focus.
.omega. om 2 ( z , f , .lamda. , .omega. om , M ) : = .omega. om 1
( 1 - z f ) 2 + [ .pi. ( .omega. om M ) 2 .lamda. f ] 2 ( 2 )
##EQU00002##
[0057] Each laser diode 102 could be placed so that the
semiconductor facet that emits the laser light is positioned at the
front focal point of its corresponding lenslet 107, and so that the
waist .omega..sub.om of the laser beam 224 is at the back focal
point of lenslet 107. However, this location is also the most
sensitive to defocus errors. The output waist location d.sub.2 as a
function of the input waist location d.sub.1 may be computed
according to Equation 3, as is shown in FIG. 13:
d 2 ( d 1 , f , .lamda. , .omega. om , M ) : = f 1 - d 1 f - f 2 d
1 2 - d 1 f + .pi. ( .omega. om m ) 2 .lamda. ( 3 )
##EQU00003##
In a preferred embodiment, the focal length of the microlens array
106 is slightly larger than the optimum focal length used in
Equation 2 to obtain the data of FIG. 12, and the focal plane
location of the laser beam is not located at the front focus, which
pushes the waist location at the output to be in the range of about
5 mm-15 mm from the front of microlens array 106. The laser beam
then expands from there so that adjacent beams 224 overlap at some
distance away from the lenslet. The image of the laser beam spots
is then transferred to the image plane 302 by the afocal optical
relay system 300.
[0058] Output image 304 of laser diodes 102 has a predetermined
magnification that is selected to satisfy the pixel pitch
requirement of the direct-write application. This is illustrated by
way of an example, in which thermochromic target label 303 is
positioned for marking at image plane 302, and a bar code marking
width of 18 mm is needed, with a desired image pixel spacing of
about 70 microns. Given that laser beam 224 diverges by about 5-10
degrees at full width, half maximum (hereinafter "FWHM") as it
propagates through microlens array 106, its Gaussian beam radius at
the output of the microlens array 106 is about 62 microns. This
translates to a FWHM laser beam spot size 308 at the output of the
microlens array 106 of about 73 microns. The overall magnification
of the afocal optical relay system 300 is given by the ratio of the
image pixel spacing (70 microns) to the laser diode array pitch, in
this example, (about 125 microns), yielding a factor of 0.562.
Applying this factor to the FWHM laser beam spot size yields a
final output laser beam spot size 308 of 41 microns.
[0059] Referring to FIGS. 14-16, a label edge sensor 350, shown in
FIGS. 15 and 16 may be provided for detecting proper centering of
the laser beam output image 304, as shown in FIG. 14, in which
output image 304 has final output laser beam spot size 308, with
respect to target label 303 positioned on bellows tip 56. In a
preferred embodiment, label edge sensor 350 may be inserted between
microlens array 106 and the input to optical relay system 300.
Turning to FIG. 16, label edge sensor 350 is preferably constructed
using a red laser beam 352 that is reflected by multi-layer
thermochromic target label 303. Red laser beam 352 is split using a
50% dichroic beamsplitter 354, so that half of the red light forms
a reference signal 355 that is deflected by 90 degrees and directed
toward a split detector 356. The other half of the red light forms
a sensing signal 358 that is reflected by a flat mirror 360 so as
to propagate alongside laser beam 224 throughout optical relay
system 300. When sensing signal 358 encounters target label 303, it
reflects and forms a return signal 362. Return signal 362
propagates anti-parallel to laser beam 224, back along the folded
path of optical relay system 300 until it again meets flat mirror
360 and dichroic beam splitter 354, which cooperate to direct the
return signal 362 into split detector 356. If sensing signal 358
and laser beam 224 are misaligned with respect to target label 303,
at least a portion of sensing signal 358 will fail to encounter
target label 303, thereby diminishing the intensity of return
signal 362. When the intensity of return signal 362 is then
compared with that of reference signal 355, a mismatch indicates
misalignment of laser beam 224 on the target label 303.
[0060] Referring to FIG. 17, a single power detector 364 for
monitoring the power level of laser beam 224 may be added to afocal
optical relay system 300. In a preferred embodiment, power detector
364 is placed behind second mirror 316, which may be specially
designed to have partial transmission, thereby allowing a portion
of the light from laser beam 224, ranging from 0.1% to 0.5%, to be
sacrificed and directed into power detector 364.
[0061] Although certain embodiments have been illustrated and
described herein, it will be appreciated by those of ordinary skill
in the art that a wide variety of alternate and/or equivalent
embodiments or implementations calculated to achieve the same
purposes may be substituted for the embodiments illustrated and
described without departing from the scope of the present
invention. Those with skill in the art will readily appreciate that
embodiments in accordance with the present invention may be
implemented in a very wide variety of ways. This application is
intended to cover any adaptations or variations of the embodiments
discussed herein. The terms and expressions which have been
employed in the foregoing specification are used therein as terms
of description and not of limitation, and there is no intention, in
the use of such terms and expressions, to exclude equivalents of
the features shown and described or portions thereof, it being
recognized that the scope of the invention is defined and limited
only by the claims that follow.
* * * * *