Note: Descriptions are shown in the official language in which they were submitted.
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TARGET ILLUMINATION DEVICE
FIELD OF THE INVENTION
The present invention relates to image readers. More particularly, the present
invention relates to an target illumination device for use with an image
reader.
BACKGROUND OF THE INVENTION
To identify certain objects, such as electronic components, many industries,
such as the automotive and electronics industries, often use indicia, such as
bar codes or data
matrix codes, etched onto the surface of the object. Typically, these indicia
represent data
used to identify the objects and, particularly in the case of electronic
components, to
accurately position the components during assembly. Generally, the indicia, or
targets, are
read by an image reader, such as a camera, positioned over the object. To
provide the camera
with a clear image of the indicia to be read, proper illumination of the
indicia is essential.
Often, the surfaces onto which the indicia are etched are shiny, or mirror-
like,
surfaces. Proper illumination of many different shiny and uneven surfaces is
critical,
especially in a an application where robotic assembly is required. However,
shiny, uneven
surfaces are difficult to illuminate for accurate imaging. The uneven
reflections from these
surfaces frequently produce erroneous images and signals within the camera,
thereby resulting
in erroneous identification or positioning of the object.
Identification of objects is rapidly becoming a critical issue in the
manufacture
2 o and sale of miniature components, particularly in the electronics
industry. Identification is
used to track faulty components during automated manufacturing processes. For
example, it
is costly to apply subsequent steps of the manufacturing process on a
component that has been
identified as faulty at an earlier step. By reading the identity of the
component before each
is step is applied, an automated manufacturing process can determine whether
the component
is faulty and, consequently, whether to apply the current step. Thus, if a
component is
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identified as faulty during one step of the manufacturing process, it can be
ignored at all
subsequent steps.
Similarly, object identification is also desirable in order to trace
components
once they have been shipped into the field. If a problem develops with a
component in the
field, the identification on the component provides a key to accessing
historical information
retained on the component at the factory. This historical information is
invaluable in
troubleshooting problems in the field.
One object identification technique that has been used with great success is
the etching ofbar codes onto the objects' surfaces. However, as components
become smaller,
it is necessary to fit more data into less surface area. In response, the
etching of data matrix
codes onto the objects' surfaces has begun to emerge as a preferred
identification technique.
Due to the large amount of data stored in such a small area, it is important
that the image
provided to the camera be as accurate as possible. Thus, to pick up the subtle
contrasts in a
data matrix code etched onto a highly reflective, uneven surface, proper
target illumination
has become even more critical.
Thus, there is a need in the art for a target illumination device that
provides the
necessary illumination of targets on highly reflective, uneven surfaces of
miniature
components such that an image reader can accurately process the image of the
target.
SUMMARY OF THE INVENTION
2 0 The present invention satisfies these needs in the art by providing a
target
illumination device comprising a radiation source, a lens, and a deflector.
The present
invention also provides a radiation directing device comprising a deflector
and a lens, or a
deflector and a wave guide. The radiation source comprises, for example, a
plurality of
light emitting diodes. The lens and the deflector are disposed in spaced apart
relationship
2 5 defining a transmission space therebetween and a reflection space
proximate the
transmission space. In a first embodiment, the reflection space and the
transmission space
are both symmetric around a central axis. In another embodiment, the
reflection space is
symmetric about a central plane, to which the transmission space is
substantially parallel.
The lens is adapted to receive incident radiation from the radiation source
3 o and to transmit at least a portion of the incident radiation in a first
direction through the
transmission space in a manner such that the transmitted radiation is
substantially uniform
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through the transmission space and such that the reflection space is
substantially devoid of
transmitted radiation.
The deflector is adapted to be placed proximate a target and to deflect at
least a portion of the transmitted radiation onto the target at an angle such
that the target
reflects at least a portion of the deflected radiation in a second direction
through the
reflection space. The deflector may have an inner wall and an outer wall, the
outer wall
forming an angle with the inner wall. The outer wall is adapted to deflect the
transmitted
radiation toward the target a predetermined angle.
In a preferred embodiment, the transmission space is contained within the
l0 inner and outer walls of a wave guide. The wave guide is adapted to receive
the
transmitted radiation and to guide the transmitted radiation to the deflector.
The deflector
has a first end adapted to receive at least a portion of the transmitted
radiation and a second
end adapted to prevent at least a portion of the transmitted radiation from
passing
therethrough. The lens and the deflector are integrally formed with the wave
guide. Also,
a trap may be disposed around a portion of the reflection space, on the inner
walls of the
wave guide and the deflector. The trap is adapted to prevent at least a
portion of the
transmitted radiation from entering the reflection space.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood, and its numerous objects
2 0 and advantages will become apparent by reference to the following detailed
description
of the invention when taken in conjunction with the following drawings, in
which:
FIG. 1 shows an embodiment of a target illumination device according to
the present invention;
FIG. 2 shows a target illumination device according to the present invention
2 5 in use in an image reader;
FIG. 3 shows a target illumination device according to the present invention
in use with a hand-held image reader; and
FIG. 4 shows an embodiment of a target illumination device according to
the present invention which is particularly suitable for reading linear
indicia.
3 0 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
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An apparatus which meets the above-mentioned objects and provides other
beneficial features in accordance with the presently preferred exemplary
embodiments of
the invention will be described below with reference to Figures 1-4. Those
skilled in the
art will readily appreciate that the description given herein with respect to
those figures
is for explanatory purposes only and is not intended in any way to limit the
scope of the
invention. Accordingly, all questions regarding the scope of the invention
should be
resolved by referring to the appended claims.
FIG. 1 shows a preferred embodiment of a target illumination device
according to the present invention. As shown in FIG. 1, a target illumination
device 100
comprises a radiation source 20 and a device 101 for directing incident
radiation 10 from
radiation source 20 to a target S0. Preferably, incident radiation 10 is
light. Radiation
source 20, comprises a plurality of radiation emitters 224, preferably LEDs.
By way of
example, target 50 may be any of several indicia, such a bar code or a data
matrix code,
etched onto the surface of an object, such as an electronic component.
Radiation directing
device 101 comprises a lens 140 and a deflector 120. Lens 140 and deflector
120 are
disposed in spaced apart relationship defining a transmission space 106
therebetween and a
reflection space 136 proximate transmission space 106.
Lens 140 is adapted to receive incident radiation 10 and to transmit at least
a portion of incident radiation 10 in a first direction through transmission
space 106 to
2 0 deflector 120 in a manner such that transmitted radiation 12 is
substantially uniform
through transmission space 106 and such that reflection space 136 is
substantially devoid
of transmitted radiation 12. Deflector 120 is adapted to be placed proximate
target 50 and
to deflect at least a portion of transmitted radiation 12 onto target 50 at an
angle 160 such
that target 50 reflects at least a portion of deflected radiation 138 in a
second direction
2 5 through reflection space 136.
In a preferred embodiment, radiation source 20 and lens 140 are constructed
such that the irradiance (i.e., power per unit area) striking deflector 120 is
nearly constant
throughout any differential cross-section of transmission space 106. This is
achieved by
using a plurality of LEDs spaced as closely together as possible and an
anamorphic lens.
3 0 The LEDs used have no optical components embodied within the LED
structure itself. Thus, they individuaiy serve as near point sources of highly
divergent
radiation. The divergence angle of the radiation from the LED source differs
in the
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meridional or tangential plane versus the sagittal plane. This divergence
angle plays a role
in determining the collection efficiency of the lens as described below.
In a preferred embodiment, lens 140 is a half toroid. It serves as an
anamorphic optical element. An anamorphic optical element is one in which the
optical
power or "light bending capability" is different in the sagittal plane versus
the tangential
plane. In the tangential plane, lens 140 collects and nearly collimates
incident radiation 10
into transmission space 106. In the sagital plane, which can be viewed as an
infinitesimal
arc segment of a circle which bisects the half toroid, lens 140 has no power
and incident
radiation 10 continues to diverge in this plane. Some of this radiation
remains in the
transmission space overlapping radiation from the sagittal planes of
contiguous LEDs and
contributes to the irradiance uniformity in this direction in transmission
space 106. (Note:
it is contemplated that lens 140 may be designed such that a radius is
constructed along the
sagittal direction to control the divergence and overlap along this direction
in transmission
space 106. This would effectively break-up the half toroid). The combinatorial
effect of
the lens is to efficiently collect the radiation and to provide a source of
irradiance which is
constant throughout any differential cross-section of transmission space 106.
As a second purpose in collecting and redirecting the radiation, lens 140
also prevents radiation from the highly divergent source from entering
reflection space
136. This effect is augmented by the addition of trap 142, described more
fully below.
2 0 Interference, or noise, would be defined as radiation reflections from
target 50 whose
initial source was from a direction other that that coming from deflector 120.
In keeping
reflection space 136 devoid of any transmitted radiation 12, this interference
is minimized,
thus increasing the signal-to-noise performance of the device 101.
As shown in FIG. 1, the device of the present invention further comprises a
wave guide 110. Wave guide 110 is adapted to receive transmitted radiation 12
and to
guide transmitted radiation 12 to deflector 120. Wave guide 110 has an inner
wall 116 and
an outer wall 119. Transmitted radiation 12 is directed through wave guide 110
between
inner wall 116 and outer wall 119. Thus, in the embodiment shown in FIG. 1,
transmission space 106 is essentially contained between firmer wall 116 and
outer wall 119
3 0 of wave guide 110 and inner wall 116 of wave guide 110 forms a boundary of
reflection
space 136. By directing transmitted radiation 12 between inner wall 116 and
outer wall
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119, lens 140 prevents at least a portion of transmitted radiation 12 from
entering
reflection space 136.
Similarly, deflector 120 has an inner wall 126 and an outer wall 129. Inner
wall 126 of deflector 120 also forms a boundary of reflection space 136. Outer
wall 129 of
deflector 120 forms an angle 131 with inner wall 126 of deflector 120 and an
angle 130
with outer wall of wave guide 110. Deflector 120 may be fixedly connected with
wave
guide 110, or deflector 120 may be integrally formed with wave guide 110.
Outer wall of
deflector 120 is adapted to deflect transmitted radiation 12 at an angle 132
toward target
50.
to In a preferred embodiment shown in FIG. 1, reflection space 136 is
symmetric around a central axis 200. Similarly, transmission space 106 is
symmetric
around central axis 200. The axial symmetry of deflected radiation 138 on
target 50
provides several advantages. Because the incidence angle from the target
surface normal
to the radiation is controlled, the specularly reflected component of the
radiation is known
and directed away from the imaging device in all directions. Thus, the
reflection from
specular or highly reflective surfaces on which codes have been marked will
not reach the
imaging device. If the specular reflection did reach the imaging device, it
could produce a
signal exceeding the dynamic range of the imaging device. This would prevent
the
imaging device from producing the desired result of successfully detecting the
marked
2 0 code, whose diffuse component of reflection is usually at a much lower
radiant intensity
(watts per solid angle).
The axial symmetry of the radiation incident on the target also increases the
consistency and uniformity of the diffuse reflected component of radiation
from uneven
surfaces. Uneven surfaces when illuminated from one particular direction at a
2 5 predetermined angle will cause shadows and other reflection
irregularities. Sometimes this
may be desired to enhance the contrast of a surface irregularity as in the
case of flaw
detection.
In the case of code reading, the marked surface as well as the background
surface may be irregular and/or uneven. The reflection irregularities caused
by the
3 0 combination of the surface irregularities and the construction of the
target illumination will
cause a noise component to be induced on the nominal contrast value of the
background
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and/or the marked portion of the code. This can reduce the overall performance
of the
imaging/reading system.
By using directional illumination which is symmetric about the surface
normal to the target, the reflection irregularities caused by surface
irregularities
(unevenness) are minimized. Shadows caused by one direction of illumination
are filled in
by the other directions uniformly. This serves to reduce the contrast noise
described above
while at the same time eliminating specular reflection in the image,
increasing the overall
signal-to-noise performance of the system.
As shown in FIG. 1, wave guide 110 is essentially a hollow cylinder and
deflector 120 is essentially a hollow frustum, each of which has an annular
cross-section.
Preferably, the inner diameter 127 of deflector 120 is equal to the inner
diameter 117 of
wave guide 110. Inner wall 116 of wave guide 110 and inner wall 126 of
deflector 120 are
each disposed symmetrically around central axis 200. Thus, inner wall 116 of
wave guide
110 and inner wall 126 of deflector 120 form a boundary of reflection space
136.
Deflector 120 has a varying outer diameter 128 and, consequently, a varying
thickness, w.
Preferably, at end 124 of deflector 120 thickness, w, is nearly zero.
Similarly, lens 140 is
disposed symmetrically around central axis 200. Preferably, lens 140 is
essentially a ring
or half toroid having an annular cross-section and radiation source 20, is a
ring of radiation
emitters, such as LEDs. It is contemplated that wave guide 110 and deflector
120 may
each be hollow frustums symmetric around central axis 200 (i.e., radiation
directing device
101 would be essentially conical in shape). In this embodiment, lens 140 may
be a ring as
described above, as would radiation source 20. Again, wave guide 110,
deflector 120, and
lens 140 each would have an annular cross-section.
In the embodiment shown in FIG. 1, the directions of travel for transmitted
2 5 radiation 12 and reflected radiation 1 SO are substantially opposite one
another. Similarly,
the directions are substantially parallel. However, this is primarily a result
of the
cylindrical overall shape of radiation directing device 101. The benefit is to
minimize the
overall size of device 101 in directions radially perpendicular to central
axis 200. In an
embodiment wherein radiation directing device 101 is, for example, conical (as
described
3 0 above), the directions of travel for trasmitted radiation 12 and reflected
radiation 150
would not be parallel.
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As shown in FIG. l, deflector 120 and lens 140 are integrally formed with
wave guide 110. Outer diameter 128 of deflector 120 varies such that outer
wall 129 of
deflector 120 forms angle 130 with outer wall 119 of wave guide 110. Thus, in
the
embodiment shown in FIG. I, deflector 120 is essentially a tapered, or
beveled, end of
radiation directing device 101. Thus, it is an advantage of the present
invention that the
embodiment shown in FIG. 1 can be formed from a tube having a suitable outer
diameter
118, inner diameter 117, and thickness, t. Preferably, the tube is made of
clear acrylic and
is lathed on one end to form deflector 120, and ground on the other end to
form lens 140.
The length of inner wall 126 of deflector 120 is optimized for each
application (i.e., set to
ensure that transmitted radiation 12 is deflected uniformly toward target 50).
The optical characteristics of the transmission medium should be suited to
the characteristics of the radiation. In the case of an acrylic tube, the
material provides a
near lossless transmission medium for the wavelength of the radiation used
(for example,
660 nm nominally).
In the embodiment shown in FIG. 1, the radius of the lens is related to the
waveguide thickness. The radius of the lens, which determines its focal length
and
therefore optical power, dictates the placement of the radiation source
relative to the device
so as to provide the desired collection and redirection of the incident
radiation.
The collection efficiency of the lens is related to the divergence angle of
the
2 0 radiation previously mentioned. For maximum efficiency, the F number of
the lens, which
is related to the ratio of the lens' focal length to its diameter, should be
low enough to
collect as much of the radiation as possible while providing the desired
redirection
characteristics. Since the lens radius, which determines focal length, is
related to
waveguide thickness in this embodiment, the desired waveguide thickness is
driven by this
2 5 relationship.
The length of the waveguide is arbitrary. It may depend, however, on the
optical characteristics of the imaging lens. The illumination device should be
of suitable
length such that it illuminates the target at the proper angle and distance
where the target is
in focus on the imaging device.
3 0 A trap 142 is disposed around at least a portion of reflection space 106.
Trap 142 is adapted to prevent at least a portion of transmitted radiation 12
from entering
reflection space 106. For example, trap 142 may be opaque or reflective.
Preferably, trap
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142 is disposed around the entire inner wall 116 of wave guide 110 and extends
beyond
the top of radiation directing device 101 as shown in FIG. 1.
It has been observed that, especially when a clear, acrylic tube is used to
form radiation directing device 101, a portion of incident radiation 10 is not
deflected, but
passes through outer wall 129 of deflector 120. Thus, to decrease the amount
of incident
radiation I 0 that passes through outer wall 129, and to increase the
efficiency of radiation
directing device 101, an opaque or reflective surface may be disposed around
outer wall
129 of deflector 120. Similarly, in an embodiment in which deflector end 124
has a
nonzero width, it has been observed that a portion of transmitted radiation 12
passes
1 o through deflector end 124. Deflector end 124 acts as a lens, focusing any
transmitted
radiation 12 that passes therethrough onto points on target 50, rather than
uniformly
distributing the radiation. These points are commonly know as "hot-spots" and
cause
interference with the image of the target as seen by an image reader, for
example. Thus,
end 124 of deflector 120 is adapted to prevent at least a portion of
transmitted radiation 12
from passing therethrough. For example, to decrease the amount of transmitted
radiation
12 that passes through deflector end 124, deflector end 124 may be made opaque
or
reflective.
FIG. 2 shows a target illumination device 100 according to the present
invention in use in an image reader, an application for which the invention is
particularly
2 0 suited. In the application shown, a plurality of objects 210, such as
electronic components,
are located on a moving surface 202, such a conveyor belt. A target 50, such
as a data
matrix code, is disposed, such as by etching, on each object 210. An image
reader 220,
such as a data matrix code reader, is situated over moving surface 202. Image
reader 220
comprises a camera lens 226 and an image sensor 228. Image sensor 228 may be,
for
2 5 example, a charge coupled device (CCD). A target illumination device I 00
is coupled to
image reader 220. Radiation source 20, shown as a ring of LEDs 224, is
disposed between
image reader 220 and radiation directing device 101. As shown in FIG. 2,
radiation
directing device 101 is positioned such that central axis 200 is substantially
vertical, with
deflector end 124 nearest to moving surface 202. In operation, as moving
surface 202
3 0 moves objects 210 past image reader 220, radiation source 20 emits
incident radiation 10.
Lens 140 receives incident radiation 10 and transmits at least a portion of
incident
radiation 10 through wave guide 110. Deflector 120 deflects at least a portion
of
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transmitted radiation 12 toward target 50. Deflected radiation 138 is
reflected off of target
SO through reflection space 136. Reflected radiation 150 is redirected by
camera lens 226
to image sensor 228. Image reader 220 interprets information contained in
reflected
radiation 150 by processes known in the art.
FIG. 3 shows a target illumination device according to the present invention
in use with a hand-held image reader. In the application shown, a target 50,
such as a
data matrix code, is disposed, such as by etching, on an object 210. Hand-held
image
reader 420 comprises a camera lens 226 and an image sensor 228. Hand-held
image reader
420 may be, for example, a data matrix code reader. Image sensor 228 may be,
for
example, a charge coupled device (CCD). As shown, a target illumination device
100 is
coupled to image reader 420. Radiation source 20, shown as a ring of LEDs 224,
is
disposed between image reader 420 and radiation directing device 101.
Radiation
directing device 101 is positioned such that central axis 200 is substantially
perpendicular
to image reader 420, with deflector end 124 farthest from image reader 420. In
operation,
a user places image reader 420 over target SO in a manner such that target 50
is
surrounded, at least in part, by deflector 120. Deflector end 124 is
particularly suited to
aid the user in positioning the target within the viewing area. The user
simply adjusts the
attitude of hand-held image reader 420 until deflector end 124 is flush
against the surface
on which target 50 is located and moves hand-held image reader 420 until
target 50 is
2 0 located beneath reflection space 136. In so doing, radiation directing
device is
substantially vertical, insuring that deflected radiation 138 will uniformly
and
symmetrically illuminate target 50.
To read the information represented by indicia 50, radiation source 20 emits
incident radiation 10. Lens 140 receives incident radiation 10 and transmits
at least a
2 5 portion of incident radiation 10 through wave guide 110. Deflector 120
deflects at least a
portion of transmitted radiation 12 toward target 50. Deflected radiation 138
is reflected
off of target 50 through reflection space 136. Reflected radiation 1 SO is
redirected by
camera lens 226 to image sensor 228. Image reader 420 interprets information
contained
in reflected radiation 150 by processes known in the art.
3 o FIG. 4 shows an embodiment of a target illumination device according to
the present invention which is particularly suitable for reading linear
indicia, such as bar
codes. As shown in FIG. 4, target illumination device 102 comprises a
radiation source 20
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and a pair of plates 300. Radiation source 20, comprises a plurality of
radiation emitters
224, preferably LEDs. Plates 300 are substantially parallel to each other and
separated by
a distance, d. Plates 300 may be coupled to one another and kept parallel to
one another
by the use of spacers, for example.
Each plate 300 comprises a deflector 320, a wave guide 310, and a lens 340.
Each deflector 320 has an inner wall 326. Inner walls 326 of deflectors 320
form
boundaries of a reflection space 336. Reflection space 336 is symmetric about
a central
plane 360. Inner walls 326 of deflectors 320 are substantially parallel to
central plane 360.
Similarly, each wave guide 310 has an inner wall 316. Inner walls 316 of wave
guides 310
are also substantially parallel to central plane 360. Preferably, inner wall
316 of wave
guide 310 is coplanar with inner wall 326 of deflector 320. Lenses 340 are
disposed
substantially parallel to central plane 360. Each lens 340 is adapted to
receive incident
radiation 10 from radiation source 20 and to transmit at least a portion of
incident radiation
10 to deflector 320 in a manner such that transmitted radiation 12 is
substantially uniform
and such that reflection space 336 is substantially devoid of transmitted
radiation. Wave
guide 310 is adapted to receive transmitted radiation 12 and to guide
transmitted radiation
12 through wave guide 310 to deflector 320. Each deflector 320 is adapted to
be placed
proximate target SO to deflect at least a portion of transmitted radiation 12
toward target 50
in a manner such that at least a portion of deflected radiation 138 is
reflected off of target
2 0 50 into reflection space 336.
In the embodiment shown in FIG. 4, wave guide 310 has an outer wall 319.
Transmitted radiation 12 is directed through wave guide 310 between inner wall
316 and
outer wall 319. Thus, a transmission space 306 is contained between outer wall
319 and
inner wall 316 of wave guide 310. Similarly, deflector 320 has an outer wall
329. Outer
2 5 wall 329 of deflector 320 forms an angle 330 with outer wall 319 of wave
guide 310.
Wave guide 310 has a thickness, t, and deflector 320 has a varying thickness,
w.
Preferably, end 324 of deflector 320 has a nonzero thickness. Thus, deflector
320 is
essentially a tapered, or beveled, end of plate 300. The length of inner wall
326 of
deflector 320 is optimized for each application (i.e., set to ensure that
transmitted radiation
3 0 12 is deflected uniformly toward target 50.
In a preferred embodiment, a trap 342 is disposed on at least a portion of
inner wall 316 of wave guide 310. Trap 342 is adapted to prevent at least a
portion of
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transmitted radiation 12 from entering reflection space 336. Thus, it is
preferred that trap
342 be disposed on the entire inner wall 316 of wave guide 310. Trap 342 may
be an
opaque or reflective surface, for example.
As shown in FIG. 4, deflector 320 and lens 340 are integrally formed with
wave guide 310. Thus, it is an advantage of the present invention that plate
300 can be
formed from a sheet having a thickness suitable for wave guide 310. The sheet,
which,
preferably, is made of clear acrylic, is lathed on one end to form deflector
320, and ground
on the other end to form lens 340.
It has been observed that, especially when a clear, acrylic sheet is used to
foam plate 300, a portion of transmitted radiation 12 is not deflected off of
deflector 320,
but passes through outer wall 329 of deflector 320. Thus, a reflective
surface, such as a
mirror, may be disposed on outer wall 329 of deflector 320 to prevent at least
a portion of
transmitted radiation 12 from passing through outer wall 329. Similarly, it
has been
observed that, if deflector end 324 has a nonzero width, a portion of
deflected radiation
138 passes through deflector end 324. Thus, deflector end 324 may be made
opaque or
reflective to decrease the amount of deflected radiation 138 that passes
through deflector
end 324.
While the invention has been described and illustrated with reference to
specific embodiments, those skilled in the art will recognize that
modification and
2 o variations may be made without departing from the principles of the
invention as
described hereinabove and set forth in the following claims.
