Note: Descriptions are shown in the official language in which they were submitted.
95/03590 PCT/US94/08088
METHOD AND APPARATUS FOR NON-CONTACT READING OF A RELIEF
PATTERN
BACKGROUND OF THE INVENTION
1. Field of the Invention.
This invention relates in general to pattern recognition
and in particular to a method and a system for non-
contact reading of product or part identification
patterns, and more specifically to reading of patterns
which are present in relief form..
2. Description of the Prior Art.
The use of bar codes for identification of parts and
products is widespread. The most common means of
including a bar code for an object is to either affix a
printed label, or to print the code directly on the part
or product. Most conventional bar code scanners require
very high contrast between the bars and the background
in order to properly read the code. In some cases,
however, affixing a label or painting a code is either
impractical or too expensive. For example, some
surfaces will not allow labels to adhere properly or the
presence of a label may interfere with the proper
operation of the part.
One way to overcome the limitations of bar code
labels and painted bar codes is.to have the code set in
relief on the surface of the object. This can be done
by stamping, engraving, etching, milling, molding, or by
other methods. The codes can be raised from the surface
or can be depressed from the surface. The primary
problem with such relief bar codes is that they cannot
be read by currently available non-contact scanning
techniques, since the contrast between the high and low
parts of the pattern is generally very low.
SUMMARY OF THE INVENTION
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It is therefore a primary object of the present
invention to provide an improved method and apparatus
for effectively reading and decoding relief patterns
using non-contact techniques.
These and other objects of the present invention are
attained by a non-contact apparatus for reading a ,
surface that has information encoded in a relief pattern
thereon, the relief pattern having relatively raised and
recessed portions. The apparatus comprises an optical
transmitter for projecting a line of light onto the
relief pattern at a first angle to a normal of the
surface; an optical receiver that views the projected
line of light at a second angle to the normal of the
surface and produces a signal that is representative of
an image of the reflection thereon. The image
incorporates apparent displacements of the line of
illumination that occur at transitions between raised
and recessed portions of the relief pattern therealong.
The displacements are detected by a pattern detector
which is coupled to the optical receiver. The output of
the pattern detector is submitted to a decoder.
In accordance with one aspect of the invention the
optical receiver and the optical transmitter are spaced
apart in a plane that is substantially parallel to the
surface.
In accordance with another aspect of the invention
the decoder and pattern detector are realized by a
digitizer and a microprocessor having a suitable program
stored therein.
The optical receiver can be realized as a plurality
of light sensors arranged in a two dimensional array,
each having an output responsive to light energy
incident thereon, and including optical means for
projecting a focused image of the reflection on the sensors.
Alternatively the light sensors can be arranged in a
linear array and the optical transmitter configured to
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sweep a spot of light across the relief pattern to
define a substantially rectangular area thereon, a cross
sectional dimension of the spot being less than the
width of the smallest feature in the relief pattern in
order to achieve a desired resolution. Apparent
displacements of the line of illumination on the surf
ace
are represented by displacements of the focused image
along the linear array.
BRIEF DESCRIPTION OF THE DRAWING
For a better understanding of these and other
objects of the present invention, ref erence is made to
the detailed description of the invention which is to
be
read in conjunction with the following drawings,
wherein:
FIG. 1 illustrates a block diagram of a relief
pattern reading apparatus in accordance with the
invention;
FIG. 2 illustrates a swept column beam of light
impinging on a flat surface;
FIG. 3 illustrates a fan beam of light impinging on
a flat surface;
FIG. 4 illustrates a swept column or fan of~
illumination impinging on a relief patterned surface;
FIG. 5 is an enlarged perspective view of a surface
bearing a relief pattern which has been exaggerated for
clarity to illustrate parallactic displacements of a
line of illumination;
FIG. 5a is an enlarged fragmentary perspective view
of FIG. 5 taken on line 5a-5a;
FIG. 6 is a diagram that illustrates a mapping of an
image of the line of illumination on the surface shown
in FIG. 5 onto a two-dimensional photodetector array in
an optical receiver;
FIG. 7 diagrammatically illustrates a mapping of an
image of a line of illumination generated by sweeping
a
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beam of light across the surface shown in FIG. 5 onto a
linear photodetector array; and
FIG. 8 schematically illustrates an alternate
optical arrangement for mapping the image onto the
photodetector array of FIGS. 6 and 7;
DESCRIPTION OF THE PREFERRED EMBODIMENT
i
With reference now to the Drawing and in particular
to FIG. 1, there is depicted a representation of a
relief pattern reading apparatus 1, in accordance with a
preferred embodiment of the invention, positioned near
an object surface containing a~relief pattern 8. The
relief pattern reading apparatus 1 includes an optical
transmitter 2, an optical receiver 3, a pattern detector
4, and a decoder 5. A preferred embodiment of the
present invention may be implemented with the components
2, 3, 4, 5 in a single unit, or with any or all of the
components 2, 3, 4, 5 in separate enclosures.
The optical transmitter 2 emits a beam of light 6
which impinges on the object surface containing the
relief pattern 8. The beam of light 6 emitted from the
optical transmitter 2 can be either a spot beam which is
swept or scanned, or the beam of light 6 may be a
stationary beam. In either case, the beam, if projected
onto a flat surface, would appear as a line of light.
Alternate methods of illuminating the surface are shown
respectively in FIGS. 2 and 3, wherein the light beams
are diagrammatically shown as originating from a source
in the optical transmitter 2, the structural details of
which have been omitted for clarity.
In FIG. 2 there is depicted a spot beam of light 10,
which is swept or scanned to form a line 11 when
projected onto a flat surface 12. Representative
positions of the beam 10 as the spot moves across the
surface are indicated by the series of dotted lines in
FIG. 2.
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Ref erring to FIG. 3, there is depicted a fan beam of
light 13, which forms a line 14 when projected onto the
flat surface 15.
Turning again to FIG. 1, the optical receiver 3 is
positioned such that its field of view 7 contains the
projected image of the line of illumination produced by
the beam of light 6 on the surface 8. The optical
receiver 3 converts the image in its field of view 7
into electrical signals which can be processed by the
pattern detector 4. The pattern detector 4 extracts the
coded pattern from the electrical representation of the
image and sends the coded'pattern to the decoder 5 for
decoding. This representation may be a digitized signal
as is known to the decoding art, or could be in other
well known formats. The decoder 5 decodes the coded
pattern to extract the character string or other
information which was encoded into the relief pattern.
The decoder 5 uses techniques well known in the art to
decode the coded pattern, and as such is not a subject
of the present invention. The decoder 5 outputs the
decoded character string or information to a display or
similar conventional device using a communications port
9. This communications port 9 may take many forms and
is also not a subject of the present invention.
In order for the components in FIG. 1 to be utilized
in reading relief patterns, the optical transmitter 2
must be separated from the optical receiver 3 by some
distance. Referring to FIG. 4, the swept spot or fan
illumination 16 generated by the optical transmitter 2
impinges upon the relief pattern 8 forming what appears
from the viewpoint of the optical transmitter 2 to be a
straight line. The optical receiver 3 is positioned
such that its direction of view 17 is as shown in
FIG. 4.
FIG. 5 depicts the image of the relief pattern
surface 8 as seen from the point of view of the optical
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receiver 3: Because of the separation of the optical
transmitter 2 and the optical receiver 3, those portions
of the illuminated surface 18 which are on the raised
portions of the pattern exhibit a parallactic
displacement 20 from those portions of the illuminated
surface 19 on the lower portions of the pattern. It is
this parallactic displacement 20 which is detected by
the pattern detector 4.
In order to better understand the apparent
displacement of the line of illumination along the
relief pattern it will be helpful to refer to FIG. 5a,
in which a line of illumination, indicated generally by
reference numeral 40, has a segment 18 that crosses a
relatively raised zone 28 of the relief pattern, and a
substantially vertical segment 41, representing a
transition between raised portion 28 and relatively
recessed zone 29. Line 40 continues horizontally along
zone 29 as segment 19. An angularly displaced observer,
such as optical receiver 3 in FIG. 1, views the relief
pattern along line 42. Such an observer would perceive
segment 18, and segment 19' (shown as a dotted line in
FIG. 5a) as collinear, as the angular displacement a of
these segments from the observer~s.optical axis 45 is
identical. Dotted line 19' of course is not actually
part of the line of illumination. This observer
perceives segment 19, the actual line of illumination,
as being displaced a distance 20 from segment 18, a
phenomenon referred to herein as parallactic
displacement. It will be evident that the degree of
parallactic displacement is a function of the difference
between the angular displacement ~i of segment 19 and the
angular displacement a of segment 18. This angular
difference is affected by the distance between the
relief pattern and the observer and of the dimension 27
that represents the relative difference in elevation of
portions 28 and 29.
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While the above explanation assumes a substantially
vertical illuminating beam and an angularly displaced .
observer, the optical transmitter and receiver can be
exchanged. An angular displacement of approximately
equal magnitude but of opposite direction will result.
Neither component need be vertical.
The optical receiver 3 can be constructed using
either a linear array of photodetectors or a square or
rectangular two-dimensional array of photodetectors.
Any conventional photodetector array such as a CCD array
is suitable. Referring now to FIG. 6, optical receiver
3, schematically enclosed by the dashed line in FIG. 6,
comprises imaging lens 21 and two-dimensional array of
photodetectors 22. Imaging lens 21 collects light from
the illuminated portions of surface 8, and focuses the
light to form an image on two-dimensional array of
photodetectors 22. The parallactic displacement 20 is
translated by imaging lens 21 into a displacement in the
direction arrow 23 on the two-dimensional array of
photodetectors 22. This image containing the
displacement is translated by two-dimensional array of
photodetectors 22 into an electrical representation of
the image which is sent to the pattern detector for
further processing. This optical receiver configuration
can use either a swept spot beam or a fixed fan beam
implementation of the optical transmitter 2.
FIG. 7 depicts an alternative embodiment of the
optical receiver in accordance with the invention. In
this preferred embodiment, optical receiver 73 comprises
a lens system 24, which can be a cylindrical lens, and a
linear array of photodetectors 25. This embodiment
requires the use of the swept spot beam implementation
of the optical transmitter 2. The lens system 24
focuses light from the relief surface onto the linear
array 25 such that each element or cell of the linear
array 25 collects light from an area of the surface that
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is approximately parallel to the direction of the
illuminating beam's image. The parallactic displacement
20 is therefore translated by the lens system 24 into a
displacement on the linear array of photodetectors 25 in ,
the direction of arrow 26. At any point along the sweep
of the spot beam, the photodetector-cell with the
highest intensity corresponds to the position of the
surface that the spot beam is incident upon. The
optical receiver 73 generates an electrical
representation of the current position of the spot
which, when sampled over the entire sweep of the spot,
directly indicates the relative height of the relief at
each sample point. This information is sent to the
pattern detector 4 for further processing.
Decoder 5 and pattern detector 4 are known in the
art, and as they form no part of the present invention,
they will r_ot be further discussed herein.
In FIG. 8 there is shown an alternate embodiment of
the arrangement of FIGS. 6 and 7. A lens system, shown
as lens 74 has an optical axis 72. The lens system can
be a single element, or can be a combination as is
appropriate for a given application. An image.plane 71'
is defined which is substantially normal to the optical
axis 72. Individual surfaces being scanned by the
system have a bar coded symbol thereon, possible
locations of which are indicated by lines 75a - 75c.
The remainder of the surfaces on which the bar code
symbols are located have been omitted for clarity. It
is understood that while three bar code symbol locations
from three different surfaces are illustrated, only one
surface is scanned at any one time. The bar code symbol
may be the same symbol as shown in FIGs. 6 and 7. A fan
beam of light, indicated in two dimensional projection
by dotted line 76, is projected, preferably by a laser,
striking the surface and the bar code symbols. In
general the bar code symbols being scanned will not lie
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on the optical axis 72, so that light rays striking
points 75a - 75c do not travel equal distances from the
light source. Reflected light from the scanned surfaces
is collected by lens 74, the principal rays being
indicated by the dotted lines 79, 79, 81, 81 and solid
lines 83, 83, and then projected generally towards a
photodetector array 71 which can be linear or two-
dimensional. In general, due to the variation in the
positions of individual surfaces being scanned, light
paths extending to the image plane 71' from points on
the surfaces that lie at a constant half angle with
respect to optical axis 72 vary in length. Another
consequence of the positional variation in the scanned
surfaces is that the planes normal to axis 72 and
passing through various bar code elements being scanned,
indicated as points 75a - 75c, lie at different
distances (shown as dimensions "a", "b" and "c"
respectively) from the lens 74.. Consequently the image
plane 71' does not include all the images 76a, 76b and
76c of points 75a, 75b, and 75c respectively, but can
include at most only one of them, here shown as point
76b. The photodetector~array 71 is tilted from the
normal of optical axis 72 at a tilt angle cp, so that
points 76a - 76c all fall approximately on the array 71.
The angle cp is readily calculated to minimize the
distances of points such as 76a - 76c from the actual
position of array 71 using the equations of lens 74, or
it may be determined empirically. Although shown for
clarity as a single dotted line 76, the laser light is
actually a fan beam, and the three dimensional surfaces
that include points 75a - 75c have been omitted for
clarity. Tilting the array 71 with respect to the image
plane 71' substantially increases the functional depth
of field of the system. It will be apparent that points
on other three dimensional surfaces (not shown) that lie
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intermediate points 75a and 75c will be focused on a
pixel element somewhere on array 71.
While this invention has been explained with
ref erence to the structure disclosed herein, it is not
confined to the details set forth and this application
is~ intended to cover any modifications and changes as i
may come within the scope of the following claims:
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