Apparatus for proximity detection of an opaque pattern on a translucent substrate

The physical separation between a rear illuminated opaque pattern on a translucent substrate and an optical pattern sensing device viewing the shadow image of the opaque pattern is increased without any corresponding loss of resolution (and/or resolution may be increased without any corresponding reduction in the physical separation between the optical pattern sensing device and the opaque pattern) by positioning a narrow spectral band pass interference filter between the opaque pattern and the optical pattern sensing device, and using as the rear illumination, light having a spectrally narrow band which substantially matches the pass band of the interference filter.

TECHNICAL FIELD 
This invention relates to apparatus for sensing or detecting an opaque 
pattern on a translucent background using rear illumination for the 
purpose of reading, recording, copying, inspecting or viewing the pattern 
and more particularly it relates to such apparatus wherein the pattern is 
sensed or detected without the use of any imaging optics. 
BACKGROUND ART 
Opaque patterns on translucent backgrounds are often sensed or detected for 
the purpose of reading, recording, copying, inspecting or viewing the 
pattern. This is done, for example, when documents are electronically read 
by an optical reading or copying device and when individual ceramic 
microcircuit packaging sheets are inspected prior to assembly into 
multilayer structures. 
It is known that a pattern may be sensed or detected in theory without 
using any imaging optics by positioning a suitable optical pattern sensing 
apparatus (such as a TV camera tube, a stationary or scanned linear or 
matrix light detector array, a scanned individual light detector, an 
optically sensitive film or layer, an image converter, etc.) sufficiently 
close to the pattern. This is sometimes called proximity pattern 
detection. 
Proximity pattern detection is not usually practical for use with front 
illumination, however, because too great a distance is required between 
the detector and the pattern in order to let in the front illumination. As 
a result, proximity pattern detection is more commonly done by forming an 
opaque pattern on a non-opaque background (or vice versa) and illuminating 
the pattern from the back (rear illumination) while positioning a suitable 
optical pattern sensing apparatus adjacent the front side of the pattern. 
Proximity pattern detection using rear illumination works very well when 
the non-opaque background is transparent, because the use of collimated 
rear illumination then allows the optical pattern sensing apparatus to be 
placed a reasonable distance away from the pattern without significant 
loss of pattern detail or resolution. 
Unfortunately, when the opaque pattern to be detected has a tranlucent 
background, the light scattering which occurs as the light passes through 
the translucent background requires that the optical pattern sensing 
apparatus be positioned extremely close to the pattern. This is generally 
not practical to do, so that projection pattern detection usually is used 
instead by inserting imaging optics between the opaque pattern and the 
optical pattern sensing apparatus. The imaging optics projects the light 
pattern formed by the opaque and translucent areas onto the optical 
pattern sensing apparatus. Because of the presence of the imaging optics, 
projection pattern detection is inherently much more complicated and 
expensive than proximity pattern detection. 
The object of this invention is to provide a practical proximity pattern 
detection technique and apparatus wherein an opaque pattern on a 
translucent background may be sensed or detected using rear illumination 
without requiring that the optical pattern sensing apparatus be positioned 
unreasonably close to the pattern and without requiring any use of imaging 
optics between the opaque pattern and the optical pattern sensing 
apparatus. 
DISCLOSURE OF THE INVENTION 
In accordance with this invention, the physical separation between a rear 
illuminated opaque pattern on a translucent substrate and an optical 
pattern sensing device viewing the shadow image of the opaque pattern is 
increased without any corresponding loss of resolution (and/or resolution 
may be increased without any corresponding reduction in the physical 
separation between the optical pattern sensing device and the opaque 
pattern) by positioning a narrow spectral band pass interference filter 
between the opaque pattern and the optical pattern sensing device, and 
using as the rear illumination, light having a spectrally narrow band 
which substantially matches the pass band of the interference filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, a transparent substrate 10 supports an opaque pattern 12 which 
is illuminated from the rear with nominally parallel (collimated) 
illumination 14. The parallel illumination 14 may be formed, for example, 
by placing a source of light 16 at the front focal point of a lens 18. 
Since the substrate 10 is transparent, illumination 14 is not scattered 
very much while passing through the substrate so that a downstream optical 
pattern sensing device 22 will see a shadow pattern 20 corresponding to 
opaque pattern 12. If the pattern features of the opaque pattern 12 are 
large compared with the spread of light due to diffraction (approximately 
equal to the square root of the product of the separation Z and the 
wavelength of the rear illumination) and the spread of light due to 
divergence of the rear illumination (approximately Z times the divergence 
angle .phi. of the illumination beam), then there is a sharp shadow at the 
optical pattern sensing device 22 and no imaging optics are required. 
On the other hand, if the opaque pattern 12 is carried by a translucent 
substrate 24, the light transmitted through the substrate is diffused or 
scattered at all angles, as shown in FIG. 2, and the shadow pattern of the 
opaque pattern 12 cannot be seen sharply by a viewing device 22 unless the 
viewing device is placed very close to the opaque pattern (much closer 
than the minimum dimensions of the pattern features in the opaque pattern 
12). The optical pattern sensing device 22 may be positioned at a 
reasonable distance away from the pattern 12 by inserting imaging optics 
26 between the opaque pattern and the optical pattern sensing device. 
Optics 26 acts to project an image 28 of the opaque pattern onto the input 
face of the optical pattern sensing device 22, as shown in FIG. 2. 
In accordance with this invention, FIG. 3 illustrates how the use of such 
imaging optics can be avoided when an opaque pattern on a translucent 
substrate is to be viewed by an optical pattern sensing device without 
positioning the optical pattern sensing device closer to the opaque 
pattern than the minimum feature dimensions of the pattern to be viewed. 
Interposed between the optical pattern sensing device 22 and the opaque 
pattern 12 is a narrow band pass interference filter 30, which acts as an 
angular filter in that it passes only the light rays which are 
substantially parallel to a particular direction. Interference filter 30 
does not pass light rays which have been scattered in other directions by 
the translucent substrate. Accordingly, a shadow pattern of the opaque 
pattern is formed by the narrow band pass interference filter and can be 
seen by a downstream optical pattern sensing device 22. An interference 
filter of this type may be fabricated in several different ways, but the 
preferred way is to form a stack of thin films, all having a similar 
thickness. Reflections from the film interfaces causes interference 
effects to occur in the films such that only a narrow band of light 
wavelengths will be transmitted through the film stack. Interference 
filters of this type are described in great detail, for example, in the 
book entitled THIN FILM OPTICAL FILTERS by H. A. Macleod (Elsevier, 1969), 
which is hereby incorporated by reference. 
In order for this desired effect to occur, however, it is important that 
the rear illumination have a narrow spectral band and that the narrow 
spectral band of the rear illumination and the narrow pass band of the 
interference filter 30 are substantially matching. "Substantially 
matching" is intended to mean that the spectral band of the rear 
illumination and the pass band of the interference filter overlap each 
other to a significant degree. Preferably these bands are substantially 
equal. However, it is possible for either band to cover a wavelength or 
frequency range which extends somewhat higher and/or lower than the other 
so long as the degree of overlap in the bands is significant. It is 
preferred that if one of the bands is narrower than the other, that the 
narrower band be the pass band of the interference filter. Preferably the 
pass bands are centered at substantially the same wavelength. 
FIG. 4 shows how a second narrow band pass interference filter may be used 
to produce the narrow spectral band rear illumination from a source of 
light having a wider spectral band. A wide spectral band source of light 
16, such as a mercury arc lamp, is first collimated by a lens 32 to 
produce wide band collimated light 36. The wide band collimated light is 
then passed through a narrow band interference filter 34. Since the light 
entering interference filter 34 is collimated, only a narrow band of light 
wavelengths will be passed. The illumination light 38 passed by the 
interference filter 34 also will be substantially collimated as shown. On 
the other side of the translucent substrate is the other narrow band 
interference filter 30, which must have a substantially matching narrow 
band pass characteristic. While the first interference filter 34 acts as a 
true band pass filter, the second interference filter acts as an angular 
filter by passing only the light rays travelling in a preferred direction 
determined by the band pass characteristic of the filter. The pass bands 
of the two interference filters must at least overlap each other to a 
significant degree (i.e., be "substantially matching"). Preferably, the 
pass bands are each centered at substantially the same wavelength and are 
substantially identical. However, it is possible for either band to cover 
a wavelength or frequency range which extends somewhat higher and/or lower 
than the other so long as the degree of overlap in the bands is 
significant. It is preferred that if one of the bands is narrower than the 
other, that the narrower band be the pass band of the interference filter 
acting as the angular filter. 
It should be apparent to those of ordinary skill in this field that this 
invention may be used in any application where an opaque pattern on a 
translucent background is being read, recorded, copied, inspected or 
viewed for any purpose at all, and that the optical pattern sensing 
apparatus may take any known form. For example, the optical pattern 
sensing apparatus may be a TV camera tube, an image converter, a scanned 
light detector or linear array of light detection elements, a two 
dimensional array of discrete light sensing elements, an optically 
sensitive film or layer or surface, such as a photoresist layer or an 
electrophotographic surface, etc.