INTIMATE SOURCE AND DETECTOR AND APPARATUS EMPLOYING SAME

Abstract: There is provided a system of detecting mark-sense indicia wherein the images of a light source (26) and the region of sensitivity (122) of a light sensor are substantially coaxial, coplanar, and coextensive throughout the working depth of field (21). This system is insensitive to the laminate and the diffusion effects. A light source (26) is aimed at a beam splitter (15) that diverts a portion of the light beam (36) through optics (14) that focus the light on a reflective target (12) containing a bar code symbol (17) to be scanned. The reflected beam (38) is returned through the optics (14) and the beam splitter (15), and a portion of the reflected beam is conducted to a light sensor (24). In this manner, the light path from the source (26) to the optical sensor (24) is split twice by the beam splitter (15). The optics (14) are configured so that the light beams that define the field of illumination and the region of sensitivity and which pass between the optics (14) and the target (12) are congruent, having identical optical axes and angles of divergence (16, 18). The region of sensitivy is congruent with the field of illumination in any plane throughout the depth of field of the optics. Field stops (20, 22) may be optionally provided for the light source (26) and the light sensor (24) in order to better define the dimensions of the field of view and field of illumination.

Description: INTIMATE SOURCE AND DETECTOR AND APPARATUS EMPLOYING SAME BACKGROUND OF THE INVENTION This invention relates to an assembly in a scanner * or imager where the field of view of a light sensor and the field of illumination of a light source are coplanar * 5 and coextensive in any plane where the target is located. In conventional mark-sense and imaging apparatus, the light source and light sensor are discrete and have optical paths which generally are not congruent in space. Such an arrangement provides a field of illumination that 10 includes optical patterns of interest but which in general is larger than the light sensor's field of view (termed herein the sensor's region of sensitivity). The light sensor generates a signal that is specifically responsive to light from the region of sensitivity that is incident 15 on the light sensor. When the field of illumination is larger than the region of sensitivity, flood illumination is said to be present. In this circumstance the process of accurately interpreting the information content of the indicia being 20 scanned is interfered with, as will now be explained. In FIG. 3 there is illustrated a typical prior art device wherein LEDs 121, 121 illuminate a field of illumi¬ nation 124 on target 123. A region of sensitivity 122, being smaller than field of illumination 124, is entirely 25 included within field of illumination 124. The region of sensitivity 122 is viewed by light sensor 127 through optics 125, and presents a signal to amplifier 129 in response to detected light. Mark-sense detectors are typically employed in 30 optical scanning devices, such as bar code scanners or readers, in which the output of the detector may be coupled to decoding circuitry. There are a number of v known optical effects that can produce errors in the determination of transitions between bars and spaces in a bar code symbol that is situated on a substrate, including the diffusion and laminate effects. The diffusion effect can be understood with refer¬ ence to FIG. 4a and 4b. It results from light, shown representatively as beams 126, that is incident outside the region of sensitivity 122 of a light sensor. Beams 126 enter the substrate 138 on which a pattern is located, and are then scattered internally within the substrate into the region of sensitivity. A fraction of the scat- tered light is thus ultimately returned to the light sensor via region of sensitivity 122 where it contributes to the received signal level. In scanning applications such as bar code scanning there is another consequence of the diffusion effect. In this application the region of sensitivity travels across dark bars that are separated by fields of light spaces. These spaces are referred to herein as white spaces; however those skilled in the art will understand that areas of high reflectivity are denoted. As the region of sensitivity 122 approaches a dark bar 139, as shown in FIG. 4b, the bar 139 will absorb more light than does a white space of similar dimension. Since some light 128 is absorbed, less is available to scatter into the approaching region of sensitivity. Thus the diffusion effect differs quantitatively when a bar is near the region of sensitivity than when it is not. This can be appreciated with reference to FIG. 5 which shows an analog waveform 130 generated by a bar code scanner utilizing flood illumination which has scanned bar code symbol 132. It will be apparent that the signal minima 137 corresponding to the wide dark bars are quite sharp, while the maxima 135 corresponding to the intervening white spaces are rounded. Furthermore the peaks 136 corresponding to the narrow white spaces between the narrow dark bars 134 are reduced in amplitude relative to maxima 135 corresponding to wide white spaces 135 between the wide dark bars 137. This appearance is predicted by the above discussion of the diffusion effect. If the narrow dark bars could not be resolved due to the modula¬ tion transfer function of the reader's optical system, the patterns would have been symmetrical about the maxima and minima. While the diffusion effect has been explained in connection with a bar code scanner, it influences non- scanning readers of optical patterns as well. In summary, the diffusion effect tends to reduce the apparent white level in the vicinity of dark bars while not affecting the black level. Another undesired effect of flood illumination in certain imaging applications, such as bar code scanning, is the laminate effect, also known as the overlay effect. Practical bar code symbols often possess protective overlaminates. The laminate effect is caused by the light that is scattered from outside the region of sensitivity being totally internally reflected at the overlaminate-to- air interface onto the region of sensitivity. Reference may be made to FIG. 6 where the laminate effect is depict- ed diagrammatically. During the lamination process an adhesive flows over the bar code symbol substrate 140, expelling the air between laminate 141 and bar code symbol 142. As a result, the laminate 141 and bar code symbol 142 are in intimate contact and are essentially index matched. When the laminated bar code substrate is illumi¬ nated, as with the diffusion effect, some of the incident light rays, such as rays 226a, 226b falling outside the region of sensitivity 122 are scattered from the top surface of layer 142 into overlaminate 141 and are inci- dent on surface 227 at an angle 144 that can exceed the. critical angle for total internal reflection. Such light rays 226a, 226b are totally internally reflected into the region of sensitivity 122. A portion of this light is then scattered back toward the overlaminate 141, pene- trates it, and finally arrives at the light sensor, where it contributes to the received signal level. As in the case of the diffusion effect, as the region of sensitivity passes over the bar code symbol, some of the incident light will be absorbed by dark bars adjacent to the region of sensitivity, thereby reducing the white space signal level. The magnitude of the laminate effect is a function of laminate thickness, the dimensions of the bar code symbol, reflectance of the bars and spaces, nominal diffusion length in the substrate, and refractive index. While laminate effects are sometimes difficult to quanti- fy, the range of this effect can be quite large. The laminate and diffusion effects are known to degrade the performance of bar code scanners, other types of mark- sense detectors, and image detecting apparatus generally. A laminated bar code symbol typically exhibits both the diffusion and the laminate effects. It will be recognized that an analogous argument can be made for the case where a small source of illumina¬ tion is used in conjunction with a large region of sensi¬ tivity, so called flood sensitivity. A number of methods have been employed in the prior art that tend to bring the beams of light from the light source and those directed to the light sensor from the target into alignment, estab¬ lishing a common operational region. The images of the light source and the defined region of sensitivity are generally not congruent throughout a relatively large depth of field. For this reason, all these systems are sensitive to the degrading effects of the diffusion and laminate effects over all or most of their operational depth of field. For example, in U.S. Pat. No. 4,346,292 to Routt, Jr. et al, there is shown an optical scanner in which source and reflected light beams are coaxially aligned at the target. In this device the region of sensitivity is substantially congruent with the field of illumination only in the immediate neighborhood of the focal plane, and it cannot achieve a larger depth of field without flood illuminating the target. U.S. Pat. No. 4,675,531 to Clark et al shows a scanner that has coaxial incident and reflective beams, achieved by a multi-surfaced lens arrangement. The beams, while having the same optical axis, could not define a light source image that is congruent to a region of sensitivity except in the focal plane. This device would not be suitable for applications requiring a larger depth of focus where flood illumination is undesirable. In U.S. Pat. No. 4,816,659 to Bianco et al an apparatus is shown in which a bar code symbol is illumi¬ nated by a lamp, and a reflected beam received by a photosensor. The illumination and detection optical axes are not coaxially aligned in the plane of the bar code symbol. SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide a system of detecting mark-sense indicia that is insensitive to the laminate and the diffusion effects. It is another object of the present invention to provide an improved system for detecting mark-sense indicia wherein the images of a light source and the region of sensitivity of a light sensor are substantially coaxial, coplanar, and coextensive throughout the working depth of field. It is another object of the present invention to make an improved bar code scanner that is insensitive to the laminate and the diffusion effects. These and other objects of the present invention are attained in an optical assembly that can be housed within a mark-sense detector, such as a bar code reader. A light source is aimed at a beam splitter that diverts a portion of the light beam through optics that focus the light on a reflective target containing a bar code symbol to be scanned. The reflected beam is returned through the optics and the beam splitter, and a portion of the re¬ flected beam is conducted to a light sensor. In this manner, the light path from the detector to the optical sensor is split twice by the beam splitter. The optics are configured so that the light beams that define the field of illumination and the region of sensitivity and which pass between the optics and the target are congru¬ ent, having identical optical axes and angles of diver- gence. By appropriately specifying the axes of rotation of images that are projected onto the field of illumina¬ tion and back projected from the region of sensitivity onto the light sensor, the field of illumination and region of sensitivity are brought into precise rotational alignment, such that the image of the light source is sub¬ stantially coplanar and coextensive with the region of sensitivity in a working depth of field in which a target is located. The region of sensitivity is thus congruent with the field of illumination in any plane throughout the depth of field of the optics. Field stops may be provided for the light source and the light sensor in order to better define the dimen¬ sions of the field of view and field of illumination. When this is done an image of the light source field stop is projected precisely onto the region of sensitivity, and the back-projection of the region of sensitivity is focused on and coextensive with the field stop of the light sensor. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of these and other objects of the present invention, reference is made to the detailed description of the invention which is to be read in conjunction with the following drawings, wherein: FIG. 1 is a schematic illustration of an assembly in accordance of the present invention shown in a bar code reading application; FIG. 2 is a side view of the apparatus of FIG. 1 shown schematically. FIG. 3 is a schematic view of an optical reader in accordance with the prior art; FIGs. 4a, 4b, and 5 are diagrams explaining the diffusion effect; FIG. 6 is a diagram explaining the laminate effect; and FIG. 7 schematically illustrates an alternate embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION Turning now to FIG. 1, there is shown generally at 10 an illustration of an optical assembly in accordance with the present invention. While not illustrated in order to preserve clarity, assembly 10 is generally contained within the housing of a bar code scanner. The housing has a port to permit the passage of light there¬ through in a desired direction. A function of the housing with respect to assembly 10 is the exclusion of unwanted light from light sensor 24. The invention may be prac¬ ticed in other applications with any suitable means for the exclusion of unwanted light, such as a shield, or a partially enclosing canopy. Light source 26 emits a beam of light 36 that in the preferred embodiment has a sec¬ tional area and configuration that are defined by light source field stop 20. Light source 26 is preferably an LED or a laser, but could be an incandescent lamp, arc lamp, or any suitable source of light. If the image of light source field stop 20 as projected onto target 12 is noncircular, then the light source field stop 20 is suitably aligned about a rotational axis so that the field of illumination will be rotationally aligned with minimal bar code symbol element 13. Alternatively the optics can be configured to achieve rotational alignment. Light source field stop 20 is preferably circular; however other field stop designs may be employed. Light beam 36 eventually strikes beam splitter 15, a conventional partially silvered mirror type splitter that is disposed at a 45 degree angle to light beam 36. Other angles could be used to accommodate specific appli¬ cations. Normally a 50% beam splitter is used, but this may be varied if desired. A proportion of beam 36 contin- ues beyond beam splitter 15 and is lost; the remainder of the beam is reflected by beam splitter 15 and is directed to lens 14 at an angle of divergence 18. Lens 14 is any suitable lens or system of lenses that focuses an image of light source field stop 20 on bar code symbol 12, also shown in FIG. 2. Aperture 28 is provided to control lens aberrations. Region 21 defines the useful working depth of field of the instrument, and it may be varied by suitable design of the aperture - lens combination, as is well known in the art. When the light beam passes through lens 14, and strikes a region 17 of the bar code symbol, the width dimension of the optical image of the light source field stop 20 formed thereon by lens 14 is of the order of the lateral dimension of the bar code symbol minimum element width 13. Reflected light from a region 17 is captured by lens 14 and returned to beam splitter 15. A portion of the light passes through the beam splitter, and an image of region 17 of the bar code symbol is projected by lens 14 onto light sensor field stop 22. Light energy reflect- ed from region 17 is thus eventually received by light sensor 24, which generates a signal in response thereto. The signal produced by sensor 24 is coupled to convention¬ al signal processing circuitry. The light beam 38 extend¬ ing from light sensor 24 to beam splitter 15 as defined by light sensor field stop 22 has an angle of divergence 16 that is equal to angle 18. Field stops 20 and 22 should be equal in size and configuration. They are positioned equidistant from the beam splitter, close to the light source and light sensor respectively, and are oriented such that their optical images in region 21 are coaxial, coplanar and coextensive. The cones of light beams defined by the field stops will thus be congruent in three dimensions between the beam splitter 15 and the optical images of field stops 20 and 22 in region 21. The light sensor 24 is conventional and can be a photodiode, avalanche photodiode, photomultiplier, photo- transistor, or a photo-FET. It is possible to operate the apparatus without field stops 20 and 22, and directly focus the images of optically sensitive areas of light sensor 24 and of light source 26 in region 21. However the field stops are useful in avoiding interference from filament images and the structural detail of the light sensor. The field stops 20 and 22 may be implemented as an integral part of the light source 26 and the light sensor 24 respectively. The light sensor, of course, is responsive to defocused light that has passed through light sensor field stop 22. It will be appreciated that the positions of light source 26 and light sensor 24 can be interchanged without affecting the function of the apparatus. Referring now to FIG. 7, there is shown an alterna¬ tive embodiment of the invention, in which reference numerals of similar elements in respect of FIG. 1 are advanced by 400. Light source 426 emits a beam of light 436, prefer¬ ably through a light source field stop 420 that defines its sectional area and configuration. Light beam 436 is now directed through a first lens 514 at an angle of divergence 418. Lens aberrations are reduced by aperture 528, associated with the lens 514. The light beam, having passed through lens 514, eventually strikes beam splitter 415, which is disposed at a 45 degree angle to the optical axis of incident light beam 436. Other angles could be used to accommodate specific applications. Normally a 50% beam splitter is used, but this may be varied if desired. A portion of beam 436 continues beyond beam splitter 415 and is lost; the remainder of the beam is reflected by beam splitter 415 and is directed to target 412, converging at an angle 419. When the light beam passing through lens 514 strikes a region 417 of the bar code symbol, the width dimension of the optical image of the light source field stop 420 formed thereon by lens 514 is of the order of the lateral dimension of the bar code symbol minimum element width 413. Reflected light from the region 417 of the bar code symbol is incident on beam splitter 415. A portion of the light passes through the beam splitter, through a second lens 414, onto light sensor field stop 422, and is eventu¬ ally received by light sensor 424, which generates a signal in response thereto. This signal is conducted to signal processing circuitry. Aperture 428, associated with lens 414, reduces lens aberrations. In this embodiment lens - aperture combinations 414, 428 and 514, 528 are designed so that the images of field stops 420 and 422 are substantially coaxial, coplanar, and coextensive in a working depth of field in which bar code symbol 412 is located. The lenses are not necessarily identical, but may be designed to accommodate geometrical constraints in the housing of a scanner that require the light paths between light source 426 and beam splitter 415 to be of different length from the path between beam splitter 415 and light sensor 424. Field stops 420 and 422 should be geometrically similar in shape, but need not be equal in size. They are positioned close to the light source and light sensor respectively. The cones of light beams defined by the field stops will be substantially congruent in three dimensions in the working depth of the instrument. Field stops 422 and 420 may be integrated into the light sensor 424 and the light source 426. It will be appreciated that the positions of light source 426 and light sensor 424 can be interchanged without affecting the function of the apparatus. Folded light paths may also be employed without departing from the spirit of the inven¬ tion, so long as the desired congruence of the light beams in the working region as discussed above is maintained. While this invention has been explained with reference 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 may come within the scope of the following claims:

Claims: What is claimed is: 1. An optical assembly for use in detecting indicia, comprising: a light source means for emitting a beam of light that illuminates a target; a light sensor means, for producing a signal in response to light energy received from a region of sensitivity on a target; and an optical means in the path of said beam of light, for establishing said region of sensitivity on said target and for creating an image of said light source on said target, said region of sensitivity and said image being defined by light beams that are substantially congruent throughout a useful working depth of field, said region of sensitivity and said image further being rotationally aligned, whereby said image and said region of sensitivity are substantially coexten- sive on said target; and a means for excluding unwanted light from said light sensor means. 2. The assembly of claim 1, wherein said light source means further comprises a first field stop means for dimensioning a sectional area and configuration of said emitted beam of light; said light sensor means further comprises a second field stop means for dimensioning a sectional area and configuration of a light beam that is directed to said light sensor means by said optical image forming means; and said optical image forming means is configured to form images of said first and said second field stop means that are substantially coplanar and coextensive throughout said working depth. 3. The assembly of claim 2, wherein said first field stop means is integral with said light source means. 4. The assembly of claim 2, wherein said second field stop means is integral with said light sensor means. 5. The assembly of claim 1, wherein said optical image forming means comprises: beam splitter means, disposed in said emitted beam of light and in said path of light that is reflected from a target, for directing a portion of said reflected light to said light sensor means, and for directing a portion of said emitted beam of light to said target; and lens means for producing optical images. 6. The assembly of claim 5, further comprising aperture means, associated with said lens means, for reducing aberrations in said lens means. 7. A bar code scanning apparatus, comprising: a housing; a light source means for emitting a beam of light that illuminates a target; a light sensor means, associated with said housing, for producing a signal in response to light energy that is received from a region of sensitivity on a target; an optical means, disposed in the path of said beam of light, for establishing said region of sensitivity on said target and for creating an image of said light source on said target, said region of sensitivity and said image being defined by light beams that are substantially congruent throughout a useful working depth of field, said region of sensitivity and said image further being rotationally aligned, whereby said image and said region of sensitivity are substantially coextensive on said target. 8. The apparatus of claim 7, wherein said light source means further comprises a first field stop, proximately disposed thereto in the path of said emitted beam of light; said light sensor means further comprises a second field stop, proximately disposed and in the path of a light beam directed thereto by said optical image forming means; and said optical image forming means is configured to form images of said first and said second field stops that are substantially coplanar and coextensive throughout said useful working depth. 9. A method of reading indicia on a substrate that is disposed in a useful working depth of field, comprising the steps of: establishing a region of sensitivity for a light sensor in said depth of field, said region of sensitivity containing indicia to be read; directing light from a light source onto said region of sensitivity; forming an image of said light source in said depth of field that is substantially coplanar and coextensive with said region of sensitivity throughout said depth of field. generating a signal in said light sensor in response to light that is reflected from said region of sensitivi- ty. 10. The method of claim 9, further comprising the step of dimensioning said directed light. 11. The method of claim 9, further comprising the step of dimensioning said reflected light.

Inventors: HAMMOND CHARLES M JR, HAVENS WILLIAM H, LONGACRE ANDREW JR, HAMMOND, CHARLES, M., JR., HAVENS, WILLIAM, H., LONGACRE, ANDREW, JR.
Applicants: WELCH ALLYN INC, WELCH ALLYN, INC.
Application year: 1993
Main classification: G06K7
IPC codes: G06K7, G06T1