US 3812374 A
Description (OCR text may contain errors)
United States Patent [191 [111 3,812,374
Tuhro [4 1 May 21, 1974 SPECULAR REFLECTION SUPPRESSION Primary Examiner-James W. Lawrence APPARATUS Assistant Examiner-T. N. Grigsby  manor: Richard Tuhm, Norwood M ass Attorney, Agent, or Firm-Joseph S. landiorio  Assignee: Computer ldentics Corporation,
Westwood. Mass.  ABSTRACT In a system for reading a diffuse reflective label, appa-  1973 ratus for suppressing specular reflections comprising a [2l] Appl. No.: 329,752 source of polarized radiation having a first polarization', means for providing relative motion between the  Us Cl. U 250/568, 235/6 n E 250/225 label and polarized radiation for scanning a diffuse re- [SI 1 1m CI G01" 21/30 Gozf "l8 006k 7/00 flective label with the polarized radiation of the first polarization; and means for receiving diffusely re-  Fwd Search 250/225 ,3 flected radiation having random polarization and specularly reflected radiation having the first polarization luding polarizing means for selectively blocking  References Cited me specular reflections having the first polarization and UNITED STATES PATENTS passing radiation having other polarization for increas- ,5ll 1/1970 Mori et al. 250/225 X ing the signal to noise ratio of the difi'usely reflected 1443972 5/ 250/225 K radiation to the specuiarly reflected radiation. 3.502.888 3/l970 Stites 250/225 X 1 Claim, 3 Drawing Figures iii i 68 5o j' 74 PHOTOSENSOR 44 SPECULAR REFLECTION SUPPRESSION APPARATUS FIELD OF INVENTION This invention relates to an apparatus for enhancing the signal to noise ratio in the reading of diffuse reflective media, and more particularly to such apparatus for blocking specular reflections.
BACKGROUND OF INVENTION In the early development of automatic reading machines the media, such as labels, read by the machines typically had to conform to very definite requirements such as size, registration marks, and contrast properties in order to insure that these relatively unsophisticated machines could recognize and distinguish the labels. As these machines have grown more sophisticated in system design as well as in the design of optical and electronic components they have become capable of recognizing and reading a wider range of labels. For example, presently there are labels in use which consist of essentially gum backed white paper labels on which a code is printed in black ink by conventional printing techniques or even by computer print-out devices. Such labels are extremely desirable for they not only begin with an inexpensive paper label which is easily available and not a special item but they are printed inexpensively and can even be printed by a computer. This latter feature is significant in many applications such as inventory control where the inventory, orders, purchases, shipments, billing, etc. are all computerized on the same computer system which can print labels as an integral part of its overall control of the flow of goods.
Such paper labels are typically regarded as diffuse reflecting surfaces. However, it has been determined that while usually 90 percent or more of incident radiation is reflected diffusely, a significant amount, anywhere from a few tenths of a per cent or less to ten per cent or more, of the incident radiation is specularly reflected. This small but significant amount of specular reflection has introduced a major source of error into the system causing a number of labels to be missed or misread. This is so because under certain conditions when the label, scanning beam and receiver are in a particular orientation the specular reflection may be reflected directly back to the receiver. Since the specular reflection is only a small part of the reflection this may not seem such a serious problem. But it is. For the specular reflection is highly eflicient; most of it will be reflected to the reader. The diffuse reflection, by its nature, is not eflicient and less than half, typically, only 0.1 to 0.0! percent, of the diffuse reflection may actually reach the reader. Thus, in many cases the specular reflection reaching the reader can overwhelm the desired diffuse reflection and distort the label reading. The problem is even greater when strong ambient light such as sunlight is present in which case the specular reflection received by the receiver can actually be greater than the diffuse reflection. The problem is also apparent when the background of the label, i.e., the surface of the object, produces specular reflection such as is the case with metal objects. This problem exists regardless of the color of the labels or contrasted coding. In a typical black on white label the inked black surface has been found to give greater specular reflection than the uninked white surface presumably because the ink fills the paper pores and makes the surface smoother. The initial reaction after the source of the problem is discovered is to rearrange the reader so that the specular reflection cannot be received by it. This is not a workable solution because typically: the size and working requirements of the reader will not permit it; existing equipment and structures at the site will not permit it; the ambient light is so strong that specular reflection is high in all arrangements; the process of precisely aligning the equipment to avoid the specular reflections is tedious and time consuming as is the continuing monitoring required to maintain the alignment and the objects are apt to be in any number of different orientations as they pass the reader.
SUMMARY OF INVENTION It is therefore an object of this invention to provide a simple, inexpensive, extremely reliable apparatus for blocking specular reflections which is easy to install and maintain.
It is a further object of this invention to provide such apparatus which blocks the specular reflections.
It is a further object of this invention to provide, in a system for reading diffuse reflective media, apparatus for increasing the signal to noise ratio relative to diffuse and specular reflections.
The invention results from the discovery that there is a sufficient amount of specular reflection from diffuse reflective media and environs to cause, under certain conditions, erroneous readings and the realization that specularly reflected radiation maintains the same polarization in the reflected ray as it had in the incident ray so that by using polarized light of a first polarization to scan the label a polarizing element can be set to block light of that polarization while passing light of other polarization thereby blocking the specular reflections derived from the scanning beam but passing a substantial portion of the diffuse reflections.
The invention features apparatus for suppressing specular reflection adapted for use in a system for reading a diffuse reflective label. The apparatus includes a source of polarized radiation having a first polarization and means for providing relative motion between the label and polarized radiation for scanning a diffuse reflective label with the polarized radiation of the first polarization. There are means for receiving diffusely reflected radiation having random polarization and specularly reflected radiation having the first polarization. Polarizing means are provided for selectively blocking specular reflections having the first polarization and passing radiation having other polarization for increasing the signal to noise ratio of the diffusely reflected radiation to the specularly reflected radiation.
DISCLOSURE OF PREFERRED EMBODIMENT Other objects, features and advantages will occur from the following description of a preferred embodiment and the accompanying drawings, in which:
FIG. I is a diagrammatic representation of a system for reading a diffuse reflective label including apparatus according to this invention for blocking specular reflections from the label or the background surrounding the label;
FIG. 2 is a diagrammatic representation similar to that shown in FIG. 1 showing more specifically the reflected radiation returning from the label; and
FIG. 3 is a diagrammatic representation of a system similar to that shown in FIGS. 1 and 2 using circularly polarized radiation.
The apparatus for blocking specular reflections according to this invention may be used in a system for reading a diffuse reflected label which is either located on a background or in an environment from which there are significant specular reflections or in which the label itself produces significant specular reflections or both. A beam of linearly polarized light is provided either by a polarized laser or an unpolarized laser or any other unpolarized light source with a polarizing element provided at its output. The linearly polarized beam is directed to strike the diffuse reflective label that is to be read. The return beam reflected from the label contains randomly polarized diffuse radiation reflected from the label plus it may also contain significant amounts of specular reflection from the label or from the background surrounding the label. The specular reflection is not randomly polarized but rather is polarized in the same way as was the scanning beam from the laser. The return beam and other radiation coming from the area of the label is received by the reader through some receiving means which may include a linear polarizing unit and may also include a lens and aperture or slot. The linear polarizing unit is oriented with its polarization axis transverse to the polarization axis of the linearly polarized light constituting the scanning beam. Typically, for optimum results the polarization axis of the polarizing element will be orthogonal to that of the polarization axis of the scanning beam. Thus, the polarizing element will block any radiation having a polarization axis which is the same as that of the scanning beam. Specular reflection causes the return beam to have the same polarization as the incident beam. Therefore, since the specular reflection will return polarized in the same way as the incident scanning beam the polarizing element oriented with its polarization axis orthogonal to that of the polarization axis of the scanning beam will effectively block all of the specular reflections returning from the label and the surrounding area. Diffuse reflections having random polarizations will be attenuated by approximately 50 percent by the polarization element, but the remaining portion of the diffuse reflection, which is passed by the polarizing element, will be essentially free of the specular reflection due to the blocking action of the polarizing element.
Alternatively, a quarter wave plate and a linear polarizing element or a source of linearly polarized radiation may be used to produce circularly polarized radiation for the scanning beam. The return beam will then contain specular reflections which are circularly polarized and diffuse reflections which are randomly polarized. Submission of the return beam to a quarter wave plate having the opposite rotation to that of the circular polarization of the return beam produces an output from the quarter wave plate which is linearly polarized and oriented so as to bisect the slow and fast axes of the quarter wave plate. Following this, in the return beam, a linear polarizing element, having its polarization axis transverse to, or optimally, orthogonal to, the axis of polarization of the linearly polarized light produced at the output of the quarter wave plate, will effectively block all of the circularly polarized light in the return beam. Since any specular reflection will contain the same polarization as the incident beam, in this case circular, specular reflection would be essentially blocked. However, diffuse reflection having random polarization passes through the quarter wave plate maintaining its random polarization and then encounters the polarizing element which attenuates approximately half of the randomly polarized diffuse reflections and passes the remaining portion.
In one embodiment, FIG. 1, an object 10 bearing a label 12 moving in the direction of arrow 14 is scanned by scanning beam 16 moving upwardly in the figure as shown by arrow 18. Beam 16 may be generated by a light source, such as laser 20, which produces light polarized as indicated by the arrows 22. If an unpolarized source is used, a polarizing element 24 may be placed at the output of the source to produce the proper linear polarization. Scanning beam 16 from laser 20 strikes mirror 26 from which it is reflected to one of a number of mirrors 28 on the periphery of a rotating wheel 30 which rotates in the clockwise direction as shown by arrow 32. After striking any particular mirror 28', beam 16 is reflected out through an aperture 34 to label 12. Return beam 36 is essentially coincident with scanning beam 16 from the label to mirror 26 via a mirror 28. In FIG. 1 the scanning beam 16 has been emphasized over the return beam 36 while in FIG. 2 the converse is true. Beyond mirror 26 return beam 36 passes through lens 38 and then through linear polarizing element 40 having its axis of polarization transverse, and optimally orthogonal, to that of the polarization axis of scanning beam 16 as indicated by arrows 42. The output from polarizing element 40 is delivered to a photosensor 44 which is connected to pulse shaping and amplifying circuits and finally to decoding circuits for decoding the information coded on label 12.
Typically, laser 20 may have a beam diameter of 0.040 inch so that mirror 26 may be very small, on the order of a tenth of an inch in diameter. Since lens 38 may be approximately 2 inches in diameter, mirror 26 affords a very small loss of the light collected from the returning beam. Alternatively, mirror 26 may be replaced by a partially reflecting mirror or prism or by a larger mirror having a small hole in the center through which the scanning beam can pass. Although polarizing element 40 is shown between lens 38 and photosensor 44 this is not a limitation of the invention as the polarizing element 40 may be placed anywhere in the system where it will affect the return beam but not the scanning beam such as between lens 38 and mirror 26, for example.
The operation of the system with respect to the return beam may be more clearly understood with reference to FIG. 2 where the return beam 36 has been emphasized over the scanning beam 16 except in the area between mirror 26 and laser 20. There are two types of radiation associated with return beam 36: the specular reflection linearly polarized in the same way as the scanning beam 16, as shown by arrows 22, and the diffuse reflection which is randomly polarized, as indicated by the bundle of arrows 50. When the specular reflection, polarized as indicated by arrows 22, strikes crossed polarizing element 40 whose polarization axis is orthogonal to the polarization axis, indicated by arrows 22, that specular reflection, having the linear polarization axis as indicated by arrows 22, is blocked and does not pass through polarizing element 40. However, the randomly polarized radiation striking crossed polarizing element 40 is discriminated: the portion of the randomly polarized radiation which is orthogonal to the axis of polarization element 40, ie, parallel to the axis of polarization of the scanning beam, as indicated by arrows 22, is blocked. The remaining portion of the randomly polarized radiation, which is aligned with the axis of polarization of polarizing element 40, is passed, as indicated by arrow 50', and is subsequently detected by photosensor 44.
Thus, specular reflection produced by the label or surrounding area, derived from the return beam 36 and directed back to the reader system, will be blocked in sofar as that specular reflection maintains the same polarization axis as imposed on the scanning beam and which the polarizing element 40 has been oriented to block. The randomly polarized radiation in the return beam derived from the diffuse reflections will pass through the polarizing element with only approximately a 50 percent loss. Therefore, while only approximately half of the useful information in the randomly polarized diffuse reflections is lost or is atentuated, effectively all of the specular reflections, derived from the incidence of the scanning beam on the label and the surrounding area, is blocked. Typically, commercially available linear polarizing elements having an attenuation rate of 1,000 to 1 are not uncommon.
Although thus far the illustrative embodiments of FIGS. 1 and 2 have described a system using a linearly polarized scanning beam, this is not a necessary limitation of the invention, for as shown in FIG. 3, a form of elliptically polarized radiation such as circularly polarized radiation may be used for the same purpose. Circularly polarized radiation is regarded as a form of elliptically polarized radiation in which the major and minor axis are equal and linearly polarized light may be considered as a form of elliptically polarized radiation in which one of the axes is zero. Circularly polarized light may be produced using a laser or other light source and first subjecting the beam to a linear polarizing element 60 and then submitting the linearly polarized beam to a quarter wave plate 62 such that the axis of linear polarization of the beam bisects the slow and the fast axes of the quarter wave plate. The circularly polarized beam 16 is then reflected as before off of mirror 26, scanner wheel and out aperture 34 to label 12. The return beam 36 contains two types of radiation associated with it: diffuse reflections having random polarization as indicated by the bundle of arrows 50 and specular reflection having circularly polarized radiation as indicated by circles 66. For purposes of this example, the circular polarization of the return beam 36 is, arbitrarily, shown as a left-handed or counter-clockwise polarization. When the return beam containing the randomly polarized, arrows S0, and circularly polarized, circles 66, radiation passes through lens 38 it first strikes a quarter wave plate 68 or other circular polarizing element. The circularly polarized radiation derived from the specular reflection is converted by the quarter wave plate 68 to linearly polarized radiation with its polarization axis oriented to bisect the angle between the slow 70 and fast 72 axes of quarter wave plate 68. This linearly polarized radiation is indi cated by arrow 74; when the randomly polarized radia tion, indicated by the group of arrows 50, derived from the diffuse reflections encounters quarter wave plate 68 it passes through and emerges as still randomly polarized radiation, as indicated by the group of arrows 50. However, both the linearly polarized radiation 70, indicated by arrow 74, and the randomly polarized radiation, indicated by bundle of arrows 50, is next submitted to a linear polarizing element 76 having its polarization axis, as indicated by arrows 78, transverse or, optimally, orthogonal to the direction of the polarization axis of the radiation emerging from quarter wave plate 68 derived from the circularly polarized radiation in the return beam. Thus, the linear polarization radiation indicated by arrow 74 derived from the circular polarization in the return beam 36 is totally blocked by polarizing element 76 and does not reach photosensor 44. However, the randomly polarized radiation, indicated by group of arrows 50, when submitted to polarizing element 76 is only partially attenuated so that the radiation, having an axis of polarization parallel to that of polarizing element 76, is passed while the remainder is not. Thus, this arrangement too decreases the radiation associated with the specular reflections effectively to zero while it attenuates the radiation associated with the diffuse reflections by a factor of approximately two,
Other embodiments will occur to those skilled in the art and are within the following claims:
What is claimed is:
l. in a system for reading a diffuse reflective label, apparatus for suppressing specular reflections comprismg:
a source of polarized radiation having a first circular polarization;
means for providing relative motion between the label and the polarized radiation for scanning a diffuse reflective label with said polarized radiation of said first circular polarization, and;
means for receiving diffusely reflected radiation having random polarization and specularly reflected radiation having said first circular polarization including polarizing means for selectively blocking specular reflections having said first circular polarization and passing radiation having other polarization for increasing the signal to noise ratio of the diffusely reflected radiation to the specularly reflected radiation.