U.S. patent number 3,812,374 [Application Number 05/329,752] was granted by the patent office on 1974-05-21 for specular reflection suppression apparatus.
This patent grant is currently assigned to Computer Identics Corporation. Invention is credited to Richard H. Tuhro.
United States Patent |
3,812,374 |
Tuhro |
May 21, 1974 |
SPECULAR REFLECTION SUPPRESSION APPARATUS
Abstract
In a system for reading a diffuse reflective label, apparatus
for suppressing specular reflections comprising a source of
polarized radiation having a first polarization; 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; and means for receiving
diffusely reflected radiation having random polarization and
specularly reflected radiation having the first polarization
including polarizing means 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.
Inventors: |
Tuhro; Richard H. (Norwood,
MA) |
Assignee: |
Computer Identics Corporation
(Westwood, MA)
|
Family
ID: |
23286852 |
Appl.
No.: |
05/329,752 |
Filed: |
February 5, 1973 |
Current U.S.
Class: |
250/568; 250/225;
235/462.06; 356/446 |
Current CPC
Class: |
G06K
7/10861 (20130101); G02B 27/28 (20130101); G06K
9/2009 (20130101) |
Current International
Class: |
G06K
7/10 (20060101); G02B 27/28 (20060101); G06K
9/20 (20060101); G01n 021/30 (); G02f 001/18 ();
G06k 007/00 () |
Field of
Search: |
;250/225,219D,568
;235/61.11E |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lawrence; James W.
Assistant Examiner: Grigsby; T. N.
Attorney, Agent or Firm: Iandiorio; Joseph S.
Claims
1. In a system for reading a diffuse reflective label, apparatus
for suppressing specular reflections comprising:
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.
Description
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 efficient; most of it will be reflected to the reader. The
diffuse reflection, by its nature, is not efficient and less than
half, typically, only 0.1 to 0.01 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. 1 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, i.e., 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 insofar 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 20 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 30 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 50, 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 indicated by arrow 74; when the
randomly polarized radiation, 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:
* * * * *