U.S. patent application number 11/378593 was filed with the patent office on 2006-08-10 for retroreflective photoelectric sensor.
This patent application is currently assigned to OMRON Corporation. Invention is credited to Yukinori Kurumado, Yoshikazu Majima, Arata Nakamura.
Application Number | 20060175542 11/378593 |
Document ID | / |
Family ID | 32510626 |
Filed Date | 2006-08-10 |
United States Patent
Application |
20060175542 |
Kind Code |
A1 |
Majima; Yoshikazu ; et
al. |
August 10, 2006 |
RETROREFLECTIVE PHOTOELECTRIC SENSOR
Abstract
A retroreflective photoelectric sensor has a light-emitting
optical system having a light-emitting element and a first
polarizer and serving to transmit light from the light-emitting
element through the first polarizer, a light-receiving optical
system having a second polarizer and a light-receiving element and
serving to convert light received through the second polarizer into
an electrical signal by the light-receiving element, the first and
second polarizers having mutually perpendicular polarizer axes, a
single lens for both emitting light from the light-emitting element
and receiving light to the light-receiving element therethrough, a
beam splitter serving to direct light received from the
light-emitting optical system to the single lens and light received
from the single lens to the light-receiving optical system, and a
phase shifter inserted between the light-emitting optical system
and the single lens.
Inventors: |
Majima; Yoshikazu;
(Fukuchiyama, JP) ; Kurumado; Yukinori; (Ogaki,
JP) ; Nakamura; Arata; (Mishima-gun, JP) |
Correspondence
Address: |
BEYER WEAVER & THOMAS LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
OMRON Corporation
|
Family ID: |
32510626 |
Appl. No.: |
11/378593 |
Filed: |
March 16, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10731526 |
Dec 8, 2003 |
7045766 |
|
|
11378593 |
Mar 16, 2006 |
|
|
|
Current U.S.
Class: |
250/225 |
Current CPC
Class: |
G01V 8/14 20130101 |
Class at
Publication: |
250/225 |
International
Class: |
H01J 40/14 20060101
H01J040/14 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 9, 2002 |
JP |
2002-357263 |
Nov 18, 2003 |
JP |
2003-388499 |
Claims
1. A retroreflective photoelectric sensor comprising: a
light-emitting optical system having a light-emitting element and a
first polarizer and serving to transmit light from said
light-emitting element through said first polarizer; a
light-receiving optical system having a second polarizer and a
light-receiving element and serving to convert light received
through said second polarizer into an electrical signal by said
light-receiving element; a single lens for both emitting light from
said light-emitting element and receiving light to said
light-receiving element therethrough; a beam splitter serving to
direct light received from said light-emitting optical system to
said single lens and light received from said single lens to said
light-receiving optical system; and a phase shifter inserted
between said light-emitting optical system and said single lens.
wherein said first polarizer and said second polarizer have
mutually perpendicular polarizer axes.
2. The retroreflective photoelectric sensor of claim 1 wherein said
phase shifter is inserted between said first polarizer and said
beam splitter.
3. The retroreflective photoelectric sensor of claim 2 further
comprising another phase shifter inserted between said second
polarizer and said beam splitter.
4. The retroreflective photoelectric sensor of claim 1 wherein said
phase shifter and said another phase shifter each serve to shift
the phase by 3/8-5/8 with respect to the wavelength.
5. A retroreflective photoelectric sensor of claim 1 wherein said
phase shifter is inserted between said beam splitter and said
single lens.
6. A retroreflective photoelectric sensor comprising: a
light-emitting optical system having a light-emitting element and a
first polarizer and serving to transmit light from said
light-emitting element through said first polarizer; a
light-receiving optical system having a second polarizer and a
light-receiving element and serving to convert light received
through said second polarizer into an electrical signal by said
light-receiving element, said first polarizer and said second
polarizer having mutually perpendicular polarizer axes; a single
lens for both emitting light from said light-emitting element and
receiving light to said light-receiving element therethrough; a
beam splitter serving to direct light received from said
light-emitting optical system to said single lens and light
received from said single lens to said light-receiving optical
system; and means disposed between said first polarizer and said
beam splitter for canceling the total rotation of the polarization
plane that is the sum of rotations caused by passing through said
first polarizer and said light-emitting lens.
Description
[0001] This is a divisional of application Ser. No. 10/731,526
filed Dec. 8, 2003, currently pending.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a retroreflective photoelectric
sensor which may be used with a reflector to together sandwich a
target detection area for detecting a light-reflecting target
object in the detection area.
[0003] A retroreflective photoelectric sensor is used generally
with a reflector to together sandwich a target detection area
therebetween and detects the presence or absence of a target object
of detection based on the difference in the character of reflected
light from the reflector and that from the target object of
detection for light emitted from the sensor. In the case of a
retroreflective photoelectric sensor for a target object, a
reflector with a reflection characteristic that affects the
polarization mode of the light emitted from the sensor is used, and
the presence or absence of a target object of detection is
determined on the basis of whether or not a polarization component
peculiar to the reflected light from the reflector is contained in
the light received from the target area of detection. For this
purpose, it has been known to use reflectors of the type, for
example, having many triangular pyramid-shaped indentations
distributed on the reflective surface such that the incident light
is reflected several times on the three surfaces around the top
point of the pyramid shape to convert the polarization mode of the
incident light which is initially linearly polarized and to return
the reflected light back in the direction from which the incident
light came.
[0004] The present inventors have earlier proposed a
retroreflective photoelectric sensor of a so-called biaxial kind.
For this sensor, a light-emitting lens and a light-receiving lens
with small polarization distortion with retardation value less than
17 nm, made by injection-molding of a resin material, were used,
and an optical system for the light emission was made by arranging
a light-emitting element, a first polarizer (say, for vertical
polarization) and the light-emitting lens in this order, another
optical system for the light reception being made by arranging the
light-receiving lens, a second polarizer (say, for horizontal
polarization) and a light-receiving element, in this order.
[0005] Similarly, the present inventors also proposed (in Japanese
Patent Publication Tokkai 2001-228260) a different retroreflective
photoelectric sensor of a so-called coaxial kind, comprising a
light-emitting optical system for emitting light from a
light-emitting element by passing it through a first polarizer
(say, for vertical polarization), a light-receiving optical system
for receiving light through a second polarizer (say, for horizontal
polarization) and converting the received light into an electrical
signal by using a light-receiving element, a single common lens for
both emitting light from the light-emitting element and receiving
light to be received by the light-receiving element and a beam
splitter placed between the light-emitting and light-receiving
optical systems and the common lens for directing both the outgoing
light from the light-emitting element to the common lens and the
incoming light through the common lens to the light-receiving
optical system.
[0006] A sensor with this structure is advantageous in that the
first and second polarizers, which used to be placed in front of
the respective lenses according to the earlier technology, are now
placed behind the lens and they may be made smaller in size and
hence that the sensor case and the lens can be integrally formed,
thereby contributing to significantly reduce the production cost of
the sensor.
[0007] With a sensor of this structure with both the first and
second polarizers placed behind the lens, however, there was a
problem of light leakage even if the two polarizers are set in the
mutually perpendicular relationship (or the cross-nicol
relationship, forming so-called crossed nicols) such that the
quantity of received light in the presence of a light-reflecting
target object could not be made sufficiently smaller than that in
its absence.
SUMMARY OF THE INVENTION
[0008] It is therefore an object of this invention in view of the
problem described above to provide a retroreflective photoelectric
sensor which can be produced inexpensively and still has a reliable
detection capability.
[0009] A retroreflective photoelectric sensor embodying this
invention may be characterized as comprising a light-emitting
optical system having a light-emitting element and a first
polarizer and serving to transmit light from the light-emitting
element through the first polarizer, a light-receiving optical
system having a second polarizer and a light-receiving element and
serving to convert light received through the second polarizer into
an electrical signal by the light-receiving element, a single lens
for both emitting light from the light-emitting element and
receiving light to the light-receiving element therethrough, a beam
splitter serving to direct light received from the light-emitting
optical system to the single lens and light received from the
single lens to the light-receiving optical system, and a phase
shifter inserted between the light-emitting optical system and the
single lens, and wherein the first polarizer and the second
polarizer have mutually perpendicular polarizer axes.
[0010] In the above, the phase shifter may be inserted between the
first polarizer and the beam splitter or between the beam splitter
and the single lens. Another phase shifter may be inserted between
the second polarizer and the beam splitter, each serving to shift
the phase by 3/8-5/8 with respect to the wavelength.
[0011] Another retroreflective photoelectric sensor according to
this invention may be characterized as comprising a light-emitting
optical system having a light-emitting element and a first
polarizer and serving to transmit light from the light-emitting
element through the first polarizer, a light-receiving optical
system having a second polarizer and a light-receiving element and
serving to convert light received through the second polarizer into
an electrical signal by the light-receiving element, the first
polarizer and the second polarizer having mutually perpendicular
polarizer axes, a single lens for both emitting light from the
light-emitting element and receiving light to the light-receiving
element therethrough, a beam splitter serving to direct light
received from the light-emitting optical system to the single lens
and light received from the single lens to the light-receiving
optical system, and means disposed between the first polarizer and
the beam splitter for canceling the total rotation of the
polarization plane that is the sum of rotations caused by passing
through the first polarizer and the light-emitting lens.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1A and 1B are schematic drawings for explaining the
principle of a retroreflective photoelectric sensor of a biaxial
type respectively when there is not and there is an object in the
target area of detection.
[0013] FIG. 2 is a structural diagram of the optical system of a
retroreflective photoelectric sensor of a biaxial type embodying
this invention.
[0014] FIGS. 3A and 3B are schematic drawings for explaining the
principle of a retroreflective photoelectric sensor of a coaxial
type respectively when there is not and there is an object in the
target area of detection.
[0015] FIG. 4 is a structural diagram of the optical system of a
retroreflective photoelectric sensor of a coaxial type embodying
this invention.
[0016] FIG. 5, consisting of FIGS. 5A, 5B and 5C, includes drawings
for explaining the change in the direction of polarization of light
by a polarizer.
[0017] FIG. 6, consisting of FIGS. 6A, 6B and 6C, includes drawings
for explaining the change in the direction of polarization of light
by a lens.
[0018] FIG. 7, consisting of FIGS. 7A, 7B and 7C, includes drawings
for explaining the change in the direction of polarization of light
by a 1/2 phase shifter.
[0019] FIG. 8, consisting of FIGS. 8A, 8B, 8C and FIG. 9,
consisting of FIGS. 9A and 9B, are drawings for explaining the
change in the direction of polarization of light by a 1/2 phase
shifter obtained by eliminating the effects of the lens.
[0020] FIG. 10, consisting of FIGS. 10A, 10B, 10C and FIG. 11,
consisting of FIGS. 11A and 11B, are drawings for explaining the
total change in the direction of polarization of light by a
polarizer, a 1/2 phase shifter and a lens.
[0021] FIGS. 12A and 12B, together referred to as FIG. 12, are
sectional views of the case of a retroreflective photoelectric
sensor of the biaxial type embodying this invention.
[0022] FIG. 13 is a sectional view of the case of a retroreflective
photoelectric sensor of the coaxial type embodying this
invention.
[0023] FIG. 14 is a structural diagram of the optical system of
another retroreflective photoelectric sensor of a coaxial type
embodying this invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The invention is described next by way of an example with
reference to drawings but this example is merely one of many that
embody the invention and hence is not intended to limit the scope
of the invention.
[0025] FIGS. 1A and 1B are schematic drawings for explaining the
principle of a retroreflective photoelectric sensor 100 of a
biaxial type respectively when there is not and there is an object
in the target area of detection. In FIGS. 1A and 1B, symbol 200
indicates a reflector having a reflecting surface 200a, symbol 300
indicates a light-reflecting target object of detection with a
reflecting surface 300a, symbol L1 indicates light emitted from the
sensor 100, symbol L2 indicates reflected light from the reflector
200 and symbol L3 indicates reflected light from the target object
300.
[0026] As can be clearly understood from FIGS. 1A and 1B, the
retroreflective sensor 100 is placed opposite the reflector 200 so
as to sandwich therewith a target area of detection where the
target object 300 is expected to pass. As explained above, the
reflecting surface 200a of the reflector 200 is formed so as not
only to reflect the emitted light L1 from the sensor 100 but also
to change its polarization mode upon reflection. If the emitted
light L1 from the sensor 100 is linearly polarized in the
horizontal direction, for example, the reflected light L2 may be
elliptically polarized including a vertical component.
[0027] If there is no object in the target area of detection, as
shown in FIG. 1A, the emitted light L1 from the sensor 100 is
reflected by the reflecting surface 200a of the reflector 200 and
hence a sufficiently large quantity of reflected light L2 is
received by the sensor 100. If the emitted light L1 from the sensor
100 is linearly polarized in the vertical direction, for example,
the reflected light L2 is elliptically polarized including a
horizontal component. Thus, the sensor 100 can determine the
presence or absence of an object in the target area of detection on
the basis of the determination whether or not a specified amount of
horizontal component is included in the light received thereby from
the target area of detection.
[0028] If there is a light-reflecting target object 300 in the
target area of detection, as shown in FIG. 1B, the emitted light L1
from the sensor 100 is reflected by its reflecting surface 300a and
hence a fairly large quantity of reflected light L3 is received by
the sensor 100. Since the reflecting surface 300a of the target
object 300 is not specifically structured to modify the
polarization mode of the light L1 from the sensor 100 made incident
thereupon, if the emitted light L1 from the sensor 100 is linearly
polarized in the vertical direction, for example, the reflected
light L3 from the target object 300 will also be linearly polarized
in the vertical direction, having no horizontal component. Thus,
the presence of an object in the target area of detection can be
detected on the basis of the absence of horizontal component in the
light received from the target area of detection.
[0029] As shown schematically in FIG. 2, the retroreflective
photoelectric sensor 100' of a biaxial type embodying this
invention comprises an optical system for emitting light (the
light-emitting optical system 1) and another optical system for
receiving light (the light-receiving optical system 2). The
light-emitting optical system 1 has a light-emitting element 11, a
first polarizer 12 and a light-emitting lens 14 arranged in this
order, and the light-receiving optical system 2 has a
light-receiving lens 21, a second polarizer 23 and a
light-receiving element 24 in this order. The first and second
polarizers 12 and 23 have their polarizer axes differently
oriented. In the present invention, the polarization axis of the
first polarizer 12 is vertical and that of the second polarizer 23
is horizontal. In other words, the first and second polarizers 12
and 23 in this example are in the so-called cross-nicol
relationship.
[0030] In addition, a half-wave phase shifter (hereinafter referred
to as the "1/2 phase shifter") 13 is disposed between the first
polarizer 12 and the light-emitting lens 14 of the light-emitting
optical system 1 and another 1/2 phase shifter 22 is disposed
between the light-receiving lens 21 and the second polarizer 23 of
the light-receiving optical system 2. Although FIG. 2 shows the
first and second polarizers 12 and 23 in direct contact
respectively with the associated one of the 1/2 phase shifters 13
and 22, this is not intended to limit the scope of the invention.
They may be arranged so as to be mutually separated with
appropriate intervals therebetween. It is preferable to use a
material with a small index of refraction for the light-emitting
and light-receiving lenses 14 and 21. Plastic lenses with little
double refraction and glass lenses may be utilized.
[0031] The emitted light from the light-emitting element 11 passes
through the first polarizer 12 and the 1/2 phase shifter 13 while
enlarging the sectional area of its flux at a fixed rate and is
made incidence on the light-emitting lens 14, propagating
thereafter as the emitted light L1 to the target area of detection.
Reflected light L2 or L3 from the target area of detection is
passed through the light-receiving lens 21 and thereafter through
the 1/2 phase shifter 22 and the second polarizer 23 while reducing
the sectional area of its flux at a fixed rate, being received by
the light receiving element 24 and converted thereby into an
electrical signal. Since the first and second polarizers 12 and 23
are in the cross-nicol relationship, the presence or absence of an
object in the target area of detection can be determined as
explained above with reference to FIG. 1.
[0032] Since the 1/2 phase shifters 13 and 22 are inserted
according to this invention respectively between the first
polarizer 12 and the light-emitting lens 14 and between the
light-receiving lens 21 and the second polarizer 23, leakage of
light can be minimized when there is a light-reflecting target
object in the target area of detection such that there will be a
sufficiently large difference in the quantity of received light
between when there is and is not a target object in the target area
of detection. This will be explained more in detail below with
reference to FIGS. 5-11.
[0033] The present invention includes retroreflective photoelectric
sensors of the so-called coaxial type. FIGS. 3A and 3B are
schematic drawings for explaining the principle of a
retroreflective photoelectric sensor 400 of the coaxial type
respectively when there is not and there is an object in the target
area of detection. In FIGS. 3A and 3B, as in FIGS. 1A and 1B
explained above, symbol 200 indicates a reflector having a
reflecting surface 200a, symbol 300 indicates a target object with
a reflecting surface 300a, symbol L1 indicates light emitted from
the sensor 100, symbol L2 indicates reflected light from the
reflector 200 and symbol L3 indicates reflected light from the
target object 300.
[0034] The principles of operation are essentially the same as for
a sensor of the biaxial type. If there is no object in the target
area of detection as shown in FIG. 3A, the sensor 400 receives
reflected light L2 from the reflector 200 but if there is a target
object 300 in the target area of detection as shown in FIG. 3B, the
sensor 400 receives reflected light L3 from the target object 300.
The difference is that the reflected light L2 and L3 and the
emitted light L1 have the same optical axis.
[0035] As shown schematically in FIG. 4, the retroreflective
photoelectric sensor 400 of the coaxial type embodying this
invention comprises an optical system for emitting light (the
light-emitting optical system 3) and another optical system for
receiving light (the light-receiving optical system 4). The
light-emitting optical system 3 serves to output the light from a
light-emitting element 31 through a first polarizer 32, and the
light-receiving optical system 4 serves to convert received light
polarized by a second polarizer 42 into an electrical signal by a
light-receiving element 43. The sensor 400 further comprises a
single lens (the "common lens 5") used for passing both emitted and
received light and a beam splitter 6 disposed between the common
lens 5 and the light-emitting and light-receiving optical systems 3
and 4 for directing the outgoing light emitted from the
light-emitting optical system 3 to the common lens 5 and the
incoming light received through the common lens 5 to the
light-receiving optical system 4.
[0036] The polarization directions (or the directions of the
polarizer axes) of the first and second polarizers 32 and 42
respectively of the light-emitting and light-receiving optical
systems 3 and 4 are different. In the present example being
described, the polarizer axis of the first polarizer 32 is
perpendicular to the plane defined by the light-emitting element
and the common lens 5, and the polarizer axis of the second
polarizer is horizontal.
[0037] In addition to the above, a 1/2 phase shifter 33 is disposed
between the first polarizer 32 of the light-emitting optical system
3 and the beam splitter 6 and another 1/2 phase shifter 41 is
disposed between the second polarizer 42 of the light-receiving
system 4 and the beam splitter 6. It is preferable to use a
material with a small index of refraction also for the common lens
5. A plastic lens with little double refraction or a glass lenses
may be utilized.
[0038] The emitted light from the light-emitting element 31 passes
through the first polarizer 32 and the 1/2 phase shifter 33 while
enlarging the sectional area of its flux at a fixed rate and is
made incidence to the common lens 5, propagating thereafter as the
emitted light L1 to the target area of detection. Reflected light
L2 or L3 from the target area of detection is passed through the
common lens 5 and thereafter sequentially through the 1/2 phase
shifter 41 and the second polarizer 42 while reducing the sectional
area of its flux at a fixed rate, being received by the light
receiving element 43 and converted thereby into an electrical
signal according to the quantity of the received light. Since the
first and second polarizers 32 and 42 are in the cross-nicol
relationship, the presence or absence of an object in the target
area of detection can be determined as explained above with
reference to FIG. 3.
[0039] Since the 1/2 phase shifters 33 and 41 are inserted
according to this invention respectively between the first
polarizer 32 and the beam splitter 6 and between the beam splitter
6 and the second polarizer 42, the difference in the quantity of
received light can be made significantly large between when there
is and is not a light-reflecting target object in the target area
of detection. This, too, will be explained more in detail below
with reference to FIGS. 5-11.
[0040] FIGS. 5A, 5B and SC explain the local rotation of the plane
of polarization (or the direction of polarization) by a polarizer.
The present inventors set a light-emitting element LT and a
stationary camera SC opposite each other and a first polarizer P11
and a second polarizer P12 in a cross-nicol relationship on the
optical path between the light-emitting element LT and the
stationary camera SC, as shown in FIG. 5A. The image thus taken by
the camera SC clearly showed the presence of light leakage at four
places corresponding to the four corners of a square. It is
believed because the light from the light-emitting element LT
propagates with its sectional area increasing and hence does not
pass through the polarizers P11 and P12 perpendicularly thereto if
its "gate angle" .theta.1 (defined, as shown in FIG. 5A, as the
angle between the optical axis of the beam and the beam under
consideration off the optical axis) is large.
[0041] Let us assume that the direction of polarization (or the
polarizer axis) of the first polarizer P1 is vertical and that of
the second polarizer P2 is horizontal. FIG. 5B shows the directions
of polarization of different portions of the emitted light from the
light-emitting element LT, the center of the circle indicating the
beam of light heading perpendicularly to the first polarizer P11.
The beams heading upward, to the right, downward and to the left
are respectively represented at positions of 12 o'clock, 3 o'clock,
6 o'clock and 9 o'clock. As shown in FIG. 5B, the vertical
direction of polarization of the emitted beam from the
light-emitting element LT does not change for beams propagation
straight as well as those propagating in the directions
corresponding to 12 o'clock, 3 o'clock, 6 o'clock and 9 o'clock.
For beams in the directions in between (each making an angle of
45.degree. with the polarizer axis) corresponding to 1 o'clock and
30 minutes, 4 o'clock and 30 minutes, 7 o'clock and 30 minutes and
10 o'clock and 30 minutes, however, there is a change in the
direction of polarization. This change increases as the gate angle
increases, as shown in FIG. 5C, causing the light leakage as
explained above.
[0042] The graph of FIG. 5C shows the relationship between the gate
angle and the angle of polarization at the position of 1 o'clock 30
minutes (Position A indicated in FIG. 5B). This graph shows that
the direction of polarization hardly changes if the gate angle is
less than 10.degree. but changes rapidly if the gate angle exceeds
10.degree. and that the change in the polarization direction is
about 2.degree. when the gate angle is 30.degree..
[0043] Thus, if two polarizers P11 and P12 are placed in front of
the light-emitting element LT and light therefrom is caused to pass
through these polarizers P11 and P12, the beams separated from the
optical axis of the emitted light pass through the polarizers
obliquely and the polarization direction changes significantly for
such beams. This is how leakage of light results.
[0044] FIGS. 6A, 6B and 6C explain the local rotation of the plane
of polarization by a lens. Let us consider a situation where a
linearly polarized beam of light is passed through a plano-convex
lens LS (having a flat surface S1 and a convex surface S2) from the
side of its convex surface S2, as shown in FIG. 6A. If the
direction of an incident light beam is indicated by symbol 51 and
the normal line at the point of incidence is indicated by symbol
52, the angle of incidence .theta.2 for this beam is the angle
between these two directions 51 and 52. This shows clearly that the
angle of incidence .theta.2 increases as the point of incidence of
the beam approaches the periphery of the lens LS.
[0045] In FIG. 6A, symbol B indicates a linearly polarized beam
making incidence obliquely at an angle of 45.degree.. The graph of
FIG. 6C indicates that the polarization direction of such a beam
changes, as explained more in detail, for example, in "Applied
Physical Engineering" by Tsuruta (published by Baiyo-kan (Fifth
Edition (1998) at pages 237-240). It is explained, in particular,
that: "If linearly polarized light oscillating in the direction of
45.degree. from the plane of incidence, there is no retardation in
phase between transmissivity tp and ts of p-polarization and
s-polarization and since tp>ts, the transmitted beam is also
linearly polarized and its plane of oscillation approaches slightly
the plane of incidence (.theta.<45.degree.)."
[0046] FIGS. 7A, 7B and 7C explain the effect of change in the
direction of polarization by a 1/2 phase shifter. Let us consider a
situation where a 1/2 phase shifter P2 is superposed on the output
side of the first polarizer P11, as shown in FIG. 7A, to form a
combination and light from the light-emitting element LT is passed
therethrough from the side of the first polarizer to the 1/2 phase
shifter P2. Then, as explained above with reference to FIG. 5B,
light being outputted from the polarizer P11 has the direction of
its linear polarization changed at positions in the directions of 1
o'clock and 30 minutes, 4 o'clock and 30 minutes, 7 o'clock and 30
minutes and 10 o'clock and 30 minutes. On the output side of the
1/2 phase shifter P2, however, there are changes in the opposite
direction. In this situation, the change in the polarization
direction by the oblique transmission of the polarizer P11 and that
by the transmission of the 1/2 phase shifter P2 are related so as
to reverse the polarity, as shown in FIG. 7C. For example, if the
change in the polarization direction by passing through the
polarizer P11 is +.DELTA..theta., the change after the passage
through the 1/2 phase shifter P2 is -.DELTA..theta.. This change is
illustrated in FIG. 7B for other beams.
[0047] Next, FIGS. 8A, 8B, 8C, 9A and 9B are referenced to explain
the experiment carried out by the present inventors regarding the
change in the polarization direction by a 1/2 phase shifter,
obtained by eliminating the effects of the lens. In this
experiment, the light-emitting element LT and the stationary camera
SC were set opposite each other and the first polarizer P11 and the
second polarizer P12 were set in a cross-nicol relationship on the
optical path between the light-emitting element LT and the
stationary camera SC, as shown in FIG. 5A. The 1/2 phase shifter P2
was further placed on the output side of the first polarizer P11,
as shown in FIG. 7A and altogether as shown in FIG. 8A. The
directions of polarization observed at the output side of the
polarizer P11 and the output side of the 1/2 phase shifter P2 are
shown in FIG. 8B.
[0048] FIG. 8B shows that changes in the polarization direction are
observed at positions in the directions of 1 o'clock and 30
minutes, 4 o'clock and 30 minutes, 7 o'clock and 30 minutes and 10
o'clock and 30 minutes according to the gate angle. As shown in
FIG. 8C, if the change in the polarization direction due to the
polarizer P11 is +.DELTA..theta., the change after the passage
through the 1/2 phase shifter P2 is -.DELTA..theta.. As a result,
the image taken by the camera SC by the light after passing through
the second polarizer P12 includes four areas A11, A12, A13 and A14
with light leakage, as shown in FIG. 9A. Symbol A2 indicates a
screened area.
[0049] FIG. 9B is a graph for showing that the change in the
polarization direction due to oblique incidence onto the polarizer
is reversed in polarity by the 1/2 phase shifter. In summary, as
shown by FIGS. 7A and 8A, the effect of the polarizer P11 on the
polarization direction can be reversed by the 1/2 phase shifter P2
placed on the output side of the polarizer P11.
[0050] Next, FIGS. 10A, 10B, 10C, 11A and 11B are referenced to
explain the combined effects of polarizers, a 1/2 phase shifter and
a lens. As shown in FIG. 10A, the light-emitting element LT, the
first polarizer P11, the 1/2 phase shifter P2, the lens LS, the
second polarizer P12 and the stationary camera SC were arranged in
this order and light emitted from the light-emitting element LT was
passed sequentially through the first polarizer P11, the 1/2 phase
shifter P2 and the lens LS to be made incidence on the second
polarizer P12, and the output light from the second polarizer P12
was observed by the camera SC. As a result, as shown in FIGS. 10B
and 10C, the polarization direction changes as the light passes
through the polarizer P11 at the positions in the directions of 1
o'clock and 30 minutes, 4 o'clock and 30 minutes, 7 o'clock and 30
minutes and 10 o'clock and 30 minutes according to the gate angle.
After the light passes through the 1/2 phase shifter P2, an
inversion in polarity takes place on the change in the direction of
polarization brought about by the oblique incidence onto the
polarizer P11. As the light further passes through the lens LS, the
polarization direction changes further, depending on the angle of
incidence. Since the change in the polarization direction by the
lens LS cancels out the change in the polarization direction
brought about by the oblique incidence onto the 1/2 phase shifter
P2, as shown in FIG. 10C, the image of the second polarizer P12
taken by the camera SC may be made entirely into the screened area
A2 such that the leakage of light can be completely prevented.
[0051] This means, as shown in FIG. 11B, that the change in the
polarization direction can be made approximately constant,
regardless of the increase in the gate angle. In other words, the
change in the polarization direction shown in FIG. 5C brought about
by the oblique incidence onto the polarizer can be inverted by
means of the 1/2 phase shifter as shown in FIG. 9B and further
cancelled by the change depending upon the angle of incidence to
the lens as shown in FIG. 9C such that the nearly flat change
characteristic as shown in FIG. 11B can be obtained finally. In
this manner, the cross-nicol relationship between the first and
second polarizers can be maintained nearly all over the area.
[0052] Although FIGS. 5-11 were referenced above to explain a
situation where light passes through a polarizer, a 1/2 phase
shifter and a lens, in this order, a similar result is obtained
also where light passes through a lens, a 1/2 phase shifter and a
polarizer, in this order. Thus, leakage of light can be reduced in
a similar way also in the light-receiving optical system.
[0053] In summary, a retroreflective photoelectric sensor of this
invention adopts an optical structure as characterized by FIG. 2 or
4 such that the leakage of light can be reduced as shown by FIG. 11
and hence that the presence and absence of a target object of
detection can be clearly distinguished.
[0054] As explained above with reference to FIGS. 5-11 above, the
basic principle of the invention is to appropriately balance the
change in the polarization direction by the passage of light
through a polarizer, the inversion of the polarization direction by
the passage through a phase shifter and the change in the
polarization direction by the passage through a lens such that they
will cancel out. Thus, the phase shift to be effected by the phase
shifter may be considered to be a matter of design. According to
the studies by the present inventors, it is preferable that the
phase shift to be effected by the phase shifter be in the range of
3/8-5/8 (with respect to the wavelength, that is, in units of
2.pi.) and more particularly preferable to be closer to 1/2. The
optimum phase shift is believed to be obtainable from the
relationship between the angles of incidence to the polarizer and
the lens. According to the studies by the present inventors, the
angle between the polarization axis of the polarizer and the
optical axis of the phase shifter should preferably be less than
about 5.degree..
[0055] FIG. 2 is referenced again to explain the invention more in
detail.
[0056] The beam of light from the light-emitting element 11 passes
through the first polarizer 12 and the 1/2 phase shifter 13 and
then continues to propagate while increasing its sectional area
such that it will be about the same as the effective surface area
of the light-emitting lens 14. The center portion of the beam
(along its optical axis) makes incidence to all of the first
polarizer 12, the 1/2 phase shifter 13 and the light-emitting lens
14 and hence the direction of its (linear) polarization does not
change. Since the optical axis of the 1/2 phase shifter is set
parallel (or perpendicular) to the direction of polarization
(polarizer axis) of the first polarizer 12, the center beam is
under the same condition as if the 1/2 phase shifter were not
present and the beam propagates to the light-emitting lens 14 with
the direction of polarization unchanged. Since this center beam
makes incidence onto the light-emitting lens 14 also
perpendicularly, the beam passes through the center of the
light-emitting lens 14 without changing the direction of its
polarization.
[0057] Next, peripheral portions of the beam of light emitted from
the light-emitting element 11 (which will pass peripheral points of
the effective area of the light-emitting lens 14) will be
considered. These portions of light do not make incidence
perpendicularly onto the first polarizer 12 and the direction of
polarization changes, depending of the angular position of the
beam, as explained above with reference to FIG. 5B. Consider, for
example, the portion propagating in the direction of 1 o'clock and
30 minutes with reference to FIG. 5B. The direction of polarization
of this beam will not change as long as its gate angle is
sufficiently small (say, less than 10.degree.) but begins to change
as the gate angle increases. When the gate angle is 30.degree., the
direction of polarization changes by +1.8.degree. C.
[0058] If the 1/2 phase shifter 13 were not present and the
aforementioned peripheral portion of the beam were passed directly
through the light-emitting lens 14, the direction of its
polarization would further change in the positive direction, as
explained above with reference to FIG. 6. If the light-emitting
lens 14 is made of an acryl resin material, the change in the
polarization direction will be as large as +4.0.degree. if the
angle of incidence is 68.degree. (corresponding to the gate angle
of 30.degree.). The total change including the change by the first
polarizer 12 would be 1.8.degree.+4.0.degree.=5.8.degree..
[0059] If the 1/2 phase shifter 13 is inserted between the first
polarizer 12 and the light-emitting lens 14 according to the
present invention, it functions to change the polarization
direction in the opposite direction by the same angle by which the
polarization direction was changed as the beam passed through the
first polarizer 12, as explained above with reference to FIGS. 7
and 8. If the gate angle is 30.degree., the polarization direction
should change from +1.8.degree. to -1.8.degree.. Experimentally,
however, it was not -1.8.degree. but -3.2.degree.. It was probably
because the 1/2 phase shifter was pasted onto the first polarizer
12 and there was an error in the arrangement of its optical
axis.
[0060] Thereafter, the beam is passed through the light-emitting
lens 14 and the polarization direction is changed again in the
positive direction and the earlier obtained change in the negative
direction is cancelled. If the angle of incidence onto the
light-emitting lens 14 is 68.degree., the polarization direction
changes by +4.00 and the total change becomes
-3.3.degree.+4.0.degree.=+0.8.degree.. This is much smaller than
the total change of +5.8.degree. in the polarization direction if
the 1/2 phase shifter 13 were not inserted. Even if the
polarization direction were -1.8.degree. after the 1/2 phase
shifter 13 is passed, the total change after the light passes
through the light-transmitting lens 114 becomes
-1.8.degree.+4.0.degree.=+2.2.degree. and it is still much smaller
than if the 1/2 phase shifter 13 were not inserted.
[0061] Next, FIG. 4 is referenced again to explain more in detail
the relationship between the directions of the polarizer axes of
the first and second polarizers 32 and 42.
[0062] Let us assume firstly that the direction of the polarizer
axis of the first polarizer 32 is perpendicular (in the
"perpendicular direction") to the plane defined by the
light-emitting element 31, the light-receiving element 43 and the
common lens 5 and that of the second polarizer 32 is horizontal. In
this case, the light emitted from the light-emitting element 31 and
passed through the first polarizer 32 is linearly polarized in the
perpendicular direction except that the direction of polarization
changes for beams propagating in the directions of 1 o'clock and 30
minutes, 4 o'clock and 30 minutes, 7 o'clock and 30 minutes and 10
o'clock and 30 minutes (or directions making 45.degree. with the
polarizer axis) if the gate angle becomes sufficiently large, as
explained above. After this linearly polarized light passes through
the 1/2 phase shifter 33, it makes incidence onto the beam splitter
6 as an s-polarized beam (that is, with its plane of polarization
parallel to the surface of the beam splitter 6). Thus, a part of
this incident beam is reflected by the beam splitter 6 according to
the Brewster's law and only the portion of the light oscillating in
the perpendicular direction (or the p-polarized beam) propagates
towards the target area of detection (as emitted light L1 of FIG.
3).
[0063] If this light is reflected by the reflector 200 (as shown in
FIG. 3A), the reflected light L2 is elliptically polarized,
including both light oscillating in the perpendicular direction and
light oscillating in the horizontal direction. Most of the light
oscillating in the horizontal direction passes through the beam
splitter 6 and is received by the light-receiving element 43 after
it passes through the 1/2 phase shifter 41 and the second polarizer
42 with horizontal polarizer axis. In other words, a portion of the
reflected light L2 is received by the light-receiving element
43.
[0064] If the emitted light L1 is reflected by a target object 300
as shown in FIG. 3B, the reflected light L3 includes only
components that oscillate in the perpendicular direction because
only light oscillating perpendicularly to the target object is made
incidence to the target object. A portion of this reflected light
is reflected again by the beam splitter 6 according to the
Brewster's law but the remaining portion passes through the beam
splitter 6. The portion that passes through the beam splitter 6
also passes through the 1/2 phase shifter 41 but cannot pass
through the second polarizer 42 and hence is not received by the
light-receiving element 43. In other words, reflected light L3 from
the target object 300 is not received by the light-receiving
element 43. Thus, the sensor can distinguish between reflected
light from the reflector and reflected light from a target object.
The beam splitter 6 may be a half mirror without any polarization
characteristic or a polarizing beam splitter with a polarization
characteristic.
[0065] Let us consider next the situation where the direction of
the polarizer axis of the first polarizer 32 is horizontal and that
of the second polarizer 32 is in the perpendicular direction. In
this case, the light emitted from the light-emitting element 31 and
passed through the first polarizer 32 is linearly polarized,
oscillating in the horizontal direction. After this linearly
polarized light passes through the 1/2 phase shifter 33, it makes
incidence onto the beam splitter 6 as the p-polarized beam, that
is, it has the plane of polarization perpendicular to the plane of
the beam splitter 6. Thus, the incident beam passes through the
beam splitter 6 according to the Brewster's law and hardly any
light is propagated into the target area of detection. This means
that the structure shown in FIG. 4 is preferable in the case of a
retroreflective photoelectric sensor of the coaxial type. This
should be kept in mind also where the positions of the
light-emitting and light-receiving elements 31 and 43 are
interchanged.
[0066] Although FIG. 4 shows an embodiment wherein the 1/2 phase
shifters 33 and 41 are intimately in contact respectively with the
first and second polarizers 32 and 42, a single 1/2 phase shifter
may be used to replace them, disposed between the beam splitter 6
and the common lens 5. FIG. 14 shows a retroreflective
photoelectric sensor 400' thus structured according to such an
alternative embodiment of this invention having a single 1/2 phase
shifter 50 placed between the beam splitter 6 and the common lens
5. Its light-emitting and light-receiving optical systems 3' and 4'
are different from those of the sensor 400 shown in FIG. 4 in that
the phase shifters 33 and 41 are dispensed with. Thus, this
embodiment is advantageous in that the number of components is
reduced and its production cost is accordingly lower.
[0067] As shown in FIGS. 12A and 12B, the retroreflective
photoelectric sensor 100 of the biaxial type embodying this
invention may comprise a case 101, a lens unit 105 forming the
light-emitting lens 14 and the light-receiving lens 21 in an
integrated form together with a transparent cover 102, a linear
polarizer sheet (as the first polarizer 12) on the light-emitting
side and another linear polarizer sheet (as the second polarizer
13) on the light-receiving side. Symbols 13 and 22 each indicate a
1/2 wave sheet (serving as a 1/2 wave shifter).
[0068] The lens unit 105 is a molded product of an acryl resin
material with retardation value of less than 17 nm/mm, produced by
an extrusion molding process. The light-emitting lens 14 and the
light-receiving lens 21 are arranged on the back surface of the
planar transparent cover 102 where protrusions 114 and 115 are
formed for engagement.
[0069] The case 101 is formed with conically shaped indentations
110 and 111 respectively for providing a space for receiving the
light-emitting lens 14 and the light-receiving lens 21 of the lens
unit 105.
[0070] The light-emitting element 11 serving as a light source, the
linear polarizer 12 and the 1/2 phase shifter 13 are mounted inside
the indentation 110, and the light-receiving element 24, the linear
polarizer 23 and the 1/2 phase shifter 22 are mounted inside the
indentation 111.
[0071] The lens unit 105 is attached to an attachment member in
front of the case 101 by engaging the protrusions 114 and 115 with
indentations 116 and 117 such that the light-emitting lens 14 and
the light-receiving lens 21 are contained inside the indentations
110 and 111, respectively. The light-emitting element 11 is
operated by a light-emitting circuit (not shown) and output signals
from the light-receiving element 24 are inputted to a
light-receiving circuit (not shown) such that the presence or
absence of an object is determined, depending on the quantity of
light received.
[0072] The quenching ratio of the lenses 14 and 21 is about 1/1000
and their retardation value is less than 17 nm/mm. They are resin
lenses produced by extrusion molding and cause only small
deformations on the linear polarization of the transmitting light.
The polarizers 12 and 23 are disposed respectively on the
light-receiving side of the light-emitting lens 14 and the
light-emitting side of the light-receiving lens 21 such that the
light emitted from the light-emitting element 11 can be passed
through the polarizer 12 to polarize it linearly and to be made
incidence onto the light-emitting lens 14 and the reflected light
from a reflector or a target object can be made incidence on the
light-receiving lens 21 and passed through the polarizer 23 to
linearly polarize it.
[0073] Since the lenses 14 and 21 are made of acryl resin and
produced by extrusion molding, they can be formed integrally with
the case 101 and hence a water-resistant and dust-resistant
structure can be easily formed and the number of individual
components to be produced can be reduced. This also serves to
provide a compact photoelectric sensor.
[0074] FIG. 13 is a sectional view of the photoelectric sensor 400
of the coaxial type. Symbols 401, 31, 32, 33, 41, 42, 43, 6 and 5
respectively indicate a case, the light-emitting element, the first
polarizer, the first 1/2 phase shifter, the second 1/2 phase
shifter, the light-receiving element, the beam splitter and the
common lens.
[0075] With this sensor 400, too, the lens 5 is of an acryl
material and is formed integrally with the case 401 and the optical
system is of a coaxial type. Persons skilled in the art will easily
understand the operations of this sensor without further
explanations.
[0076] In summary, the present invention provides retroreflective
photoelectric sensors that can be produced inexpensively and are
capable of detecting a light-reflecting target object reliably.
* * * * *