PHOTOSWITCHR Photoelectric Sens ors
Introduction
Transmitted beam sensors provide the
longest sensing distances and the
highest level of operating margin. For
Transmitted beam sensing may not be
suitable for detection of translucent or
transparent targets. The high margin
The maximum available sensing
distance of a sensor and reflector will
depend in part upon the efficiency of the
example, PHOTOSWITCH
levels allow the sensor to “see through” reflector or reflective tape. These
Series 4000B Transmitted Beam
sensors are capable of sensing
distances of up to 274 m (900 ft).
these targets. While it is often possible
to reduce the sensitivity of the receiver,
retroreflective or diffuse sensing may
provide a better solution.
reflective materials (page 1--306) are
rated with a reflective index.
The PHOTOSWITCH standard 78 mm
(3 in.) diameter round reflector (catalog
number 92--39) is used to determine the
maximum sensing distance of most
PHOTOSWITCH sensors.
Transmitted beam application margins
at ranges of less than 10 m (3.1 ft) can
exceed 10,000X. For this reason,
transmitted beam is the best sensing
mode when operating in very dusty or
dirty industrial environments.
Retroreflective
Retroreflective (reflex) is the most
popular sensing mode. A retroreflective
sensor contains both the light source
and receiver in one housing. The light
beam emitted by the light source is
reflected by a special reflective object
and detected by the receiver. The target
is detected when it breaks this light
beam (Figure 8).
The 92--39 reflector has a reflective
index of 100. The 92--99 reflective tape
has a reflective index of 77 meaning
that it will reflect only 77% as much light
as a 92--39 reflector.
Another example: Series 9000
Transmitted Beam photoelectric
sensors offer 300X margin at a sensing
distance of 3 m (9.8 ft). At this distance,
these sensors will continue to operate
even if 99.67% of the combined lens
area of the light source and receiver is
covered with contamination.
Retroreflective sensors are easier to
install than transmitted beam sensors.
Only one sensor housing must be
installed and wired. However, margins
when the target is absent are typically
10 to 1000 times lower than transmitted
beam sensing, making retroreflective
sensing less desirable in highly
Figure 8
Retroreflective Sens ing
The “effective beam” of a
transmitted beam sensor is equivalent
to the diameter of the lens on the light
source and receiver (Figure 6). Reliable
detection occurs when the target is
opaque and breaks at least 50% of the
effective beam.
Retroreflective
Object
to be
Sensed
contaminated environments.
Target
Caution must be used when applying
standard retroreflective sensors in
applications where shiny or highly
reflective targets must be sensed.
Reflections from the target itself may be
detected. It may be possible to orient
the sensor and reflector or reflective
tape so that the shiny target reflects
light away from the receiver. However,
for most applications with shiny targets,
polarized retroreflective sensing offers a
better solution.
Sensor
Figure 6
Effective Beam
Special reflectors or reflective tapes are
used for retroreflective sensing. Unlike
mirrors or other flat reflective surfaces,
these reflective objects do not have to
be aligned perfectly perpendicular to
the sensor. Misalignment of a reflector
or reflective tape of up to 15_ will
typically not significantly reduce the
margin of the sensing system (see
Figure 9).
Field of View
Field of View
Effective Beam
Detection of objects smaller than the
effective beam can best be achieved by
reducing the beam diameter through
means of apertures placed in front of
the light source and receiver (Figure 7).
Apertures are available for most 42KL,
42KB and 42EF transmitted beam
Polarized retroreflective sensors
contain polarizing filters in front of the
light source and receiver. These filters
are perpendicular or 90_ out of phase
with each other (Figure 10, on page
1--23).
Figure 9
Retroreflective Materials
sensors. Some users have created their
own apertures for other sensor families.
The sensor cannot see light reflected by
most targets. The reflected polarized
light cannot pass through the polarizing
filter located in front of the receiver.
Figure 7
Effective Beam with Apertures
Reflectors depolarize reflected light.
Some of the reflected depolarized light
can pass though the polarizing filter in
front to the receiver and can be
detected by the sensor.
Mirror
Reflector or
Retroreflective Tape
Aperture
Field of View
Field of View
In summary, the sensor can “see” the
reflection from a reflector, and it cannot
“see” the reflection from most shiny
targets.
Reduced
Effective Beam
“Corner-Cube” Reflector
Glass Bead Reflectors
Aperture
A wide selection of reflectors and
reflective tapes are available.
The most reliable transmitted beam
applications have a very high margin
when the target is absent, and a margin
of zero (or close to zero) when the
target is present.
Visit our website: www.ab.com/catalogs.
Preferred availability cat. nos. are printed in bold.
1--22
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