Cut Sheet

Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com V8-T12-31
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
Reflex
Calculating the excess gain
for a reflex (retro-reflective)
sensor is similar to the
method used for diffuse
reflective sensors.
With this type of sensor,
excess gain and range are
related to the light
bouncing back from the
reflector. Maximum
operating range also depends
upon lens geometry and
detector amplifier gain.
The effective beam is defined
as the actual size of the
reflector surface. The target
must be larger than the
reflector before the sensor
will recognize the target and
switch its output.
Effective Reflex Sensor Beam
Light Curtain
A light curtain is a
specialized reflex sensor
head. It has four transmitters
and four detectors side by
side behind a cylindrical lens.
The light curtain emits a fan-
shaped beam, which provides
a wide viewing area.
The distance from the lens to
the reflector strip, together
with the length of the
reflector, serve to define the
effective detection area.
Effective Light Curtain Sensor Beam
Effective Detection Area Graph
Source Radiation
Retroreector
Detector Field of View
Effective Beam
+3
+2
+1
–
–1
–2
–3
2 ft 4 ft 6 ft 8 ft 10 ft
Curtain of Light Beam Prole
Deviation from
Beam Center
in ft
Corner Cube Retroreflector and Reflex
In the case of the corner cube
reflector, range and excess
gain depend upon on
reflector quality.
Corner cube reflectors
provide the highest signal
return to the sensor. Cube
style reflectors have 2000-
3000 times the reflectivity of
white paper.
Corner cube reflectors
consist of three adjoining
sides arranged at right angles
to one another.
Corner Cube Reflector
When a ray of light strikes
one of the three adjoining
sides, the ray is reflected to
the second side, then to the
third, and then back to its
source in a direction parallel
to its original course.
Thousands of these cube
shapes are molded into a
rugged plastic reflector or
vinyl material.
Glass Bead
Glass bead retroreflectors are
available in tape form for use
in dispensers for package
coding on conveyors. They
are also available in sheet
form that can be cut to size.
The bead surface is typically
rated at 200 to 900 times the
reflectivity of white paper.
Only corner cube reflectors
can be used with polarized
reflex sensors. The light
returned from the cube’s
surface is depolarized with
respect to the light it
received. Glass bead
reflectors cannot be used
with polarized retro-reflective
sensors.
Light Ray
Light Ray
Opaque
Material