Optical
Proximity Switches - Diffuse
The photoelectric proximity switches combine
transmitter and receiver in one housing. The light emitted by the transmitter
is reflected by the target object. The receiver evaluates this reflected light.
The advantage of this method is that no reflector is required.
As the proximity switch evaluates the reflected light
and its energy, the scanning range of conventional sensors (also called
energetic optical sensors) depends to a great extent on the colour of the
target object and on its surface.
Due to the poor reflection properties of dark
materials, the sensing distances of energetic sensors are reduced on those
surfaces. The type of reflection depends on the structure of the surface. Very
rough, inhomogeneous surfaces reflect in a diffuse way, i.e. in all directions.
Only a small amount of the light sent out is reflected to the receiver. In this
case, the scanning range is very short.
Proximity switches, which are based on energetic
evaluation, are ideally suited to the detection of larger objects or of objects
whose colour and surface structure are constant. Furthermore, attention must be
paid to the fact that the quantity of light reflected by the background should
not be greater than the quantity of light reflected by the object itself. This
effect occurs, e.g. with a black object in front of a white background. In this
case, detection with an energetic switch is impossible. Here we recommend the
use of a switch with background suppression.