Photoreflectors

Photoreflectors (or sometimes called reflective interrupters)
are composed of an infrared LED and a phototransistor
in one housing, similar to the previously discussed transmissive
photointerrupters. The main difference is, that the IR emitting
LED and the phototransistor are not positioned face to face like
in the transmissive interrupters, but the emission and detection
direction of both devices are oriented parallel to each other in
the same direction. If there is no reflecting surface positioned
above the device, the IR radiation emitted by the IR-LED “disappears”
in the space above the device and there is no photo
current through the phototransistor. If there is a reflecting surface
present within distances smaller than the detection range,
the infrared radiation is reflected to the phototransistor and a
photocurrent is generated. The magnitude of the photo current
not only depends on the radiant intensity of the IR-LED, but also
on the reflection properties of the reflecting surface and on the
distance between the reflecting surface and the photoreflector.
Approaching the reflecting surface to the photoreflector the
photocurrent increases with decreasing distance. At a certain
distance d* the photocurrent reaches a maximum value. If the
distance of the reflecting surface is decreased below d* the
photocurrent decreases with decreasing distance because the
window of the IR-LED is shut more and more. At d=0 the IRPR-
LED is shut completely and there is no photocurrent through the
phototransistor.
In sensor applications where photoreflectors are used, the engineer
should very carefully consider the role of mechanical tolerances
of the distance between the reflecting surface and the
photoreflector and estimate the consequences of these tolerances
on the photocurrent. This has to be compared with the
tolerance of the device sensitivity. Last, but not least, the optical
properties of material used for the reflecting surface play an
important role. Polished metal surfaces usually are well reflecting
materials, whereas most plastic materials were more or less
transparent for infrared-radiation (even if appearing non-transparent
for the human eye) and hence were not recommended
as reflecting surfaces.