handbook of modern sensors

6.5 Optoelectronic Motion Detectors 239
(A)
(B)
Fig. 6.9. General arrangement of an optoelectronic motion detector. A lens forms an image of a
moving object (intruder). When the image crosses the optical axis of the sensor, it superimposes
with the sensitive element (A). The element responds with the signal, which is amplified and
compared to two thresholds in the window comparator (B).
The operating principle of the optical motion detectors is based on the detection
of light (either visible or not) emanated from the surface of a moving object into
the surrounding space. Such radiation may be originated either by an external light
source and then reflected by the object or it may be produced by the object itself
in the form of natural emission. The former case is classified as an active detector
and the latter is classified as a passive detector. Hence, an active detector requires an
additional light source, (e.g., daylight, electric lamp, an infrared light-emitting diode
(LED), and so forth. The passive detectors perceive mid- and far-infrared emission
from objects having temperatures that are different from the surroundings. Both types
of detector use an optical contrast as a means of object recognition.
First, we must consider the limitations of the optoelectronic detectors as opposed
to such devices as microwave or ultrasonic devices. Presently, optoelectronic detectors
are used almost exclusively to detect the presence or absence of movement
qualitatively rather than quantitatively. In other words, the optoelectronic detectors
are very useful for indicating whether an object moves or not; however, they cannot
distinguish one moving object from another and they cannot be utilized to accurately
measure the distance to a moving object or its velocity. The major application areas
for the optoelectronic motion detectors are in security systems (to detect intruders),