handbook of modern sensors

6.5 Optoelectronic Motion Detectors 241
required, it is quite all right; however, in the majority of cases, a much wider field of
view is desirable. This can be achieved by several methods described in the following
subsections.
6.5.1.1 Multiple Sensors
An array of detectors may be placed in the focal plane of a focusing mirror or lens.
Each individual detector covers a narrow field of view; however, in combination, they
protect larger area. All detectors in the array either must be multiplexed or otherwise
interconnected to produce a combined detection signal.
6.5.1.2 Complex Sensor Shape
If the detector’s surface area is sufficiently large to cover an entire angle of view,
it may be optically broken into smaller elements, thus creating an equivalent of a
multiple-detector array. To break the surface area into several parts, one may shape
the sensing element in an odd pattern like that shown in Fig. 6.10A. Each part of the
element acts as a separate light detector. All such detectors are electrically connected
either in parallel or in series while being arranged in a serpentine pattern. The parallel
or serially connected detectors generate a combined output signal, (e.g., voltage v),
when the image of the object moves along the element surface crossing sensitive and
nonsensitive areas alternatively. This results in an alternate signal v at the detector
terminals. Each sensitive and nonsensitive area must be sufficiently large to overlap
with most of the object’s image.
6.5.1.3 Image Distortion
Instead of making the detector in a complex shape, an image of an entire field of view
may be broken into several parts. This can be done by placing a distortion mask in
front of the detector having a sufficiently large area, as is depicted in Fig. 6.10B. The
mask is opaque and allows the formation of an image on the detector’s surface only
(A)
(B)
Fig. 6.10. Complex shape of a sensing element (A); an image-distortion mask (B).