handbook of modern sensors

276 7 Position, Displacement, and Level
(A)
(B)
(C)
Fig. 7.25. Four-quadrant photodetector: (A) focusing an object on the sensor; (B) connection
of the sensing elements to difference amplifiers; (C) sensor in a packaging. (From Advanced
Photonix, Inc. Camarillo, CA.)
Fig. 7.25. A four-quadrant photodetector consists of four light detectors connected in
a bridgelike circuit. The object must have an optical contrast against the background.
Consider a positioning system of a spacecraft (Fig. 7.25A). An image of the Sun
or any other sufficiently bright object is focused by an optical system (a telescope)
on a four-quadrant photodetector. The opposite parts of the detector are connected
to the corresponding inputs of the difference amplifiers (Fig. 7.25B). Each amplifier
produces the output signal proportional to a displacement of the image from the optical
center of the sensor along a corresponding axis. When the image is perfectly centered,
both amplifiers produce zero outputs. This may happen only when the optical axis of
the telescope passes through the object.
7.5.2 Proximity Detector with Polarized Light
One method of building a better optoelectronic sensor is to use polarized light. Each
light photon has specific magnetic and electric field directions perpendicular to each
other and to the direction of propagation (see Fig. 3.48 of Chapter 3). The direction
of the electric field is the direction of the light polarization. Most of the light sources
produce light with randomly polarized photons. To make light polarized, it can be
directed through a polarizing filter, (i.e., a special material which transmits light polarized
only in one direction and absorbs and reflects photons with wrong polarizations).