A four-quadrant photocell is used in the system to replace photoelectric cell, and it ACTS both as centering detector and light intensity detector.
测试系统中使用四象限光电池代替一路光电池进行差动运算，四象限光电池同时兼有对光和光强检测的功能。
Photocell: or photoelectric cell, or electric eye; Solid-state device with a photosensitive cathode that emits electrons when illuminated and an anode for collecting the emitted electrons.
光电管：一种固体探测器，有一个对光灵敏的阴极，受到光照时能发射电子，还有一个用于收集所发射电子的阳极。
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