A new semiconductor photocell

A new differential photocell utilizing lateral photocell effect at a semiconductor junction is described. It can detect the position of a light spot to less than 100 A. Through an associated lens, one can measure an angular motion smaller than 0.1 second of arc. The output voltage of the cell is a linear function of the position of the light spot, with zero output for the light spot in the center, reversing in sign when the light spot changes from one side to the other of the center position. The linearity is better than 1.5 per cent over a distance of 0.030 inches. The equivalent noise resistance of the cell is equal to its output resistance, approximately 100 ohms. The sensitivity of the cell is approximately 200 micro-amperes per lumen and its frequency response is about the same as that of junction transistors. The response curve can be shifted electronically, constituting an electronic equivalent to a mechanical movement of the cell. Also, a square wave applied to the junction gives an electronic equivalent to a light chopper. Some applications are suggested.