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The effect of illumination of a semiconductor junction is, as is well-known, a photovoltage between the two sides of the junction. In this article it will be shown that a nonuniform illuimination gives a lateral photovoltage parallel to the junction in addition to the (transverse) photovoltage mentioned above. A photocell will be described that uses the lateral effect and can detect the position of a light spot to less than 100 Ã . By utilizing an associated lens or aperture, 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 inch. The equivalent noise resistance of the cell is equal to its output resistance, approximately 100 ohms. The sensitivity of the cell is approximately 200 microamperes per lumen and its frequency response is about the same as that of junction transistors. The response curve can be shifted by the application of a voltage between the base contacts. This is an electronic equivalent of a mechanical translation of the cell. It is also possible to do the equivalent of "chopping" the light by applying a modulating voltage to the alloyed dot.