An optical potentiometer is disclosed having first and second variably light transmissive windows, a source of light mounted on one side of the windows and corresponding first and second light responsive devices mounted on the other side of the window in such a fashion that only one of the light responsive devices receives energizing light at a time, and a handle for moving first and second variable light transmissive windows past the corresponding first and second light responsive devices. A dwell period may be provided between the times when the first and second light responsive devices receive energizing light.