A system which automatically processes optical excitation data is described. This system is capable of producing data faster and of better quality than the old method. The apparent cross section for excitation by electron impact of a particular state is proportional to the light intensity of a radiative transition from this level divided by the product of the electron beam current times pressure. The light intensity is measured from the output current of a photomultiplier tube. When the pressure in the collision chamber is maintained constant, the apparent cross section is proportional to the ratio of photomultiplier current to electron beam current. This ratio is continuously computed by an analog divider while the electron accelerating voltage power supply is slowly swept through its range by a motor and gear reduction system so as to give a visual display of the excitation function on an oscilloscope. Permanent records of the function are made by photographing the trace. Absolute values of the excitation function can be obtained by calibration against a standard lamp. In order to suppress noise and light signals not originating in the collision chamber, the light beam from the collision chamber is square wave modulated; this then is converted into a sinusoidal wave by means of a tuned amplifier. This in turn is rectified by a phase‐sensitive detector which produces a dc voltage proportional to the signal light intensity. Circuits of the phase‐sensitive detector and modulation unit are presented.