A Fast-Response Dynamic Lightmeter and Its Application

A dynamic lightmeter is described. It has high gain, fast response, and is linear over a range of several decades. It is useful to present instantaneous light values on an oscilloscope or average values on a dc voltmeter. The instrument uses photodiodes in a differential circuit which balances the variation of dark current with temperature. In the high-gain position the overall frequency response is 125 kHz. Its sensitivity can be varied from 2 to 20 mV/fc. The application of the lightmeter is demonstrated on a special-purpose fluorescent ballast, operating from a 25-kHz square-wave source. The meter is used to observe start and decay times and steady-state operation of the lamp. Suggestions are made for calibration of the instrument and its application for closed-loop control.