In order to detect the circular fringes of a Fabry–Perot interferometer with high sensitivity the electronic scanning of an optical multichannel analyzer (OMA) has been modified. Compared to the common scanning Fabry–Perot photomultiplier combination this modification yields a gain in sensitivity of more than a factor of 10. Furthermore, single pulses or strongly fluctuating sources may be measured without the noise problems of a scanning interferometer. The modified OMA is capable of resolving about 90 circles. The system is applied to the detection of the fine structure of weak laser‐excited phosphorescence spectra.