A device has been built and tested to check the overall energy in a double-beam infra-red spectrometer, especially when solvents are used in the reference beam. The apparatus consists of a synchronous motor driving a specially shaped chopper into the sample beam of the spectrometer at regular intervals. The spectrum of energy against wavelength is recorded and regions of low energy can easily be seen by the decrease in the amplitude of oscillations. Similar systems can be used for other types of absorption spectrometers.