A 0.3–40 &mgr; wavelength‐modulation spectrometer, utilizing dual beam optics and a single detector, has been designed which is free of most of the difficulties present in previously reported instruments. Over the wavelength range of the photomultiplier tube this system, limited by photon noise, is capable of an optical resolution of 6 Å. For purposes of evaluating the performance of the system, the measured derivative spectra of a holmium oxide filter and of a single crystal gallium arsenide plate are reported. Measurements at liquid nitrogen temperatures, in gallium arsenide, produce spectral details that are substantially more pronounced than in electroreflectance or thermoreflectance spectra. The additional resolution permits new optical structures in gallium arsenide to be observed. The values of measured transition energies in this semiconductor confirm many of those earlier reported. New values for the temperature dependence of the band structure are reported.