A sensitive visible spectrometer has been designed to detect very small differences in the transmission or the reflection of two adjacent areas of a substrate. A probing light beam of variable wavelength is deflected between the two areas by a galvanometer‐type oscillating mirror in order to obtain a differential absorption or reflection. The absorption spectra of a dye incorporated in a Langmuir film, a monolayer of dye molecules covalently bonded to glass and molecules oriented in a stretched polyethylene film are presented as examples. The modification of the instrument to cover the near UV and IR regions is straightforward and will be discussed.