Abstract: A spectrum analyzer is disclosed which is of the type that utilizes an electrically scannable array of optical detectors. The array is selectively illuminated, i.e. irradiated, by at least two portions of a spectral band image of radiation. One of the portions is associated with a first spectral range and the other portion is associated with a second spectral range. The first and second ranges include mutually exclusive first and second spectral components, respectively, and a common third spectral component which is intermediate of the other two components. The detectors are calibrated by means which include a pair of optical filters. One of the filters is a single narow band transmission which has a transmission characteristic corresponding to one of the three aforementioned spectral components. The other filter is a double, i.e. two, narrow band transmission filter which has a transmission characteristic corresponding to the other two spectral components.