For the MSS, this involves 6 fiber optics leads for the six lines scanned simultaneously to 6 detectors for each of the four spectral bands, or a total of 24 detectors in all. Most detectors are made of solid-state semiconductor metals or alloys. A semiconductor has a conductivity intermediate between a metal and an insulator. The resistance to flow varies inversely with the number of incident photons. The process is best understood by quantum theory.
Different materials respond to different wavelengths and are thus spectrally selective.
In the visible light range, silicon metal and PbO are common detector materials. Other detector materials are also used and perform under specific conditions. This next diagram gives some idea of the variability of semiconductor detectivity over operating wavelength ranges. Other detector systems, less commonly used in remote sensing function in different ways. Pre-amplification may be the first stage.