A single beam spectrometer measures the light transmitted through samples one at a time. It first adjusts to read zero transmittance without light, then 100% transmittance with light. Samples are then measured and the absorbance read, maintaining the same light source and detector settings. Dispersive IR spectrometers have sample compartments before the monochromator to reduce stray light. Radiation sources include thermal sources like tungsten filament lamps and electric discharge sources like deuterium and mercury lamps. Monochromators like prisms and gratings disperse radiation by wavelength. Cuvettes to hold samples are usually made of quartz, glass or plastic. Common detectors include phototubes, photomulti