The use of infrared measurements to characterize impurities in doped semiconducting silicon wafers at room and low temperatures is reviewed. Examples include the detection of antimony, arsenic, boron, phosphorus, carbon, and oxygen impurities; the spectral features of these elements are qualitatively discussed. In particular, the design and implementation of a cryostat suitable as an accessory to an infrared interferometer for nondestructive characterization of large-diameter silicon wafers at liquid nitrogen and liquid helium temperatures are also given.