The Wood anomalies are dark or bright bands in the spectrum produced by a diffraction grating. They occur at observed wavelengths for which light of the same wavelength is diffracted in another order so as to graze the surface of the grating. The equations describing the position of the anomalies are discussed for a variety of grating arrangements. Some experimental observations of Wood anomalies are described, including a new finding concerning the reluctance of two anomalies to coincide. Finally, a discussion is given of spectrophotometric errors introduced by Wood anomalies.
James E. Stewart and William S. Gallaway, "Diffraction Anomalies in Grating Spectrophotometers," Appl. Opt. 1, 421-429 (1962)