Diffraction of the general angle of incidence by dielectric surface-relief gratings is analyzed by use of a differential method. The method is applied to sinusoidal, rectangular, and triangular gratings. In all our results, error in energy conservation is of the order of 10−4 to groove depths as deep as 2.5 grating periods. It is also shown that, in conical diffraction mountings, when conditions of incidence upon these gratings are controlled, the gratings function as if a polarizing beam splitter had been cascaded with a rotatable half-wave retardation plate.