Change of blaze wavelength as a function of position on the surface of a concave grating

Conventional ruling of a concave diffraction grating with blaze angle constant relative to the optical axis produces a grating whose blaze wavelength changes continuously with displacement across the face of the ruled area. A simple explanation of the effect is given in terms of geometrical optics, and quantitative expressions are developed that relate the expected blaze-wavelength shift to the grating constant and aperture ratio. Results are compared with experimental efficiency measurements, and the magnitude of the effect in a typical grating is discussed.