A glass substrate carrying a layer of deposited aluminum that has been pressure-ruled with a large number of fine equidistant grooves, using a diamond edge as a tool. Light falling on such a grating is dispersed into a series of spectra on both sides of the incident beam, the angular dispersion being inversely proportional to the line spacing. By proper shaping of the diamond edge, however, the grooves can be formed in such a way as to concentrate most of the energy into a single spectral order; such a grating is said to be blazed. Cast replica gratings can be made in plastic or another substrate, the replica being used either as a transmitting grating or by aluminizing it, as a reflecting grating. Plane gratings require external optics to focus the spectral lines, but a grating ruled on a concave surface is self-focusing. See concave grating.