An array of fine, parallel, equally spaced grooves ("rulings") on a reflecting or transparent substrate, which grooves result in diffractive and mutual interference effects that concentrate reflected or transmitted electromagnetic energy in discrete directions, called "orders," or "spectral orders." Note 1: The groove dimensions and spacings are on the order of the wavelength in question. In the optical regime, in which the use of diffraction gratings is most common, there are many hundreds, or thousands, of grooves per millimeter. Note 2: Order zero corresponds to direct transmission or specular reflection. Higher orders result in deviation of the incident beam from the direction predicted by geometric (ray) optics. With a normal angle of incidence, the angle , the deviation of the diffracted ray from the direction predicted by geometric optics, is given by
where n is the spectral order, is the wavelength, and d is the spacing between corresponding parts of adjacent grooves. Note 3: Because the angle of deviation of the diffracted beam is wavelength-dependent, a diffraction grating is dispersive, i.e., it separates the incident beam spatially into its constituent wavelength components, producing a spectrum. Note 4: The spectral orders produced by diffraction gratings may overlap, depending on the spectral content of the incident beam and the number of grooves per unit distance on the grating. The higher the spectral order, the greater the overlap into the next-lower order. Note 5: By controlling the cross-sectional shape of the grooves, it is possible to concentrate most of the diffracted energy in the order of interest. This technique is called "blazing."