The free aperture, also called the clear aperture , of a grating is the maximum area of the surface that will be illuminated. Also, it is important to ensure that the free aperture specifies an area that is completely circumscribed by the ruled area, so that the illuminated area never includes part of the grating surface that does not have grooves. Ruled Area . The ruled area of a grating is the maximum area of the surface that will be covered by the groove pattern. The ruled area is assumed to be centered on the substrate face unless otherwise indicated.
Most rectangular gratings have their grooves parallel to the shorter blank dimension.
Substrate Dimensions . If the grating is designed to be front-mounted, the blank specifications can be somewhat looser than if the grating surface will be positioned or oriented by the precise placement of the substrate. A grating blanks should have bevels on its active face, so that it is easier to produce and to reduce chipping the edges while in use. Substrate Material . The particular substrate material should be specified.