At Newport, we can replicate transmission gratings from almost any of our plane ruled gratings. Designed to direct as much light as possible for a specific wavelength into lower orders, transmission gratings can be used in spectrographs or as beam splitters.Transmission gratings have special uses in spectrometry. Any optical imaging system, such as a camera or telescope, can be converted into a spectrograph by placing a transmission grating in the system, typically in front of the objective lens. Transmission gratings also serve as convenient beam-splitters for monochromatic light sources, such as lasers. Gratings for use in the visible are typically manufactured on special-quality BK-7 substrates with an anti-reflective coating on the back face of the blank to reduce light loss and internal reflections. Gratings for the ultraviolet are typically manufactured on transmission grade fused silica substrates, these gratings do not have an anti-reflective coating on the back surface of the blank. Geometrical optic considerations require relatively coarse groove spacing (usually, no more than 600 g/mm). Finer grating pitches are possible, but at sharply reduced efficiencies.Light incident on a transmission grating is often normal to the back surface of the grating. Since no metals are present on the grating surface, and the angles of diffraction are usually small, the efficiency behavior of transmission gratings is simpler than that of reflectance gratings. The peak efficiency of a blazed (triangular-groove) transmission grating occurs when the refraction of the incident beam through the groove lies in the same direction as the diffraction given by the grating equation. Unlike reflection gratings, the groove angle is much larger than the blaze angle for transmission gratings. The groove angle for transmission gratings is defined to be the angle at which a normally incident beam at the blaze wavelength is diffracted. The corresponding transmission blaze wavelength is calculated from a theoretical extrapolation. Transmission gratings can serve as beam dividers. When two beams are required, a coarse, low blaze angle transmission grating will direct most of the incident light into the zero and first orders. When three beams are required, a lamellar groove transmission grating divides light between the zero order and two equal first orders in almost any intensity ratio desired.Standard transmission gratings range in size from 30 x 30 x 10 mm to 110 x 110 x 12 mm. Special sizes are available upon request. When requesting special sizes, please be sure to indicate if an anti-reflective (AR) coating is required on the back face of the blank. In applications where high transmission efficiency is not required, lower quality materials may be used without an AR coating to reduce production costs.
Transmission gratings can be replicated onto the face of a prism (forming a grism), which produces a straight-through spectrum, undeviated at one central wavelength. In such cases, the groove angle is often chosen to be approximately equal to the prism angle. Technical Note 4, identified on this page under the title, "Grisms", offers additional details on the use of grisms.