The information below is provided to assist you in specifying your particular requirements for a special grating order.
Grating size
Grating size is usually dictated by the desired throughput, which is a function of the source and detector characteristics, the resolution of the optical system, and the required data-acquisition rate. The *Product tables* list the ruled area of each plane grating as the groove length followed by the ruled width (for example, 65 x 75 mm indicates a groove length of 65 mm and a ruled width of 75 mm). The dimensions of the ruled area and the substrate may be altered from the regular catalog sizes at an additional cost. Substrates of special configuration can also be supplied.
Substrate materials
The standard substrate material for small and medium-sized gratings is specially annealed borosilicate crown glass (BK-7). Float glass (plate glass) may be used for small cut-up gratings. However, low expansion material, such as Zerodur® or fused silica, can be supplied on request. For certain applications, it is possible to furnish replicas on metal substrates (such as copper or aluminum) that act as good heat sinks.
UV and IR enhancement
Aluminum (Al) is the standard reflectance coating. Fast-fired aluminum with an over-coating of magnesium fluoride (MgF2) can be used to enhance reflectivity in the region of 120-160 nm. For the extreme ultraviolet region below 50 nm, gold (Au) replicas are recommended. Platinum (Pt) is recommended for the 80-110 nm region. Gold replicas also have higher reflectivity in the infrared spectrum.
Groove spacing
The angular dispersion of a grating for a given wavelength is a function of the angles of incidence and diffraction. Once these angles have been determined, the corresponding groove spacing becomes a function of the order in which the grating will operate. Most gratings are used in the first order, thereby minimizing the effects of overlapping wavelengths, and resulting in high efficiency over a wide range. Many systems, however, operate successfully in the second or even higher orders, although this usually requires order-sorting of some kind.
Blaze angle
The gratings in this catalog are listed with their blaze angles and corresponding first-order Littrow blaze wavelengths, even though a few high-blaze angle gratings are not intended for first-order use. For practical reasons, blaze angles are usually chosen to favor the short end of the spectral region to be covered.
