Grating efficiency measurements are generally performed with a double monochromator system. The first monochromator supplies monochromatic light derived from a tungsten lamp, mercury arc, or deuterium lamp, depending on the spectral region involved. The grating being tested serves as the dispersing element in the second monochromator. In the normal mode of operation, the output is compared with that from a high-grade mirror coated with the same material as the grating. The efficiency of the grating relative to that of the mirror is reported (relative efficiency), although absolute efficiency values can also be obtained (either by direct measurement or through knowledge of the variation of mirror reflectance with wavelength).
For plane reflection gratings, the wavelength region covered is usually 190 to 2200 nm. Diffraction gratings blazed farther into the infrared are often measured in higher orders. Concave reflection gratings focus as well as disperse the light, so the entrance and exit slits of the second monochromator are placed at the positions for which the grating was designed (that is, concave grating efficiencies are measured in the geometry in which the gratings are to be used). Transmission gratings are tested on the same equipment, with values given as the ratio of diffracted intensity to the intensity falling directly on the detector from the light source (i.e., absolute efficiency).
Curves of efficiency vs. wavelength for plane gratings are made routinely on all new master gratings produced by MKS, both plane and concave, with light polarized in the S and P planes to assess the presence and amplitudes (if any) of anomalies. Such curves are available on the Richardson Gratings website, http://www.gratinglab.com/ (for an example, see Figure 11-3).
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