Measurement of Groove Density of Diffraction Gratings

The use of diffraction gratings with variable groove density is becoming increasingly common. This is because it has become possible to preserve the beam divergence, reduce aberrations and improve the focal characteristics of such gratings. The demands in terms of optical performance are becoming even greater and, to be sure that a grating as manufactured is close to that required, techniques to measure accurately the groove density variation have had to be developed. In this chapter, one such method, arguably the most accurate, is described, although it has some limitations which will also be discussed.