We assume, until proven otherwise, that the bulk material is homogenous and is not affected by strain (resulting either from the growth/melt process or from thermal heating and/or clamping). The optical performance of such a perfect component would then relate to its form specification (shape or angle). However, although we would like the surface profile to be perfect, in practice we find that practical surfaces have texture; see Fig. 3.1:
Roughness—deviations from the desired form,
Waviness—deviations from the measured roughness,
Imperfections—digs, cuts, and sleeks.
The definitions of each of these texture parameters depend on the spatial density and the spatial frequency of the measurement.
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