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Chapter 2: From Extended Light Source to Collimated Illumination
The collimation obtained in the manner shown in Fig. 1.10(b) uses a suitable projection lens with diameter-to-focal-length ratios (d/F ) that are limited to about 1:10 and do not yet require spherical correction. Many low cost monochromats are available for optical components having large diameters. Optical component diameters must be compatible with the work they are expected to perform. A 10-cm diameter needs a 1-m-long mechanical structure. Z folding makes the structure more compact. Antireflection coating of surfaces that are not involved in interference formation is required.
A simple modification to the Newton testplating setup that is operated with collimated light is shown in Figs. 2.1(a) and (b). Figure 2.1(a) shows a SiC wafer under a reference at 0.25-mm distance under collimated oblique incidence. The collimator is indicated only in Fig. 2.1(a). The sample and the adjusting feet of the reference rest on the same base. The base is a granite plate, a glass plate, or a chuck. The adjustment to the sample distance uses a removable lens-cleaning tissue, a cigarette paper, or a nylon string, which is removed as the distance is increased.
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