The past few decades have seen an explosion of dedicated products with optical surface specifications that are flat with tight tolerances. For example, computer memory storage disks started with a 14-in. diameter, then briefly went to 8 in., then had a surge of 5¼-in. substrates for magnetic data storage. Silicon wafers started to be mass produced with 2-in. diameters, then 4 and 5 in., and currently are being produced at 450 mm. Polygon mirror wheels combining optical and mechanical tolerances for their drive bearings and motors became a real challenge to precision engineering. These developments prompted efforts in both automatic sorting and fast visual sorting of bad samples. (The fast-changing market was a welcome development to small businesses supplying dedicated hardware.)
The amount of effort involved in optical/instrumental testing can be
reduced if the mere view of the reflection off of a substrate surface (e.g., of a collimated beam) produces a light distribution on a screen, as in Fig. 8.1. Sunlight or a bright beam of light from a tungsten lamp suffices when visually judging the sample’s quality by reflected light distribution on the spot. The light distribution, as viewed in Fig. 8.1, might offer a quick qualitative in situ check without the need for handling and aligning. Also, adjusting the chucking vacuum will quickly produce evidence of elastic deformations (dirt on chuck) directly on the machine.
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