An account of the early history of the manufacture, testing, and use of optical components can be found in Twyman's book. Of the four surface metrics worthy of measurement, form, as we have seen, is probably the most important due to its direct influence on system performance. If two easily abradable surfaces are rubbed together in all directions, their area of contact increases to the point where both surfaces assume a spherical form of the same radius. This basic process is used in surface generation, but we still need means for determining the end point depending on acceptance tolerances.
A flat contacting metal template can be made to the desired shape and placed against a ground surface to assess the accuracy of form produced by viewing variations in the width of gap. Skill is needed to achieve a precision of 0.01 mm.
Polished surfaces require a much higher degree of measurement accuracy. This can be obtained by using some form of interferometer. In its simplest form, a carefully made reference plate can be put into close contact with the surface under test. When illuminated from above with a diffuse monochromatic source, as shown for flat surfaces in Fig. 2.1, Newton's fringes can be seen at the interface.
The advantage of this method is that the whole surface can be seen, so that areal cover as opposed to line cover, using the template method, is available. High sensitivity of 50 nm is typically achieved if the fringes are analyzed by eye, due to the small unit of optical wavelength employed, but surface damage can arise if the surfaces are brought into contact. Skill is needed in this case to avoid dust in the interface zone.
A third technique for form measurement involves probe gauging. A proximity probe is scanned in a straight line over the surface to be measured and height variations are recorded. The method, which is slow to achieve areal cover, is more usually applied in coordinate measuring machines using a contacting probe. It typically achieves an accuracy of 0.001 mm and requires some skill to apply.
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