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Whatever the intended application for an optical component an integral part of its manufacturing process involves inspection and testing of that component to ascertain its conformance with design tolerances. Very often that inspection and testing can conveniently be carried out interferometrically. Past NPL research in this area has included development of the Fizeau interferometer for the accurate measurement of flatness; common path interferometers for the examination of large optical elements including mirrors; the use of computer generated holograms to facilitate null-testing of aspheric surfaces and, more recently, oblique incidence interferometers for the examination of non-optical components. Space permits just two examples to be discussed in detail. Lenses manufactured at NPL as standards for measurement of the Optical Transfer function are assessed after assembly by interferometric measurement of their wavefront aberration. A Twyman-Green interferometer is used to generate interferograms which are measured and computer analysed to elicit the shape of the wave-front, for comparison with that predicted from the design data. A polynomial representation of that wave-front may then be used in calculation of the lens OTF, again for comparison with theoretical predictions. For these comparisons with theoretical performance to be meaningful considerable care must be taken in the design and operation of the interferometer and in the measurement of the interferograms. Certain aspects of these operations will be discussed in detail. The assessment of optical flatness has traditionally been an interferometric measurement, the high sensitivity of normal incidence interferometers, approximately 0.25µm per contouring fringe, coupled with display of the flatness error of complete surfaces in a single interferogram being important features. However such interferometers are limited to optically polished surfaces whereas flatness is also often specified for other components that are not finished to the same degree. For the inspection of these components two distinct forms of oblique incidence interferometer have been developed with desensitised fringe contour intervals of 2.5 µm. These interferometers are able to indicate departure from flatness of lightly lapped or fine-ground metal and glass surfaces covering areas of up to 75 mm square in one design and surface areas up to 750 mm long by 75 mm wide in a single interferogram in the second design. Evaluation of departure from flatness of even larger surface areas is accomplished by computer assisted assembly of overlapping interferograms.
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