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Traditionally, the key component design parameters such as radius, lens thickness, size and shape of most types of optical components are measured using optical techniques. There are several reasons for this, but in particular: the form of the entire surface is generally revealed in one testing set up, the optical functionality of the component is almost always the performance defining factor, and the heritage of many of the optical methods, i.e. the huge investment and expertise that has in the past been brought to bear on perfecting the testing methods. There are however, alternative non-optical instruments, such as CMMs, for measuring the form of optical components that are becoming increasingly attractive for conformal optical components, off-axis optics, and aspherical lenses and components, for example corrector plates, grisms, and so on. The main reasons for the increased acceptance of such techniques are that: the asphericity of some of the surfaces is often too great to be handled satisfactorily by interferometric methods at optical wavelengths, or even at infra-red wavelengths; the probing force of modern, special- purpose probes is remarkably small; the cost of producing computer-generated holograms required for optical testing can be very high and often the numbers of components to be tested do not justify the expense; the speed of production is such that the component cannot be repeatedly removed and replaced in the manufacturing machine and/or the manufacturing process is not so conductive to optical testing because of the presence of cutting fluids etc. and, finally, the level of accuracy required cannot be achieved using optical techniques for unorthodox shapes.
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