Due to the demanding specifications required by the modern optics industry a large number of instruments and methods have been developed to establish an appropriate metrology infrastructure. When measuring surface texture (profile, waviness and roughness), the form of the test object is removed, either by some type of mechanical skid device or by digital filtering techniques. In the field of large optics, deviations from ideal form can be many orders of magnitude larger than the surface texture specifications - this makes the metrology difficult. The most common forms of instrument used to measure optical surfaces are based upon non-contact optical probing techniques for which there are no international specification standards. These problems, along with the multitude of pick-up systems available, mean that it is difficult to obtain traceability to the definition of the meter. This paper discusses the issues that plague the surface metrology community and presents the results of a number of comparisons.
|