The prime purpose of surface metrology is to ensure the quality of manufacture defined as fitness for the intended purpose of the component. Hence, there is the need for tolerances relating to parameters that can be measured and used to characterize the performance or appearance of a component. It is an advantage if these metrics can also provide diagnostic information on the processes of manufacture.
Whereas deviations from the ideal form, or surface macrotopography, of a component computed by the designer have a direct and predictable influence on the performance of the system involved, the effect of residual finish, or surface microtopography, is not so obvious. Finish, not usually considered by the designer as it is not amenable in the same way as form to computation, is dealt with by defining a range of different metrics that will be discussed in the remaining chapters.
Ideally, all optical surfaces should be finished to the state where further surface treatment results in no improvement in surface quality. In practice, however, defects of atomic dimensions can now be visualized and so thresholds of acceptance of measurable parameters are required. As we have seen, it is convenient to analyze finish into metrics called texture, which extends over the whole surface, and imperfections that are highly localized. Texture can involve a random component of surface height variations called roughness, together with a periodic component termed waviness. This chapter deals with the most common methods for the measurement of surface roughness.
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