Most tolerances on optical elements can be derived or calculated from the application requirements using computeraided
optical design programs. For surface quality and surface roughness, however, there are few guidelines or tools for
calculating appropriate tolerances. Typically, we simply use a legacy specification (e.g. 60-40 and 3 A RMS) with little
thought for either the cost of achieving the specification or the penalty for failing to achieve it. Often these legacy
specifications are ambiguous, unnecessarily costly and in some cases completely meaningless. This paper provides some
basic rules and equations for calculation of the real or perceived impact of these specifications, and some guidelines for
the initiate (and for some of us veterans as well) as to how to compose a meaningful tolerance.