Just How Smooth is That Surface?
Charles Langhorn, Coherent Optics
For many people, optics are but one of the many components that make up a system. They understand the specifications the optics must meet but, many times, they do not know nor are they concerned about the broader performance characteristics of the optics. However, for those applications in which the optics' performance is crucial to the operation of a system, making a distinction between levels of precision optics becomes important.
Optical specifications as generally seen on prints frequently are only a partial definition of what the optic must actually do. And, sometimes even for specifications that are quantitatively defined, such as with a certain damage threshold, performance can vary because of variations in measurement methodology. Additionally, a significant part of optics' performance is related to fabrication and coating processes that specifications cannot easily define.
Optical manufacturing processes are largely proprietary to the manufacturer and therefore are not open to review. However, a key indicator to the processes, and as a consequence, the performance of the optic, is the opticis surface morphology: its form and structure. Under an atomic force microscope, variations in processes become more obvious. You can learn a lot about the performance of an optic by measuring the surface morphology of the substrate and coating.