Surface imperfections (optics)

Surface imperfections on optical surfaces such as lenses or mirrors, can be caused during the manufacturing of the part or handling. These imperfections are part of the surface and cannot be removed by cleaning. Surface quality is characterized either by the American military standard notation (eg "60-40") or by specifying RMS (root mean square) roughness (eg "0.3 nm RMS"). American notation focuses on how visible surface defects are, and is a "cosmetic" specification. RMS notation is an objective measurable property of the surface. Tighter specifications increase the costs of fabricating optical elements but looser ones affect performance.

While surface imperfections can be labeled "cosmetic defects", they are not purely cosmetic. Optics for laser applications are more sensitive to surface quality as any imperfections can lead to laser-induced damage. In some cases, imperfections in optical elements will be directly imaged as defects in the image plane. Optical systems requiring high radiation intensity tend to be sensitive to any loss of power due to surface scattering caused by imperfections. Systems operating in the ultraviolet range require a more demanding standard as the shorter wavelength of the ultraviolet radiation is more sensitive to scattering.

There are many different standards used by optical element manufacturers, designers, and users which vary by geographic region and industry. For example, German manufacturers use ISO 10110, while the US military developed MIL-PRF-13830 and their long-standing use of it has made it the de facto global standard. It is not always possible to translate the scratch grade by one standard to another and sometimes the translation ends up being statistical (sampling defects to ensure that statistically, the percentage rejected elements will be similar in both methods).

Examining surface quality in terms of 'Scratch & Dig' is a specialized skill that takes time to develop. The practice is to compare the element to a standard master (reference). Automated systems now replace the human technician, for flat optics, but recently also for convex and concave lenses. In contrast, 'Roughness' characterization is done with more precise and easier-to-quantify methods.