Surface quality is a parameter that allows manufacturers and customers to specify a limit (tolerance) on the number and severity of imperfections, such as scratches, in a defined surface region.
The term ‘imperfection’ rather than the term ‘defect’ is used because an imperfection is defined as any deviation from a perfect optical surface, while a defect is an imperfection that has been determined to be outside of the ‘tolerance’ set by the designer.
Why specify surface quality?
Imperfections can be introduced during handling or manufacture of an optical component and carry the potential to impact both cosmetic quality and performance.
Meanwhile, the importance of individual surface imperfections can vary greatly depending on application. For example, for any surfaces that reside close to imaging planes, there is likely to be less of a tolerance for imperfections, because of the direct impact on imaging performance. On the other hand, surfaces that are to be placed far from any imaging planes will tend to have a greater tolerance for imperfections due to the reduced impact.
The challenge for manufacturers
When considering surface quality, the key challenge for manufacturers of precision optics is the balancing of quality and cost. It is important to ensure an appropriate surface quality specification has been agreed because setting the imperfection tolerance too tight will unnecessarily increase manufacturing cost, while setting it too loose will adversely affect product quality and performance.
Second to this, it is also crucial to have a reliable process for evaluating and controlling optical surface quality, so that manufacturers and customers can agree whether a particular component meets the surface quality specification. This is where the standards come in.
The standards for optical surface quality
Several standards have been developed to provide the supply chain with set methodologies for specifying and controlling optical surface quality. Of these standards, MIL-PRF-13830B (MIL-O-13830) and ISO 10110-7 are the most widely adopted, while ANSI/OEOSC OP1.002 and MIL-C-48497A are also notable in the frequency of their use.
MIL-PRF-13830B takes the visual approach to the inspection and characterisation of imperfections and is therefore considered to be a cosmetic surface quality standard.
ISO 10110-7 is the most recent international standard to provide a system for the assessment of optical surface imperfections. It allows both visual and dimensional approaches to the characterisation of imperfections.