Imperfections are described in ISO 10110-7:1996 as "localized defects within the effective aperture of an optical surface produced by improper treatment during or after the fabrication process." They degrade the quality of a surface finish. Since modern methods of surface examination can resolve atomic dimensions, it can be assumed that no surface is completely free of imperfections. But most surfaces generated are of acceptable quality and so there arises the need to define imperfection tolerances related to application.
The need to specify these tolerances on optical drawings has been recognized for a long time. Countries including the USA, the UK, and Germany developed standard scratch and dig plates that provided reference imperfections with a wide range of severities. These were used to identify the severity of a test imperfection - by eye - by comparing its visibility with the reference values. After a period of time it became clear that the operation of national standards was always going to be variable, as the reference plates were difficult to manufacture consistently and results depended on the observer's technique and visual performance. Even worse, correlation between different national standards was completely lacking.
The reason for this was traced to the fact that the microtopography of real scratches is extremely complex; therefore, it is not practically possible to suggest a statistical function that could be taken as a measure of severity. Moreover, since the eye is so sensitive to imperfections on a high-quality substrate, it was impossible to devise a deterministic approach likely to be appropriate for use by industry. After much research, it was concluded that imperfection severity could only be defined by creating a parameter related to practical use and which had traceability to national standards. The dark-field visibility metric was subjective and lacked traceability. Since optical imperfections are only of concern due to their ability to interact with optical radiation, scattering methods were nevertheless obvious candidates for such a metric.
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