Proceedings PaperAssessment And Specification Of The Scatter And Residual Reflectance Of Coated Optical Components In The Visual Region
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Optical images are invariably degraded by glare light and it has been known for many years that a substantial amount of glare can arise from multiple reflections. In fact, by far the principal source of glare from a polished, uncoated refracting surface is the reflection. The existence of these unwanted reflections led to the development of antireflecting coatings which have been very successful in suppressing the reflections, but such coatings nearly always introduce scatter. In making such coatings it is the usual practice to introduce plane-parallel witness pieces into the coating chamber and then to measure their reflectance, which is assumed to be typical of the reflectance of the batch. Investigations at Sira and elsewhere have shown that it is also important to control the scatter as this can have a considerable effect on the glare. However, many manufacturers state they are "not called upon" to measure scatter and specifications for antireflection coatings frequently make no mention of scatter. It is the purpose of this paper to suggest methods of testing coated optical components and witness pieces so as to produce readings which are closely related to the resulting glare in the image plane. Thus the relative importance of scatter and residual reflectance can be determined and coatings on components with curved surfaces can be assessed independently of the coating contractor, possibly after a period of use. Although this paper is mainly concerned with scatter and reflectance it is also relevant to discuss the assessment of localised defects such as digs and scratches. The assessment of such defects is usually undertaken by the manufacturer's optical inspectors. As inspectors get more skilled in detecting scratches, they also become more critical and it is known that many components are rejected for defects which can have no possible effect upon the performance. Much effort has been expended in recent years in producing graded standard defects with which to assess component faults by making visual comparisons but, in the opinion of this investigator, a functional test is much to be preferred.