ABSTRACT
Optical tolerances can be discussed in several different ways. On the most elementary level we can take a geometrical optics approach and consider the spread of the rays in the image of a point as indicating the degradation of the image by aberrations. Thus the spot diagram or the ray intersection density would be compared with the scale of structure to be imaged. The geometrical optics approach is appropriate mainly to systems of moderate performance and to the earlier stages of design of higher quality systems. Sometimes it is more appropriate to consider extended objects rather than single points; these extended objects may be single lines, edges or periodic structures; in each case the question of the nature of the illumination, that is, whether it is coherent or incoherent, has to be considered. It can be seen from the above that the subject of optical tolerances is very extensive.
