Annular Pupil Arrays

Any modification of the uniform amplitude distribution in the pupil of an optical system by a non-uniformly absorbing filter leads to changes in its spread function. Radially symmetrical pupil masks with a continuously varying transmittance have been used for this purpose. These can be made by vacuum deposition or photographic integration, but the manufacturing process is difficult and involves the use of rotating masks. An alternative is to use a binary mask consisting of a concentric array of annuli which approximates to the continuously varying absorption. The potential of such annular pupils for modifying the point spread of a confocally scanned imaging system is discussed, and some experimental results are presented.