Abstract

While Hopkins¹ showed that pure phase objects can be seen in bright field microscopy with coherent illumination, very little quantitative study of such system performance has been carried out to date. We evaluate intensity distributions of images, in and out of focus, from planar phase and amplitude objects in bright field microscopy. This includes numerical calculations, based on the theory of partial coherence, and their quantitative comparison to experiments with test objects. Objects that we consider are slits or bar charts with known transmission properties. They are fabricated by photolithographic methods and their structure is verified by electron microscopy. In particular, we observe the effect of defocus under partially coherent illumination for objects near in size to the resolution limit of the light microscope. We observe that images of phase objects in partially coherent illumination are asymmetric about the focal plane, in agreement with theory, although the contrast is often lower than predicted. Also, we calculate spatial frequency content of the image intensity patterns and analyze the effects of scalar theory breakdown at high aperture angles.

© 1986 Optical Society of America