Bright-field microscopy of semitransparent objects.

We examine theoretically and experimentally the characteristics of in-focus and out-of-focus images of simple, well-defined phase objects. Theoretical calculations are based on the theory of partial coherence, and a simple calculation for imaging with coherent light demonstrates distinctive aspects of bright-field images. Experiments are performed with a well-corrected microscope, equipped for the precise control of illumination conditions and focus position. Theoretical and experimental results agree, although the contrast in the experimental images is often lower than expected. Also verified by experiment is a (to our knowledge) previously uninvestigated linear response in the intensity modulation of defocused, coherent images of thin, phase objects. The near-focus behavior of phase object images differs in symmetry from the more-familiar behavior of opaque object images.