Resolution of Phase Particles in Phase Microscopy

This paper deals with the question of two-point resolution for pure phase particles in a phase contrast microscope (no amplitude contrast is considered) illuminated with partially coherent light. It is seen that the phase particles in phase microscopy behaves in a manner equivalent to that of amplitude particles in ordinary microscopy. The limit of resolution as estimated on the basis of Sparrow criterion is seen to be independent of Δ, i.e. the magnitude of phase particles. In other words, the limit of resolvable separation with any finite contrast may be expressed as K = K _s+δK, where K _s, the limit obtainable on the basis of Sparrow criterion, is independent of the magnitude of Δ and δK is an incremental term that depends on the magnitude of Δ and the final required image contrast. Both terms, however, depend on the degree of coherence between the object particles. Results are given for the phase particles both leading or lagging in phase with respect to the background. The question of low contrast or small phase approximation is also discussed.