The adjustments of a variable phase microscope control two variable experimental parameters, specifically the phase difference δ between and the amplitude transmission ratio h of the conjugate and complementary areas of the diffraction plate or its polanret equivalent. It is shown that the center of the sharply focused diffraction image of a single unresolved particle can be brought to zero intensity by a definite choice of the parameters h and δ. The relations which exist among the required choice of h, δ, the area and the optical properties of the particle are derived and specialized to various classes of particles. These relations permit the computation of the area and one or more optical properties of the particle from the observed values of the parameters h, δ. Observations made upon opaque particles with a modified polanret microscope are described and found to be in accord with the theory. The radii of these particles ranged from 0.5 to 0.15 times the Airy limit of the test objective.
HAROLD OSTERBERG and GILBERT E. PRIDE, "The Measurement of Unresolved, Single Particles of Uniform Thickness by Means of Variable Phase Microscopy," J. Opt. Soc. Am. 40, 64-65 (1950)