Abstract

A liquid-crystal variable retarder inserted into a differential-interference contrast video microscope switches image highlights into shadows and vice versa in alternate frames. Synchronous computation and display of the difference between alternate frames yield a stream of images with doubled contrast and reduced fixed-position noise because of the automatic background subtraction. The measured signal-to-noise ratio (SNR) peaks when the modulation ±Γ of the retarder equals the phase shift δ of the sample. A Jones calculus model of the central ray in the polarization-modulated differential-interference contrast microscope yields

SNR=sin Γ sin δ1-cos Γ cos δN,
where N is the rms time-dependent photon noise. This expression fits the experiments closely for 1.8° ≤ Γ ≤ 115°.

© 2000 Optical Society of America

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