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 δ / (1−cos Γ cos δ)<i>N</i>, where <i>N</i> is the rms time-dependent photon noise. This expression fits the experiments closely for 1.8° ≤ Γ ≤ 115°.
© 2000 Optical Society of America
George M. Holzwarth, David B. Hill, and Ethan B. McLaughlin, "Polarization-Modulated Differential-Interference Contrast Microscopy with a Variable Retarder," Appl. Opt. 39, 6288-6294 (2000)