Polarization-modulated differential-interference contrast microscopy with a variable retarder

摘要

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)N' where N is the rms time-dependent photon noise. This expression fits the experiments closely for 1.8°≤Γ≤115°.