RESUMO
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 +/- Gamma of the retarder equals the phase shift delta of the sample. A Jones calculus model of the central ray in the polarization-modulated differential-interference contrast microscope yields SNR = (sin Gamma sin delta)/((1 - cos Gamma cos delta)N), where N is the rms time-dependent photon noise. This expression fits the experiments closely for 1.8 degrees < or = Gamma < or = 115 degrees.