Differential interference contrast (DIC) light microscopy, particularly when coupled with digital image processing, is a powerful tool for the high-resolution microscopy of unstained, transparent biological specimens and can equally well be applied to semiconductor measurements. We show analytically, and with images of diatoms, plant cells and protoplasts, that switching the polarization of the incident light by 90 degrees, changes the image highlights found in conventional DIC images into shadows and vice versa (1). Using a ferroelectric liquid-crystal modulator, this switching can be done at frame rates, synchronized to the camera. By subtracting alternate frames, a stream of difference DIC images is created. We call this technique Pol Mod DIC. Subtraction of alternate images is carried out efficiently by frame buffer operations and amounts to massively parallel synchronous detection. A similar method has been applied to confocal microscopy (2).