In this paper we investigate the use of a geometric phase- shifting (GPS) technique which allows us to convert conventional transmission or reflection differential interference contrast (DIC) microscopy into a quantitative mode. A phase-shifting algorithm is employed to extract the specimen phase gradient from the mixture of phase and amplitude information which is common in DIC. Fourier techniques are then used to recover the exact phase (i.e. optical path length variations) throughout the biological specimen viewed. In addition to this quantitative 'phase map,' we demonstrate that the GPS process simultaneously yields an 'amplitude-only' representation in which various absorption and transmission properties of the specimen are displayed as intensity variations in the image, similar to brightfield microscopy. These two resulting images can then be analyzed or further processed in a number of ways that are not possible with conventional DIC and which improve the microscopist's ability to correctly identify, interpret and measure features in the specimen.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.