Using liquid crystal variable retarders for fast modulation of bias and shear direction in quantitative differential interference contrast (DIC) microscope.

Conventional DIC microscope shows the two-dimensional distribution of optical path length gradient encountered along the shear direction between two interfering beams. It is therefore necessary to rotate unknown objects in order to examine them at several orientations. We built new DIC beam shearing assembly, which allows the bias to be modulated and shear directions to be switched rapidly without any mechanically rotating the specimen or the prisms. The assembly consists of two standard DIC prisms with liquid crystal cell in between. Another liquid crystal cell is employed for modulating a bias. All components do not require a special design and are available on the market. We describe techniques for measuring parameters of DIC prisms and calibrating liquid crystal cells. One beam-shearing assembly is added to the illumination path and another one to the imaging path of standard microscope. Two sets of raw DIC images at the orthogonal shear directions and two or three different biases are captured and processed within a second. Then the quantitative image of optical path gradient distribution within a thin optical section is displayed on a computer screen. The obtained data are also used to compute the quantitative distribution of optical phase, which represents refractive index gradient or height distribution. It is possible to generate back the enhanced regular DIC images with any desired shear direction. New DIC microscope can be combined with other techniques, such as fluorescence and polarization microscopy.