differential interference contrast

Contrast formed by the difference of optical density or refractive index. It is noted that a substance with different refractive indices does not show any contrast in the transmission microscope. In the case of an optical microscope, "differential interference contrast" is obtained by the following way. Light is split into two rays with different polarizations by a polarizer and these two rays are passed through different specimen areas. At this event, a phase difference arises between the two rays depending on the difference of the optical density of the areas. Then, the two rays with the different phases are put back unpolarized by the polarizer to make interference between the two. As a result, the differential interference contrast is produced. When an additional bias phase is given between the two rays, an image with a shadow on the object edge is formed. In the case of TEM, an attempt has been made to obtain a sharp contrast image by giving an additional bias phase to a half of the electron beam that passes through an object specimen.