DIC Microscopy

The physical principle of DIC microscope is completely different from that of phase contrast microscope, and the technical design is much more complicated. DIC uses polarized light and has four special optical components: a polarizer, a DIC prism, a DIC slider, and an analyzer. Polarizers are mounted directly in front of the condenser system to linearly polarize the light. A quartz Wollaston prism, DIC prism, is installed in the concentrator. This prism can decompose a beam of light into two beams of light (x and y) with different polarization directions, and the two form a small angle. The condenser aligns the two beams of light parallel to the microscope optical axis. Initially, the phases of the two beams of light are consistent. After passing through the adjacent area of the specimen, due to the difference in the thickness and refractive index of the specimen, the two beams of light have an optical path difference. A second Wollaston prism, a DIC slider, is installed at the back focal plane of the objective, which combines the two beams of light into one. At this time, the polarization planes (x and y) of the two beams of light still exist. Finally the beam passes through a second polarizing device, the analyzer.

Before the beam forms the DIC image in the eyepiece, the analyzer is at right angles to the direction of the polarizer. The analyzer combines two perpendicular beams of light into two beams with the same plane of polarization, causing them to interfere. The optical path difference between the x and y waves determines how much light is transmitted. When the optical path difference is 0, no light passes through the analyzer; when the optical path difference is equal to half the wavelength, the light passing through reaches the maximum value. Therefore, on the gray background, the structure of the specimen presents a difference between light and dark. In order to achieve the best image contrast, the optical path difference can be changed by adjusting the longitudinal fine-tuning of the DIC slider, and the optical path difference can change the brightness of the image. Adjusting the DIC slider can make the fine structure of the specimen present a positive or negative projection image, usually one side is bright and the other side is dark, which results in an artificial three-dimensional impression of the specimen