Microscopy technique used to view unstained transparent or translucent specimens. Phase contrast takes advantage of different thicknesses within a specimen. For example, the nucleus of a cell is thicker than other parts of a cell. Light is retarded when passing through these thicker parts. The phase contrast microscope exploits the interference effects that take place when the two types of light recombine. If the waves complement each other the light is brighter, while if they are 180 degrees out of phase they cancel each other and the image is dark. Other such techniques to increase contrast include, Differential Interference Contrast (DIC) and Hoffman Modulation Contrast (HMC).
Phase contrast microscopy was first described in 1934 by Dutch physicist Frits Zernike and is a contrast-enhancing optical technique that can be utilized to produce high-contrast images of transparent specimens such as living cells, microorganisms, thin tissue slices, lithographic patterns, and sub-cellular particles (such as nuclei and other organelles). Phase contrast uses an optical mechanism to translate small variations in phase into corresponding changes in amplitude, which can be visualized as differences in image contrast. One of the major advantages of phase contrast microscopy is that living cells can be examined in their natural state without being killed, fixed, and stained. Phase contrast objectives can be added or purchased on many high power compound, or laboratory microscopes. Phase is only useful on specimens that do not absorb light (they are called "phase objects") and it is very useful in showing details in certain specimens such as cell parts in protozoans, bacteria, sperm tails and other types of unstained cells.
