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PHASE CONTRAST MICROSCOPY

brightfield and phase contrast compared

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Phase contrast is a type of microscopy employing an optical mechanism to translate minute variations in the phase of light waves(invisible to the naked eye), into corresponding changes in amplitude, which can be visualized as differences in image contrast. This allows details that would not otherwise be visible with ordinary transmitted light microscopy to be easily and quickly visualized. It was developed by Fritz Zernicke in 1934, for which he won the Nobel prize.

One of the major advantages of phase contrast microscopy is that living cells and unstained specimens that have minimal or slight contrast differences, show up with much more contrast and can be examined quickly in their natural state. In the case of living material this also means that the specimen can be examined without being killed. Phase contrast is a relatively inexpensive technique to view unstained specimens with much greater detail quickly and easily. It utilizes a special condenser and special objectives. There is a phase plate in the condenser and phase rings at the back focal plane of the objective. For the technique to work properly the rings must be aligned. A small accessory eyepiece to view an enlarged image of the rear focal plane of the objective called a phase telescope is used for this purpose.

One of the big disadvantages of ordinary phase contrast is the creation of halos around objects. Recently this has been eliminated with the relatively new technique of apodized phase contrast. Another way to eliminate halos and yet provide increased contrast is with the more complicated and expensive technique of Differential Interference or Nomarski Interference Microscopy, DIC or NIC. This technique will be discussed in future web pages when time permits.