Phase contrast is a method used in microscopy and developed in the early 20th century by Frits Zernike. Zernike discovered that if you speed up the direct light path, you can cause destructive interference patterns in the viewed image. These patterns make details in the image appear darker against a light background. To cause these interference patterns, Zernike developed a system of rings located both in the objective lens and in the condenser system. When aligned properly, light waves emitted from the illuminator arrive at your eye 1/2 wavelength out of phase. The image of the specimen then becomes greatly enhanced. Phase is only useful on specimens that are colorless and transparent and usually difficult to distinguish from their surroundings. We call these specimens "phase objects". Examples of phase objects include cell parts in protozoans, bacteria, sperm tails and other types of unstained cells. This phase contrast technique proved to be such an advancement in microscopy that Zernike was awarded the Nobel prize (physics) in 1953.

The Phase Contrast Microscope
The phase contrast microscope is widely used for examining such specimens as biological tissues. It is a type of light microscopy that enhances contrasts of transparent and colorless objects by influencing the optical path of light. The phase contrast microscope is able to show components in a cell or bacteria, which would be very difficult to see in an ordinary light microscope.

Altering the Light Waves
The phase contrast microscope uses the fact that the light passing trough a transparent part of the specimen travels slower and, due to this is shifted compared to the uninfluenced light. This difference in phase is not visible to the human eye. However, the change in phase can be increased to half a wavelength by a transparent phase-plate in the microscope and thereby causing a difference in brightness. This makes the transparent object shine out in contrast to its surroundings.

The Invisible Can Be Seen
The phase contrast microscope is a vital instrument in biological and medical research. When dealing with transparent and colorless components in a cell, dyeing is an alternative but at the same time stops all processes in it. The phase contrast microscope has made it possible to study living cells, and cell division is an example of a process that has been examined in detail with it. The phase contrast microscope was awarded with the Nobel Prize in Physics, 1953.

Phase contrast is preferable to bright field microscopy when high magnifications (400x, 1000x) are needed and the specimen is colorless or the details so fine that color does not show up well. Cilia and flagella, for example, are nearly invisible in bright field but show up in sharp contrast in phase contrast. Amoebae look like vague outlines in bright field, but show a great deal of detail in phase. Most living microscopic organisms are much more obvious in phase contrast.

Using phase contrast
Phase contrast condensers and objective lenses add considerable cost to a microscope, and so phase contrast is often not used in teaching labs except perhaps in classes in the health professions and in some university undergraduate programs. This is unfortunate since the images obtainable in phase contrast mode can be very dramatic.

To use phase contrast the light path must be aligned. An element in the condenser is aligned with an element in a specialized phase contrast lens. This usually involves sliding a component into the light path or rotating a condenser turret. The elements are either lined up in a fixed position or are adjusted by the observer until the phase effect is optimized. Generally, more light is needed for phase contrast than for corresponding bright field viewing, since the technique is based on a diminishment of brightness of most objects.