phase contrast microscope

n.
A microscope that uses the differences in the phase of light transmitted or reflected by a specimen to form distinct, contrasting images of different parts of the specimen.

A microscope used for making visible differences in phase or optical path in transparent or reflecting specimens. It is one of the most important instruments available for studying living cells and is widely used in biological and medical research.

The essential features of a phase-contrast microscope are shown in the illustration. The practical problem is to find some way of separating the incident or direct light from that diffracted by the object. This is done by placing a diaphragm D of easily recognizable shape, such as an annulus, at the front focal plane of the substage condenser C. Light from each point of the focal plane passes as a parallel pencil of rays through the specimen S and is brought to a focus at the rear focal plane P of the objective O. Thus, on removing the eyepiece, an image of the annulus will be seen at the back of the objective lens. This image corresponds to the incident light. In addition, when a specimen is present, some light is diffracted by it and spreads out to fill the whole of the back lens of the objective. Thus, apart from the small area of overlap over the image of the annulus, the direct and diffracted waves are essentially separated at the plane P. A phase plate is now inserted at this level. This can be a transparent disk with an annular groove of such dimensions that it coincides exactly with the image of the diaphragm D. All the direct light now passes through the groove in the phase plate, whereas the diffracted light passes mainly outside the groove. Since the diffracted light has to pass through a greater thickness of transparent material than the direct light, a phase difference, depending on the refractive index of the phase-plate material and on the thickness of the groove, is introduced between them. If this phase difference is about one-quarter of a wavelength, the basic conditions for phase contrast will have been achieved. If the phase plate is made to retard the incident wave by a quarter of a wavelength, the crests and troughs of the two waves will coincide, giving a resultant of greater amplitude. Refractile details will appear bright (negative contrast) instead of dark (positive contrast).

Diagram of a phase-contrast microscope.

The phase-contrast microscope is the routine instrument for the examination of living cells because it is possible to study the cell structure under excellent optical conditions and with no loss in resolving power. The method is also useful for the study of unstained tissue sections and has found considerable use for the comparison of material in the electron and optical microscopes. See also Electron microscope; Microscope; Optical microscope.

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n

A specially constructed microscope that has a special condenser and objective containing a phase-shifting mechanism whereby small differences in refraction can be made visible to intensity or contrast in the images. The phase-contrast microscope is particularly helpful in examining living or unstained cells and tissues. This microscope is an excellent aid in the education and motivation of patients in the understanding and control of dental plaque.

n.

A microscope that uses the differences in the phase of light transmitted or reflected by a specimen to form distinct, contrasting images of different parts of the specimen. Also called phase microscope.