Phase Contrast Microscopes (PCM)

Phase contrast microscopes, or phase contrast microscopy (PCM), employ a special illumination technique known as phase contrast to improve the contrast in unstained biological samples without significantly sacrificing image resolution. Similar to darkfield illumination, phase contrast is ideal for use with transparent samples that seem to blend into a light background, and results in specimens that contrast from a darker background. The phase contrast effect is achieved by superimposing a circular phase annulus located in the sub-stage condenser with a corresponding circular phase ring located in the back focal plane of the objective lens. Specimens subjected to phase contrast illumination often appear to be surrounded by halos, and appear against a gray-ish background if used with plain white light, and appear against a green background if used with a green filter in the light path. Phase contrast is one of the most, if not the most, commonly used biological contrast enhancing technique today. Phase contrast microscopy can be performed on upright microscopes and inverted microscopes, and is especially useful in the study of living cells and tissues, aquatic and other microorganisms, microbiological features such as cell nuclei, glass fragments and fibers, mineral fibers such as asbestos, bacterial cultures and more.