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Published online by Cambridge University Press: 31 January 2011
Zernike invented the phase-contrast microscope in 1935, and was awarded the 1953 Nobel prize in physics for this achievement. In an ordinary optical microscope, an object that imparts a phase modulation to the incident light will produce only a faint image. This faint image may be attributed to the diffraction of a small amount of the light out of the entrance pupil of the objective lens. To improve this image, Zernike in effect extracted a reference beam from the light collected by the objective lens and produced an interferogram of the object at the image plane of the microscope, thus converting phase information into amplitude (or intensity) modulation.
The principles of operation of the phase-contrast microscope are by now fully understood. Both spatially coherent and spatially incoherent light may be used in this type of microscopy. For best results, a quasi-monochromatic light source with a reasonable coherence time must be employed. Our goal in the present chapter is to give a simple explanation of the main ideas behind the method and to provide a pictorial survey of this important branch of modern optical microscopy.
The phase-contrast microscope
The diagram in Figure 38.1 shows the main elements of a phase-contrast microscope. The light source may be a coherent source (e.g., a laser) or an incoherent one (e.g., a tungsten lamp or an arc lamp); monochromaticity may be achieved by means of a colored glass filter. The condenser lens projects the source onto the object, whose image is formed by the objective lens.
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