phasecontrast

Phase-contrast microscopy is an optical technique that enhances contrast in transparent or nearly transparent specimens by converting phase shifts of light, caused by variations in refractive index and thickness, into detectable differences in image intensity. It is widely used to observe living cells and unstained microorganisms that are difficult to see in conventional brightfield illumination.

The method relies on manipulating the phase of light in the microscope’s optical path. Light that passes through a specimen acquires a phase shift relative to background light. In a typical phase-contrast system a phase plate in the objective rear focal plane shifts the phase of unscattered light by 90 degrees, while an annular diaphragm in the condenser creates a hollow cone of light. The scattered light interferes with this shifted background, converting phase differences into intensity differences. The result highlights edges and fine structures against a relatively neutral field.

Variants include Zernike phase contrast, the original form, and Rheinberg illumination, which uses colored filters. Phase contrast is related but distinct from differential interference contrast (DIC), which provides a pseudo-3D relief image using polarized light.

Applications include observation of live cells and unstained microorganisms in biology and medicine. It is fast and non-destructive but can produce halo artifacts and is not a quantitative measure of phase. Performance depends on wavelength, numerical aperture, and precise alignment of the phase components.

History: Frits Zernike developed phase contrast in the 1930s; he received the Nobel Prize in Physics in 1953 for this work.