fasecontrast

Fasecontrast, or phase-contrast microscopy, is a light microscopy technique designed to enhance image contrast of transparent specimens without staining. It exploits the fact that light passing through different parts of a sample undergoes small changes in optical path length, or phase, that are invisible in standard bright-field images. By converting these phase differences into variations in light intensity, phase-contrast microscopy reveals cellular structures and processes in living cells that would otherwise be difficult to see.

Principle and implementation. The method uses a phase plate in the objective and an annular diaphragm in the condenser. The phase plate shifts the phase of the undiffracted light by about a quarter of a wavelength relative to the light that passes through the specimen. Oblique illumination via the annulus causes the two beams to interfere when recombined, converting phase differences into brightness differences and producing high-contrast images of transparent structures.

History and impact. Fasecontrast was developed by Dutch physicist Frits Zernike in the 1930s, with a description published in 1934. It enabled routine visualization of living, unstained cells and contributed significantly to cell biology. For this work, Zernike was awarded the Nobel Prize in Physics in 1953.

Applications and limitations. It is widely used in biology and medicine to observe cells, bacteria, pollen, and other transparent specimens without staining. It provides real-time observation of dynamics but has limitations: best suited for thin samples, and halos or shading artifacts can occur at edges or thicker regions. The technique requires precise alignment and specialized optics and does not add color to the specimen.