Phase Contrast and Differential Interference Contrast Microscopy

Unstained living cells are transparent and hard to see under an optical microscope. Phase-contrast and differential-interference contrast or DIC microscopes allow their visualization by creating higher resolution and sharper contrast images.

When a sample is illuminated in-phase-contrast, some light is diffracted by the cell components, while the rest directly passes through. The extent of diffraction depends on the refractive indices of the cell components.

As the diffracted and the direct light interfere, parts of the sample appear bright if the waves are in-phase and dark if they are out-of-phase. This creates high-contrast images of large organelles and thin tissue sections.

Sharp changes in refractive indices produce a halo around the image.

Such artifacts are absent in DIC, which uses a polarized light source to modulate the light waves passing through the sample.

The image contrast depends on the rate of change of the refractive indices across the specimen. Steep changes generate high-contrast images with an apparent three-dimensional effect.

DIC helps visualize thick specimens like brain slices, eggs, or embryos.