MV 60: The Principle of Phase Contrast Microscopy
Simply Explained

Unstained living cells absorb practically no light. Poor light absorption results in extremely low contrast between the cells and the surrounding media. This makes unstained cells barely visible in a brightfield microscope. But when light passes through cells, small phase shifts occur as well, which are invisible to the human eye. A phase contrast microscope, transfers these small phase shifts into intensity shifts to enhance the contrast of non-pigmented cells.

The optical key components of a phase contrast microscope are an annular ring located in the condenser and a phase ring located in the back focal plane of the objective. The annular ring creates a tube-shaped illumination that is matched with the phase ring in the detection path, to apply an ±90° (ʎ/4) phase shift. However, scattered light from the sample mostly bypasses the phase ring. Now, interference between the scattered light and the tube-shaped and phase-shifted light, changes the detection lights amplitude and by this creates a good contrast.