Polarized illumination

OPTICAL ILLUMINATION TECHNIQUE
- TRANSMITTED AND POLARIZED LIGHTING

A polarizing filter is an oriented crystal that transmits only the light that travels in a fixed direction. Polarized light doesn’t keep its polarization when it diffusely reflects.
However, when polarized light specularly reflects, it keeps its polarization. Using this principle, it is possible to use a polarizing filter, with adjustable orientation, to remove specularly reflected light.

Schematic of a polarizing filter. The light that travels in the same direction as the crystal transmits through the filter. The light that travels in a different direction from the crystal can't transmit through the filter.