Polarised Illumination
Optical Illumination Technique - Transmitted and Polarised Lighting
A polarising filter is an oriented crystal that transmits only the light that travels in a fixed direction. Polarised light doesn’t keep its polarisation when it diffusely reflects.
However, when polarised light specularly reflects, it keeps its polarisation. Using this principle, it is possible to use a polarising filter, with adjustable orientation, to remove specularly reflected light.