Polarizing filters reduce reflection, enhance contrast and detect imperfections in transparent materials. They’re best used for applications that have overwhelming reflection when inspecting objects on shiny surfaces.
Light reflected from a non-metallic surface becomes polarized, sometimes to the degree that a strong glare is created. A linear polarizer rotated to pass only light polarized in the direction perpendicular to the reflected light (the glare) will absorb much of it. Aside from reducing reflections, polarizers help to improve contrast by reducing dynamic range, allow for evaluation of stress in transparent plastic and glass, and can be used in pairs to form a variable neutral density filter.
For smooth surfaces or surfaces covered with grease, oil, or liquid, achieve the best results by using a polarizing filter over the lens in conjunction with polarizing film or polarizing glass sheet over the light source. Most polarizers are wavelength range specific; visible or infrared polarizing filters or filter sheet materials are offered.
Light used to image any nonmetallic surface will produce polarized light in reflection, therefore the use of linear polarizers can often result in appreciable benefit in industrial imaging applications.