Polarizer between the led and the diffuser helps to delete the reflection of the lighting on the workpieces to be controlled. The camera, equipped with a polarizer filter, analyzer, can then start the analysis of the parts, without being disturbed by shiny effects. Polarizer can be very useful to lead a quality control of workpieces packed in transparent plastic bags.
Only available for visible wavelengths, the polarizer highly reduces the lighting power (about 50%).
Normal unpolarized light rays have many different "orientations". A polarizing filter only lets through light with a certain "orientation" and proportionally filters out light rays with different orientations. The further the orientation is away from the orientation of the polarizer, the less light makes it through, up 90 degrees where no light makes it through.