Circular Polarizer.
Circular polarizers are mainly used in consumer applications, such as with camcorders or SLR cameras that have a beamsplitter that send part of the light coming through the lens to a viewfinder and part to a light meter. Light entering the exposure meter is partially polarized by the beamsplitter. A linear polarizer over the lens would act as a secondary polarizer and block light going to the meter (but not the sensor or film), resulting in incorrect exposures. A circular polarizer ovecomes this problem with the addition of a ¼-wave retarder, which rotates the linearly polarized light 90° so that it appears unpolarized to the light meter, resulting in proper exposure.