How it Works: Visible Light Circular Polarizer
Circular polarizers are used to improve the viewability of emissive displays. In bright light environments, Electroluminescent (EL), Field Emissive Displays (FED), Cathode Ray Tube (CRT), Light Emitting Diode (LED) and Organic Light Emitting Diode (OLED) benefit greatly from the use of a circular polarizer.
Circular polarizers are used in camera, video and sensor applications where sensitivity to linearly polarized light is an issue, but glare reduction is required. Circular polarizers are also used in 3D applications as an alternative to linear polarizers.
How does a circular polarizer work?:
API circular polarisers are a combination of a linear polarizer and a quarter wave retarder. The linear polarizer absorbs the randomly polarized light that passes through it, with the exception of the light that is polarized parallel to the transmission axis.
This linearly polarized light then passes through the quarter wave retarder. The quarter wave retards the velocity of one of the polarization components (x or y) one quarter of a wave out of phase from the other polarization component. At this point, the light becomes circularly polarized (see Figure 1). One sometimes refers to the action of the quarter wave as “twisting” the polarized light. Note that depending on which polarization component is retarded, one will have either a left handed or right handed circular polarizer.
The usefulness of a circular polarizer occurs when light that has passed through the filter reflects off of a non-depolarizing surface (glass, metal, conductive coatings, acrylic, etc.). The handedness (left or right) of the reflected circularly polarized light switches to the opposite handedness. If light emitted from the circular polarizing filter is left handed, the reflected light becomes right handed.
The nature of a circular polarizer is that left handed circular polarized light cannot pass through a right handed circular polarizer and vice versa. This occurs because as the reflected opposite handed circularly polarized light passes through the wave retarder, it becomes linearly polarized again, but 90º to its original orientation. Therefore light passing through a circular polarizer filter, which then reflects off another surface, cannot pass back through the circular polarizer (see Figure 2). Thus making circular polarizers excellent glare reduction filters for displays.