3D polarizer. Circular polarizer. Neutral gray circular polarizer
Image Credit: American Polarizers | Knight Optical | American Polarizers
Circular polarizers block all photons rotating in one direction while allowing those rotating in the opposite direction to pass. Circular polarization is considered a special case of elliptical polarization where the horizontal and vertical electric fields are added together in equal amounts and have ninety degrees of phase difference. Circular polarizers are commonly used in photography and the lenses of 3D glasses for viewing stereoscopic movies. These optical polarizers are assemblies that consist of a linear polarizer and a wave retarder, usually a quarter waveplate or half waveplate. The difference between circular polarizers and linear polarizers is that linear polarizers are optical components that transmit light waves along one axis and absorb them along the other. The transmitting and absorbing axes of linear polarization are oriented at 90 degrees to each other.
Circular polarizer. Image Credit: American Polarizers, Inc.
How Circular Polarizers Work
Normally, circularly polarized light is created when one linear electric field component is phase-shifted in relation to the orthogonal component. Circular polarizers may be also be used as optical isolators, transmitting left-circular light or right-circular polarized light for an input beam of any polarization state. By aligning the linear polarizer component with the quarter-wave retarder component, circular polarizers can turn linearly-polarized light into circularly-polarized light. Since specular reflection causes circularly polarized light to exhibit shift in orientation or handedness, the reflected circularly-polarized light becomes linearly-polarized and rotates 90 degrees from the light leaving the isolator. As a rule, horizontally-polarized light that falls upon a vertically-aligned polarizer is rejected. Circular polarized light that emerges from the polarizer is reflected by the partially-reflecting mirror with an intensity which is independent of the angle at which the polarizer is set. This eliminates metering errors caused by the polarization angle. In general, there is little difference between a linear and circular polarizer.
Circular polarizer animation. Video credit: 0ruff0
Circular Polarization Waveplates
Waveplates and retardation plates are optical elements with two principal axes, one slow and one fast, which resolve an incident polarized beam into two mutually perpendicular polarized beams. Selecting circular polarizers requires an understanding of half-wave and quarter-wave retardations.
- Half-wave retardation plates are made of a double-refracting crystal having such a density that a phase difference of one-half cycle is formed between the ordinary and extraordinary elements of light passing through.
- Quarter waveplates are made of a double-refracting crystal having such a density that a phase difference of one-quarter cycle is formed between the ordinary and extraordinary elements of light passing through. They are attached to the back of the linear polarizer component in a specific orientation so the light emerging from the quarter waveplate is circulary polarized. As components of circular polarizers, they are used for conversion of linearly polarized radiation into circularly polarized radiation when the input polarization is oriented at an angle of 45 degrees to the waveplate optical axis.
Types of Circular Polarizers
There are two basic types of circular polarizers: circular dichroic polarizers and circular beam separators.
- The term "dichroic" refers to how light rays with different polarizations are absorbed in different amounts. Circular dichroic polarizers consist of a dichroic linear polarizer and a quarter-wave retarder. The dichroic lens can be made of glass or polymers. Polymer polarizers are the least expensive and are usually made out of a thinly stretched polymer sheet that is doped with microcrystals that align parallel to the stretch direction. This allows the electromagnetic waves that are parallel to the alignment to be absorbed. They are easily customizable and can be found in flat panel displays in television and monitors. The disadvantages of polymer dichroic polarizers include their fragility and thermal performance. They can be damaged by exposure to too much light and the polymer may decompose when exposed to a laser.
- Circular beam separators also consist of a quarter-wave retarder, but feature an assembly that is oriented with the fast axis at 45 degrees to the transmission axis of a laser line beam-splitting cube. With these circular polarizers, the transmitted beam is circularly-polarized regardless of the input beam's polarization state.
Extinction Ratio- Extinction ratio describes the ratio between the transmittance of the desired polarization direction and the undesired orthogonal polarization direction. This is also referred to as the contrast ratio.
Image Credit: Alpine Research Optics
Transmittance- Transmission is the percentage of light that passes through the polarizer. All of the light goes through the polarizer at 100% transmittance, which only occurs when the light is polarized. Polarizers are not ideal and therefore will not reach 100% transmittance; however, maximum transmittance occurs when a polarizer is placed to ensure the polarization axis and incoming light are parallel. Minimum transmittance occurs when the axis and incoming light are perpendicular.
Clear aperture- Clear aperture describes the surface area of an optical filter which is free of any defects or obstructions. The boundary of the clear aperture is often a metal or opaque material around the outside edge of the filter. It is important that the clear aperture does not restrict the overall aperture of the microscope, and that there is no leakage of unfiltered light around the edge.
Diameter- The diameter of the polarizer is important to consider when selecting a circular or linear polarizer. Sizing depends on the manufacturing process of the polarizer.
Wavelength range- Wavelength range is the spectral region over which the spectral engine and detectors are operated. Light waves correspond to a particular wavelength range (i.e. visible light is 400 - 700 nanometers)
Image Credit: Wavelength image from Universe by Freedman and Kaufmann.
Linear and circular polarizers are used differently with a camera, but do not alter the final image on the film. Circular polarizers are used to improve the view ability of emissive displays, as well as in camera, video, and sensor applications where the device is sensitive to linearly polarized light, but glare reduction is required. They can also be used in 3D applications.
Effect of polarizer on light reflected from a water surface. The polarizing filter is used on the right.
Image Credit: Wikipedia