Linear Polarizers Information
Linear polarizers transmit light waves along one axis and absorb them along the other. Polarization is the direction in which the electric field of a light wave oscillates. A polarizer is a device which uses selective absorption, refraction, or transmission to turn unpolarized light into polarized light. Unpolarized light travels in a random direction; after it travels through a linear polarizer, it has an electric field in one specific direction and can be analyzed as a linear combination of two vectors (vertical and horizontal) with the same phase.
Linear polarization. Image Credit: American Polarizers, Inc.
How Linear Polarizers Work
The basic function of a linear polarizer is to absorb, scatter, or reflect light in the unwanted polarization direction. Linear polarizers allow the transmission of only one polarization state. The transmitting and absorbing axes of the device are oriented at 90 degrees to each other. The output polarization axis orientation is independent of the input beam polarization state. Rotating the linear polarizer about its beam axis changes the plane of polarization. An optimal linear polarizer will transmit 50% of an unpolarized input beam. Two perfect polarizers with their transmission axes crossed will totally extinguish an incident beam. There are several methods of producing linearly polarized light.
Polarizing of Light. Video Credit: Rapid learning center / CC BY-SA 4.0
Double refraction or birefringent polarizers use natural crystals to divide a single beam of unpolarized light into two separate polarized beams of equal intensity. The two polarized beams can be split, creating very efficient linear polarization.
Reflection polarizers use a flat, smooth, non-metallic surface. The light beams hit the surface at an angle and the reflected beam is partially or completely polarized. The degree of polarization depends on the angle of incidence and refraction index on the reflecting surface. The Brewster angle, also known as the polarizing angle, is the angle at which the degree of polarization is 100%.
Dichroic absorptive polarizers use the property, dichroism, to absorb light that is polarized in a particular direction. This type of polarizer has an indicated absorption and transmission axis (polarizing axis). Stretched Polyvinyl Alcohol (PVA) is most commonly used in dichroic polarizers.
When choosing between different linear polarizers, there are several key specifications to consider, including:
Extinction Ratio- Extinction ratio describes the ratio between the transmittance of the desired polarization direction and the undesired orthogonal polarization direction. The ratio is the power of a plane-polarized beam that is transmitted through a polarizer. The polarizing axis is parallel to the beam's plane, as compared with the transmitted power when the polarizer's axis is perpendicular to the beam's plane. This is also referred to as the contrast ratio.
Image Credit: Alpine Research Optics
Transmittance- Transmittance 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 perfect 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.
Transmission vs. Extinction Ratio. Image Credit: Meadowlark.com
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). Linear polarizers can be used to polarize light in the infrared, visible, and ultraviolet wavelengths ranges.
- Infrared- Infrared wavelengths are in 750nm to 2500nm wavelength range.
- Visible- Visible wavelengths are in the 380nm to 750nm wavelength range.
- Ultraviolet- Ultraviolet wavelengths are in the 4nm to 380 nm wavelength range
Image Credit: Wavelength image from Universe by Freedman and Kaufmann.
Beam deviation- Beam deviation is the deviation of the polarized beam from normal. The deviation is determined by observing the difference in angle of an incoming beam to a transmitted, reflected or refracted beam through an optic. This is particularly relevant to cubes and dispersive prisms.
Thickness- The thickness of the retardation plates used in the polarizer devices.
Operating temperature- The temperature range in which the device is designed to operate.
Polarizer Surface Specifications
Surface quality describes the level of defects that can be seen on the optic. A scratch is considered a defect whose length is many times its width, while a dig is nearly equal in terms of length and width. The specification is usually noted by MIL-0-13830A or 60-40, where the first number is the visibility of the scratches and the second is the visibility of the digs or small pits. The lowest number refers to the highest quality.
Image Credit: Edmund Optics
Surface flatness is a specification used to describe the deviation of a flat surface such as a mirror, window, prism, or plano-lens. The flatness can be measured using an optical flat and are measured in valves of waves, which are multiples of the wavelength of the testing source. For example, 1λ flatness is considered typical grade and 1/20λ us considered high precision grade.
Linear polarizers are used in a wide range of applications in the electronics, photographic, scientific and industrial field. The largest single application for linear polarizers is in liquid crystal displays (LCDs). In LCDs, the linear polarizers are placed on each side of the LCD with their axes crossed at 90 degrees. In this manner, the polarizers do not allow light to pass. However, in the power-off state, the liquid crystal cells in the LCD panels between the crossed polarizers rotate light passing through them 90 degrees, defeating the effect of the crossed polarizers. This causes the display to appear transparent. When the surface of an individual liquid crystal cell in the LCD panel is electrically charged or activated, the cell ceases to rotate the polarized light and the segment appears as a dark spot on the screen. By electronically controlling the activation state of the individual cells in the LCD panel, information may be displayed. The same basic principal applies to all basic black and white, and color LCDs.
Effect of a polarization filter (left). Polarized sunglasses (right). Image Credit: Wikipedia | fitbodynow.wordpress.com
Linear polarizers are also popular in camera filters, sunglasses, and in machine vision systems when glare reduction is required. By placing two polarizers over top of each other and rotating one against the other, the brightness and intensity of a light source can be controlled.
Finding the Optimal Polarizer (pdf)
Visible Light Linear Polarizers