Inorganic polarizers: Technology and mechanisms

What is polarization?

One of the key optical components used in devices such as LCDs and projectors is a polarizer. Dexerials has been developing optical components since 2008 when it first launched its inorganic polarizers with excellent heat resistance and durability. This article covers the main functions of polarizers and its improvements over time.

First, let’s explore what polarized light is. Light is a type of electromagnetic wave  that travels in the direction the waves move. Natural light, such as sunlight, moonlight, and fluorescent light, is a random mixture of vertical, horizontal, and diagonal waves. Polarized light, on the other hand, refers to light in which the waves oscillate in a specific direction.

A polarizer is an optical  component designed to artificially create polarized light out of randomly oscillating light. By placing a filter on top of a substrate and allowing only light vibrating in a specific direction to pass through, it makes it possible to extract polarized light from natural light. Below is a diagram of unpolarized natural light where the vertical component has been filtered out by a polarizer, leaving only the horizontal component. Hence, a polarizer plays the role of a filter to block certain light waves.

Requirements for polarizers used in projectors

Although polarizers essentially do the same thing in both LCDs and projectors, those used in projectors have different required features. This section describes the features and functions of the Dexerials inorganic polarizers used in projectors.

High transmittance : The first requirement is to be able to transmit specific light waves. The light that is not transmitted through the polarizer is absorbed and converted into heat. The laser light used in projectors has higher energy levels than the backlight of LCDs. Thus, more energy is converted into heat, meaning that a certain level of heat resistance is necessary. Optimizing the use of light also helps the projector save power.

Low reflectance : When light is reflected by a polarizer, a phenomenon called “stray light” occurs. This is the light scattered in various directions inside the housing. When stray light enters other optical components, it can cause a phenomenon called “ghosting” or “double image”,  the image that is projected twice. For this reason, polarizers must have low reflectance capabilities.

The graph below shows the transmittance and reflectance of Dexerials’ inorganic polarizers (green  line) vs wavelength (horizontal axis). Transmittance refers to the  percentage of light transmitted in the targeted direction, while absorbance refers to the rate at which unwanted light is transmitted through the polarizer. The graph shows that transmittance is over 90% at a wavelength of around 550 nm, while absorbance is close to zero. This indicates that the polarizer does not interfere with the passage of “wanted” light, while firmly blocking “unwanted” light. Also, when reflectance on the transmission axis and the absorption axis is too high, it can cause stray light as mentioned above.

Heat resistance : As mentioned earlier, light that does not pass through the polarizer is absorbed by the polarizer and converted into heat.  Recent projectors with remarkably high luminance can reach temperatures  of 250 to 300℃. For this reason, polarizers are required to be heat-resistant enough to withstand such high temperatures.

In order to meet the market needs, Dexerials has developed a lineup of inorganic polarizers with improved heat resistance. The following photo shows a comparison of the standard product and the highly heat-resistant product.

The graph below shows the change in the reflectance on the absorption axis of  Dexerials’ polarizers when subjected to a heat resistance test at 300°C for 3000 hours. The change in reflectance of heat-resistant products is smaller than that of the standard product over time.

Moisture resistance : Projectors are widely used in a variety of settings, both indoors and outdoors. In locations  with high humidity, condensation can occur, which can corrode the metal materials used in inorganic polarizers. To prevent this, moisture-resistant protective film is applied to the Dexerials’ inorganic polarizers.

High contrast : The image quality of projectors can be improved by increasing the contrast ratio. Polarizers can assist with this as they increase the rate at which certain types of light pass through and prevent the transmission of other unwanted types of light.

The graph below shows the transmittance of “wanted” light on the horizontal axis and the transmittance of “unwanted” light on the vertical axis. The blue line indicates conventional polarizers  while the green line indicates the inorganic polarizers developed by Dexerials.   Dexerials’ inorganic polarizer can achieve a higher contrast ratio than conventional polarizers.

The continued innovation of Dexerials polarizers

The above contrast ratio is a direct indicator of the image quality of projectors these days, but until about 10 years ago, inorganic polarizers with a transmittance of about 88% were the norm. Dexerials has continued to improve polarizers by applying the alternative substrate materials and developing new polarizer structures. As demand for laser light projectors continues to increase, Dexerials will strive to meet wide range of customer needs by supplying broad  lineup of inorganic polarizers.