Choosing an LCD Display Polarizer

When designing a Liquid Crystal Display, including character, graphic and segment displays, a LCD display polarizer needs to be selected that will optimize the ambient lighting conditions the display will operate in.

The primary job of a LCD Polarizer

The primary job function of a LCD display polarizer is to improve definition, color and to control how light is reflected, transflected or transmitted; without the polarizer it would be impossible to read the display.

To understand the placement of the polarizer, it is necessary to explain the construction of a basic Liquid Crystal Display. (This does not apply to TFT, LED, VFD or OLEDs.)

An illustration of a Liquid Crystal Displays is like a sandwich. Take two pieces of ITO (Indium Tin Oxide) treated glass, add a little adhesive to hold them together then fill the void between the layers with a twisted nematic fluid such as a TN, STN, FSTN or UWVD.

What is the difference between TN / STN / FSTN and UWVD?

  • TN = Twisted Nematic (Lowest cost, smallest viewing angle)
  • STN = Super Twisted Nematic (Medium cost and wide viewing angle)
  • FSTN = Film Super Twisted Nematic (More expensive than STN, offers a sharper contrast than STN)
  • UWVD = Ultra-Wide Viewing Display Technology (Bright backlight, wide viewing angle, fast response)

Applying the two polarizers is the next step: One polarizer is applied to the top layer of glass; this must always be a Transmissive polarizer. The second polarizer is located on the back of the bottom layer of glass (farthest away from the person reading the LCD). This polarizer is selectable by the designer.

The drawing below illustrates the location of the two polarizers with respect to the layers of glass and Nematic fluid.

Types and uses of LCD display polarizers

Details of the three types of polarizers are covered in more detail in another article, but below is a quick summary:

  • Reflective Polarizer: For use in environments of bright ambient light such as indoors and direct sun light. This is a great choice for battery powered instruments where there will be sufficient ambient light.
  • Transmissive Polarizer: Provides the brightest backlight, but the display is difficult to read when the backlight if off. This is a poor option for battery powered products as the backlight can use a lot of power. This is also a poor choice for direct sun light as the display will look dark if the sunlight reflection overwhelms the backlight.
  • Transflective Polarizer (most popular): Combines functions of both the Reflective and Transmissive polarizers. It allows the display to be readable whether the backlight is on or off. In high ambient light conditions its contrast is not as good as the Reflective type and in low ambient light conditions, with the backlight on, the display is not as bright as the Transmissive type.

Cost differences of the three polarizers:

There is no difference in cost or lead time of the three polarizers; one variable that does affect the cost of the LCD module is the size of the glass.

The larger the size of the glass, the higher the cost to build the display, but when the glass size becomes too small the cost of the module increases. The reason is that the polarizer needs to be attached by hand which increases both the labor cost and the fall out rate

Not sure which LCD display polarizer to select? Need help with your new product design?

This article on choosing an LCD display polarizer is a guest journal from: John Keenan, MicroReady Inc., Electronic Product Design located in Phoenix, AZ.

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