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LCD Polarizers: Reflective, Transflective and Transmissive

Posted by Eric Hawkins on Nov 24th 2017


When asking us questions about LCD polarizers, you may be pummeled with questions like:

  • “Will the LCD operate in direct sun light?”
  • “Or will it operate only in the dark?”
  • “Or will it be a combination of both?”
  • “Is it battery powered?”

You may feel like you are being cross examined in front of a judge and jury. I don’t mean to come across that way, but sometimes when working with a design engineer, I fire off these questions one after another with the ultimate goal to provide a LCD display that meets ALL of their needs.

What are these questions leading to? The polarizer There are three choices of LCD polarizers; and selecting the correct one is critical.

Why are LCD polarizers required?

Light must be present for the liquid crystal display (LCD) to be readable. It illuminates the display, either by entering the display as ambient light passing through the front of the glass (office light, sun light), or it is supplied from a backlight that is placed behind the LCD. In some cases light is a combination of both ambient light and backlight.

These are the three types of LCD polarizers: Reflective, Transflective, and Transmissive. The correct polarizer is determined by the different lighting conditions your product will operate in or be exposed to.

Let’s begin this with the most basic type of polarizer: Reflective

LCD modules with a Reflective Polarizer:

                              

The reflective LCD polarizer is nothing more than a mirror that is applied to the bottom layer of glass. It reflects 100% of the ambient light entering through the top layer of glass back towards the user.

ADVANTAGES OF REFLECTIVE LCD POLARIZERS:

The brighter the ambient light (the light that is available from the sun, office lights, etc.) the sharper the contrast and the easier it is to read.

DISADVANTAGES OF REFLECTIVE LCD POLARIZERS:

A backlight will not illuminate the display since any light placed behind the polarizer will be blocked from passing through to the front piece of glass.

Note: It is possible to use a reflective polarizer with a side-lit LED Display, or edge-lit LED Display. A side-lit display positions the LEDs above the reflective polarizer. This is a popular option since it provides a sharp contrast with the added benefit of being thinner (in the Z axis) than a LCD with a backlight.

LCD Modules with a Transmissive Polarizer:

A Transmissive LCD polarizer, unlike the reflective polarizer, allows all the light from the backlight to pass thought the bottom and top layer of glass towards the user. The backlight must be on for the display to be readable.

Ambient light from office lights or sun light will pass through the bottom layer of glass and not reflect back towards the top glass; so ambient light will add very little illumination to the display.

ADVANTAGES OF TRANSMISSIVE LCD POLARIZERS:

The backlight behind a Transmissive polarizer will be brighter than the same backlight behind a Transflective polarizer, making the overall display brighter. It is also possible to reduce the LED driving current with a goal of extending the half-life of the backlight.

DISADVANTAGES OF TRANSMISSIVE LCD POLARIZERS:

The display is difficult to read when the backlight is off, making this a poor choice for battery powered products requiring a low current draw. The driving current of the backlight can range from 15mA and higher. Also, when turned on, the image will be ‘washed’ out by direct sunlight.

The final option is a Transflective LCD polarizer.

LCD Modules with a Transflective Polarizer:

The Transflective LCD polarizer is a combination, or hybrid, of the Reflective and the Transmissive polarizer. It reflects some of the ambient light passing through the front of the glass and at the same time allows some of the light from the backlight to pass through to the front layer of glass.

It’s the best of both worlds and is the most popular of the three LCD polarizers.

ADVANTAGES OF TRANSFLECTIVE LCD POLARIZERS:

A Transflective polarizer allows the display to be read with or without the backlight on. If the Liquid Crystal Display is located in an area with good ambient light, the backlight can be turned off and the display is still readable. When the LCD is moved into poor ambient light, the backlight can be turned on and the display can still be read.

Note:

  • Most battery powered products requiring a backlight integrate a Transflective polarizer
  • Another issue to take into account when selecting a polarizer is the half-life of its backlight.

DISADVANTAGES OF TRANSFLECTIVE LCD POLARIZERS:

A backlight placed behind a Transflective polarizer will allow some light to pass through, but the display will not be as bright as the same backlight placed behind a Transmissive polarizer. If you need your backlight to be brighter and your display to operate in both strong and poor ambient light, there are additional methods to increase the brightness of the LED. Contact our US based technical support for ideas.

LCD Polarizer Summary:

In your choice of LCD Polarizers: there is no difference in cost or lead time between a Reflective, Transflective or Transmissive polarizer. Choose the polarizer based on the type of lighting conditions your display will operate in. If you have any questions contact Focus Displays Solutions for US based support. You can call 480-503-4295.