LCD Efficiency

Efficiency
LCDs are relatively inefficient in terms of power use per display size, because the vast majority of light that is being produced at the back of the screen is blocked before it reaches the viewer. To start with, the rear polarizer filters out over half of the original un-polarized light. Examining the image above, you can see that a good portion of the screen area is covered by the cell structure around the shutters, which removes another portion. After that, each sub-pixel's color filter removes the majority of what is left to leave only the desired color. Finally, to control the color and luminance of a pixel as a whole, the light has to be further absorbed in the shutters. 3M suggests that, on average, only 8 to 10% of the light being generated at the back of the set reaches the viewer.

For these reasons the backlighting system has to be extremely powerful. In spite of using highly efficient CCFLs, most sets use several hundred watts of power, more than would be required to light an entire house with the same technology. As a result, LCD televisions end up with overall power usage similar to a CRT of the same size. Using the same examples, the KV-40XBR800 dissipates 245 W,[2] while the LC-42D65 dissipates 235 W.[1] Plasma displays are worse; the best are on par with LCDs, but typical sets draw much more.

Modern LCD sets have attempted to address the power use through a process known as "dynamic lighting" (originally introduced for other reasons, see below). This system examines the image to find areas that are darker, and reduces the backlighting in those areas. CCFLs are long cylinders that run the length of the screen, so this change can only be used to control the brightness of the screen as a whole, or at least wide horizontal bands of it. This makes the technique suitable only for particular types of images, like the credits at the end of a movie. Sets using LEDs are more distributed, with each LED lighting only a small number of pixels, typically a 16 by 16 patch. This allows them to dynamically adjust brightness of much smaller areas, which is suitable for a much wider set of images.

Another ongoing area of research is to use materials that optically route light in order to re-use as much of the signal as possible. One potential improvement is to use microprisms or dichromic mirrors to split the light into R, G and B, instead of absorbing the unwanted colors in a filter. A successful system would improve efficiency by three times. Another would be to direct the light that would normally fall on opaque elements back into the transparent portion of the shutters. A number of companies are actively researching a variety of approaches, and 3M currently sells several products that route leaked light back toward the front of the screen.

Several newer technologies, OLED, FED and SED, have lower power use as one of their primary advantages. All of these technologies directly produce light on a sub-pixel basis, and use only as much power as that light level requires. Sony has demonstrated 36" FED units displaying very bright images drawing only 14 W, less than 1/10 as much as a similarly sized LCD. OLEDs and SEDs are similar to FEDs in power terms. The dramatically lower power requirements make these technologies particularly interesting in low-power uses like laptop computers and mobile phones. These sorts of devices were the market that originally bootstrapped LCD technology, due to its light weight and thinness.