Backlight

Animation of LCD, both unlit and with electroluminescent backlight switched on.

A backlight is the form of illumination used in an LCD display. Backlights differ from frontlights because they illuminate the LCD from the side or back, whereas frontlights are in front of the LCD. They are used in small displays to increase readability in low light conditions, and in computer displays and LCD televisions to produce light in a manner similar to a CRT display.

Light source types

The light source can be:

• Incandescent light bulb
• One or more light-emitting diodes (LEDs)
• Electroluminescent panel (ELP)
• One or more cold cathode fluorescent lamp (CCFL)
• Hot cathode fluorescent lamps (HCFL)

An ELP gives off uniform light over its entire surface, but other backlights usually employ a diffuser to provide even lighting from an uneven source.

Backlights can be any color. Monochrome LCDs usually have yellow, green, blue or white backlights, while color displays use white backlights that cover most of the color spectrum.

Usage

LED backlighting is most commonly used in small, inexpensive LCD panels. The light is usually colored, although white LED backlighting is becoming more common. ELP backlighting is often used for larger displays or when even backlighting is important; it can also be either colored or white. An ELP must be driven by relatively high voltage AC power, which is provided by an inverter circuit. CCFL backlights are used on large displays like computer monitors, and are usually white in color. These also require the use of an inverter and diffuser. Incandescent backlighting can be used when very high brightness is desired, but a drawback is the limited life of incandescent bulbs, and the amount of heat generated, which often means that the bulb needs to be mounted away from the display.

LED backlighting in larger displays is a recent innovation that helps to improve the color gamut of the LCD display. LED white light is created by three separate LEDs to produce a color spectrum that closely matches the color filters in the LCD pixels themselves. In this way the filter passband can be made more narrow, so that each color component lets only a very narrow band of spectrum through the LCD. This improves the efficiency of the display since little light is blocked when white is displayed. Also, the actual Red, Green, and Blue points can be moved farther out so that the display is capable of reproducing more vivid colors. Many current LCD TVs can generate only 70-80% of the colors specified in the NTSC specification while newer displays based on LED backlights will be able to generate all the colors in the NTSC specification plus some additional colors.

Challenges

However, there are challenges with LED backlights. Good uniformity is harder to achieve especially as the LEDs age with each LED possibly aging at a different rate. Also the use of three separate light sources for Red, Green, and Blue means that the white point of the display can move as the LEDs age at different rates. Power also can be a challenge. Though it is possible for an LED display to be more power efficient, this is not a given and many first generation implementations may use the same or more power than their CCFL counterparts.

LED backlights on the rise

The usage of LED backlights on notebooks has been growing and is set to increase in 2008. Sony has used LED backlights on some of its higher end slim VAIO notebooks since 2005. Fujitsu has also introduced notebooks with LED backlights in 2006. As of 2007, Asus, Dell and Apple have also introduced LED backlights into some of their notebook models, and other companies like HP will also be marketing LED backlit notebooks in the near future.

How a backlight diffuser works

In order for a non-ELP backlight to produce even lighting (which is critical for LCD displays), the light is first passed through a specially-designed layer of plastic that diffuses the light through a series of evenly-spaced bumps whose density increases according to a set mathematical formula. The diffused light then travels to either side of the diffuser: one side contains the actual LCD panel, the other a simple (often aluminium foil) reflector to guide otherwise wasted light back towards the LCD panel.