Backlight

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This article is about backlights in LCD displays. Backlight is also used to refer to the rear window of an automobile. A third meaning for backlight is the practice of lighting from the back in the context of lighting design.

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, where 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.

The light source can be an incandescent light bulb, one or more light-emitting diodes (LEDs), an electroluminescent panel (ELP), one or more cold cathode fluorescent lamp (CCFL) or 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.

LED backlighting is 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 helps to improve the color representation of the LCD display. LED light is created by three separate LED's to produce a color spectrum that closely matches the color filters in the LCD pixels themselves. In this way the LCD color filters can be made more narrow so that each color component lets only a very narrow band of spectrum though 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 TV's 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.

However, there are challenges with LED backlights. Good uniformity is harder to achieve especially as the LED's 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 LED's 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.

[edit] 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.