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

A backlight is a form of illumination used in liquid crystal displays (LCDs). Backlights illuminate the LCD from the side or back of the display panel, unlike frontlights, which are placed in front of the LCD. Backlights 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.

Simple types of LCD displays are built without an internal light source, requiring external light sources to convey the display image to the user. Modern LCD screens, however, are built with an internal light source. Such LCD screens consist of several layers. The backlight is usually the first layer from the back. But in order to create screen images, a mechanism is needed to regulate the light intensity of the screen's pixels. For this, light valves are used that vary the amount of light reaching the target by blocking its passage in some way. The most common element is a polarizing filter to polarize the light from the source in one of two transverse directions and then passing it through a switching polarizing filter, to block the path of undesirable light.

LCD Back Light

[edit] Light source types

The light source can be made up of:

• Incandescent light bulbs
• light-emitting diodes (LEDs)
• An electroluminescent panel (ELP)
• One or more cold cathode fluorescent lamps (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.

[edit] Usage

LCD with CCFL backlight

Colored LED backlighting is most commonly used in small, inexpensive LCD panels. 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.

[edit] LED backlights

LCD with LED matrix backlight

LED backlighting in color screens comes in two flavors: white LED backlights and RGB LED backlights^[1]. White LEDs are used most often in notebooks and desktop screens, and in virtually all mobile LCD screens. A white LED is actually a blue LED with yellow phosphor to give the impression of white light. The spectral curve has big gaps in the green and red parts. RGB LEDs consist of a red, a blue, and a green LED and can be controlled to produce different temperatures of white. RGB LEDs for backlighting are found in high end color proofing displays such as HP DreamColor LP2480zx monitor or selected HP 8730w notebooks, as well as newer consumer grade displays such as Dell's Studio series laptops which have an optional RGB LED display. The LED Apple Cinema Display for example uses white LEDs.

RGB LEDs can deliver an enormous color gamut to screens. When using three separate LEDs (additive color) the backlight can produce a color spectrum that closely matches the color filters in the LCD pixels themselves. In this way, the filter passband can be narrowed 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. CCFL backlighting has also improved in this respect. Many current LCD models, from cheap TN-displays to color proofing S-IPS or S-PVA panels, have wide gamut CCFLs representing more than 95% of the NTSC color specification.

There are several 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. Aging also occurs with White LEDs, with changes of several 100K being recorded. White LEDs also suffer from blue shifts at higher temperatures varying from 3141K to 3222K for 10°C to 80°C respectively.^[2] Power efficiency 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.^{[citation needed]}

The use of LED backlights in notebook computers has been growing. Sony has used LED backlights in some of its higher-end slim VAIO notebooks since 2005. Fujitsu introduced notebooks with LED backlights in 2006. In 2007, Asus, Dell, and Apple introduced LED backlights into some of their notebook models. As of 2008^[update], Lenovo has also announced LED-backlit notebooks, and other companies like HP will also be marketing LED-backlit notebooks in the near future. In October 2008, Apple announced it will be using LED backlights for all its notebook and its new 24-inch Apple Cinema Display, and one year later it introduced a new LED iMac, meaning all of Apple's computer screens, save the 30-inch Cinema HD Display, are now LED. Almost every laptop with 16:9 display introduced since September 2009 uses LED-backlit panels.

[edit] Backlight diffusers

In order for a non-ELP backlight to produce even lighting (which is critical for LCDs), the light is first passed through a lightguide - a specially-designed layer of plastic that diffuses the light through a series of evenly-spaced bumps. The density of bumps increases further away from the light source according to a diffusion equation. The diffused light then travels to either side of the diffuser; the front faces the actual LCD panel, the back has a reflector to guide otherwise wasted light back towards the LCD panel. The reflector is sometimes made of aluminum foil, sometimes merely a white-pigmented surface or, as in the 3M Vikuiti ESR, consisting of hundreds of polymer layers of alternating low and high index. Between the lightguide and the LCD panel is usually placed a reflective polarizer film which vastly increases efficiency by repeatedly reflecting any unpolarized light back, which would otherwise be absorbed by the LCD's rear polarizer.

[edit] References

[edit] External links

• Animated tutorial of LCD and Backlight technology by 3M