Liquid Crystal Display - Illumination

Illumination

LCD panels produce no light of their own; they require external light to produce a visible image. While passive-matrix displays are usually not backlit (e.g. calculators, wristwatches); active-matrix displays almost always are (with a few exceptions, such as the display in the original Game Boy Advance).

Currently, there are several common implementations of LCD backlight technology:

• CCFL: The LCD panel is lit (usually) by two cold cathode fluorescent lamps placed at opposite edges of the display. A diffuser and two polarizers then spread the light out evenly across the whole display. For many years, this technology has been used almost exclusively. Unlike white LEDs, most CCFLs have an even-white spectral output resulting in better color gamut for the display. However, CCFLs are less energy efficient then LEDs and require a somewhat costly inverter to convert whatever voltage the device uses (usually 5 or 12v) to the high voltage needed to light a CCFL. The thickness of the inverter transformer also limits how thin the display can be made.
• EL-WLED: The LCD panel is lit by a row of white LEDs placed at one or more edges of the screen. A light diffuser is then used to spread the light evenly across the whole display. As of 2012, this design is the most popular one in desktop computer monitors. Some LCD monitors using this technology have a feature called "Dynamic Contrast" where the backlight is dimmed to the brightest color that appears on the screen, allowing the 1000:1 contrast ratio of the LCD panel to be scaled to different light intensities, resulting in the "30000:1" contrast ratios seen in the advertising on some of these monitors. Since computer screen images usually have full white somewhere in the picture, the backlight will usually be at full intensity, making this "feature" mostly a marketing gimmick.
• WLED array: The LCD panel is lit by a full array of white LEDs placed behind a diffuser behind the panel. LCD displays that use this implementation will usually have the ability to dim the LEDs in the dark areas of the image being displayed, effectively increasing the contrast ratio of the display. As of 2012, this design gets most of its use from LCD televisions.
• RGB-LED: Similar to the WLED array, except the panel is lit by a full array of RGB LEDs. While displays lit with white LEDs usually have a poorer color gamut than CCFL lit displays, panels lit with RGB LEDs have very wide color gamuts. This implementation is most popular on professional graphics editing LCD displays. As of 2012, LCD displays in this category usually cost more than $1000.

Today, most LCD screens are being designed with an LED backlight instead of the traditional CCFL backlight.