Backlight

A backlight is a form of illumination used in liquid-crystal displays (LCDs) that provides illumination from the back or side of a display panel. LCDs do not produce light by themselves, so they need illumination (ambient light or a special light source) to produce a visible image. Backlights are often used in smartphones, computer monitors, and LCD televisions. They are used in small displays to increase readability in low light conditions such as in wristwatches.^[1] Typical sources of light for backlights include light-emitting diodes (LEDs) and cold cathode fluorescent lamps (CCFLs).

Simple types of LCDs such as those used in pocket calculators are built without an internal light source, requiring external light sources to convey the display image to the user. Most LCD screens, however, are built with an internal light source. Such screens consist of several layers. The backlight is usually the first layer from the back. Light valves then vary the amount of light reaching the eye, by blocking its passage in some way. Most use a fixed polarizing filter and a switching one, to block the undesired light.

Many types of displays other than LCD generate their own light and do not require a backlight, for example, OLED displays, cathode ray tube (CRT), and plasma (PDP) displays.

A similar type of technology is called a frontlight, which illuminates an LCD from the front.

A review of some early backlighting schemes for LCDs is given in a report Engineering and Technology History by Peter J. Wild.^[2]