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backlight

LCD displays are passive in principle and therefore non-illuminating. They only emit or reflect the input light when an additional light source illuminates the LCD display. In the case of backlighting, light is projected from behind evenly and over the entire LCD display.

Cold cathode fluorescent tubes( CCFL), but also organic light emitting diodes( OLED) and light emitting diodes( LED) are used as light sources for backlighting. There are two technical approaches for backlighting: Light sources that are attached to the edge of the housing and illuminate the LCD display by means of diffusers, or light source arrays that are attached behind the LCD screen and illuminate the display. Both techniques can be implemented with cold cathode tubes, light-emitting diodes or OLEDs. The LED technology with LED strips attached to the edge of the housing is the edge LED technology. It is relatively complex because it requires light guide elements that evenly couple out the light via micro-optics and project it through the Liquid Crystal Display (LCD). The second technology, known as full-LED or direct-LED technology, has the advantage that you can control the brightness of the LED array locally and thus increase the contrast. Hence the term local dimming.

Color space for different backlights with CCFL and tri-color LED

Color space for different backlights with CCFL and tri-color LED

The light sources of the backlight have their own color space, which can limit or expand the color space of the display. In the case of CCFL technology, which produces a white light, it is the case that the CCFL color space has a much smaller scope than that of LEDs. The CCFL color space does not expand the color space, rather it restricts it. For this reason, white LEDs, WLEDs, and also three-color LEDs in RGB are increasingly being used for backlighting. The color space becomes much larger which has led to the name Wide Color Gamut(WCG). Since the light spectra of the LEDs are narrower than the filter curves of the color filters, the colors appear more brilliant.

The HDR display with high contrast

Background illumination in the HDR display

Background illumination in the HDR display

InHDR displays, High Dynamic Range ( HDR), the backlighting significantly increases the dynamic range of the screen display by not illuminating the background with the same brightness over the entire area, but rather partially controlled depending on the image content. Bright parts of the image are backlit with brighter light, darker parts with darker light. Newer methods use polarization films such as Enhanced Specular Reflector( ESR) or Collimating Multilayer Optical Film (CMOF) to distribute the light evenly across the LCD screen.

Informations:
Englisch: backlight
Updated at: 15.10.2020
#Words: 420
Links: lost calls delayed (LCD), light, display, indium (In), cold cathode fluorescent lamp (display) (CCFL)
Translations: DE
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