Backlight

A backlight (English backlight ) is used in liquid crystal displays and indicators used. The light is arranged through the front, individually controlled liquid crystal cells absorbed to different degrees, which creates the impression of a self-luminous image.

• 3.1 electroluminescent film
• 3.2 cold-cathode tube
• 3.3 LEDs 3.3.1 Backlight
• 3.3.2 Edge - LEDs

History

In the context of liquid crystal displays ( Liquid Crystal Displays LCD) device internal lighting was first used in electronic calculators of the company North American Rockwell, USA, sold under various brand names such as Lloyds IBICO 086 or 40 from 1972 ( see calculator). It is not yet involved the now common backlight for LCDs, because this first LCDs used the dynamic scattering mode ( DSM); while the liquid crystal display was transparent at rest and became milky electrical voltage applied. To further improve the readability of LCDs, one that the viewer has not been affixed visible, white light source above the display, which wrong incident light on the back of the display generated so that only when light scattering capitalized digits segments by the scattered light as a light on a dark background were visible.

After Twisted Nematic LCDs had replaced the dynamic scattering mode LCDs, Ableseschwierigkeiten were shown on these purely reflexive, working with ambient light displays with little or unfavorable incident ambient light. This was pronounced in the now emerging digital wrist watches with TN- LCDs at dusk or at night the case because liquid crystal displays itself not produce light, but only transmit the incident light or lock.

After preliminary work at the pharmaceutical company Roche in Basel and at the Research Center of Brown, Boveri & Cie ( BBC ), Baden TN- LCDs were primarily created for watches in mass production in a newly created factory in Lenzburg ( Switzerland ) from 1974. In the accompanying application laboratory backlighting techniques have been developed under the direction of Peter J. Wild. As a result of this work a backlight for watches with a laterally positioned point light source and a diffuser plate was made ​​of plastic. The light distribution was fitted with special coatings and placed behind the liquid crystal display for illumination at your fingertips. Similar arrangements are now referred to in LED televisions as edge-lit, in contrast to watches the backlight is permanently necessary because the liquid crystal display is operated solely in transmitted light.

For monitoring of industrial processes in the so-called waiting panel meters are used with vertical display area. In this type of ambient light are purely reflexive TN- LCDs, as on the left side of the picture aside, rather difficult to read. Therefore an arrangement with a fluorescent tube ( CCFL type CCFL) as a light source and a diffuser was developed as a light diffuser for backlighting a TN- LCDs in transmitted light at BBC in the 1970s, which can be seen on the right side of the figure and was excellent to read.

From the 1990s such lighting arrangements were generally used with cold cathode tubes for operation of colored LCD flat screens before alternatives were developed with LEDs as light sources for LED TV and are preferred since about 2011.

Operation

The backlighting will illuminate the image area of liquid crystal cells as evenly as possible from behind. For color displays, the backlight must contain all basic colors. They are different then strongly filtered by the color filter of the individual liquid crystal cells in order to achieve the desired color effects. Monochrome liquid crystal displays can also have a colored backlighting.

Light guide

The light emitted by a point or rod-shaped light sources has to be distributed as evenly as possible over the surface. With relatively small surfaces, the light is often injected on the end faces of an optical waveguide. In practice, the light guide is a flat plate made ​​of a transparent plastic such as acrylic glass. This includes extractors, the light coupled out from the light guide. The coupling can be realized by distributed in the light guide material scattering structures, specific fine surface structures or fine printed pattern. The inhomogeneous distribution of the outcoupling structures causes the uniform illumination of the surface is also achieved, for example, with only one frontally radiating cold cathode tube. To increase the luminance of the light sources may also be attached to two or all four end faces. A backlight according to this principle is referred to in English as edge-lit backlight.

With increasing size of the light source ( and constant aspect ratio, eg 16:9) the sum of the side lengths of the surface, however, only grows in proportion to the length of one side, square. Since the power of the light sources can not be increased arbitrarily, the edge-lit backlights are limited in principle here. For larger sizes, designs are used which are derived from the known light boxes. The light sources are thereby located in a shallow pan, which diffusely reflects the light within them and can only emerge towards the open side. For fluorescent lamps often specially shaped reflectors and diffuser lenses for LEDs are used so that despite a low depth of the backlight, the light emerging from the bath light is approximately homogeneous. Remaining inhomogeneities of the illumination manifest themselves in the form of cloudy representations (English Clouding ).

Diffuser

The distributed from the light guide or light bath light has possibly even a spatial structure and must be equally distributed, so that it comes close to an absolutely uniform white light-emitting surface. A simple solution is an opal diffusing plate between the light guide or light bath and LCD screen. Usually, however, it is a film that homogenize the light more efficiently than is possible with opal:

By the 3M Company such as the so-called Vikuiti films have been developed which take advantage of the light over an opal diffuser better by a factor of two. These films reflect those light components back to the light guide that is with regard to the direction of polarization and not suitable for the backlighting of the LCD. This light is scattered in the optical waveguide, usually mixed in direction and polarization, and returns again towards LCD. Like a geometric series, the process is repeated, resulting in a better utilization of the light.

Light sources

LCD - backlit cold cathode tubes

LCD - Backlight Edge - LEDs

LCD backlight full- array LED with local dimming

Electroluminescent film

The surface bright electroluminescent film (EL - foil) came to about 2008 especially for watches to use, there usually in blue or blue - green tones. Due to the necessary high voltage (approx. 200 V) to control, their very low efficiency compared to today and available around since 2008 she plays LEDs as a backlight but even in such simple ads no longer matters. As display backlighting, it was never used due to their low luminance, low long-term stability and low in comparison to CCFL efficiency.

Cold-cathode tube

The compact and efficient cold cathode tubes ( tubes ) provide better light output compared to the EL sheet. To illuminate a larger area more of them must be mounted parallel behind the scene. A scattering film, the light distributed to the tubes then the image area. The desired uniform light distribution can only be achieved by a minimum distance to the picture plane, which monitors with CCFL lamps must have a minimum depth of about 35 mm. Also, the diameter of the tubes themselves can not be arbitrarily reduced. In the example in the picture, the diameter is 3 mm. In addition, relatively high electrical voltages and powers are necessary.

LEDs

Backlight

The use of many small point sources in the form of light emitting diodes (LED, light emitting diodes) for backlighting has the advantage that they can be selectively controlled in brightness. The selective reduction of the backlight in darker areas ( jargon Local Dimming ) appears the overall picture contrast and brightness distance between the darkest black and the brightest white representable is improved. At the same time, however, the achievable brightness value within the darkened regions significantly reduced, so that, for example, bright highlights are darkened in dark areas. This problem is addressed by a growing number of LEDs for backlighting, but what drives the price of the equipment clearly upwards, because the increase in the number of LEDs requires a more complex electronics to their control. These LED arrays are referred to in the jargon as a full -array LED or Direct.

Edge - LEDs

A special form of backlighting with LEDs are the so-called Edge - LEDs. In this form of the backlight plurality of LEDs are mounted on the side edge of a light guide and scatter the light transmitted over the optical fiber to the front ( see the light guide ). Advantage of this design is the very small height, which can be achieved with it. Instead of side-mounted LEDs is an elongated light source at a Swiss start-up companies in the development, which promises a more uniform lateral illumination with lower power requirements. Here, in an ultra-thin fiber with novel nanostructure distributed light is generated. However, in these edge-lit backlights the screen size limitations. From around 50 - inch screen size, the more expensive full -array LED array is preferred.

Other bulbs

In very simple liquid crystal displays small bulbs or small glow lamps are used, the light is scattered over a textured background and reflecting the viewer. Theoretically, a Planon lamp from Osram be used for backlighting, which has not enforced because of the high price and the comparable thick design for the mass market.

Other applications

The principle of the backlighting is used, inter alia, in the light advertising to produce self-luminous logos, writing or photographic recordings. Other examples are backlit doorbell and nameplates and house numbers. Again, most are fluorescent, but occasionally also used LEDs or incandescent lamps. The ultraviolet - filtering and UV- resistance of the materials and motifs is of particular importance, since the useful life is much higher than that of electronic devices.

Light tables and boxes for viewing image and developed film material also work on the principle of the backlighting.