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Developments in LCD Backlights - TvTechnology

Developments in LCD Backlights

The cathode ray tube that until recent years served as the principal television and video display device generates its own light.
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The cathode ray tube that until recent years served as the principal television and video display device generates its own light.

The CRT's electron beam bombards electroluminescent phosphors, which absorb the energy in the electron beam, exciting the electrons in their atoms to a higher energy level. When an energized electron returns to its previous lower energy level, it emits its excess energy as a photon of light. The materials comprising the phosphors are chosen to emit photons of the desired light colors.

In 2006, the CRT's days appear numbered as consumers choose among various advanced displays. One of today's most prevalent advanced displays, the direct-view liquid crystal display, does not emit its own light, but rather must be illuminated from behind. One possible source of illumination for such a display is a xenon lamp, as is frequently used in projection displays.

While xenon lamps furnish a lot of light with good spectral content, they are not ideal for direct-view LCD panels because in addition to generating a considerable amount of heat, they are close to being point sources, making it difficult to spread their light evenly over an entire display screen.

A fluorescent tube is more optimally shaped for direct-view LCD backlighting. It may be made in a length that spans the entire display horizontally or vertically, and it may have a diameter sufficiently small to fit within the LCD's thin depth.

Fluorescent lights bear a similarity to CRTs in that their light is also generated by phosphor coatings, but the mechanism used to energize the electrons in fluorescent phosphors is different. The atoms in a fluorescent tube's gases are excited to the point where they cause some of their outer electrons to separate, and a state is reached where some electrons absorbing quanta of energy are jumping out of atomic orbit, while some previously freed electrons rejoin their atoms, with the release of quanta of energy in the form of photons. This roiling stew of electrons and positive ions is called a "plasma."

The photons released from the plasma are in the ultraviolet region of the electromagnetic spectrum. They bombard the tube's phosphor coating, raising some of the phosphors' electrons to higher energy levels. When these electrons return to their lower energy states, they release photons in the visible light region.

The fluorescent tubes used to light offices use "hot cathodes,"-coils of wire coated with materials that enhance ther-mionic emission, or the "boiling off" of electrons, to generate their plasmas. There is such a cathode at each end of the tube, and the light is started by connecting the two cathodes in series, and passing an alternating electric current through them.

The thermionic filament-cathodes become sufficiently hot to boil off electrons in the same way a vacuum tube cathode does. When this point is reached, the series connection between the two filament-cathodes is opened, and this causes an inductive voltage surge from the lamp's ballast to strike an arc through the now conductive plasma in the tube, igniting it.

COLD CATHODES

The fluorescent tubes used as LCD backlights are cold cathode fluorescent lights. In a CCFL tube, electrons are not boiled off a heated filament-cathode by thermionic emission.

Instead, electrons are released from two parallel cathodes within the CCFL tube by ion and electron bombardment. As ions and electrons bombard the cathode material, secondary electrons are knocked loose and emitted from it. CCFL lamps are also driven with alternating current, so their cathodes also serve as anodes.

The cathode material is typically a metal such as iron. Although this is a called a "cold cathode" device, heat is of course generated in the bombardment process and the excitation of the plasma within the tube, so heat is emitted by the CCFL lamp, albeit less heat than a hot cathode lamp emits.

One disadvantage of the CCFL is that the constant bombardment of the cathodes causes them to deteriorate over time. A relatively recent development that addresses this problem is the external electrode fluorescent lamp.

This EEFL does not contain internal electrodes. Rather, the electrodes are external to the lamp itself, and the electron energy they generate is capacitively coupled to the gases within the tube.

ADVANTAGES OF EEFL

Claimed advantages of the EEFL include high efficiency and brightness, less heat and less deterioration of the phosphors that heat causes, as well as the elimination of the electrode deterioration caused by ion bombardment.

Further, capacitive coupling imparts the ability to operate a number of EEFL lamps in parallel from a single set of electronics, as opposed to the CCFL, where each lamp or pair of lamps requires its own set of electronics.

Because there are no electrodes within the lamp, no wires pass through the glass tube, eliminating the need for a glass-to-metal seal, which is claimed to reduce gas leakage problems.

EEFL is a relatively new development in fluorescent lamp technology, and as such is not prevalent in current display products. We may expect to see it in an increasing number of future products, however.

Another significant new fluorescent backlight development is the flat fluorescent lamp. The FFL is a thin, flat, rectangular lighting source that has the appearance of being an adaptation of the EEFL.

The FFL is sufficiently thin and large to cover the entire display area. Electrodes are arranged on the back surface of the FFL fixture, phosphors coat the front, while the gas plasma occupies the space between. This permits the backlight source to be spread across the entire display area, rather than being located along its edges with distribution by reflectors, as is the case with round fluorescent tubes.

One recent survey of backlighting techniques indicates that today, in 2006, backlighting for direct-view LCDs consists almost entirely of CCFL sources. Four years from now, CCFL is projected to be the second-largest source of backlighting. The No. 1 light source is projected to be LED lighting, as has previously been mentioned in this column.

The No. 3 and 4 backlighting sources are projected to be EEFL and FFL fixtures respectively. New technologies are being applied not only to advanced displays themselves, but also to methods of backlighting them.