Some Were Over the Rainbow...

[James Morrow]

LED backlighting is becoming ever more common with LCD panels – potentially offering significant power saving and contrast enhancements over cold cathode based backlights, as well as faster warm-up, more stable colour reproduction, wider colour gamut and, with spatially addressable backlights and vastly increased contrast.

Available LED light output has now increased to a level at which a trio of red, green and blue LEDs is sufficient to drive a large rear-projection display in place of the conventional lamp. For example, Samsung announced four new big-screen rear projection displays recently at CES 2006, including the 56” 1080p single DLP HL-S5679W, which replaces the usual colour wheel/lamp with a trio of LEDs. Benefits include allowing a wider colour gamut, a three to six times longer light source life, and a much shorter turn-on time – seven seconds, as well as elimination of the rainbow effect that causes some viewers of single chip DLP displays to see brief trails of colour in certain circumstances.

Although featuring dual 1080p-compatible HDMI inputs, the DLP chip in the current generation displays is often thought of as being a “quasi” 1080p solution, because it only has 960 by 1080 actual pixels, which are made to scan twice using a process called wobulation. What this does to the timing and fluidity of motion has to be considered – as well as any filtering that may have to be applied to produce an acceptable image. For example, conventional vertically interlaced signals have to have their vertical resolution almost halved in order to avoid a number of unpleasant and distracting visible artefacts. Anything which has the potential to reduce the fluidity and simplicity of progressively scanned source material for display is likely to significantly reduce the illusion of reality that the best of 1080p can approach.

Interestingly, the Samsung spokesperson claimed that the new light engine (three LEDs) can produce brighter images than current lamps. Certainly LEDs are more efficient light generators than a conventional lamp and so, given high enough output (or enough) LEDs, any given light modulator (DLP/LCD, etc.) is likely to be able to support a higher light output whilst running at the same or a lower temperature. Just as spatially modulated LED backlights can be used in flat panel LCD displays to greatly increase contrast (dynamic range), higher-powered variants could be used in rear – and later front – projection systems, providing that the LED conversion efficiency is sufficient to allow the temperature of the primary “spatial modulator” (LCD or DLP) to remain within specified operating limits. Note that six or eight colour LED light engines (or more) could be used to further enhance brightness, colour control and colour resolution.

See http://www.cnet.com/4831-11405_1-6413134.html?tag=next and
http://reviews.cnet.com/Samsung_HL_R...4.html?tag=toc for more details.