Solid-state lighting for Advanced Display applications

The advantages of solid-state lighting for the illumination of high definition displays are both numerous and compelling. Solid-state light sources are inherently reliable and therefore never have to be replaced, so they don't contribute to the waste stream. Each solid-state light source provides a single saturated color, which when combined, provides access to a wider range of display colors. Since the light does not need to be broken up into separate colors like with a basic white lamp, no filters or moving parts are required, reducing the complexity of the optical display system. Solid-state light sources also do not require any warm-up time (unlike, for example, projection lamps used in microdisplay rear-projection TV systems). Finally, solid-state light sources turn on and off instantly. When synchronized with the display image, this can be used to improve contrast and image quality, and to consume less power.

Despite these advantages, the use of solid-state lighting in advanced displays has been limited to demonstrations, with no real products making it into the retail channel. The reason for the slow adoption of solid-state light sources has primarily been brightness and cost.

For example, although bright, visible, solid-state lasers have been available for more than a decade, their cost has been prohibitive for large volume display applications in consumer electronics applications. Light Emitting Diodes (LEDs) on the other hand, while inexpensive, just don't provide enough lumens (optical power as seen by the human eye) to illuminate advanced displays. As a result, LED illumination has been limited to very small screens, such as cell phone displays.

The combination of multiple LEDs to provide more lumens for larger screen displays is limited by the growing optical complexity (and cost) associated with collecting and combining light from multiple LED sources. In addition, multi-LED systems require "binning" of similar color LEDs so that the combined source maintains its color saturation. Unfortunately, selective binning increases the effective LED cost since some LEDs don't make the cut and go to waste.

Innovations in LED design such as chip shaping, wafer-bonding, surface roughening and flip-chip methods have led to substantial improvements in brightness and efficiency, enabling new High Brightness LEDs (HB-LEDs). HB-LEDs have many useful applications outside of displays, but are also currently being designed into lower-brightness pocket projectors. In fact, the first LED-based mini-front projectors are now on the market but with light output limited to only 10-25 lumens. Still, even these lumen levels may be enough for certain applications. Widespread market acceptance, however, of mini-front projectors and even smaller "pocket projectors" will require light output of at least l00 lumens, pressing the limits of technological advancement for HB-LEDs.

Bringing Solid-State Lighting to Advanced Displays
The television market is in the middle of a considerable transformation. The long awaited advent of high-definition TV is finally taking place, feeding demand for new, large screen displays. In turn, several new technologies have emerged to replace the venerable Cathode Ray Tube (CRT) TV. Consumers are faced with options like never before " from several "flavors" of microdisplay RPTVs to plasma (PDP) and liquid crystal display (LCD). While opinions vary over which technology is best, there is no argument that the adoption of solid-state light sources would help RPTV maintain, or even grow, market share for larger screen sizes (where RPTV is much less expensive than Plasma or LCD) and help accelerate LCD domination at smaller screen sizes.

There is an immediate need for solid-state light sources to reinvigorate the RPTV market, as the Achilles heel of the microdisplay RPTV has always been the light source. The incumbent light source used in today's microdisplay-based projectors and televisions is the arc lamp. These high-pressure lamps are operated under extreme conditions, becoming very hot during operation and even sometimes exploding. Therefore, arc lamps have lifetimes limited to a few thousand hours before needing to be replaced, at a typical cost of a few hundred dollars. An additional downside of arc lamps is that they take up to a minute to warm up and cool down. Arc lamps also contain mercury, a toxic, environmentally hazardous substance that risks entering the waste stream when these bulbs are replaced.

Early attempts at constructing RPTVs using LEDs did not produce enough lumens and required complex combination optics to collect light from the multiple LED sources used. Recently, laser-based RPTVs have been demonstrated that overcome many of the technical challenges that have plagued the technology in the past, but cost continues to limit their commercialization. In addition, the public perception of safety risk with lasers may provide a substantial marketing challenge.

There is also growing interest in solid-state illumination in LCD displays, for many of the same reasons as for RPTV. The cold-cathode fluorescent lamps (CCFLs) used in LCD displays also contain mercury and have limited color range. Moreover, since solid-state light sources can turn on and off instantly, blurring of images in LCD screens can be eliminated and black levels improved.

Early attempts at commercializing large screen LCD screens illuminated with hundreds of LEDs were not successful, due to cost limitations and technical complexities. More recent LCD backlighting approaches rely on thousands of lower cost (and lower brightness) LEDs. Color control and reliability (due to the large number of potential LED failures) are technical challenges that must be overcome for these approaches to be commercialized.