Design Article
Rapid progress in high-brightness LEDs for Projection TVs
Christian Hoepfner
4/16/2008 12:30 PM EDT
The increase in the brightness of microdisplay-projection light-emitting-diode (LED) light engines has significantly accelerated during the past 3 years, to a level beyond many industry expectations. This acceleration has enabled the emergence of LED projection TVs. Here, we review the progress made with LEDs for projection applications and the products being enabled. Design considerations for LED-based light engines are discussed, and an outlook for further brightness improvements and new products is presented.
Introduction
LIGHT-EMITTING DIODES (LEDs) have long held the promise as a better light source for microdisplay projection, offering advantages such as long lifetime, narrow optical emission spectrum (and therefore a high degree of color saturation and large color gamut), and environmental friendliness compared to that of conventional ultra-high pressure (UHP) lamps. But there are also other less obvious advantages. The ability to pulse LEDs with very high speed allows for the implementation of sophisticated power and color-management schemes. The ruggedness typical of solid-state technology enables compact solutions that can withstand harsh environments.
Despite all those advantages, it has taken many years for the idea of using LEDs as projection light sources to become reality. The single most important challenge has been to provide sufficient brightness from the projection lens. LEDs are not as bright as the arc of a discharge lamp " and may never be. Therefore, a careful optimization of all aspects of the projection system, including LEDs as well as illumination and projection optics, is required to design a high-brightness microdisplay-projection system based on LEDs.
Since 2005, several small-form-factor front projectors with LEDs have been released to the market, often called pocket projectors or pocket imagers. Toshiba, Mitsubishi and Samsung launched such projectors, using LEDs from OSRAM and Lumileds Lighting. Projectors in this category delivered 15"25 lm from the projection lens. Newer models introduced in the Spring of 2007 have up to 50 lm.
In the Summer of 2006, the first microdisplay projection TV was launched by Samsung, followed by a model from NuVision. These TVs used a new class of LEDs from Luminus Devices called PhlatLight LEDs (see Fig. 1), the first LEDs specifically designed for microdisplay projection, and the first to exceed the brightness threshold required for high-quality projection TV. In the Spring of 2007, Samsung released six new projection-TV models into the U.S. market, with screen sizes of 50, 56, and 61 in. (see Fig. 2). In addition, LG Electronics has launched in July 2007 a pocket projector utilizing PhlatLight LEDs, the first commercial LED pocket projector to break the 100-lm barrier.
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