Rapid Progress in High-Brightness LEDs for Projection

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.

by Christian Hoepfner

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.

In the following, we will explain the design requirements of LEDs for projection applications, and then focus on the design of PhlatLight LEDs that enabled the launch of the first LED-based projection TVs. The considerable effort in light-engine design accompanying this breakthrough will be discussed only as far as it impacts the design of the LED light source.

Small Étendue Light-Engine Requirements

Modern microdisplay-TV light engines use microdisplays between 0.45 and 0.95 in. on the diagonal. The cost of microdisplays, the most expensive item of the light engine, provides a significant incentive toward smaller microdisplay sizes, and therefore small étendues. Cost and size of optical components such as projection lenses also favor small microdisplays.


Christian Hoepfner is Vice President of Products at Luminus Devices, Inc., which develops and manufactures high-performance solid-state light sources customized for a variety of illumination applications, including high-definition TVs, video projectors, avionics displays, and lighting systems. He can be reached at 1100 Technology Park Drive, Billerica, Massachusetts 01821; telephone 978/528-8000, e-mail: choepfner@luminus.com.

The small étendue of such light engines dictates the emitting area of the LED to be small, in the range of 8–24 mm2. Therefore, the available emitting area of LEDs needs to be utilized efficiently. This will favor LEDs that have surface emission only, since LEDs that exhibit edge emission require additional optics to collect this light, thus increasing the actual light-source étendue. A good LED light source will be a surface emitter with exactly the emitting area required to illuminate the microdisplay. The aspect ratio of the emitting area will be matched to the micro-display, corrected for any projection angles; this is typically 16:9 for HDTV. Any gaps in the emitting area caused by tiling smaller LEDs chips will cause a loss of actual brightness. Therefore, single large LED chips of the correct size and aspect ratio will be ideal.

LEDs for Projection

The progress in LED light-engine brightness can be attributed to three main factors: (a) performance improvements of LED technology in general, (b) the design of LEDs to specifically match light-engine requirements, and (c) the emergence of Photonic Lattice LEDs.

Today, the LED industry is a multi-billion dollar business, with cell-phone screen and keypad illumination as the largest market. Competitive pressures paired with techno-logical advances have led to a steady improvement of LED luminous efficiencies, which is the metric most widely used as the figure of merit in the LED industry. For projection TVs, however, actual luminance, not luminous efficiency, is the most important performance parameter. Nevertheless, many improvements in luminous efficiency will also improve system luminance. Therefore, projection applications have benefited from many general improvements of standard LEDs.

To meet the requirements of small-étendue light engines, a single monolithic LED chip with emitting-area dimensions matched to the microdisplay will be ideal. For example, the PT120 PhlatLight projection chipset has been designed for a DLP® xHD5 microdisplay, with an emitting area of 12 mm2 and an aspect ratio of 16:9. This allows optimal utilization of the microdisplay étendue.

PhlatLight LEDs for Microdisplay Projection

PhlatLight LEDs are the first LEDs specifically designed for microdisplay projection. In order to achieve the high brightness required, a suite of innovations was implemented on the chip and package level.

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Fig. 1: Shown is a photograph of the PhlatLight PT120 chipset, consisting of one red, green, and blue monolithic 12-mm2 emitting-area chip in a low-thermal-resistance package.

 

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Fig. 2: Samsung's 61-in. 1080p DLP HDTV with a slim LED engine (HL-T6187S).

 

Photonic lattices, often also referred to as photonic crystals or photonic band-gap materials, are created by embedding periodic structures into dielectric or semiconducting materials. The lattice constants for these periodic structures are on the order of the wavelength of light, with critical dimensions well below 100 nm. The periodic structures (Fig. 5) create an optical band gap that forbids the propagation of light with certain frequencies and directions, and therefore can fundamentally change how light propagates in the host material. The patented photonic-lattice design in PhlatLight LEDs suppresses lateral propagation of light along the quantum wells. Light is therefore forced into states perpendicular to the surface, forcing it out of the chip through the surface. Hence, PhlatLight LED chips have no edge emission; all light is emitted from the surface. Because light traveling parallel to the quantum wells can easily be reabsorbed, forcing the light out of the chip through the surface will also enhance the extraction efficiency. In addition, the photonic lattice can be designed to force more light into states close to the surface normal, resulting in collimated emission. While this collimation effect does not nominally increase the total flux of the LED, it will increase light collection into the light-engine étendue and make the optical transmission through lenses and dichroic mirrors more efficient. All three features of the PhlatLight photonic-lattice design (surface extraction, increased extraction efficiency, and collimation) contribute to improved brightness of the projection system.

In order to further increase system brightness, PhlatLight LEDs also have been designed to be operated at high drive currents. Every engineer using LEDs for high-brightness requirements will be tempted to increase the drive current beyond the typical operating conditions of 0.35 or 1 A/mm2. However, conventional LEDs are not designed to be operated reliably at higher current densities because the resulting electrical and thermal stress would reduce the lifetime of the LED.

The semiconductor junctions typically used in LEDs are more than capable of being operated at very high current densities, up to hundreds of amperes as illustrated by lasers. However, many LEDs rely on current spreading from small metal contacts at the side of the LED, which can cause current crowding at these metal contacts, thereby limiting the reliable operating current of such LEDs.

Increased current densities also mean more power and heat dissipation. In order to keep the junction temperature at levels suitable for highly reliable long-term operation, the thermal resistance from junction to heat sink needs to be lower than with standard LEDs.

Beside the implementation of photonic-lattice technology, PhlatLight LEDs have pioneered a vertical chip design to address both electrical and thermal challenges. Most high-brightness blue and green LED material is grown on sapphire wafers. While sapphire enables the growth of high-quality contact layers and quantum wells, it is electrically isolating and has very poor thermal conductivity. By bonding the epitaxial wafer to a metal submount and then removing the sapphire substrate, the epitaxial layers with the quantum wells are transferred to a superior substrate material. The new metal submount has very low thermal resistance, and it provides a full-area metal contact to the LED. Consequently, PhlatLight LEDs can be operated at current densities far exceeding operating conditions typical for standard high-brightness LEDs. They have been designed and qualified for long lifetimes at current densities of 2.5 A/mm2. For example, a PT120 chip with an emitting area of 12 mm2 can be operated at an 18-A continuous current, corresponding to an electrical power of up to 100 W. In pulsed operation, the device can be driven at even higher currents. These improved operating limits help to provide more brightness, while maintaining the high reliability of LEDs. For example, a green PT120 LED yields more than 3300 lm peak at a 30-A forward current with 50% duty cycle.

Figure 3 shows a cross section of a PhlatLight LED chip. The active layers, including quantum wells, have been bonded to the metal submount. The two-dimensional photonic-lattice structure is embedded into the surface of the LED chip.

An often overlooked advantage of Photonic Lattice LEDs is that they do not require encapsulants for light extraction. The photonic lattice has been designed to extract light from the semiconductor chip directly into air, thus eliminating the encapsulant, which is one of the most unreliable parts of the LED. PhlatLight LEDs are assembled with a proprietary epoxy process to bond the LED chip submount directly to a copper heat spreader, resulting in record low thermal resistance. These advantages, combined with the highly efficient electrical and thermal design make PhlatLight LEDs extremely reliable solid-state light sources, with median lifetimes above 120,000 hours at TV operating conditions.

LED Performance Improvements

Along with the introduction of the innovations discussed above, the brightness of PhlatLight LEDs has improved considerably during the past 2 years. Figure 4 shows the improve-ment in the measured brightness of green PhlatLight LEDs over time. Green is actually the most challenging LED color to produce for visual displays and almost always presents the bottleneck in providing color-balanced white flux. Since early 2006, when the first projection TV using PhlatLight LEDs was designed, the brightness of the green LEDs has doubled. Most of this additional brightness was used to reduce the cost of the TV system, for example, by using a smaller microdisplay.

 

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Fig. 3: Cross section of the PhlatLight chip. Green and blue chips are based on GaN materials, red chips on AlInGaP. While the fabrication processes for these materials are different, the general construction of the chips is the same.

 

Brightness improvements in all LEDs, including PhlatLight LEDs, have not yet reached a plateau. Progress is fast and PhlatLight brightness is expected to double again within 18 months. This will enable yet more novel and innovative projection applications.

LED Light-Engine Design Considerations

Although the brightness of projection LEDs has increased significantly, achieving sufficient flux from the projection lens still requires careful light-engine design. Typically, the f/# of LED light-engine optics will be smaller than for lamp-based models, increasing étendue and therefore enabling the use of a larger LED source with higher total flux. The potential loss of contrast ratio can be compensated by good optical design and by taking advantage of the fast-switching behavior of LEDs to implement dynamic-brightness management. Although tapered or parabolic light guides are a good choice to collect light from LEDs, they typically will not result in as much brightness as a single or double aspheric collection lens. This is because light rays with emission angles far off the LED surface, which can nominally be collected into the light guide, are not efficiently transmitted through the following projection optics.

Dichroics for color combination need to be designed for large angles of incidence typical for LED illumination optics. LEDs do not emit UV radiation. Therefore, all lens coatings can be designed to have larger transmission in the blue spectral range compared to lamp-based systems.

The thermal management of LEDs is very important. A very compelling feature of LEDs is that they run at low temperatures. The LED active area, including the quantum wells, is typically operated at temperatures around 70–120°C, in some cases higher, but always below 200°C. This is in stark contrast to UHP lamps where the active area (the arc) is at ~6000°C, and even the outside of the quartz tube is at ~800°C. On the other hand, because the junction temperature of LEDs is so low, it will vary with ambient temperature. Most cooling systems in pocket projectors and projection TVs use simple heat sinks with fans for air-cooling. In some TVs, heat pipes are being employed to transfer the heat to a location inside the TV cabinet where heat dissipation is convenient. Liquid cooling of the LEDs is typically not required.

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Fig. 4: Brightness improvements of green PhlatLight LEDs. The flux was normalized to 100% in January 2006 when the first TV launch decision was made. All numbers are for the same chip size and same drive current. Roadmap numbers are estimates based on ongoing development programs. Green light is usually the bottleneck color in visual display systems and the green LED brightness will determine the total white brightness.

 

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Fig. 5: Illustration of a photonic lattice microstructure. Feature size on the order of hundreds of nanometers.

 

Cooling is typically designed to keep the LED junction temperature below a maximum limit: in the case of blue and green PhlatLight LEDs, below 120°C and red PhlatLight LEDs below 80°C. These temperatures are different for blue, green, and red LEDs because they are made from different semiconductor materials with different temperature sensitivities. Actual LED junction temperature for the most part will be below these maximum temperatures and fluctuate with ambient temperature. Therefore, a color-management system needs to compensate for temperature-induced changes of relative intensities between the different colors.

Future LED Projection Products

Initial market feedback shows that LED-illuminated projection TVs are very popular because they eliminate the need to replace expensive UHP lamps and because of the superior color reproduction. It is therefore expected that most projection-TV product lines will be converted from UHP lamps to LED illumination within the next 18 months. In 2009, the large majority of all micro-display-projection TVs will use LEDs as light sources.

Based on high-end TV light engines, it is expected that very-high-image-quality home-theater projectors with LED illumination will enter the market in 2008. These home-theater projectors will use larger microdisplays to enable higher brightness and contrast ratio required for front projection and have luminous flux between 500 and 1000 lm from the projection lens.

Pocket projectors with 120–150 lm will become available in 2008. Additional LED brightness increases will be used to reduce the cost of these projectors, in order to gain a larger market share.

Even smaller projectors, often called nano- or pico-projectors, are in development and will enter the market in 2008. Most models will use LEDs as the light source because LEDs are a mature technology and readily available.

The LED brightness and efficiency improvements gained during the past few years in the pursuit of projection TVs will eventually benefit other visual display applications as well. Highly efficient green LEDs are also needed for LED backlights (BLU) for LCD TVs. For example, the collimation benefits of PhlatLight LEDs can be used to couple light efficiently into light guides, enabling very large (> 52 in.) edge-lit BLUs.

Conclusion

LEDs have a bright future as microdisplay-projection light sources. Rapid improvements in LED performance driven by the LED industry in general and the emergence of PhlatLight photonic-lattice LEDs specifically designed for projection applications have enabled a new level of LED light-engine performance. LEDs already represent a mature, multi-billion dollar industry. Very large demand for general-lighting applications, a very competitive industry, and continued innovation by established manufacturers and start-up companies will continue to push LED performance to enable ever-more exciting display applications. •