This paper describes the status and trends of visible LED (light-emitting diode) technology. LED technology is compared to conventional incandescent and fluorescent lighting technology. Performance and cost, the key issues which need to be improved to enable LEDs to substantially penetrate the illumination market are discussed.

Electric lighting of buildings in the United States consumes over 20% of the nation's primary electricity and is second only in magnitude to heating, ventilation and air conditioning systems. This installed lighting base is generally inefficient and is characterized by relatively low performance especially when compared to other building systems. While substantial opportunities for improving overall lighting system efficiency exist, the pathway to achievement of this goal is less clear. Lighting research and development conducted by the US Department of Energy's (DOE), Office of Energy Efficiency and Renewable Energy's (EERE), Building Technologies Program (BT) addresses this national issue and aggressively pursues a number of broad research areas that promise to yield significant increases in overall lighting system efficiency. Implementation of a successful program in lighting energy conservation depends upon a detailed assessment of energy consumption trends by lighting technology. The results of several years of research are presented that describe electricity consumption by market sector, application and lamp type. Following this lighting market assessment, an overview of the DOE's ongoing lighting research and development (LR&D) program portfolio linked to the market assessments is provided. Individual program contributions toward achieving ambitious lighting energy conservation goals are described. The BTS portfolio includes research in three broad areas: (1) light source and electronics, (2) fixtures, controls and distribution systems, and (3) human factors. An overview of each technical objective is provided, as well as a timeline for achieving specific energy conservation goals.

High quality InAlGaN alloys, quantum wells and associated light emitting diodes have been grown by metalorganic chemical vapor deposition for ultraviolet (UV) emitters. In-situ reflection and ex-situ atomic force microscopy measurements show that InAlGaN epilayers and structures have good surface morphology. InAlGaN epilayers have also a narrow (0006) reflection X-ray diffraction rocking curve linewidth of ~ 340 arcsec and a strong band edge photoluminescence (PL) emission peak from 320 nm to 355 nm at room temperature. Several X-ray satellite peaks were observed from InAlGaN based quantum well structures, revealing that they were periodic with good interfaces. PL mapping measurements of the quantum well structures show excellent wavelength uniformity over a 2" wafer with a standard deviation of ~ 0.4% for structures emitting from 351-372 nm. Ultraviolet light emitting diodes (UV LEDs) based on the same InAlGaN quantum well structures have an electroluminescence (EL) emission at ~ 375 nm with a linewidth of ~10 nm and an excellent wavelength uniformity of less than 1 nm across a 2" wafer. Temperature dependent study of EL spectrum from an UV LED shows "blue jump" from a broad blue emission at <170 K to a narrow UV emission at higher temperatures.

White LED is well known as a promising device for solid state lighting. It has the advantages of long life, good endurance of heavy impact, no mercury containing and potentially high efficiency. However, light output from a commercial LED lamp is usually less than 0.1W, which is very small if compared with traditional light sources. In order to get enough light output from LED devices, a big-lamp or cluster, which is packed from dozens or hundreds of LED lamps, is necessary not only for outdoor display but also for lighting applications. Some problems were encountered during high-density assembly in a big-lamp. Because of the bad heat conductivity of the epoxy, the more LED lamps are used in the big-lamp, the poorer heat dissipation will be. On the other hand, it is difficulty to manufacture a big-lamp in good alignment of every LED¡¦s during the package. One of the ways to solve this problem is to use a big LED chip instead of many small lamps. Nevertheless, to use those big chips always companions 10-25% loss in the external quantum efficiency. And, the bigger the chip is, the larger the loss will be. Highly concentrated heat will be another issue for the big chip package. In this article, we will present a new design for the package of multi-chips LED module, which could assemble dozens of LED chips with the normal size for lighting purpose. Optical devices, including reflector and focusing lens, and heat considerations are considered in the design of the new module. The prototypes of the new modules have been made. The current of maximum light output (C_max) is about 2 to 3 times superior to conventional lamps. Under the condition of normal driving current, a 4.5 W LED module have an output intensity of 164 cd. Combining 4 LED modules will be comparable with a conventional 20 W Halogen lamp.

A review of the lighting requirements and associated specifications for the aerospace environment is presented. The impact of solid-state lighting is assessed for both aircraft and airport/obstruction lighting with emphasis on new concepts in obstruction lighting. Strict optical beam profile and photometric requirements present an ideal situation for widespread introduction of solid-state lighting in aerospace.

HBLEDs bring a new dimension to architectural lighting design: color. Compared to traditional "white" light sources such as fluorescent and metal halide lamps, HBLEDs offer lighting designers unprecedented control over color temperature and lamp chromaticity. This raises two important questions: 1) How should the correlated color temperature metric used by lamp manufacturers be applied to HBLED-based light sources; and 2) What are appropriate limits on chromaticity variances for LED clusters and arrays? The answers to these questions will influence the acceptance of HBLEDs by architectural lighting designers.

A laboratory human factors experiment was conducted to understand the color rendering properties of various LED-based reading lights. Human subjects rated their preference for a given scene illuminated by a test light source compared to an identical scene illuminated by a reference light source. In another experiment the same subjects viewed and rated the scenes individually when illuminated by a test light source. LED-based light sources were preferred more than halogen and incandescent light sources for overall color appearance. However, phosphor-based white LED light sources were rated poor for the appearance of human skin tones. This study shows that RGB mix white LED light sources have the best overall characteristics to be used as reading or task lights. This study has shown that CRI has no correlation to peoples' color preference. Therefore, the authors of this manuscript do not believe that CRI should be used as a target metric for color rendering properties in the development of solid-state light sources because it could negatively impact overall performance. A better metric is long overdue to quantify light source color rendering and preference properties.

The rapid development of high brightness light emitting diodes (LEDs) has triggered many applications, especially in the area of display lighting. This paper focuses on the application of white LEDs in refrigerated display cases. The fluorescent lighting presently used in commercial refrigerators is inefficient in the application and also it provides poor lighting for merchandising. A laboratory human factors experiment was conducted to assess the preference for the different lighting systems, namely, fluorescent and LED. Two refrigerated display cases, one with the traditional fluorescent lighting system and the other with a prototype LED lighting system, were placed side-by-side in a laboratory setting. Illuminance measurements made within the two display cases showed that the lighting was more uniform with the LED system compared to the traditional fluorescent system. Sixteen human subjects participated in this study and rated their preference for the two lighting systems. The results show that human subjects strongly preferred the display case with the LED lighting. The authors of this manuscript believe a field study would be greatly beneficial to further confirm these results and to understand the relationship between preference and sales. Considering the luminous efficacy of white LEDs presently available in the marketplace, it is possible to develop a LED based lighting system for commercial refrigerators that is competitive with fluorescent lighting system in terms of energy use. The LED based lighting would provide better lighting than traditional fluorescent lighting.

We review two complementary approaches to the development of white light solid-state sources. The first approach, which involves polychromatic LED modules, is targeted at advanced optimization of spectral power distribution in order to establish an optimal trade-off between luminous efficacy and color rendering. We apply a stochastic method of optimization of a white-light source that relies on additive color mixing of the emissions from colored primary LEDs. We present the results on optimized spectra for all-semiconductor lamps composed of four primary LEDs with the line widths typical of present AlGaInP and AlInGaN technologies. We point out the problem of the lack of efficient yellow-green (570 nm) emitters required for polychromatic lamps with four and more primary LEDs. The second approach is based on the development of AlInGaN-based UV emitters that can be tailored to directly excite different phosphors without sensitizers. AlInGaN materials system demonstrated potential for making UV LEDs with a high power and short wavelengths required for such applications. This has been achieved by using Strain Energy Band Engineering (SEBE) and Pulsed Atomic Epitaxy (PALE) techniques. SEBE relies on quaternary AlGaInN compounds for controlling strain and band offset and for producing UV emitters with improved device performance. PALE allows us to incorporate the required significant amount of indium (few percent) in AlGaN, since it can be performed at lower growth temperatures required for In incorporation. Further improvements in materials quality of AlInGaN layers with a high molar fraction of Al will be achieved by using bulk AlN substrates.

The growth of homoepitaxial GaN, AlGaN layers, and GaN/AlGaN multiple quantum wells (MQWs) on Ga- and N-faces of bulk GaN single crystal substrates prepared by pressure-controlled solution growth (PC-SG) has been performed by radio-frequency molecular-beam epitaxy (RF-MBE). It was determined that homoepitaxial GaN layers grown on both Ga- and N-faces had good crystallinity with narrow full-width at half maximum (FWHM) of 150 and 94 arcsec for the (0002) plane and 119 and 106 arcsec for the (10-12) plane in x-ray rocking curve measurements, respectively. Crack-free AlGaN epilayers with Al mole fraction up to 30% were obtained on both faces. AlGaN epilayers on Ga-faces with higher Al mole fraction than those on N-faces under the same Al flux condition were obtained. Furthermore, phase separation existed only in the AlGaN epilayers grown on N-faces. The 5 K photoluminescence spectra for the GaN/AlGaN MQW structures grown on Ga-faces at peak energy of 3.419 to 3.686 eV can be obtained by varying the well thickness from 18 to 2 ML.

The microstructural, electrical and optical properties of GaN/InGaN light emitting diodes (LEDs) with various material quality grown on sapphire have been studied. Burger's vector analyses showed that edge and mixed dislocations were the most common dislocations in these samples. In defective devices, a large number of surface pits and V-defects were present, which were found to be largely associated with mixed or screw dislocations. Tunneling behavior dominated throughout all injection regimes in these devices. The I-V characteristics at the moderate forward biases can be described by I = I₀ exp (eV/E), where the energy parameter E has a temperature-independent value in the range of 70 -110 meV. Deep level states-associated emission has been observed, which is direct evidence of carrier tunneling to these states. Light output was found to be approximately current-squared dependent even at high currents, indicating nonradiative recombination through deep-lying states in the space-charge region. In contrast, dislocation bending was observed in a high quality device, which substantially reduced the density of the mixed and screw dislocations reaching the active layer. The defect-assisted tunneling was substantially suppressed in this LED device. Both forward and reverse I-V characteristics showed high temperature sensitivity, and current transport was diffusion-recombination limited. Light output of the LED became linear with the forward current at a current density as low as 1.4x10^-2 A/cm², where the nonradiative recombination centers in the InGaN active region were essentially saturated. This low saturation level suggests optical inactivity of the edge dislocations in this LED.

InGaN-based light-emitting diode with a micro-roughened top surface using the metal clusters as wet etch masks was investigated. The forward voltage, V_F, at 20 mA for InGaN/GaN MQW LED chip with a mirco-roughened top surface was improved compared to that of the conventional InGaN/MQW LED chip. This result could be attributed to the improved metal contact on p-GaN due to an increased contact area between the metal and p-GaN layer. Furthermore, the light-output power for InGaN/GaN MQW LED with micro-roughened top p-GaN surface was increased compared to that for the conventional InGaN/GaN MQW LED chip. This indicate that the scattering of photons emitted in the MQW active layer was much enhanced at the micro-roughened top p-GaN surface of LED due to the angular randomization of photons inside the LED structureu, resulting in an increase in the probability of escaping from the LED structure.

Phosphor conversion of blue radiation from InGaN LEDs has recently gone to a new quality: white light, which is illumination grade in the sense of high color rendering and color temperatures below 4000°K. At the same time power packages taking inputs of 1 Watt or even 5 W and surpassing in efficiency small size halogen lamps have been introduced. After a short review of alternative solutions state of the art devices are described and open questions outlined.

Green phosphor-converted LEDs using a blue pump InGaN diodes have advantages over the direct green InGaN LED with regards to color stability with drive and/or temperature. Added manufacturing steps are outweighed by higher color yield, as a range of pump colors can be used without changing the final chromaticity. The conversion losses can be smaller than the decrease in wall-plug efficiency from blue towards green, which has been reported by many sources. A distinct disadvantage of the concept is due to only one color and phosphor proven - SrGa₂S₄:Eu²⁺ and 535 nm peak wavelength.

In order to understand the luminescence mechanism and luminescence centers in LaOCl, LaOBr and LaOBr:Tm, Thermally stimulated luminescence(TSL) and photoluminescence(PL) studies have been made on unirradiated and irradiated samples at room temperature. LaOCl revealed three glow peaks having their maxima at 355,390 and 410k while in LaOBr a shoulder at 335K and two glow peaks at 365 and 420k are observed. Incorporation of Tm in LaOBr resulted in significant changes in intensity. In addition,the shoulder at 335K gets suppressed and the 365 and 420K glow peaks shifted towards high temperatures to 380 and 430K. The shoulders at 355, 335 K in LaOCl and LaOBr have been attributed to impurities while the glow peaks at 390 and 380 K might originated due to radiative electron - hole recombination due to detrapping from chlorine and bromine ion vacancies. The high temperature glow peak at 420 and 430k might belong to F⁺ centers being formed due to charge transfer between oxygen ion vacancies and excited electrons. The reflectance and photoluminescence studies supported these attributions as they revealed different emissions which may be responsible for color centers as well as luminescence centers.

Deposition and characterization of ZnS, Sm₂S₃ and ZnS:Sm films are described. The films have been prepared by chemical vapor deposition using new volatile complex compounds, dithiocarbamates of Zn and Sm, as precursors. Characterization includes X-ray diffraction, chemical analysis of the film composition, ellipsometry and spectrophotometry. It has been shown that at relatively low temperatures (about 380 °C) monophase crystalline Sm₂S₃ films can be fabricated. Doping of ZnS by Sm with dopant concentration up to 2 at. % has been achieved. Effects of Sm doping on structural and optical properties of the film are presented.

A laboratory experiment was conducted to understand the acceptability of different white light emitting diodes (LEDs) for outdoor landscape lighting. The study used a scaled model setup. The scene was designed to replicate the exterior of a typical upscale suburban restaurant including the exterior facade of the building, an approach with steps, and a garden. The lighting was designed to replicate light levels commonly found in nighttime outdoor conditions. The model had a central dividing partition with symmetrical scenes on both sides for side-by-side evaluations of the two scenes with different light sources. While maintaining equal luminance levels and distribution between the two scenes, four types of light sources were evaluated. These include, halogen, phosphor white LED, and two white light systems using RGB LEDs. These light sources were tested by comparing two sources at a time placed side-by-side and by individual assessment of each lighting condition. The results showed that the RGB LEDs performed equal or better than the most widely used halogen light source in this given setting. A majority of the subjects found slightly dimmer ambient lighting to be more typical for restaurants and therefore found RGB LED and halogen light sources to be more inviting. The phosphor white LEDs made the space look brighter, however a majority of the subjects disliked them.

Due to the rapid increase in flux performance from High Power LED's, illumination is an exciting growth market for solid state lighting. Today a white LED is 100+ Lm per device. This is approximately an order of magnitude below the kLm metric used for illumination applications. The radiation pattern from the LED is key in increasing the usable flux resulting in improved systems optical performance. This advancement in radiation pattern will allow new market opportunities, which were not yet feasible. In the future this effect of usable lumens will become more important as the flux per package increases. The radiation pattern of the LEDs can be controlled to optimize performance, appearance, and shape of the secondary optics. This advantage is unique to LEDs and can greatly improve system performance, control, and cosmetic appeal for the application. This paper will review the side emitting lens design, the integrated performance of this technology to secondary optics and how the Luxeon side emitter enables improved performance by creating more useable lumens.

If InGaN LEDs are going to replace current lighting solutions, they must have several money saving characteristics such as: high efficiency, low cost, and long reliability. We will take a look at the reliability aspect of the LEDs based on a comparison of different types of die-attach epoxies used to package the LED's.

The use of laser technology for the separation of gallium nitride-based light-emitting diodes (LEDs) on sapphire substrates overcomes many of the problems associated with standard separation techniques. Scribe-and-break and sawing have many drawbacks and limitations due to the hardness of the sapphire substrate and to its wurtzite crystal lattice. Laser ablation has an inherent advantage over other laser separation techniques that heat the crystal to the point of causing damage. That difference can be used to enhance throughput and yield without sacrificing valuable wafer area. This work illustrates the process advantages of using the laser die separation technology. These advantages include the flexibility of chip shaping and surface modification, minimization of street and kerf width, and chip aspect ratio. A discussion of the wider process window and ease of use of the laser separation system will be demonstrated. In addition, the electrical and optical characteristics of laser separated die will be compared with die separated by competing technologies.

Uniform current spreading is desirable for both light emitting diodes (LEDs) performance and reliability. It enhances optical efficiency because the joule losses due to current crowding in some parts of the die would be eliminated. The LED design for optimal light extraction and uniform current spreading is therefore a necessity. In this paper we report on preliminary current spreading results obtained from circuit simulation, using Pspice and Aimspice, for LED designs with and without an n-metal ring as well as the epi-up and flip chip LEDs. For the epi-up, both the lateral and vertical resistances of the transparent metals were taken into account. Whereas in the flip chip, the lateral resistance was negligibly small thus only the vertical component contributed to the total p-lump resistance. The n-lateral resistance in the active mesa was critical to uniform current spreading. It was found that the lower the n-lateral resistance, the more uniform the current spreads and flows through the active region. In both the epi-up and flip-chip structures, the contact resistance of the p-metal (including the thin Ni/Au transparent metal) dominated the total p-lump resistance. The larger this value, with fixed n-layer lateral resistance, the more uniform the current spreads in the device. However, high p-contact resistance is not desirable as it reduces the overall efficiency of the device due to excessive heating and increased leakage current. Therefore, for uniform current spreading, the n-lateral resistance should be made small while maintaining an optimum p-lump resistance to achieve a high efficiency.

Considerations for the use of white light emitting diode (LED) sources to produce illumination for automotive forward lighting is presented. Due to their reliability, small size, lower consumption, and lower heat generation LEDs are a natural choice for automotive lighting systems. Currently, LEDs are being sucessfully employed in most vehicle lighting applications. In these applications the light levels, distributions, and colors needed are achievable by present LED technologies. However, for vehicle white light illumination applications LEDs are now only being considered for low light level applications, such as back-up lamps. This is due to the relatively low lumen output that has been available up to now in white LEDs. With the advent of new higher lumen packages, and with the promise of even higher light output in the near future, the use of white LEDs sources for all vehicle forward lighting applications is beginning to be considered. Through computer modeling and photometric evaluation this paper examines the possibilities of using currently available white LED technology for vehicle headlamps. It is apparent that optimal LED sources for vehicle forward lighting applications will be constructed with hereto undeveloped technology and packaging configurations. However, the intent here in exploring currently available products is to begin the discussion on the design possibilities and significant issues surrounding LEDs in order to aid in the design and development of future LED sources and systems. Considerations such as total light output, physical size, optical control, power consumption, color appearance, and the effects of white LED spectra on glare and peripheral vision are explored. Finally, conclusions of the feasibility of current LED technology being used in these applications and recommendations of technology advancements that may need to occur are made.

An experimental study was conducted to investigate the possible use of light guides as mixing elements for mixed color white LED systems. In this study two types of light guides, one with a square cross section and the other with a circular cross section, were systematically analyzed for color mixing. Past literature suggested that square shaped light guides are better color mixers than circular light guides. This study was comprised of two parts: a computer simulation using a commercial ray tracing software package; and an experimental study verifying the results obtained from the simulation. Beam uniformity, in terms of illuminance and color, did not improve significantly with the light guides. System efficiency dropped as a function of length. The measured results matched the simulation results well. Circular and square light guide geometries showed similar performance, contrary to what was suggested in previous literature. Significant improvement of the illuminance and color uniformity was noted when the output ends of the light guides were diffused. This introduced only a small additional loss (6%) in system efficiency.

Light Emitting Diodes (LEDs) have progressed in recent years from emitting indicator level lighting to emitting enough light for illumination applications. This has opened a new field for LED applications, resulting in significant advantages over conventional light sources and creating some application challenges unique to LEDs. Methods used in the past for packaging LEDs, such as the T 1 3/4 or 5mm through-hole package, were suitable for the indicating lamp industry but have proved poor for high brightness LEDs driven at relatively high power due to temperature rises. Conventional lighting methods provide little guidance for LED thermal problems since these usually involve a very high temperature source, such as a filament or an arc, and radiant heat transfer dissipates the thermal energy. A typical incandescent bulb, for example, radiates 85-95% of the thermal energy away; the remainder is dissipated by conduction to the socket or naturally convected to the atmosphere. LED junction temperatures are limited to much lower values and hence the heat transfer system cannot depend upon radiant energy transfer. This means the cooling methods for lighting now shift from primarily radiation to conduction and natural convection, and this is paradigm shift that lighting designers must recognize when moving to LEDs. In this presentation, the thermal challenges facing LED lighting applications are discussed. Developments in packaging of die into Level 1 products are shown, and the thermal challenges of high brightness LED applications caused by the paradigm shift of heat transfer methods for lighting are discussed.

In modern GaN-based light-emitting diodes (LEDs) structures, total internal reflection (TIR) limits light extraction, and consequently, overall efficiency of the light source. Proper chip and package material combinations as well as surface property modifications offer the opportunity to reduce the luminous flux lost due to TIR and absorption. Different sepa-ration techniques are taking influence on substrate surface properties and thus on light extraction improvement. Imple-menting all these factors in a flexible ray tracing model and applying effective mathematical optimization, helps to refine a chip design in a fast and accurate way to achieve a significant increase of the light extraction. Based on experimental data and ray trace modeling, the effects of chip size scaling, surface roughness and encapsulation on light extraction val-ues will be demonstrated.

Great strides have recently been made in the development of white light emitting diodes (LEDs), although perceptible variations remain in the color and brightness of nominally identical products. The objective of this study was to examine color and brightness discriminability between different white LEDs when used as illuminants of colored and achromatic objects. A method of successive comparisons was used to assess discriminability rather than the more typical simultaneous (side-by-side) comparisons using a "same-different" response protocol. Three-dimensional "tolerance zones" were developed based upon discriminability in a chromaticity (u', v') and luminance when illuminating the colored and achromatic objects. These "tolerance zones" could be used to establish specification tolerances for different lighting applications.

The effect of N₂O plasma treatment on the reverse leakage currents of InGaN/GaN multiple-quantum well (MQW) light-emitting diodes (LEDs) was investigated. The reverse leakage current of MQW LED chip treated with an N₂O plasma was decreased by about 3 orders of magnitude at low reverse voltages compared to that of untreated sample. This could be attributed to the passivation of surface and sidewall damages that were produced by the dry etching process to obtain a reliable pattern transfer. These results suggest that the nonradiative leakage current MQW LED chip can be greatly reduced by N₂O plasma passivation, resulting in an improvement in the performance and reliability of MQW LED chip.

Solid state lighting presents a new paradigm for lighting - controllability. Certain characteristics of the lighting environment can be manipulated, because of the possibility of using multiple LEDs of different emission wavelengths as the illumination source. This will provide a new, versatile, general illumination source due to the ability to vary the spectral power distribution. New effects beyond the visual may be achieved that are not possible with conventional light sources. Illumination has long been the primary function of lighting but as the lighting industry has matured the psychological aspects of lighting have been considered by designers; for example, choosing a particular lighting distribution or color variation in retail applications. The next step in the evolution of light is to consider the physiological effects of lighting that cause biological changes in a person within the environment. This work presents the development of a source that may have important bearing on this area of lighting. A circadian light source has been developed to provide an illumination source that works by modulating its correlated color temperature to mimic the changes in natural daylight through the day. In addition, this source can cause or control physiological effects for a person illuminated by it. The importance of this is seen in the human circadian rhythm's peak response corresponding to blue light at ~460 nm which corresponds to the primary spectral difference in increasing color temperature. The device works by adding blue light to a broadband source or mixing polychromatic light to mimic the variation of color temperature observed for the Planckian Locus on the CIE diagram. This device can have several applications including: a tool for researchers in this area, a general illumination lighting technology, and a light therapy device.

In this work we have developed white-emitting phosphors that can be activated at long UV wavelengths. A blend of EuAlO₃:Eu³⁺ (red), EuAlO₃:Eu²⁺ (green) and Y₂SiO₅:Ce³⁺ (blue), prepared by the combustion synthesis technique, yielded a white luminescence spectrum that can be stimulated with long wavelength UV photons. White emission from rare earth doped single-host yttrium oxyorthosilicates, has been also investigated. (Y_0.9625Ce_0.0075Tb_0.03)₂SiO₅ exhibits an efficient, nearly D₆₅, white emission with chromaticity x=0.225, y=0.320 due to a non-radiative (inductive) energy transfer between Ce³⁺ and Tb³⁺.