including recessed cans, where the directional quality of the emitted light is important, and roadway lighting, in which a premium is placed on durability and low maintenance. In 2010 LED luminaires had achieved a 4.3 percent penetration in this latter application. SSL products for downlighting have efficacies of 35 to 85 lm/W compared to 10 to 30 lm/W for fluorescent and halogen luminaires in the same application.
Role of Standards and Testing
Because of the different spectral, electrical, and thermal characteristics of LEDs, OLEDs, and SSL products, existing standards to measure the photometric properties (i.e., measures of perceived light intensity) and colorimetric properties (i.e., measures of perceived color characteristics) of other lighting technologies frequently cannot effectively be employed. A number of standards development organizations are involved in recommending test procedures for the measurement of LEDs, OLEDs, and SSL products.4 The United States has taken early leadership on several influential standards, such as IES LM-79-08 “Electrical and Photometric Measurements of Solid-State Lighting Products,” which specifies the procedures for measuring total luminous flux, electrical power, luminous efficacy, and chromaticity of SSL lamps and luminaires. Despite rapid progress, a number of important test and measurement standards still need to be developed for SSL to be successful. For example, there is currently no way to measure or estimate the lifetime of SSL luminaires.
The committee found that cost is the biggest obstacle to the widespread deployment of SSL based on LEDs. The high cost relative to conventional light sources is due to a combination of costs associated with the LED device, heat sink, electronics, and packaging, each of which is the subject of substantial R&D activity. All categories of cost will need to be addressed along the value chain to improve the value proposition of higher-quality light, longer product life, and overall lower life-cycle cost compared to current lighting products on the market. Thermal management is particularly challenging because the LED chip must be kept at a temperature below 200°C. The small size of the chip means that even a watt or two of dissipation will raise its temperature well beyond this limit if adequate heat sinking is not provided.
Cost of the LED device is primarily driven by two issues: (1) the mismatch in thermal expansion between the sapphire substrate on which the LED is grown5 and the nitride LED material, resulting in thermal stresses that decrease device yield; and (2) because of process variability, the variability in the emission characteristics (color) among individual LEDs, which necessitates they be sorted (i.e., “binned”) and grouped for consistency. The efficacy of the LED is limited by both physical mechanisms within the semiconductor material and the limited ability to access light trapped in the substrate and emissive layers (i.e., improved outcoupling). Increasing efficacy not only improves energy savings, but also has a strong leveraging effect on the cost of LED lamps and luminaires because, as less heat is generated, smaller and less complicated thermal management and packaging systems are required. The committee recommends that the Department of Energy continue to make investments in LED core technology, aimed at increasing yields, and in fundamental emitter research to increase efficacy, including improvements in the controlled growth and performance of the emitter material.
In addition to cost, consumer acceptance of SSL will depend on an understanding of its unique characteristics and the new vernacular used to specify it. To this end DOE has created the Lighting Facts label for SSL lamps, which provides specifications for luminous output (lumens), power (watts), efficacy (lumens per watt), color temperature, and color rendering index (CRI). The Environmental Protection Agency (EPA) and ENERGY STAR® program are also engaged in developing informative labeling for SSL products. But consumers must be made aware of the significance of label parameters, and to this end EISA 2007 authorized $10 million a year to advance public awareness. This money has yet to be appropriated. The committee recommends that DOE and lamp manufacturers and retailers work together to ensure that consumers are educated about the characteristics and metrics of these new technology options.
Poor experience with spiral CFL lamps has made consumers skeptical of new lighting technologies. But unlike spiral CFLs, SSL turns on to full brightness instantly, is unaffected by low temperatures, has good color quality, and is inherently dimmable with properly designed lighting controls. However, a number of SSL performance characteristics may jeopardize consumer acceptance if not addressed. The most significant of these is the incompatibility of SSL lamps with many existing dimming controls, precluding a simple SSL retrofit, particularly in residential applications. Although unlike CFLs LEDs are in principal
4 These include, but are not limited to, the following: the Illuminating Engineering Society of North America (IES or IESNA), a professional organization; the International Electrotechnical Commission (IEC), an international consensus standards organization for electrotechnology; the International Commission on Illumination, an international standards body; the National Electrical Manufacturers Association; and the Underwriters Laboratory, which sets safety standards. The American National Standards Institute also provides accreditation and serves as the U.S. national member organization to the IEC.
5 Some devices are grown on a silicon carbide (SiC) substrate, which some manufacturers believe to be a better, albeit more expensive, alternative.