Solid-state lighting (SSL) technology is one of the fastest-evolving illumination sources. Use of light emitting diodes (LEDs) is well-established in a variety of applications that many people take for granted: automobile interiors and dashboards, cellular phone keypads and screens, televisions, computers, and other electronic devices, as well as theatrical stage lighting, street lighting, and architectural lighting.
Lately, the SSL industry has been focused on developing LED lamps and luminaires for general lighting applications in buildings as building owners and occupants grow increasingly aware of and concerned about energy use and costs and seek more efficient solutions. In the past five years alone the technology has progressed so rapidly that architects and lighting designers are tested to constantly keep up with all the latest developments. The challenge for the SSL industry lies in developing general-use LED luminaires and lamps that will deliver high-efficacy, high-quality (warm) white light at a cost-effective price point.
COLOR QUALITY, COST, AND EFFICACY Currently, the SSL industry is working toward producing lower-cost, daylight-equivalent white light LED sources that exhibit minimal color shift and degradation, longer life, and greater light output for general use. While the costs of LED sources are decreasing continually, the price for white light is not yet low enough to make LED lamps or luminaires cost-effective for most applications. “Step one is to get the cost down so there's not such a huge gap between incandescent, fluorescent, and LED,” according to Kraig Kasler, vice president of product management and marketing for GE Illumination.
Lighting quality is an equally important aspect from the designer's viewpoint, but as Kasler points out, even if high-quality LED light becomes available, few designers will be able to use it unless costs also have fallen to acceptable levels.
On the other hand, Kathy Abernathy of North Providence, Rhode Island-based Abernathy Lighting Design and chair of the International Association of Lighting Designers' (IALD) Energy and Sustainability Committee, points out that using LED luminaires, even in limited applications, allows peripheral savings in HVAC load and costs because of their minimal heat output, which is attractive both to designers and their clients.
Delivering the warm-white light end-users are accustomed to receiving from incandescent lamps at high enough outputs, combined with low color temperature variation and high efficacy has been a big challenge. While cool-white LEDs provide stable, efficient light output, warm-white LEDs have greater color instability and color variation, and lower efficacies.
Warm-white LED technology is still new, and manufacturers of luminaires and LED chips still have a lot of work ahead of them. Because LED light is not naturally white, developers have had to find ways to create white light, and the two main processes available deliver different types of white light with different benefits and drawbacks. Applying different phosphor coatings is one method, but certain phosphors decrease LED efficacy. Color-mixing RGB systems are another method, but because each LED color degrades at different rates in application, color quality and efficacy are inconsistent. Some lighting companies are experimenting with different phosphors to discover which can deliver the best mix of desired light color with high efficacy, while others are developing better RGB color-mixing systems.
GE Lighting's Lumination VIO high-power LED uses a proprietary chip and phosphor system to effect a color shift of less than 100K within the overall Kelvin scale over a 50,000-hour rated life, which the manufacturer says also overcomes color control issues and provides high efficiency at warmer color temperatures. Philips Lumileds has developed a new phosphor technology, Lumiramic, that allows specific colors and temperatures of white LEDs to be produced. The company plans to introduce Luxeon products incorporating Lumiramic phosphor technology early in 2008. Ledon's Tempura LED spotlight, utilizing Philips TIR Systems' Lexel RGB color-mixing technology, allows the color temperature of the light to be adjusted from 2500K to 6500K while providing a constant luminous flux of 1000 lumens and consuming between 40 and 75 watts.
Heat in application also affects LED performance. When too much heat is generated in an LED fixture, the lamp's lumen output and useful life decrease. Managing heat in the fixture and understanding the environmental conditions of the lighting application are critical to the success of the LED product. Manufacturers use conductive materials to create a heat sink in their LED products that pull heat away from the light source; the better the heat sink, the more stable the LED's lumen output, color, and life.
Both higher efficiencies and lower costs will deliver the payback that designers, end-users, and building owners are looking for. According to Kevin Dowling, vice president of technical innovation for Philips Solid-State Lighting Solutions, consistency, quality, and amount of light output in warm-white LEDs have all improved dramatically in the past 18 months.