For general downlighting, as at the Duke Energy Center in Cincinnati, OH, fixture specifications must account for the inherent directionality of LED sources. Courtesy of Prescolite, Inc.

 

Taking advantage of the directionality and energy efficiency of LEDs, manufacturers of lighting fixtures and systems have identified numerous suitable applications, including some that are ideal for LED sources, and have created fixtures to serve those niches. New LED fixtures for general illumination applications now include direct and indirect products including:

Indoor architectural lighting, including downlights, wall washers, accent lights, cove lighting, pendants, wall sconces, desk and workstation task lamps.

Outdoor decorative and architectural lighting, including landscape and path lighting, floodlighting, area and roadway fixtures, wall-mounted and step-mounted products, as well as decorative pole-mounted and canopy-mounted models.

LED light sources require unique attributes in fixture designs, including special componentry and materials, as well as installed performance. Two examples-one for interior and one for exterior lighting-illustrate these special needs.

LED downlights. The LED downlight provides a useful example of these special considerations in fixture design and performance, characteristics that merit close attention by architectural designers and specifiers. For example, for interior downlighting, most SSL products group together multiple LEDs into clusters. Set within a relatively small downlight package, the LEDs create significant heat. Effective thermal management within the fixture is critical for maintaining light output. This is especially important in interior spaces with insulated ceilings. If the heat is not adequately handled, the rising temperature of the LED device will cut light output, as well as the effective life of the fixture.

In terms of lighting system design, LED downlight products vary significantly in terms of delivered light output. Some models deliver only about half the light output of downlights using 65W R incandescent or 15W reflector CFLs, according to the DOE. This may mean that the designer includes more downlights on the reflected ceiling plan, or looks for more efficacious LED sources. LED sources and fixtures are improving rapidly, however, and several new models already compete favorably with traditional light sources for interior downlighting applications.

As far as energy efficiency and lumens-per-watt efficacy, LED downlights already meet or exceed such rigorous standards as California's Title 24 and EPA's Energy Star program. Luminaires may be rated for standard efficacy (typically 25-35 lumens/W) or high efficacy (typically 40-55 lumens/W or greater). In some cases, the fixtures contribute to the certification of buildings or commercial interiors by the U.S. Green Building Council's LEED® rating system.

In terms of building operations, many fixtures feature LED boards and power supplies that are easy to replace with a screwdriver and other common tools.

Last November, the DOE concluded that "recessed downlighting is potentially a good application for LEDs," as long as the specifier keeps in mind the light output of the selected fixture and heat management of the installed product."

Other interior applications were previously considered unsuitable for LEDs, but new fixtures now allow for the effective use of SSL. For example, wall-washing applications for LED fixtures were not possible due to the source's inherent directional output, which resulted in unwanted unevenness and "hot spots." Now, however, advanced LED arrays and new diode construction allow for broader, more uniform beam spreads with the color rendering and reliability required for wall washing. This is true of luminaires for both interior and outdoor applications.

LED outdoor fixtures. The use of SSL systems for outdoor applications also presents areas of special consideration for the architect, engineer, landscape architect and lighting designer. Because of the LED's long life, they are suited especially well for difficult-to-service outdoor applications. White and colored LEDs can also be interchanged among fixtures for creativity and impact in the landscape and building exterior lighting design. Compared with other lamp sources, LEDs may incur a higher up-front cost, so high-quality fixtures and LEDs with long-rated life spans and high lumen maintenance should be selected.

Accepted outdoor lighting techniques suggest smaller is better, as long as performance is not compromised. Compact LED arrays that generate comparable lumens per watt compared to traditional sources have been easily engineered into smaller luminaire envelopes that are ideal for landscape and structural illumination. A number of fixtures on the market match LEDs with relatively small luminaires designed for use in aisle, in-grade, accent, wall-wash and path applications.

Regardless of the light source, nighttime outdoor optics are different from indoor optics. The human eye adjusts to the dark by using more of the eye's rods, which are monochromatic and allow us to see even in dim light levels (scotopic vision), rather than the cones, which let the human eye perceive color (photopic vision). Rods contain a pigment that perceives light only in the green part of the spectrum, while cones can detect red, green and blue areas of the spectrum and distinguish very fine wavelengths. Because scotopic vision is dominant at night, a metal halide light will appear markedly whiter than a sulfur lamp to the human eye. This change in perception occurs in the human eye and cannot be measured, although the Illuminating Engineering Society (IES) is working on a method for measuring this effect.

In addition, because outdoor nighttime lighting is often projected toward the sky and distant background objects in addition to illuminating target objects, it is best measured by candlepower (candela), which measures how much light is produced by the source LED, rather than illuminance, or how much light falls on a target object. Forpathway and egress lighting, for example, choosing an effective source with a suitable beam spread is important to ensuring wayfinding clarity and safety. Good specifications for egress lighting include broad beam spreads and warm colors, two characteristics that have been made possible only in the latest generations of LEDs.

Outdoor lighting designs should also feature consistent and reliablein-grade fixtures, which tend to be hard to access and maintain. High-profile placement LEDs have been found to work well as ground-mounted up-lights, offering 50,000 hours or more of lamp life at 70 percent lumen maintenance. The low heat output and durability of recent fixture designs have combined with the power needed to illuminate monuments, trees and architectural elements just as well as traditional sources, whether in spot or narrow flood distributions.

In outdoor applications, the main advantage of the new LED technology is energy efficiency, as well as long life of the light source. These characteristics make LEDs appealing to building owners and managers, a fact that underlies the increasing popularity of LED applications for general illuminating.