In recent years, the lighting market has seen new general lighting products, introduced for niche applications, become mainstream. As a result, it is increasingly important for electrical contractors to understand the basics of light-emitting diodes (LEDs) to successfully work with this rapidly growing lighting technology.
While LEDs must be considered as another light source that can get a lighting job done, the technology is unique, even if it comes in familiar packaging. LEDs are both different than and, in some cases, similar to traditional light sources.
With conventional sources, electric energy is converted to light by heating a filament (incandescence) or passing current to excite a gas (fluorescence and high-intensity discharge). With LEDs, current is passed through crystalline solids (which makes LEDs a solid-state light source) to produce visible light. This yields products that are more robust, impervious to vibration and immune to extreme cold.
LED devices used in architectural light fixtures typically are high-output packages that contain diodes (the LEDs themselves) and some form of encapsulated enclosure mounted on a base to facilitate mounting to circuit boards and heat sinks. High-output LED devices can produce as much as 250 lumens in packages measuring just 7-by-9 mm.
Traditionally, luminaires and components are separate items built to widely accepted standards. Items from different manufacturers can be mixed and matched. In contrast, LED luminaires usually are integrated systems consisting of the LED device, optics, a heat sink, drive electronics and a light fixture. The “lamp” is integral to the system and cannot be removed or replaced.
LED product performance is dependent on the environment. The only valid performance data for LED products, therefore, is based on total fixture performance, not the base LED output. This is true of conventional sources as well, where fixture efficiency often presents a significant loss of performance. With LEDs, it is even more critical.
Colored LEDs emit narrow bands on the color spectrum, which makes them ideal for generating saturated colors. The combination of red, green and blue (RGB) LEDs can be mixed to create white light, although the quality is questionable. Most white-light LED products use phosphor-coated blue or ultraviolet LEDs to create white light the same way fluorescent lamps do, with similar results in both color quality and selection.
Like traditional light sources, the color temperature (color appearance or tone of the light source and the light emitted) of LEDs can be specified as warm white (2,800K–3,000K) or cool white (4,000+K) but with several differences. As with fluorescent sources, warm-white phosphor--coated LEDs typically have a lower efficacy than cool-white. It is likely that a given light fixture will have good LED-to-LED color consistency, but there still may be slight differences in color temperature between fixtures, which may or may not be noticeable (more when fixtures are very close to each other). An expert on LEDs explained that one can expect to see color variations between individual LED fixtures similar to that experienced with today’s compact fluorescent lamps, making an interesting, larger point that we must be careful not to hold LEDs to a perfect performance standard.
Regarding color rendering, a color rendering index (CRI) rating of greater than 80 is recommended for commercial applications, which is now available in products using quality LEDs. In fact, CRI ratings as high as 92 are now available and will continue to improve with the technology. CRI ratings of RGB LED systems, however, can be as low as 13 CRI.
There are arguments that CRI is an imperfect metric, particularly when it comes to LEDs and especially with RGB. When in doubt, test drive the light source.
While LED light contains no direct heat or ultraviolet light, heat is still an issue. LEDs generate as much heat as any other source with one large difference: all of the heat is emitted from the back of the device and must be removed, or light output, efficiency, and useful life of the LEDs and related electronic devices will be degraded. This requires the LED light fixture to contain significant thermal-management features. Products without apparent and significant components to extract heat from the LEDs should be avoided.
Like conventional sources, LED devices suffer from lumen depreciation over their life. In fact, being an electronic device means failure modes are different from standard lamps, which burn out long before their decline in light output becomes an issue. LEDs may last as long as 100,000 hours before failure, but at some point, the amount of light being produced is so low that replacement is necessary. Because of this, service life of an LED product is rated at the time in hours of operation where the product is producing 70 percent lumen maintenance (about 15,000-50,000 hours for an LED device, depending on whether the LEDs are overdriven and by how much) for general lighting applications and 50 percent lumen maintenance for decorative and other noncritical applications (up to 70,000 hours in well-designed products).
Owners get a double maintenance benefit from LEDs: long service life plus no spot replacement of failed lamps. The downside is keeping track of when to change LEDs. In other words, they won’t have an obvious sign, such as a failed lamp.
With the right driver, LEDs can dim from 100 to 0 percent with no loss in efficiency and no negative effect on service life; in fact, it may extend it. Note, however, that line-voltage incandescent dimming controls and LED products must be rated as compatible, or the LEDs may not operate properly or become damaged by current spikes. Even with a compatibility rating, there may be an element of risk, as one LED expert explained to me. All the driver manufacturer can do is test third-party dimmer products, convince themselves that the test devices are representative of the product line, and, in the absence of a written agreement, pray the dimmer manufacturer will not change its product design in the future.
LED drivers are available that are compatible with commercially available 0–10V DC control devices and systems, such as occupancy sensors, photosensors, theatrical controls, etc. LEDs also can work with devices governed by the DMX and Digital Adressable Lighting Interface protocols. The range of effects is extraordinary. You can do static or dynamic effects, including reactive and interactive walls and other surfaces, and if dynamic, you can control the sequence, transitions and schedule. However, be aware of the type of controls offered, and make sure all products are compatible with the controls being used.
LEDs are increasingly common and will continue to grow as technological advancement opens new commercial lighting applications. However, LEDs can be somewhat complex to design and specify, so electrical contractors who make an effort to become familiar with this potentially transformative technology will be more likely to capitalize on it, including avoiding costly errors and field problems.
For more information, go to the U.S. Department of Energy’s Web site, which includes eye-opening independent product testing reports and a series of “Using LEDs for General Illumination” publications.
DILOUIE, a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at www.zinginc.com.