Solid state lighting (SSL) is the general descriptor for forms of illumination in which light is “emitted from a solid object - a block of semiconductor - rather than from a vacuum or gas tube, as in the case of incandescent and fluorescent lighting,” (as defined by Sandia National Laboratories, a DOE-sponsored research lab). SSL presently comes in two flavors: inorganic LEDs and organic light-emitting diodes (OLEDs). We’ve all seen LEDs, which are typically smaller than a pea. OLEDs, however, may be ‘painted’ onto flat areas, creating luminous surfaces. While OLED applications are still in the development stage, LEDs are all around us in instrument lights, exit signs, traffic lights, decorative and novelty lighting, automotive tail lights, and a variety of other uses.
LED 101LEDs offer great promise for general illumination as well as specialty lighting:
- Lifetimes of 50,000 to 100,000 hrs
- Extremely sturdy
- Potentially very efficient (but not yet)
- May be fitted into many shapes
- Color can be varied over a wide range
- Some may be dimmed
- No mercury or lead disposal issues
The technology is developing rapidly, and it is supported by both industry and government. Find a good FAQ at www.sandia.gov/lighting/XlightingoverviewFAQ.htm.
For those familiar with lighting quality indicators, white LEDs shine at about 3,000 K (a slightly ‘warm’ color) and offer a color rendering index (CRI) up to about 70 (roughly as good as low quality T8 fluorescent lamps). Present efficacy (the amount of light emitted per Watt of power input) is almost double that of incandescent, but only about a third that of T8 fluorescents on electronic ballasts. There’s hope, however, that efficacy may be boosted to equal (or possibly double) that of fluorescents down the road.
Recently, we have begun to see tightly packed white LEDs inside standard light fixtures such as over-the-counter, task, and downlights. Some designs are innovative, but a few have come under scrutiny for failing to provide claimed illuminating ability.
All Not Rosy in LED landAs with some early compact fluorescent lamps (CFL) and fixtures, several LED units presently being sold do not provide the light output claimed and may have other problems. A recent study by a DOE lab of a LED task light, an over-the-counter strip light, and two retrofit downlight kits showed some disappointing results. The summary of the study (published December 2006) is available at www.netl.doe.gov/ssl/PDFs/CPTP%20Pilot%20Testing%20Results%20Summary--draft-12-06-06.pdf. The four lab test results may be requested (there is no charge) at http://www.netl.doe.gov/ssl/comm_testing_request.htm.
In the tests, fixture output was disappointing with luminaire efficacy (i.e., the amount of light escaping the fixture divided by watts going in) significantly less than many commonly available CFLs pulling the same or less power. Part of the problem lies in the difficulty in concentrating LED light output in commonly shaped fixtures. In a typical down light, for example, the ability to concentrate light is related to the ratio of reflector size to that of the light source size. Light emitted from a small incandescent filament located at the focal point of an 8-in. parabolic reflector is easily concentrated, but trying to do the same with light coming from a 3- by 3-in. flat plate of LED point sources is another story. The same lesson was learned with early CFL PAR lamps and track lights that performed miserably, disillusioning many to that technology for nearly a decade.
One surprising finding of the DOE tests was that light output dropped (in one case) by over 20% as the fixtures warmed up after about two hours. Many lighting efficiency experts are accustomed to letting fluorescent and HID fixtures warm up for five to 15 minutes before taking light readings, but few expect to wait two hours to get an accurate result.