Jake JacobsenOne of the biggest barriers to mass adoption of solid-state lighting is the large installed base of fixtures designed for what is a fundamentally different source. LEDs differ optically, electrically and thermally from traditional sources.

LEDs are much more directional than incandescent or fluorescent sources, and have a much smaller étendue than either, especially the fluorescents. Étendue characterizes the spread of the light in both area and angle. It can be thought of as the area of the beam at any given location multiplied by the angular spread of the beam at that location. It is a conserved quantity throughout an optical system. It can be increased, through scattering, but can never be decreased. Thus starting with a small étendue source allows lighting designers much greater flexibility in the design of fixtures and gives them the ability to put light only where it’s needed. Conversely, the directional nature of LEDs makes it more difficult to create an omni-directional fixture of the type that is very simple to design using traditional sources (e.g., a table lamp).

In order for designers to be really successful in displacing traditional lighting with solid-state lighting, they must develop fixtures that are specifically designed for LEDs, rather than a simple retrofit of a traditional fixture. This is not going to be easy, especially for lighting designed for home use. Walk into almost any lighting store in the country and you will likely see that most of the fixtures look like they were designed 120 years ago for candles or kerosene lamps. Incandescent bulbs were a good replacement for open flame because their étendue structure is quite similar, but LEDs are decidedly not.

Deliberate, bold designs that take advantage of the lower étendue of LEDs to shape the light can produce striking results that create better, more attractive and productive environments at a lower total cost. Such designs require skill on the part of the designer and effective optical design software for modeling and optimizing the performance of the fixture prior to developing prototypes.

Traditional commercial and civic outdoor lighting, and especially street lighting, is often quite egregious in its waste of light. One of the worst offenders is the traditional acorn fixture. Much, if not most, of the light from these fixtures is thrown everywhere but where you actually want it: on the ground near the fixture. This is the optical equivalent of watering your driveway. Yet companies are offering LED retrofit bulbs to put in these acorn fixtures. While this is laudable in that it makes the waste less costly, how much better would it be, and how much more compelling for the consumer, to design fixtures deliberately for solid-state lighting? This would eliminate the waste altogether and put the light only where it’s really needed.

The advantages of solid-state lighting are manifest and manifold: higher efficiency, longer life and greater design flexibility. But the installed fixture base is designed for other kinds of sources. That traditional base is a fact that isn’t going away anytime soon. However, the adoption of LED-based lighting should be accelerated by deliberate optical design and the rethinking of how we deploy that lighting in a way that best takes advantage of this new source.

For more information on Synopsys' LED design technology, visithttp://www.synopsys.com/Tools/OpticalDesign/Pages/default.aspx