Although there are many benefits associated with using LEDs for lighting applications, there are also many issues to resolve. Factors such as cost and luminous efficacy, and the lumens per dollar figure-of-merit, are always mentioned at LED conferences, but other factors can be equally important to lighting designers. These include standards, lifetime and reliability, color variations over time and from batch to batch, what types of fixtures to use with LEDs... the list goes on.

Take lifetime, which is routinely quoted as one of the main benefits of using LEDs. Lifetimes of 100,000 hours (more than 11 years) are often mentioned, which effectively means that the LED is unlikely to fail, unlike an incandescent lamp. However, in reality, the lumen output of an LED degrades over time, to an extent determined by many factors including its color, the operating conditions and the manufacturer. As pointed out by Jonathan David, secretary of the Society of Light and Lighting, the effect of this lumen depreciation depends on the application. "It can be relatively unimportant for decorative lighting, where the changes go largely unnoticed, but very important for task lighting," he said.

At present, there is no standard definition of lifetime. Keith Scott, market development manager with Lumileds, says that his company specifies its white and colored Luxeon LEDs as having an average lumen maintenance of 70% (i.e. the lumen output drops by 30%) at 50,000 hours. This is a useful starting point, although this level of degradation is unsuitable for some applications. (It should be pointed out that other light sources also experience lumen degradation.) Conversely, some applications simply don't require such long lifetimes.

Carsten Schaffarz, manager of product management and marketing with Vossloh-Schwabe, identified the lack of standardization in the LED industry as a problem for companies trying to evaluate the cost benefit of using LEDs rather than other light sources. "There are few standards, and performance varies from one manufacturer to another," he said. "The cost benefit is difficult to define and it is down to the end-user to undertake the final evaluation." LEDs will only be used to replace existing light sources if the advantages are clear and if higher costs are justifiable.

Fortunately, acceptable benefits can be psychological as well as economic, especially in Europe, where in general more value is placed on light quality. An example was given by Alan Oliver, sales and marketing director for Telectra, a company that has installed lighting systems in aircraft for airlines including Virgin Atlantic and KLM. The principal advantages of LED-based lights in this application are ease of maintenance, safety, and the ability to adjust the appearance of the light (for example color and intensity) to maximize passenger comfort.