Lighting community outlines challenges for LED industry

Lighting designers are eager to increase their adoption of LED technology, but are looking for device suppliers to remove some of the hurdles, writes Tim Whitaker.

LEDs in architecture

While LED manufacturers concentrate on improving the brightness and efficacy of their devices, many designers and specifiers in the lighting community remain unsure of how to adopt this new technology in their products and projects. This discontinuity was highlighted at the "LEDs: Meeting the Design and Performance Challenges" conference, which was held in London, UK, in late January. Organized by Lighting Equipment News magazine and the UK-based Lighting Industry Federation, the one-day event was slanted more towards end-users in the lighting industry than device manufacturers, although speakers from Lumileds and Osram Opto Semiconductors represented the latter group.

Challenges and benefits

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 h (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 h. 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.

Metrics and standards

Kevin Dowling, vice-president of strategy and technology at the specialist LED lighting company Color Kinetics, described a metric for comparing the relative costs of different lighting technologies. The "cost of light" metric encompasses not only the cost of the light source and fixtures, but also the cost of maintenance and electricity over the lifetime of the light source. Although incandescent lamps have a very low cost and high lumen output compared with LEDs, the LED source has a much longer lifetime and consumes far less power. In fact, typical LEDs already have a lower "cost of light" than incandescent and halogen sources, according to Dowling, although this is not widely appreciated. "Most Color Kinetics installations are not driven by energy savings, although the benefits are still there," he said.

Speakers and panelists kept returning to the issue of standards. Richard Hall of Thorn Lighting commented: "There is a very high need for standardization to allow luminaire [lighting fixture] manufacturers to manage their customers' expectations." In fact, a number of standards are being developed by the CIE, the International Commission on Illumination, relating to areas such as the measurement of LED flux, color rendering index, and other properties. In some respects, LEDs will need to fit in with existing standards. "There are a number of internationally recognized standards, for example covering transport-related lighting applications, and LEDs will have to adapt themselves to fit around these," said Jonathan David. With regard to photobiological safety, Telectra's Oliver pointed out that the regulations covering laser eye safety also cover LEDs.

Color variation

One issue raised frequently by lighting designers is the variation in color within batches of white LEDs. The human eye is highly sensitive to differences in hue, and can also perceive differences in intensity as a difference in color. The eye is particularly sensitive to edges, for example where two beams of slightly different color tints are projected side by side onto a wall.

Gordon Routledge of UK company Lumidrives described some of the challenges of using white LEDs for illumination, starting with product binning. Older binning methods tend to use correlated color temperature (CCT), but LEDs with the same CCT can have a different color tint depending on their position relative to the blackbody locus. Suppliers have now started to use new binning strategies that reduce the variability of LEDs within each bin, but this remains an issue.

Routledge pointed out that the problem is less acute on relatively small projects requiring small numbers of LEDs, which can all be obtained from the same bin, and in applications such as edge illumination, where mixing effectively averages out product variations. In large-scale applications, such as attempting to produce a consistent color for signage for a particular brand that will be seen in many locations, color matching can be a major challenge. The problems are most acute in applications where the LEDs are viewed directly, side by side without mixing.

Color Kinetics' Dowling emphasized the problem of color variation. "For those of us buying large quantities of LEDs, it's very frustrating not to be able to buy all the devices from the same bin," he said. LED manufacturers require constructive criticism on this subject, as Keith Scott from Lumileds pointed out. "LED makers need constant feedback from their customers regarding binning requirements, otherwise they will produce bins that work best for them in terms of maintaining high yield," he said.

Thermal management

Effective heat sinking is a key factor in ensuring stable LED performance over a long lifetime. The LED junction temperature influences the luminous flux of the device, its lifetime, wavelength and efficacy. As described by James Hooker of Lighting Equipment News, the junction temperature is influenced by the drive current, the provision of heat sinking, and the ambient temperature. LEDs are generally rated in terms of their output at a junction temperature of 25 ºC, although several people questioned why this was the case when that value was unlikely ever to be attained in practice. The actual junction temperature can be calculated by adding the temperature of the mounting board to the product of the board's thermal resistance and the wattage of the LED.

Hooker gave a practical demonstration of the effect of temperature on LED brightness, using Luxeon devices from Lumileds. A few minutes after switching on the LEDs the package became hot to the touch and the intensity of the light output dropped appreciably. When the LED package was taped to a large aluminum plate that acted as a heat sink, the light output returned to its original level.

Packaging improvements

LED fountain

Colin Beale of Osram Opto Semiconductors gave an historical perspective of his company's progress in developing high-brightness LEDs. There has been steady progress in the efficacy of LEDs (measured in lm/W), with the average value across the range of different colors roughly doubling every two years. However, much more rapid progress has been made in the total luminous output (measured in lumens) - this has increased by at least a factor of 100 in the past eight years. This can be largely attributed to the development of advanced packages such as Osram's 1.5 W Golden Dragon, which enables white LEDs with a total flux of 34 lm.

Crucially, although the costs of similar types of LED have remained roughly constant over the past two years, improvements in lumen output have halved the cost per lumen. "I urge everyone to look at the cost per lumen when sourcing LEDs," said Beale.

Osram Opto produced more than 4.5 billion LEDs in 2002-2003, and has a total capacity of 11 billion chips per year at its recently opened facility in Regensburg, Germany. The company is particularly successful in the automotive market, having supplied LEDs for interior lighting in many Volkswagen, BMW, Audi and Rover cars. In fact, Beale estimated that around 2.5 billion Osram LEDs are being driven around in European cars at present.

One very interesting automotive application is in head-up displays (HUDs), which project information such as speed onto the windscreen. Monochrome HUDs have been used in General Motors vehicles for a number of years, and some of these now use green LEDs for higher brightness. BMW recently unveiled the first full-color HUD, built using Osram LEDs, which will be offered as an option on its 5 Series cars.

Attendees at the London conference saw a whole host of other applications for LEDs, exploiting the particular benefits of these devices compared with other light sources. This seems to be crucial for the continued success of LEDs in lighting, rather than simply trying to replace existing light sources. As Kevin Dowling said, lighting designers should "think outside the bulb culture" - and at the same time, LED manufacturers have a lot of work to do to enable the transition to the new lighting technology.

About the author

Tim Whitaker is the consulting editor of Compound Semiconductor