SAN JOSE, CA-As the nascent high-brightness LED industry chases an emerging market for what Steve Landau, worldwide marketing communications manager at Philips Lumileds, describes as “high fidelity lighting,” the development of industry-wide standards is likely to play an important role. Currently, there is no standard industry-wide definition of what constitutes a high-brightness LED, he said.
Landau says that 350 mA at 1 W is generally considered a “sweet spot” that defines high-power LEDs. But those numbers are not grounded in principles of physics or the nature of light. Lumileds “made them up”, based on what the company was capable of six years ago, when they introduced the first high-brightness Luxeon LED. They drove the Luxeon I at 350 mA to get 1.23 Watts of output. “If you drive the same device at lower current, you’ll get less than 1W out, however, and if you drive it at higher current, you’ll get more than 1 W out,” he said. So simply saying 1W, 3W or 5W does not define LED performance. Lumileds recently redefined its own standard with the introduction of Luxeon K2, which can be driven at currents up to 1500 mA, due to a maximum junction temperature that has jumped to 185ºC (depending on color) over a previous maximum of 135ºC.
Even through brighter illumination becomes possible, the actual objective is broader engineering latitude. For instance, the same level of brightness can be obtained from 6 K2s mounted on an FR4 circuit board, without the need for heat-sinking and driven at a relatively low current to maximize efficiency, as can be obtained from two K2s driven at high current, Landau said. The latter solution would cost less, because it uses fewer LEDs. It would also require heat sinking and provide fewer lumens per Watt. So it often becomes useful to think of LED illumination in terms of luminance desired for a particular application and of system approaches to providing the desired illumination, rather than single device specifications, he said. For instance, efficiency or lumens/Watt may be higher at lower current, but an application, such as the power LED headlights that will begin appearing in high-end automobiles next year, may require simply maximizing lumens for higher light output. For projection applications on the other hand, the key parameter is controlled distribution of illumination, or lumens/mm2.
Some applications call for simultaneous optimization of lumens/Watt for efficiency; lumens/$ for cost; and lumens/mm2 for illumination control, according to Robert Steele, director of optoelectronics at Strategies Unlimited (Mountain View, CA), which Landau described as probably the closest thing to an industry association for makers of high-brightness LEDs.
Steele listed five categories that Strategies Unlimited uses to classify high-brightness LEDs, and even that categorization remains dynamic because four of them have been around for a while and the fifth is just emerging. Also, while many factors go into the categorization, chip size actually seems to be one of the most important.
The first category, which Steele called “standard”, consists of chips with surface dimensions on the order of 5 mm x 5mm that enable them to be driven by currents of up to 20 mA. A second category, which Steele called “high current”, consists of standard-sized chips within thermally conductive packaging that enable them to be driven up to current levels on the order of 150 mA. A third category, which Steele called “high power”, consists of larger chips (such as the Luxeon line from Lumileds) that can be therefore driven at higher currents and at power levels that tend to range from 1/2 to 5 W.
The 5 W figure, as with the 350 mA and 1 W figures that Landau mentioned, represents available technology rather than physical limits, however. “The top end might actually be 10 W, or there may not be a top end,” Steele said. A fourth category consists of multiple-chip combinations of red, green and blue chips for producing white light. And a fifth “just emerging” category consists of multiple-chip packages in a single color (usually white) in which manufacturers package numerous small chips together to obtain both high brightness and high efficiency. “Efficiency in excess of 60 lumens/W has been achieved for cool white illumination, and lumens/W in the low 50s have been achieved for warm white,” Steele said. “Of course, the drawbacks are that it is harder to gather, collect and focus the light with multi-chip collections.” So very high luminance applications would require packaging such small chips very tightly together, presenting additional engineering challenges.
Of course one engineer’s challenge, might well be viewed as another engineer’s latitude, particularly in an industry that has set itself the goal of providing hi-fidelity lighting that allows users to essentially “dial in” an optimal quality and quantity of light for a specific application. Commenting on the proliferation of approaches to illumination that one currently finds in the nascent LED industry, Steele said, “There are a number of tradeoffs to be made depending on demands of a particular user application.”
-Hassaun A. Jones-Bey