This has very important implications for an LED’s lifetime. Maintaining an LED die’s temperature at 60°C allows it to emit at least 70% of its initial light output after 100,000 hours, an uncommon temperature for most high-brightness lighting applications. And if the die’s temperature reaches 100°C, that lifetime is cut down to just 30,000 hours or less. If anything, it’s more common for a high-brightness LED’s die to reach 120°C.
The type of package used, the materials and type of construction of the light source, and the heatsinking employed can influence how heat is managed and the light source’s lifetime. Fortunately, new materials like Honeywell’s LTM6300-SP are coming to the rescue. According to the company, this family of phase-changing thermal-management materials improves LED lifetimes by changing phase during use to more efficiently transfer heat away from the device. It replaces silicone-based pastes, which pump out and degrade at high temperatures, with a polymer matrix containing thermally conductive metals.
“This material enhances the performance of energy-efficient LEDs, extending their lifetimes and making sure their colors do not become muted due to overheating,” says Brian Daniels, CTO for Honeywell’s Electronic Materials sector.
The LED industry’s hype about 100,000- hour lifetimes is a bit exaggerated. Technically speaking, an LED might be able to emit light for 100,000 hours. However, its output would become so degraded after such time that it would not be considered a useful light source, especially for highbrightness applications.
“The technology for 100,000-hour lifetimes for high-brightness LEDs does not exist,” says Pat Goodman, an application engineer at Philips Lumiled Lighting Co. “We have published what is probably the longest database on lifetime data and it only goes out to 9000 hours.”
Nevertheless, the U.S. Department of Energy (DoE) is providing incentives for LED lighting improvements. Its Bright Tomorrow Lighting Competition will award a $10 million “L Prize” for the best LED replacement of a common 60-W A-19 incandescent bulb (see “$10 Million Light Bulb Prize Remains Unfunded” and “Beyond The $10 Million Light Bulb”). This competition was founded by the Energy Independence and Security Act of 2007. In fact, Philips Electronics has already submitted an entry.
There is a proviso, though. The winning device must perform similarly to the incandescent lamp it’s intended to replace in terms of color appearance, light output and distribution, lamp shape and size, form factor, and operating environment. It must also be reliable and available through normal market channels. And, it must be competitively priced.
Recently, the DoE’s National Technology Laboratory (NETL) selected four projects for solid-state lighting in its sixth round of funding. These awards will be used to examine high-priority R&D activities that will advance state-of-the-art solidstate lighting used for general illumination applications. Three of the projects are funded under the mandate of the American Recovery and Reinvestment Act, and one is funded by appropriation.