With their pure spectral hues, light-emitting diodes (LEDs) intended for illumination have excelled in applications that require colored light, such as traffic signals and mood lighting. Because their output falls entirely within the desired color range, LEDs require no filters, greatly boosting their efficiency relative to that of conventional filtered incandescent sources. The availability of low-cost indium gallium nitride (InGaN) blue emitters has poised illumination-style LEDs for use in even inexpensive consumer items. Now, in one of the first such uses, Forever Bright (Yardley, PA) has developed LED holiday lights that span the color gamut (see figure).
The company had previously produced and marketed light strings containing red, green, and yellow LEDs; the limited appeal of less-than-full-color lighting was partially countered by long LED lifetimes that eliminated the dreaded searches for dead bulbs in ordinary light strings. The blue LEDs chosen by Forever Bright to flesh out the color range have optical outputs of from 2.5 to 5 mW and emit at 470 to 475 nm, a wavelength range highly visible to the human eye while remaining blue in color.
Although high-brightness green-emitting InGaN LEDs exist, the green lights in the holiday light strings sold for consumer use are the older-technology aluminum indium gallium phosphide green. "InGaN green is a significant breakthrough, but still has a way to go," says Dave Allen, president of Forever Bright. "There are two reactor manufacturers today; both reactors make good InGaN blue, but only one makes a decent InGaN green. The green emitters are still a bit pricey for consumer applications, but are gorgeous in commercial light sets."
The company also manufactures strings of white-light InGaN LEDs; new developments are forthcoming here too. The latest and greatest in white-light technology is the development of 380-nm-emitting ultraviolet (UV) LED chips, says Allen. "White LED lamps are commonly manufactured by coating a blue-emitting chip with a yellow phosphor," he notes. "Arduous color sorting is then required as you get many renderings of 'white.' Using presorted LED chips helps this process, but yields of usable lamps are still quite low. This coloration issue is compounded when white lamps are in close proximity to each other in applications such as light strings or arrays. The eye can detect subtle differences and color uniformity is difficult to achieve."
Allen notes that white-light lamps manufactured using UV-emitting chips show excellent color uniformity because the light pumping the phosphor is not visible. "The only drawbacks at this time are availability, cost, and lower radiated power. Time will cure this," says Allen.
The radically different properties of the LED will inspire a transformation in the way these light emitters are used, but old ways of thinking die hard. The LED lamps in this year's Forever Bright light strings are replaceable, though they have a 200,000-hour average lifetime. This is changing, says Allen. The models for 2003, already developed, are permanently sealed in place and impervious to moisture.