this number fell to 200; today’s signals contain only 10 to 20 high-powered LEDs. Each signal head uses 15 watts and has a lifetime of 120 months; by comparison, incandescent bulbs use 135 watts and last about six months, a 9x energy savings and a 20x maintenance savings.
While the natural advantages of LEDs are easily suited to these early applications, the next stages of application will require major technological advances. Dr. Craford summarized some technological trends in LEDs that are leading to the next generation of applications.
Chips are getting larger. In the early 1990s, the standard phosphide chip was 0.25 mm square, drawing just 0.1 watt and putting out a few lumens of light. Now a bigger chip, 0.5 mm square, emits tens of lumens. In the newer gallium nitride technology the trend is similar, from smaller to larger, from low to high power; GaN chips now emit up to 50 lumens, and in some research devices, over 100 lumens.
Chips are being shaped. LED chips, which have a high index of refraction, suffer from the problem of light trapping, which limits the extraction of light from the chip. Recently, chips with tapered sides or roughened surfaces have become available. These shapes break the symmetry of the chip and allow more light to escape.
More white-light LEDs are being made. White LEDs have generally been 5 mm in diameter and put out about a lumen of light. These LEDs are not made by mixing red, green, and blue (R-G-B) layers, but by starting with a blue chip and adding a yellow phosphor.1 This is a simple system, but some efficiency is lost by going through the phosphor. The superposition of R-G-B LEDs to produce white light can yield higher performance, but the blue light/phosphor system is simpler and more convenient for low-power uses, such as in glove compartments and stairway night lights. Higher power LEDs emitting around 20 lumens were recently (late 2001) introduced commercially.
High cost is a major challenge for commercialization. The selling price of a 5mm white-light LED is about 50 cents to $1 per lumen; that same dollar can buy four 100-watt tungsten bulbs. Each bulb emits 1,500 lumens. We thus have up to a 6,000-fold difference in lumens per dollar. Considerable work must be done in manufacturing technology and other fields to drive that cost lower. Costs have been dropping about 10× per decade, and light output per package has been rising by 30× per decade. The rate of cost reduction needs to be accelerated to enable