What CMOS circuits did for the world of power-hungry electronics, LEDs may be doing for the world of lighting. Just as CMOS-based ICs reduced the energy necessary for a given electronic function, LEDs have slashed power requirements in many lamp applications. Thanks to advances in chip material and package design, these solid-state lamps, long viewed solely as status indicators, have shined their way into a host of applications that traditionally relied on incandescent lamps and other light sources.
Of course, lower power consumption is just one reason for the change. LEDs also last longer and are more rugged than the incandescents that they replace. Plus, despite their higher cost per bulb, LEDs can cut system-level lighting costs over the life of the application by their ability to reduce product maintenance and downtime. (For more on the economics of replacing incandescents with LEDs, see "Cost-Effective LEDs Fit Snugly In Today's Energy Conscious World" by Jordan P. Papanier of LEDTronics Inc., Electronic Design, Jan. 24, 2000, p. 93.)
Evidence of LED benefits abound. In the transportation and automotive realms, LED-ready products are showing up in traffic signals and vehicle brake lights, as dashboard backlights, and as roadside messaging displays. LEDs also are catching on in the aerospace industry, where they illuminate airport runways, provide in-cabin lighting, and identify flight obstructions, such as antenna and water towers. Closer to the ground, they light up exit signs, and they're being considered as both indoor and outdoor decorative lighting for many architectural applications. Those large channel letters that spell out names of department stores and other businesses now signify further uses for LED backlights.
In these wide-ranging applications, LEDs have become practical due to several factors. First, LED semiconductor materials are being honed to produce greater levels of light at the standard levels of drive current.
Meanwhile, vendors are developing high-flux chips that can be driven at much higher current levels than previously possible. Light output is rising significantly. Many of these new applications take place outdoors, where the ambient lighting is often intense sunlight. Daylight visibility of the lamp is a must, which means a very bright LED.
Packaging also accounts for much of the progress. It's possible to build very bright light sources by either of two methods. Individually packaged LED chips—discrete LEDs—may be clustered together in one lamp assembly. Alternatively, multiple LED chips can be combined in a single housing. In both methods, but especially in the multichip approach, the buildup of heat within the semiconductor threatens to cut short the LED's long operating life. Creative packaging designs, however, are applying the necessary heatsinking to prevent heat buildup in these LED arrays.
Aside from brightness, there's the issue of color. The availability of high-intensity blue and white LEDs has helped to fill out the LED color spectrum. White LEDs can generate illumination or backlighting akin to incandescents, while blues can complete the red-green-blue (RGB) trifecta needed in full-color displays. But all of these multicolor pyrotechnics would be of little value in the real world without the steady reductions in device cost fostered by developments in the lab, the fab, and the competition-driven marketplace.
Anatomy Of An LED
The traditional discrete LED has a fairly simple structure. Typically, a semiconductor diode chip is mounted within a reflector cup that sits atop the device's two-wire leadframe. Wire bonds connect the chip with the leadframe, which also carries heat away from the chip. In the standard device, the chip, wirebonds, and a section of the leads are encased in a solid epoxy lens (Fig. 1).
Among discrete through-hole LEDs, the T1-þ and T1 package styles are the most common. The numbers in these designations refer to lamp diameter in eighths of an inch. So, the T1-þ package has a diameter of 0.219 in. or approximately 5 mm. Similarly, the T1 package has about a 3-mm diameter. In addition to these popular types, other through-hole packages exist, as well as a range of standard surface-mount options. SMT LEDs can be found in 0603-, 0805-, 1206-, and 1210-sized chips, plus in SOT-23 packages. Many versions of nondiscrete LEDs, like seven-segment displays are available, too. (A more comprehensive list of LED-related terms and their meanings, along with a primer on device operation, appears in "Light-Emitting Diodes 101" on the MCD Electronics Web site: www.mcdelectronics.com/led101.html.)
But there are other variations as well. In some cases, chips of different colors are packaged together to create different color effects. For example, the combination of red, green, and blue semiconductor chips creates an LED capable of producing light in 256 colors.
Other packaging variations are employed in higher-flux LEDs, which can incorporate significantly larger dies than the typical 5-mm discretes. Consequently, the parts can operate at higher drive currents. But they additionally need more heatsinking than the leadframe and pc board affords. In some devices, a copper heatsinking slug helps keep the die cool.
Aside from these packaging effects, the package plays a large role in determining the brightness, or luminous intensity, and viewing angle of the LED. Furthermore, these two parameters are intrinsically related. Brightness in a visible product also is a function of the sensitivity of the human eye to the different wavelengths that make up the light spectrum.
It isn't surprising, therefore, that with LEDs, specifications for luminous intensity often appear in different units that reflect differences in measurement approaches. Before an LED chip is packaged, its light output might be rated in terms of radiant power (mW) or radiant flux. That value reflects the total light output. But once packaged, LEDs are typically rated in lumens (lm) or millicandellas (mcds).
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