LEDs 101

March 21, 2002
Light-emitting diodes have been around for years, but there remains a lot of confusion in regards to how they stack up against incandescent bulbs.

Light-emitting diodes have been around for years, but there remains a lot of confusion in regards to how they stack up against incandescent bulbs. An LED is essentially a PN junction semiconductor diode that emits light when current is applied. Such solidstate devices control current without heated filaments and are, therefore, very reliable. Performance is based on a few primary characteristics.

Color: LEDs emit monochromatic light in a narrow frequency range. Color is identified by peak wave-length (lpk), nanometers, which is a function of chip material. LEDs are made from gallium-based crystals that contain additives such as phosphorous to produce a distinct color and intensity. The human eye is most sensitive to the 565 to 600-nm wavelengths, so it is easiest to perceive color variations in yellow and amber LEDs.

White light: When light from all parts of the visible spectrum overlaps, the mixture appears white. However, combining primary colors red, green, and blue produces much the same effect. LEDs require a sophisticated electro-optical design to control, blend, and diffuse colors and produce white light.

A single LED can produce white light using a phosphor layer (yttrium aluminum garnet) on the surface of a blue (gallium nitride) chip. Although this approach produces various hues, white LEDs are appropriate for illuminating opaque lenses or as backlights. However, using colored LEDs to illuminate similarly colored lenses produces better visibility and overall appearance.

Intensity: Light output varies with the type of chip, encapsulation, efficiency of individual wafer lots, and other variables. Several LED manufacturers use terms such as "superbright" and "ultrabright" to describe intensity. Such terminology is entirely subjective, as there is no industry standard for LED brightness.

Light emitted from an LED is quantified by a single-point, on-axis luminous intensity value (Iv), specified in millicandela (mcd). This on-axis measurement is not comparable to mean spherical candlepower (MSCP) values for incandescent lamps. Within design limits, luminous intensity is roughly proportional to current. LEDs generally operate at 20 mA.

Designers must, however, be cognizant of the heat generated in an application. For example, single-chip LEDs produce less heat than six-chip LEDs. The latter incorporate multiple wire bonds and junction points more affected by thermal stress than single-chip designs. Similarly, LEDs that operate at higher voltages generate more heat.

Visibility: Luminous intensity (Iv) does not represent the total light output from an LED. Designers must take into account both luminous intensity and the spatial radiation pattern (viewing angle). If two LEDs have the same luminous intensity, the lamp with the larger viewing angle generates more light.

0⁄2 is the off-axis angle where luminous intensity is half the intensity at direct on-axis view. is considered an LED's full viewing angle even though light is visible beyond this angle.

Viewing angle is a function of the chip type and the epoxy lens that distributes the light. The highest luminous intensity (mcd rating) does not equate to the highest visibility. Light output from an LED chip is very directional. Concentrating the light in a tight beam produces a higher light output. Generally, a higher mcd rating means a narrower viewing angle.

Encapsulation also plays a role. Its shape can act as a lens and magnify light from the LED chip. Tint also affects visibility. If the encapsulation is diffused, the light emitted by the chip is more dispersed. If nondiffused or water clear, the light is more intense but has a narrower viewing angle.

Visibility is enhanced by increasing the number of LED chips in the encapsulation, increasing the number of individual LEDs, and using secondary optics to distribute light. In each case, the amount of visible light depends on the application. A single chip may be appropriate for direct viewing in competition with high ambient light. A six-chip design may be better suited to backlight a switch or small legend, while a cluster or lensed LED may be best to illuminate a pilot light or larger lens.

Operating life: LEDs are solid-state devices not subject to catastrophic failure when operated within design parameters. Data Display Products LEDs, for example, typically operate upwards of 100,000 hr at 25°C ambient temperature. Operating life is characterized by the degradation of LED intensity over time. When output gradually drops to half of its original intensity (after 100,000+ hr) the LED is considered to have reached the end of its useful life — even though it continues to operate. Unlike standard incandescent bulbs, LEDs resist shock and vibration and can be cycled on and off without excessive harm.

Voltage and current: LEDs are currentdriven devices. Although drive current and light output are directly related, exceeding the maximum current rating produces excessive heat. This reduces light output and operating life.

Manufacturers often add built-in safeguards. For instance, Data Display Products LEDs are designed to operate at a specific voltage and contain a built-in current-limiting resistor. Additional circuitry may include a protection diode for ac operation or a full-bridge rectifier for bipolar operation. The operating current for a particular voltage considers heat dissipation and other degradation factors, and is designed to maintain LED reliability and ensure long life.

Information for this story provided by Data Display Products, El Segundo, Calif. For more information, visit www.ddp-leds.com.

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