From Christmas tree lights and flashlights, to traffic lights and brake lights, LEDs are showing up everywhere these days, and some predict that LEDs will begin replacing conventional incandescent light bulbs in the home before the end of the decade. In the realm of indicator lights for appliances and electronics, the steady replacement of incandescent bulbs with LEDs in product designs has been underway for a long time.

The reason is no mystery. Designers already know that LED advantages include longer life, lower energy consumption, higher durability, and ability to operate at lower temperatures. In some of the basic colors, LEDs even offer a cost advantage. What designers may not know is that LED technology continues to improve at a rapid rate and some previously held assumptions may now be wrong. For example, take the notion that only a red LED is bright enough to be used in an outdoor application exposed to bright sunlight.

“That was true five years ago, but not today,” says William Chu, regional sales manager, Kingbright, City of Industry, Calif. “The sun is no longer an issue, as evidenced by the full-color outdoor billboard displays that now use LEDs.”

The brightness of an LED is a function of the semiconductor materials used to make it, and new developments in materials are constantly boosting the light output to new levels.

“For red we can now go up to 12,000 mcd,” notes Man Yu, engineering manager at Kingbright. “For high bright green we can go up to 22,000 mcd. For blue we can only go up to 5,000 mcd.”

Those figures, however, are for a single LED. Output can boosted by simply combining multiple LEDs to achieve desired brightness. That, of course, will increase cost, so designers need to know what level of output they need, in what color, and what they are willing to pay for it. As a general rule, higher brightness means higher cost due to more expensive materials, and some colors may cost more to achieve a particular output.

In addition to brightness, desired viewing angle is an important design consideration when specifying LEDs. Unlike incandescent bulbs that radiate light in all directions, LEDs emit light in a single direction, which typically necessitates using a diffuser lens to achieve a desired beam pattern. Standard packages can provide viewing angles from 15 Deg up to 180 degrees, and custom optics packages can provide a wider range.

Beyond brightness, a broader color pallette has been the other big development in LED technology. In the beginning, there was only red. Then came green. Blue, the most difficult color to achieve, only arrived in the late 90s, but its arrival also permitted the presentation of white.

White light from an LED is achieved one of two ways. One is to combine red, green and blue on a single chip. The other way, which is less expensive and more common, is to use a blue LED with a phosphor-coated lens that glows white when hit by the blue light of the LED.

The quest for bright white light from LEDs revolves around applications involving general illumination, backlighting, or full-color LED displays. For equipment indication functions, colors other than white are generally preferred. While red and green may say on and off in the same way a traffic light says stop and go, new color choices give designers the opportunity of differentiating their products by using LEDs as both functional and aesthetic components.

Kingbright design combines three ultra-bright LED chips (red, green, and blue) within a single package, allowing the component to produce a full color spectrum of light, including white. The package has a 100 Deg viewing angle and can operate on current as high as 150 mA.

“By putting red, green, and blue LED materials onto a single chip, and varying their specific intensity levels, we can theoretically provide any shade of color in the visible spectrum,” says Yu. “For example, we now have an LED with a 505 nm wavelength, which gives you a teal green, and 525 nm, which gives you a true green, neither of which you had five years ago.”

Phospher-coated lenses are also playing a greater role in expanding color choices. “If a particular shade of color is not available in a single-chip version, phosphors can be employed to achieve the desired effect,” notes Chu. “For example, if someone wanted an odd shade of violet or pink, we can provide that with the appropriate phosphor package. We can tweak it to meet the designers’ needs.”

Color choice does not affect the viewing angle when color is determined solely through LED semiconductor materials themselves, but viewing angle issues need to be examined when achieving a specific color requires a particular optics package.

In some cases, color choice can also affect circuit design. LEDs in the most common colors operate in the range of 2 VDC to 2.5 VDC. But other colors, such as blue and bright green, require a slightly higher operating voltage that can go up to 3.6 VDC. Those ranges may change in the future, as the trend for LED operating voltage is downward.

While LEDs are physically more robust than incandescent and neon alternatives, there are there are two protection issues that need to be considered, one electrical, the other thermal.

An LED driving circuit typically requires a current-limiting resistor to protect the LED from over-current, and some type of diode to protect the LED from over-voltage, reverse voltage, and electrostatic discharge.

Heat can be another issue; after a certain point, LED light output decreases as temperature increases. For some LEDs, this decline begins at about 55 DegC (131 DegF). With Kingbright LEDs, the fall off begins at 65 DegC (149 DeF). Those ratings are good enough for most applications, but there are a number of factors that can push temperature past the ratings: over-driving the LEDs, packing them too close together, placing them too close to a hot microprocessor, or exposing them to a high temperature operating environment, like an indicator light on an oven, or an outdoor device exposed to the sun.

These factors can generally be handled in standard fashion, with proper circuit board design, use of heat sinks, or providing active air flow in the application where necessary.

“In most applications, the LEDs are not the most heat-sensitive components on the circuit board,” notes Chu. “So if you have taken steps to protect the other components from excess heat, you won’t have to worry about the LEDs.”

For designers thinking about incorporating LEDs into future products, the most important thing to remember is to stay abreast of the technology.

“We have not reached a plateau yet,” says Chu. “LEDs will continue to get brighter and more efficient, and the cost will continue to come down. New developments in phosphor technology will provide more color options, and white LEDs will become more cost effective in terms of lumens per dollar. The technology is moving forward at a tremendous pace.”