Lighting the 21st century

Here's the reason the lamps in your living room probably have incandescent or fluorescent bulbs rather than LEDs: Incandescents and fluorescents still generate more visible light per watt than an equivalent LED. However, in applications such as brake and taillights, display panels, industrial controls, and traffic signals, LEDs outshine incandescents in energy efficiency.

An LED lighting system from Color Kinetics lights this Northwest Airlines 800-ft passenger tunnel at Detroit Metro Airport. The walls and ceilings are comprised of sculpted art-glass panels and translucent vinyl stretch ceilings onto which over 9,000 ft of specialized LED lighting fixtures project one of three synchronized sound and light shows.

One-third of all vehicle third brake lights are red LED clusters. Many carmakers use LEDs because the third brake light is often inaccessible and replacement is difficult. LEDs have a long life and probably will outlast the car.

It's helpful to compare a red LED with a red-filtered incandescent lamp. A red LED might be as much as 10 times more visible than the red-filtered incandescent lamp because the red filter reduces the incandescent's light output. Unlike an LED, which is monochromatic, an incandescent light emits the full spectrum and therefore requires filtering to produce light of a specific color.

Placed under a lens, an LED lamp cluster can suffer up to a 30% reduction in brightness depending on the lens color and thickness. The lighter the lens pigmentation, the brighter the LED appears. Additionally, most of an incandescent's energy, 80 to 90%, is wasted as heat.

One new application for LEDs is in computer-controlled lighting. The fact that LEDs are fundamentally digital in nature means they can be connected to a computer and controlled to produce amazing color and lighting effects. That's what one company is doing to gain a foothold in the lighting market. Boston-based Color Kinetics, (www.colorkinetics.com), developed a patented lighting technique it calls Chromacore. It uses a microprocessor to control the mixing of multicolored LEDs, generating millions of colors and an array of color-changing effects.

Demonstrating the dramatic effects of LED lighting, Goodman Theatre in downtown Chicago is lit with lighting fixtures from Boston-based Color Kinetics. The front side of the building has 96 windows each containing ColorBlast fixtures. The rotunda at the top floor has cove fixtures whose colors can be programmed and controlled via computer.

Chromacore integrates a controller, LED modulation and drive electronics, the LEDs themselves, and a network. The network can be any physical layer or protocol, but the initial choice was one called DMX because it was widely used in the theatrical industry. However, systems can interface with other networks such as Ethernet. There are also devices that take serial RS-232, parallel ports, and USB and connect to the lighting systems. Similarly, other architectural control systems such as Lutron, Vantage, and LiteTouch are compatible with the system.

The wizardry behind Chromacore starts with the light source, consisting of clusters of LEDs which are then arranged into channels of control. For example, a channel might contain a group of LEDs of one color, such as red. Others might be green or blue. A microprocessor controls individual channels.

Light intensity is controlled by rapidly switching the channels of LEDs on and off at a kilohertz rate via pulse-width modulation. Switch speeds in the kilohertz range are enough to avoid flicker. Varying the output of each LED channel creates a vast array of lighting effects. For instance, this method of additive light mixing will produce any of 16.7 million colors via three LEDs (red, green, blue) and 8-bit control for each color.

"Driving LEDs with constant not varying current prolongs lifetime and creates no color shift or adverse thermal issues," says Kevin Dowling, vice president of Strategy and Technology for Color Kinetics. The company adjusts the ratio of on-time to off-time to change apparent brightness.