Creating White Light by Adding - Not Subtracting - Color

For light-emitting diodes (LEDs) to grow as a lighting option, they’ll need to meet consumer expectations by coming close to the color rendering of traditional lighting solutions like incandescent bulbs.

LEDs have numerous advantages over incandescent bulbs, compact fluorescent lighting or even high-intensity discharge solutions. Those include greater efficiency, solid-state durability and longer life. But the quality of light that LEDs generate — particularly white light — often is described as "cold" compared to older and more familiar light sources.

Additive properties

Fortunately, light has additive properties. Equally intense red and green LED beams pointed at a white surface will overlap, with the resulting color perceived by humans as yellow. Likewise, equal intensities of red, green, and blue light from LEDs will produce white light. By varying those respective intensities, lighting engineers can produce LED-based luminaries – or lighting solutions – that generate light very similar to what consumers have come to expect from an old-fashioned incandescent light bulb. To further make this point, it’s helpful to explain these additive properties in the context of what they are not.

The idea that mixing all of these colors together will produce anything white may be hard to grasp for some consumers. Nearly every school child who finger paints is told, for example, that light blue paint and yellow paint blended together will make green paint. In the context of art class, color mixing tends to create darker hues, not lighter ones. Similarly, mixing lighter colored inks in a color printer produces darker colors. For instance, magenta and cyan combine to make blue.

When we are describing inks and paints —the more familiar form of color mixing — we are actually dealing with light's subtractive properties. People perceive the combination of magenta and cyan as blue because those colors absorb or subtract green and red light, respectively. So when natural light bounces off of "blue" ink, we perceive the color because only blue light is returned to the eye.

In LED lighting design, we focus on adding colors together in varying intensities to produce white light.

Even better white light

Initially, this may seem like a lot of work for lighting designers, particularly since subtle variations in operating conditions or even the LED manufacturing process can affect the specific wavelengths an LED emits.

To minimize the impact of these variations, lighting engineers may use ASSP integrated circuits capable of varying the intensity of individual LEDs in an RGB array to produce a specified quality or temperature of white light. In fact, the color temperature of white light emitted from an LED-based system could change based on the levels or colors to ambient light because a light measuring sensor will likely provide input to the ASSP IC. In any environment the LED light would appear white.

This sort of flexible color lighting solution can actually be a significant competitive advantage for LEDs. In this way, a perceived weakness in color quality could actually become the advantage of color adaptive LEDs.

Specifically, people tend to see wavelengths of daylight in the green portion of the spectrum best. In twilight, people tend to see blues better. Traditional lighting sources only produce a single brand of white light, which appears differently based on the amount of ambient sunlight it blends with. However, an adaptive LED-based lighting solution could vary intensity to accommodate for ambient light and human light perception. In this way, the quality of light available to a person working in an office would be consistent throughout the work day.

Similarly, automotive interior or even instrument-panel lighting could be designed to adapt to provide a reliable and consistent color, regardless of environmental lighting conditions.

Finally, LED luminaries might offer homeowners more specific control over the quality of light in a room. Traditionally, a homeowner might use a dimmer to set the mood for a dinner party. With light's additive properties and RGB LED arrays, it conceivable that future homeowners could change the actual color temperature in a room, adding more warming yellow to the light without necessarily dimming it.

Advances in LED systems are creating new advantages over – and a realistic alternative to – traditional lighting.