Color Mixing

LEDs naturally lend themselves to producing colored light by mixing primary-color sources, but there are many details in which the devil can hide.

LEDs solve problems that were nearly intractable with earlier lighting technologies. LEDs are more efficient, last much longer, and have fast response times. LEDs produce blue light with relative ease and efficiency. In contrast, incandescent sources struggle to develop any significant blue content. Just try distinguishing a pair of blue socks from a pair of black ones under an incandescent lamp.

The LED's small physical size, rapid response time, relatively low heat dissipation, and availability as monochromatic sources make color-changing applications a foregone application. But the LED's unique characteristics -- binning, long life and lumen maintenance, and color shift over time and with temperature -- introduce their own new set of challenges.

Color-changing lighting did exist before LEDs came on the scene. It was anything but simple and was limited to certain applications. For pure colors, neon lights could be used. Color-changing theatrical lighting relied on a combination of incandescent or halogen sources coupled with color filters. Some used dedicated color fixtures. Others were more sophisticated and mechanically complex solutions, relying on DMX control of servos to control rotating color wheels and dichroic lenses.

The ready availability of LEDs in monochromatic primary colors facilitates color mixing. The degree of fixture design difficulty naturally depends on the application. For disco or party lights, holiday lights, and many transient consumer applications, there are no significant lighting requirements other than perhaps a minimum luminance. These are easy to design, and today's technology is more than adequate. At the other end of the spectrum are high-end architectural, marine, and aerospace applications with demanding requirements.

High-end applications

These demanding applications often require precise color and luminance matching from fixture to fixture. Beyond that, they may require replacement fixtures to install and look exactly the same as those replaced with no apparent difference in the light output. And in these applications there is no room for color artifacts.

A color artifact is any aesthetic feature that influences the color appearance of the light source or objects illuminated by the light source. Color artifacts are effects such as color fringing and perceptible color differences between light fixtures in a system, or between light sources within a fixture. In high-end applications, these are definitely unacceptable conditions.

Fixture designers must account for initial variations in LED flux and color binning. Furthermore, they must contemplate more complex situations arising in the intended environment, particularly changes in operating flux (lumen depreciation) and wavelength over time and temperature. The luminous flux and wavelength of red LEDs in particular, especially those based on aluminum gallium indium phosphide (AlGaInP) are prone to fluctuation as a function of operating temperature.

In some applications, metameric matches of adjacent light sources would be problematic. In a metameric match, the color appearance of two sources is the same, but their spectral power distributions differ. While light sources will appear alike in metameric matches, colored objects illuminated by these sources might not.

Color fringing occurs in close physical proximity to a fixture (near field) whenever the individual color sources are offset physically. This is a significant consideration when mixing the light from red, green, and blue LEDs to create other colors. Objects placed close to the fixture can cast shadows outlined by a rainbow of colors.

So while LEDs might well be the catalysts of (color) change, they are temperamental as well, and demand the proper respect for color applications.