CHAPTER 10 ■ LEDS
■ Fun If you want to start trouble, assemble some artists, physicists, philosophers, and psychologists into a
locked room and ask them “What are the primary colors?” and “What are their wavelengths?”
You can experience the problem for yourself: The next time you see a rainbow, mentally try to slice it into neat
chunks of colors. Where does blue end and green begin?
Indicating Color via the CIE Standard Colorimetric System
The Commission Internationale de l'Eclairage (CIE) is headquartered in Vienna, Austria. In 1931, the
organization published a scientific method for specifying a color based on a standard observer. In their
colorimetric system, any color can be described exactly by three values: X, Y, and Z.
Although LEDs are rarely advertised using the CIE system, the specification sometimes appears in
datasheets. Instead of absolute XYZ, usually relative x and y chromaticity coordinates are indicated. Software
can convert the CIE values to wavelengths or on-screen colors.
LED Brightness
LED brightness is often advertised in beam millicandela (mcd). This refers to how intense the light is at a
peak angle. To increase a mcd rating, the manufacturer need simply shape the reflector cup and lens to aim
the light like a spotlight. More of the light is then concentrated at a narrow angle, making it more intense
(brighter).
The mcd rating is barely useful if you really need a bright spot, since you don’t know how wide and evenly
distributed the bright spot is. Lower-quality LEDs often have a halo shape with a dark hole in the center.
The mcd rating is completely misleading if you’re concerned with overall light output. Lumens (lm) is
the measurement unit that sums up all of the light, regardless of direction. The manufacturer can’t play with
lumen numbers by redirecting the light in a single direction because the total light remains the same.
■ Note The United States Environmental Protection Agency (EPA) now requires that consumer light bulbs
indicate their value in lumens. Unfortunately, this mandate doesn’t apply to discrete LEDs.
Table 10-1. Approximate Color Ranges
Color or Hue Approximate Wavelength Range
Infrared (IR) above 700 nm
Red 700 nm to 630 nm
Orange 630 nm to 590 nm
Yellow 590 nm to 570 nm
Green 570 nm to 500 nm
Blue 500 nm to 450 nm
Violet 450 nm to 390 nm
Ultraviolet (UV) Below 390 nm