The CIE Chart

• In 1931 the International Commission on Illumination (known as CIE, the initials of its French name) adopted a framework for understanding color perception. The CIE system is visually represented as a horseshoe-shaped map of colors on an x-y grid. Colors on the curved side of the horseshoe represent light from a single wavelength. This portion of the chart is known as the spectral locus, and individual LEDs will lie somewhere on the curve. Colors elsewhere in the chart are the result of mixing more than one wavelength.
  
  

Predicting a Color Blend

• Separate individual colors can be combined to produce new blended colors.Jupiterimages/Brand X Pictures/Getty Images
  
  Human visual perception combines the inputs from three (or more) different types of receptor cells into a signal sent to the brain. The brain then defines the mix of signals as a color. The CIE chart represents the response of the brain to different wavelengths. The response to two wavelengths together will lie somewhere on the line connecting the two different individual colors. The spot on the line is determined by the relative power of the two individual wavelengths. For example, if 480 nanometer blue light is mixed with 620 nanometer red light, the perceived color will be aqua, light purple or burnt orange, depending on the relative output of the two sources.
  
  

Making White

• White is a mix of colors; within the brain, white is created by an equal output from each of the three different color receptors in the eye. White, like just about all mixed colors, is a metameric color, which means it can be created by more than one mix of pure wavelengths. So, for example, one way of making white with LEDs relies on combining light from red, green and blue LEDs; another way is by combining light from a blue LED with yellow light, usually from a phosphor material that absorbs some blue light and emits yellow light. White can be made by combining other colors on the spectral locus as well.
  
  

Making Other Colors

• The same color mixing process used to make white can be used to create any color, as long as the sources are available. Most effort in the high-brightness diodes is going into red, green and blue LEDs, but amber, aqua, orange, pink and violet LEDs are available as well. Any one of these LEDs can be paired with another. Drawing a line on the CIE chart between a 507 nm aqua and a 420 nm violet, for example, shows that those two colors can create a variety of blue-greens, but no yellows or oranges. Adding a third source opens up many more possibilities, because then the final color is a result of all three individual colors, which is a triangle on the CIE chart instead of just a line.