White LEDs
As previously discussed, when the three primary light colors (red, green, and blue) are combined, the result is white light. Mixing light from red, green, and blue LEDs (either discrete LEDs or combined in one package) can produce white light. It is also possible to use a single LED, either UV or visible blue light, to excite phosphors to generate white.
The Creating White LEDs Image depicts four techniques for producing white light when using HB-LEDs. Two of these techniques rely on phosphors in addition to LED light. The two other techniques, the RGB-Chip and Blue Chip + Yellow Chip use only LED light. No matter which technique is used, the goal is to transform monochromatic LED light into white light containing as much of the visible light spectrum as possible.| Creating White LEDs |
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Colors of White
When HB-LEDs are applied to lighting applications, the differences between LED white light and incandescent white light become apparent. Incandescent light contains a wide range of frequencies, whereas LEDs are essentially monochromatic and produce a particular color or warmth of white. On the CIE Chromaticity diagram, the range of white colors lies on what is called the Planckian locus or Black Body Locus.
| Planckian or Black Body Locus |
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The Planckian locus, also called the Black Body Locus (BBL), is the path (or locus) that the color a Black Body takes as the temperature of the Black Body changes. Figure 11 depicts the previously discussed CIE 1931 Chromaticity Diagram with the addition of the Planckian Locus. The path (locus) goes from deep red at low temperatures through orange, yellowish white, white, and finally bluish white at very high temperatures. The Planckian Locus (BBL) can also be thought of as the “white color line.”
Correlated Color Temperature (CCT)
Correlated Color Temperature (CCT) is a value that relates to the nearest color temperature of light on the Planckian Locus that visually matches the light of a particular LED light source. For example, a 100W incandescent lamp has a color temperature of 2700°K, meaning that a black body radiator at 2700°K produces the same chromaticity as the 100W incandescent lamp. The light emitted from the incandescent lamp contains a wide range of frequencies; therefore, it has a continuous color spectrum. However, LED white light contains a limited range of frequencies, and therefore, it produces spikes in the color spectrum and its chromaticity may not lie exactly on the Planckian locus. This requires LED manufacturers to calculate a Correlated Color Temperature (CCT) for all their LED products.
