[ReNae Stueve]

the following is takend from GE's lighting specification index.

I hope it helps

Warm," Cool," Daylight," etc.


Yellowish white light, reminding people of a fireplace, is called "warm" while bluish white light is called "cool." These are based on associations with these colors. "Daylight" is supposed to mimic light coming in from a window. These are crude, but useful classifications. However, we can have differing degrees of "cool" and "warm": and therefore we need a quantitative measure, the Correlated Color Temperature, described below.

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Correlated Color Temperature (CCT) measured in Kelvins (K)

Figure 3
Correlated Color Temperature (measured in Kelvins)-or simply Color Temperature-is a scientific scale to describe how "warm" or how "cool" the light source is. It is based on the color of light emitted by an incandescent source. As a piece of metal (a theoretical Blackbody) is heated, it changes color from reddish to orange to yellowish to white to bluish-white. The color of light emitted by an incandescent object depends only on the temperature. We can use this scale to describe the color of a light source by its "Color Temperature."

When we say a lamp has a Color Temperature of 3000 Kelvins, it means a glowing metal at 3000 Kelvins would produce light of about the same color as the lamp. Instead, if the metal is heated to 4100 Kelvins, it will produce a much whiter light. Direct sunlight corresponds to about 5300 Kelvins while daylight, which has the blue from the sky mixed in, is typically 6000 Kelvins or above. A standard incandescent lamp has a filament at 2700 Kelvins, and therefore (by definition) a Color Temperature of 2700 Kelvins.

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Color Rendering Index (CRI)

Figure 4
Color Rendering Index (maximum =100) is a measure of how closely the lamp renders colors of objects compared to a standard source. Implied is that the standard source is ideal, which may not always be true. Daylight is considered a standard but then so also is any "Blackbody," i.e. any incandescent object, no matter what its temperature. Based on this definition, daylight and all incandescent and halogen sources have CRI's of 100. For a warm lamp, CRI is a measure of how close to incandescent color it is; for a very cool lamp it is how close to daylight it is. Sources with very distorted colors will have low CRI. In general, the higher the CRI the more natural the appearance of the source and the richer colors appear. More information on CRI is provided in another section.
Color Rendering

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C.I.E. Chromaticity Diagram


The C.I.E. (Commission Internationale de l'Eclairage, the International Commission on Color) diagram is based on the idea that mixing varying proportions of three hypothetical primaries (not necessarily red green and blue) can create the sensation in the human observer, of any color of light. The three "primary" colors are dubbed "X," "Y," and "Z." If we are merely concerned about color and not about brightness, we can specify just the relative strengths of these three colors, denoted by x, y and z. Since x + y + z must add up to 1 (i.e. 100%) just providing x and y is sufficient to specify lamp color; the z value is implied. Lamp color can then be represented on a two-dimensional plot of x and y. All possible colors then fall under a "guitar-pick" shaped triangle in which the perimeter encompasses spectrally pure colors (seen in nature only in rainbows and prisms) ranging from red to blue. Moving toward the center "dilutes" the color until it ultimately becomes "white". Specifying the x,y coordinates locates a color on the color triangle.

The color points traversed by an incandescent object as its temperature is raised can be plotted on the CIE Chromaticity diagram as the "Blackbody curve" and occupies the central white region. Two lamps whose x,y co-ordinates fall one above the Blackbody curve and one below could have the same CCT. However, the one above will appear slightly greener, and the one below slightly pinker. This is why two lamps having the same color temperature can still show differences in color as seen by the human eye. Color is complex; attempting to describe the lamp color with just one number (or even with two numbers) does not provide total information on how different materials will appear under that light.

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Spectral Power Distribution (SPD) Curves


The most complete description of lamp color characteristics can only be provided by a detailed plot of relative power emitted in the different regions of the spectrum. Such a plot with color shadings to indicate the colors corresponding to the different wavelengths is very useful in providing a visual feel for the color balance in a lamp. Such spectral curves for a number of lamps are provided in another section. See Spectral Power Distribution Curves.