Measuring color temperature in Kelvin (once degrees Kelvin) is a system of designating a light source's spectral distribution. The basis of this measuring system is the "Theoretical Black Body Radiator" as shown in the "Black Body Locus" on the CIE Chromaticity Chart (see illustration A).

The spectral distribution of the "The Theoretical Black Body" is continuous as it varies from 1800K (candle light) to 28,000K (north sky). That is the spectral energy is not "interrupted" in the visible range from 300 to 700nm (see illustration C).
However, the balance or ratio of red to blue is shifting - the higher the Kelvin, the "bluer" the light.

Let's examine the spectral distribution curve of a typical fluorescent lamp (see illustration D). Note the high peaks of energy at certain wavelengths and the areas of "interrupted spectrum." These published charts can be misleading because it is acceptable to shorten the high peaks of energy and widen the bar to represent the total energy (it saves paper). In reality, these bars represent high peaks of energy at certain points in the spectrum, and the smoothed out continuous line is artistic license with areas of little or no energy. The Kelvin scale should not be used to describe light sources with an "interrupted spectrum." Sadly, the term is often misused, and by those who should know better: the very lamp manufacturers being depended on for this important information.

Once upon a time, the term "Apparent Color Temperature" was used to describe light sources that "looked like" but did not behave exactly like a certain Kelvin. The use of the term "Apparent Color Temperature" was a warning that this was not an accurate description but only a guide. Unfortunately this useful description has been dropped and often replaced with "Correlated Color Temperature" (CCT) - read "Apparent Color Temperature." My objection to the use of this language is that it no longer suggests the warning. Even worse, since Correlated Color Temperature is a mouthful, lamp manufacturers will just describe a light source with an interrupted spectrum (all fluorescent and gas discharge lamps) as having its color temperature in a factor of Kelvin. This is a misuse of the Kelvin temperature scale, and can lead to many problems in film and television production or any application that requires accurate color reproduction.

Misuse of the Kelvin scale has grown with the use of light sources that do not behave in a manner similar to the "Theoretical Black Body Radiator." I can appreciate the need to use the Kelvin scale to describe the proximity of these newer light sources' color temperature to the well established incandescent and day light equivalents. However, there is no excuse for abandoning the caution "Apparent Color Temperature". "Correlated Color Temperature" does not imply as clear a caution as "Apparent Color Temperature." Worse yet, it suggests an accurate measurement, which it is not.

When one encounters a light source with an "interrupted spectrum" (all fluorescent and gas discharge lamps), problems with color reproduction will arise. Just look at the full spectral distribution chart for that lamp. The peaks, often in yellow green range and elsewhere will cause problems. Color filters can be used to eliminate spikes. However, if the color is not in its spectrum, it cannot be added with a filter. Colors that fall in this interrupted area of the spectrum will be dulled or lost. If using these light sources is a must, it is best to augment lighting with a full spectrum source.