Optical coatings and most tinted surfaces are not self-luminous. In order to see them, we need a light source. Obviously, any evaluation of color will include the properties of the light source. When calculating color, we usually use standard light sources, most of which are defined by the CIE in terms of their relative spectral output, and represent as closely as possible the characteristics of ordinary light sources, such as daylight (D65, etc.) or tungsten (light source A). The ideal actual light source is a black body. The spectral variation of the output is very smooth and is entirely determined by temperature. Therefore, the quality of a blackbody light source can be specified by simply stating its temperature, as shown in Figure 1. 
Figure 1The relative output of several different blackbody light sources. As normal, the curve is normalized to 100 at 560 nm. Unfortunately, it's not that simple for other types of light sources. Ultimately, the spectral distribution is what determines the quality of the light source, but this involves a lot of data. A very useful technique is to compare a light source to a blackbody. If the spectrum of the light source is proportional to the spectral output of a given blackbody anywhere in the visible region, then the statement of the blackbody temperature is sufficient. This temperature is called distribution temperature. The chromaticity coordinates of the light source will exactly match the chromaticity coordinates of the black body, and all color measurements will produce exactly the same values. This chromaticity coordinate is located on the Planckian Locus, as shown in Figure 2
Figure 2The chromaticity diagram showing Planck locus from 1000K to 7000K has many light sources, such as discharge lamps, and although the spectral distribution is quite different from that of a black body, the chromaticity coordinates are located on the Planck locus. The corresponding blackbody temperature is known as the color temperature of the light source. However, it is uncommon for chromaticity coordinates to correspond precisely to a point on the trajectory. Typically, the point is close but not actually on the trajectory. In this case, the temperature of the blackbody of the color closest to the light source is used, and is called the associated color temperature, abbreviated as CCT in the software. Correlated color temperature is usually measured in Kelvin. Note that a specific correlated color temperature does not guarantee that any color measurement using a light source necessarily corresponds to the color of a blackbody light source using the same temperature. How do we derive the associated color temperature?CIE 1960 (U, V)-Diagram is an attempt to create a uniform chromaticity scale in which the perceived chromatic aberration between any two points is proportional to the distance between them, anywhere in the diagram. When Planck trajectories are plotted in (u,v)-plots, the minimum perceived chromatic aberration between the light source and the blackbody will correspond to the vertical direction from the trajectory to the point. Therefore, the line perpendicular to the Planck trajectory is called an isotherm. These isotherms can be replicated in the (x,y)-chromaticity diagram (Figure 3), but, of course, they are no longer perpendicular to the trajectory. The use of techniques (u,v)-diagram in MacLeod for calculating correlated color temperatures, based on Wyszecki, Günter and Ws Styles, Color Science 2nd ed.1982, New York: John Wiley & Sons). Note that although the CIE 1960(u,v)-plot has been replaced by the 1976(u',v') graph, in order to ensure continuity, the CIE has decided to keep the (u,v)-graph for the correlated color temperature calculation. None of these differences are significant. 
Figure 3Isotherm Figures 4 and 5 plotted in (x,y)-plot illustrate this with a color correction filter designed to convert a 3000k blackbody source to a color temperature of 3300k. In the range of 2500K to 6500K, the correction is almost constant at -30RMK, as shown in Figure 7. 
Figure 4The transmittance of a color correction filter designed to convert 3000k blackbody characteristics to 3300k. This corresponds to a change of -30RMK (Reciprocal Megakelvin) at RCCT (Reciprocal Correlated Color Temperature). 
Figure 5A range of blackbody light sources with and without color temperature correction filters. Almost constant at -30rmk