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The color temperature of a light source is taken from the temperature of a perfect black body radiator that radiates light with an appearance similar to that of the light source. It is measured in absolute temperature units; Kelvin (K). Interestingly,
Although red is associated with warm colors and blue is associated with cool colors, blue appears higher on the black body curve
The temperature is higher than red. A more visual example of this apparent color temperature contradiction can be seen in candlelight, which emits a warm reddish-orange light but is actually cooler in Kelvin at 1850K. Therefore, higher color temperatures (above 5000K) are called cool colors (blue-white); lower color temperatures (2700 – 3000K) are called warm colors (yellow-white to red).

The Color Rendering Index (CRI or Ra) is a quantitative measure used to evaluate the ability of a light source to faithfully reproduce the color of an object. To objectively compare the color rendering characteristics of any light source, the CIE's standardized measurement uses a scale of 0 to 100 (poor to excellent). Calculates the color change of 14 standard colors when an object is exposed to a specific light source and then compares it to a reference light source of the same color temperature (black bodies* for color temperatures up to 5000K and daylight above 5000K). CRI for a pair of light sources Comparisons can only be made with the same color temperature. The CRI scale is chosen so that, by definition, an ideal blackbody light source such as an incandescent or halogen lamp has a CRI rating of 100. For light sources that emit a discrete spectrum, such as LEDs and discharge lamps, the CRI can be anywhere between 0 - 100. As a rule of thumb, the more the spectrum is filled in all wavelengths (380–760nm), the better the color rendering of the light source Well, but a high CRI measurement essentially means less efficiency (since less efficient wavelengths are also represented in the spectrum). * A black body is a theoretical object that absorbs all incident electromagnetic radiation and, due to its ability to absorb all wavelengths, is the best possible emitter of thermal radiation. It radiates a continuous spectrum that depends on the temperature of the body.