Color Rendering Index(CRI)
Because the term CRI tends to cause a fair amount of confusion among everyday consumers when it comes to choosing the best light source for their needs we will do the best that we can to explain the term so it can be easily understood and used appropriately. The technical definition of Color Rendering Index, or CRI for short, from Wikipedia is: "Color rendering index, or CRI, is a measure of the quality of color light, devised by the International Commission on Illumination (CIE). It generally ranges from zero for a source like a low-pressure sodium vapor lamp, which is monochromatic, to one hundred, for a source like an incandescent light bulb, which emits essentially black-body radiation.
It is related to color temperature, in that the CRI measures for a pair of light sources can only be compared if they have the same color temperature. A standard "cool white" fluorescent lamp will have a CRI near 62. CRI is a quantitatively measurable index, not a subjective one. A reference source, such as black-body radiation, is defined as having a CRI of 100 (this is why incandescent lamps have that rating, as they are, in effect, black-body radiators), and the test source with the same color temperature is compared against this. Both sources are used to illuminate several standard samples. The perceived colors under the reference and test illumination (measured in CIE 1931 form) are compared using a standard formula, and averaged over the number of samples taken (usually eight) to get the final CRI. Because eight samples are usually used, manufacturers use the prefix "octo-" on their high-CRI lamps. The standard formula consists of taking the color differences, between the test color and the eight samples, on the 1964 W*U*V* uniform color space (which is now obsolete). The color rendering index Ri is calculated for each of the eight samples: Ri=100-4.6, which gives the color rendering index with respect to each sample. The general color rendering index Ra is then the average of these eight separate indices."
A more simple explanation is how an artificial light source shifts the location of eight specified colors on a version of the C.I.E. color space as compared to the same colors lighted by a reference source of the same Color Temperature. If there is no change in appearance, the light source is given a CRI of 100 by definition. From 2000K to 5000K, the reference source is the Black Body Radiator and above 5000K, it is an agreed upon form of daylight. Incandescent bulbs have a CRI rating of 100, yet are far from ideal for color rendering and matching. Why? With a color temperature of only 2700k they are far too weak at the blue end of the spectrum making it next to impossible to distinguish between various shades of blue.
The CRI rating of 100 simply means that the 8 samples look exactly the same as they would under a black body radiator at 2700k. The same can be said for lamps that exceed 6000k in color temperature as they are too weak in the red end of the spectrum, making reds and oranges appear too similar creating a "washed out" appearance. The northern sky with a color temperature of about 7500k and a CRI of 100 is not necessarily the ideal color rendering light source either. An ideal light source for color rendering will have both a color temperature similar to daylight and a high CRI value.