Color Rendering Index (CRI) Defined12-11-2014
A simple definition of Color Rendering Index (CRI) would measure the ability of a LED light source to accurately render all frequencies of its color spectrum when compared to a perfect reference light of a similar type (color temperature). It is rated on a scale from 1-100. The lower the CRI rating, the less accurately colors will be reproduced. Light sources that are incandescent radiators have a CRI of 100 since all colors in their spectrum are rendered equally. As stated earlier, light sources that are not incandescent radiators will have Correlated Color Temperatures.
Examples of light sources with Correlated Color Temperatures, having CRI levels that are less than 100 would include: HMIs, and also most photo quality fluorescent lamps, as well as LED lights. With lower CRI ratings these sources may also have too much green or magenta in their spectrums. An acceptable Color Rendering Index level for professional imaging is considered to be 90 or above.
Comparison of High & Low CRI Fluorescent Lamps
These 2 images are worth really studying. Both were shot with daylight colored fluorescent lamps and a camera with daylight preset white balance. The image on the left was shot with Lowel 27w day-flo lamps (CRI 92+), and the image on the right side was shot with a household day-flo lamps (CRI not listed, but assumed to be aprox. 80). Compare the details of each image, noting where the colors are pretty similar, such as the red & orange peppers, and the radishes. Then look at the items with colors rendered differently, such as the floor, cutting board, carrots, cabbage, and lettuce leaf, for example. A low Color Rendering Index does not mean all colors will shift, and no 2 lamps with the same low CRI rating will necessarily have the same errors in rendering.