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Color Rendering Index

What is CRI and how does it apply to photography?

Color Rendering Index or CRI is an objective quantification of the color rendering ability of a light source, in comparision to an ideal or natural light source. The color rendering indices (Ri) consist of 8 sample colors (R1 to R8) which are unsaturated. CRI or Ra is the average of these 8 colors only. There are also 7 additional colors (R9 to R15) which include realistic colors like red,yellow, green, blue, caucasian skin color, tree leaf green, and asian skin color respectively. Test color samples R9 through R15 do not contribute to the computation of CRI or Ra. One number can not tell you evrything, yet this is the only number specified by most manufacturers. Some LEDs score poorly on R9, which is a saturated red. Red is a common color we see daily. If  a light can not accurately render red, things that are reddish will look dull. Skin tones may look pale. The Sweet Sixteen LED light source yields a CRI of 96 for R9. Caucasian skin color is represented by R13. The Sweet Sixteen LED source scores a CRI of 98 for R13. Asian skin color R15 scores a CRI of 96.

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CRI measurements taken with a Konica Minolta CL-500A Illuminance spectrophotometer. The bar graph on the left are actual readings taken on a Sweet Sixteen LED light source with a color temperature of 5600K. The bar graph on the right are actual readings taken from a typical LED source. Note the huge difference in CRI of R9 between the Sweet Sixteen LED light source and the typical LED source. For reference, the bar graph below are actual readings taken outdoors with daylight under overcast conditions. The color temperature was 6000K.

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