CRI AND R9: WHAT'S THE DEAL?
Colour Rendering Index, or CRI, is a term used to describe the measure of how well light sources render the colour of the objects they illuminate. The CRI of a light source is calculated by comparing the appearance of eight reference colours (R1 to R8) under the light source in question with a reference light source (incandescent light or sun light). The smaller the difference between the reference colours when viewed under the two light sources, the higher the R value will be. The first eight R values are then averaged, and the resulting number is the CRI (or Ra) value.
Although still a valuable resource, CRI values do not give an accurate indication of some colours that are predominant in clothing, food, art, and skin tones. A good way to judge the colour quality of a light source is by taking a more saturated colour reference, such as R9 (red), into consideration. The R9 often has a lower value in LED or fluorescent technologies, yet a high R9 value means a much better colour rendering result for the majority of applications.