What is the Colour Rendering Index and why is it so Important?

Firstly let me explain the basic's of the CRI System and how it works.  CRI = Colour Rendering Index developed in 1964 by the Commission Internationale de l'Eclairage (CIE).  Used by Lighting professionals to guide us on the Colour Spectral Reflectance Distribution in a light source.  The light source is measured against the first 8 pigment colour samples.  These eight colours are pastels and the measurement taken is an average number.  The average differences measured are subtracted from 100 to get the CRI. So small average differences will result in a higher score, while larger differences give a lower number.

Traditionally the CRI of a Light source has been fixed depending upon technology chosen. The application and desired affect would lead your choice of technology.

Now the problem with LED's is that they are manufactured using a Blue LED.  The bit that tunes the LED to reflect colour is the yellow phosphor mix that is blended and added on top and can vary between manufactures.  This notoriously gives strong blues and weak reds, but interestingly would still give a CRI80 measure.  To produce higher R9 Values which reflect Red's is a tricky, expensive process.  To add to this manufacturers have a trade off to judge, efficacy vs high CRI.  

So is the R9 Value really that important?  In certain applications I believe that it is essential!  Retail there is no question, to miss intense reds from your store with not only effect your merchandise it will change skin tone and texture of materials.

I believe that CRI is still a viable measurement but you need to ask a little more information from your manufacturer - What is the R9 Value!  A guide would be a CRI of 90+ and R9 Value above 60.  There are now alternative systems being considered one being The Colour Quality Scale (CQS), developed by NIST in collaboration with Industry experts and the CIE (International Commission on Illumination).  This is in response to a cry for better Colour Rendering Measurements with no room for error.  Based on the CRI system it will be a measurement from 1-100 like the CRI System.  But unlike the CRI system we will be given a measurement taken from a Scale of 15 colour reference oppose to 8.  It will only penalise a lamps score for hue shifts, lightness shifts and reductions in chroma.  Colour temperature will also play it's part in the calculations.  Rather than taking an average score despite being low in one or two colours the CQS will calculate the root-mean-square of colour shifts of all the individual samples used.  Experiments are under way to improve and validate the CQS before it is proposed as a new international standard.