The Color Rendering Index (CRI), is a measure of the ability of a light source to represent the colors of some objects faithfully comparing it into an ideal or a natural light source.
It represents how these light source are able to display the object colors “realistically” or “naturally” compared to a familiar reference light like the day light for example. So, the highest value is 100, and would only be given to a source identical to standardized daylight or a incandescent light.
All of these issues with the LED and CRI results come from the LED manufacturing standards and that is still-young technology. The CRI value of LEDs can range between less than 70 and over 90. This is why it is important to be able to make the distinction between good and bad quality LED.
The value often quoted as ‘CRI’ on commercially available lighting products is properly called the CIE Ra value, ‘CRI’ being a general term and CIE Ra being the international standard color rendering index.
CRI is calculated from the differences in the chromaticities of eight CIE standard color samples when illuminated by the light source and by a reference illuminant of the same correlated colour temperature (example: 3000K); the smaller the average difference in chromaticities, the higher the CRI.
As specified in CIE (1995), the original test color samples (TCS) are taken from an early edition of the Munsell Atlas. These eight samples are employed to calculate the general color rendering index Ra.
The last six samples provide supplementary information about the color rendering properties of the light source; the first four for high saturation, and the last two as representatives of well-known objects.
In below image you can see a real report measured with a spectrometer of an LED lamp with CRI Ra 85 (average of R1-R8) and also the results of the supplement colors R9-R15 for reference.
One of the things that is not so well known by consumers and that this measuring system is sometimes criticized is because it does not account for the R9 value colors between others (R9-15). For example, in our test R9 is only scoring 15 out of 100. The R9 value produces strong and vibrant reds, so it is really important to include a strong red when measuring color rendering. However, LED technology limitations have still troubles to reproduce this color. For this reason is important not only to pay attention to the CRI ra results but also on their overall results (R1-R15).
Finally, it is important to note that current EU Regulation 1194/2012 requires a minimum Index CRI Ra of 80 for directional and non directional LED lamps, although there is exception for outdoor and industrial applications being allowed a Ra of 65 .