What is the difference between CRI and Ra?

When comparing lighting products, you will undoubtedly come across the metrics CRI and Ra to describe color quality. You may assume that there is no difference between CRI vs Ra, but read on to find out how this could be a mistake!

 

CRI defined

CRI is an acronym for Color Rendering Index, and is the world's most widely accepted metric to describe a light source's ability to accurately reproduce color.

The general concept involves using a set of 15 predefined colors called test color samples (TCS) and determining how accurate a light source would make each of these colors appear.

Below are the 15 test color samples reproduced:

 

"Accurate" is defined as similarity to natural daylight or an incandescent bulb, depending on its color temperature. (This is a bit of a simplification - for detail, see here).

Each of these TCS scores is called Ri, where R stands for Rendering Score, and i is the TCS index number. For example, the score for TCS4 ("Moderate yellowish green") would be calculated and labeled as R4. Once each of the R values is calculated, two types of CRI, called General CRI and Extended CRI, can be calculated.

 

General CRI

General CRI is calculated as the average value of R1 through R8. Formulaically, this is often referred to as Ra, where a is an abbreviation for "average."

Note that only R1 through R8 are used, and R9-R15 are NOT used in the calculation of Ra.

 

Extended CRI

Extended CRI is calculated as the average value of R1 through R14. Sometimes the symbol "Re" is used, where the letter "e" represents "extended."

Notably, extended CRI captures the influence of saturated colors such as deep red (R9) and strong blue (R12) that general CRI does not.

This is one of the criticisms of the general CRI, and it is therefore always a good idea to look at extended CRI and the specific R values when working on a project where color quality matters.

 

What is Ra?

Technically, Ra is just a symbol in the formulae for general CRI calculations, but has become widely used as a synonym for general CRI.

In other words, Ra is also the average value of R1 through R8.

 

Lost in translation?

In the United States, the term CRI is used to refer to general CRI (R1-R8), while this is not necessarily the case in other regions of the world. In China and Europe, for example, CRI is typically used to describe extended CRI (R1-R14).

Depending on who you are speaking to, CRI can take on a very different meaning.

Our recommendation is to be explicit when discussing these metrics with manufacturers and customers. When discussing general CRI, it is best to use the term "CRI (Ra)" or general CRI (R1-R8). When discussing extended CRI, use the term "CRI (e)", "Re" or extended CRI (R1-R14).

Typically, extended CRI is used less frequently than general CRI, but when in doubt, it is always best to clarify!

At Waveform Lighting, to avoid confusion, we explicitly state when CRI is used to refer to CRI Ra. For example, below is a screenshot from our NorthLux T5 LED tube product page.