What distinguishes CRI from Ra?

In the evaluation of lighting products, one will inevitably encounter the metrics CRI and Ra to denote color quality. One could presume that CRI and Ra are synonymous; nevertheless, continue reading to discover how this assumption could be erroneous!

Definition of CRI

CRI stands for Color Rendering Index, the most universally recognized metric for assessing a light source's capacity to accurately reproduce colors.
The overarching principle entails utilizing a collection of 15 predetermined colors, referred to as test color samples (TCS), to assess the fidelity with which a light source renders each of these colors.
 

The following are the 15 reproduced test color samples:

"Accurate" is characterized as resembling natural daylight or an incandescent bulb, contingent upon its color temperature. (This is somewhat of an oversimplification - for further details, refer to this source).
Each TCS score is designated as Ri, with R representing Rendering Score and i indicating the TCS index number. The score for TCS4, designated as "Moderate yellowish green," would be computed and identified as R4. Upon calculating each R value, two variants of CRI, namely General CRI and Extended CRI, can be derived.
 

General CRI

The General CRI is determined by the mean of R1 to R8. This is commonly denoted as Ra, with 'a' representing "average."
Only resistors R1 through R8 are utilized, but resistors R9 through R15 are excluded from the calculation of Ra.
 

Prolonged CRI

The Extended CRI is determined by the mean of R1 to R14. Occasionally, the symbol "Re" is employed, with the letter "e" denoting "extended."
Significantly, extended CRI accounts for the impact of saturated hues like deep red (R9) and vibrant blue (R12), which are overlooked by general CRI.
This represents a critique of the general Color Rendering Index (CRI), hence it is advisable to consider the extended CRI and specific R values in projects where color quality is paramount.
 

What is Ra?

Ra is merely a symbol in the equations for general CRI calculations, although it has become often synonymous with general CRI.
In other terms, Ra represents the mean value of R1 to R8.

Misunderstood in interpretation?

In the United States, the word CRI denotes general CRI (R1-R8), although this may not hold true in other global regions. In China and Europe, CRI commonly refers to extended CRI (R1-R14).
CRI might assume markedly various interpretations based on the interlocutor.
We advise being explicit when communicating these KPIs to manufacturers and customers. When addressing general CRI, it is preferable to utilize the term "CRI (Ra)" or general CRI (R1-R8). When referring to extended CRI, utilize the terms "CRI (e)", "Re", or extended CRI (R1-R14).
Extended CRI is often utilized less often than standard CRI; however, it is advisable to seek clarification when uncertain.