By Scott McMahan
The Illumination Engineering Society plans to stop using CRI. CRI as a standard has been around since 1964. The usefulness of CRI as a standard is considerably less than what most people in the general public and even many in lighting world realize.
CRI serves as a quantitative measure of a light source’s ability to reproduce the various colors objects accurately compared to an ideal or natural light source.
Now, CRI is determined from the differences in the color appearance (chromaticities) of eight CIE standard color samples when the light source illuminates them compared to a reference luminaire of the same Correlated Color Temperature (CCT).
The smaller this difference in color appearance, the higher the CRI.
Natural light is said to have the best possible CRI of 100. Incandescent lamps can have a CRI above 95, while fluorescent lamps of just 62.
Using the current CRI measure, it is possible of a light to have a good color rendering index of 80 but still produce reds poorly.
Another issue is that high color rendering is not always what people prefer.
Even if according to CRI’s RA measurement comparing the reproduction of reds shows a poor result, people may still prefer light with more saturated colors, according to a National Institute of Standards of Technology (NIST) study.
The Illumination Engineering Society is creating a new suggested measurement standard to address these issues called TM-30. Its final form is not out yet. But what is out appears to offer some improvement compared to CRI.
The U.S. Department of Energy has answered some frequently asked questions (FAQs) about TM-30-15 (the full name includes the 15 at the end).
To find out more about TM-30 watch the National Lighting Bureau Video Below:
It features two metrics, color fidelity (which is similar to CRI) and Gamut which attempts to measure color saturation. It has been shown in an NIST study that color saturation of certain colors can be related to user preference.
However, the NIST study found that not all saturation was viewed as favorably as red saturation. To a lesser extend the study found that people preferred saturated Green. However, little difference in preference was found related to saturated yellow.
So while CRI is out as a measurement and TM-30 could be a more useful measurement with its two metrics, providing a scientifically objective measure of what people prefer for lighting may be more complicated just color fidelity and color saturation.
If Manufacture Has Color Spectrum, They Have Information to Calculate TM-30
The official TM-30 measurement standard is not out yet, and the part of that metric which is more complicated “gamut” will take longer to develop. The TM-30 does not require any new tests. In fact, if the manufacturers have a color spectrum of the output, this information can be used to calculate the two metrics of TM-30.
It could be that they only use the metric for the saturation of red and green and not the other colors, because these were found to have a closer correlation with preference according to an NIST study.
One thing that is certain though is that lighting preference is specific to a particular application. Lighting for your bathroom will probably not need and you will probably not want the same color rendering that is required in a surgical theater. Looking in the mirror, people may not want to see every line, wrinkle and skin discoloration in the perfectly accurate light.
So, while no single measurement will give you an objective measure of user preferences in all applications, the goal of TM-30 is really to give people more information about the performance of lighting for a consumer or specifier. With that goal in mind, TM-30 will likely succeed.
Eventually, the TM-30 will be refined, and the refined and more evolved version could become an international standard.