By Scott McMahan
The Illumination Engineering Society intends to cease using CRI as metric. CRI as a standard for lighting quality has been around since 1964. The value of CRI as a standard is significantly less than what most in the general public and even many in lighting community understand.
CRI offers a quantitative measure of a light source’s ability to reproduce a particular set of colors of objects accurately compared to an ideal or natural light source.
Now, CRI is calculated by defining the differences in the color appearance (chromaticities) of eight of the CIE standard color samples illuminated by the light source compared to a reference luminaire with the same Correlated Color Temperature (CCT).
The less these combined differences in color appearance, the higher the CRI.
Natural light has the best possible CRI of 100. Incandescent lamps can have a CRI above 95. Fluorescent lamps have a CRI of just 62.
One problem with CRI as a measurement is that it only defines a small set of colors, and using the current CRI measure, it is still possible to have a light with a good color rendering index of 80 that still produces reds poorly.
Another issue is user preference. Like film photography of companies such as Kodak, the industry has discovered that perfect color reproduction is not always what people want. While such color reproduction might be necessary for a surgical theater, it may not be desired for illuminating a bathroom vanity.
Even if according to CRI’s RA measurement comparing the reproduction of reds shows a poor result, people may still favor light with that shows more saturated colors than the way the would appear under sunlight, according to a National Institute of Standards of Technology (NIST) study.
The Illumination Engineering Society is developing a new suggested measurement standard to address these issues known as 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 extent 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.