Visitors to the Lighting for Museums and Galleries conference at last week’s LuxLive heard a strong case for a new way to measure how lights render colour.
Roger Sexton of Xicato argued that the new TM-30 metric, designed to replace the decades-old colour rendering index (or CRI), is a more useful and robust way of telling how colours will look under a particular light.
CRI is a key piece of information on the data sheet for any lighting product. But lighting engineers and designers have always known that CRI is a pretty blunt instrument. It gives the light source a score between 0 and 100 for how faithfully it shows colours – but this still leaves a lot of uncertainty.
The old CRI metric takes the average of eight colour samples. But when it comes to colour rendering, “the devil is in the detail”, says Sexton. It also doesn’t cover colour saturation – only colour fidelity. Sexton described CRI as a “reasonable” way of measuring fidelity, “but not, I would argue, robust enough”.
As any lighting designer can tell you, two lights with the same CRI can render colours very differently. It comes down to the spectral power distribution of the light source – the balance of the different colours that come together to make what we see as white. The rise of LED lighting brought new challenges, because many LED lights perform poorly at rendering reds, but can still get a good CRI score if they render other colours well.
The new TM-30 metric sets out to change all this, Sexton told LuxLive. First published in 2015 and due to be updated this year, the new TM-30 metric uses 99 colour samples taken from real-world objects and grouped into categories such as skin, textiles, nature and paint. This provides much more consistent coverage of the visible spectrum than the old system based on just eight colours.
It also allows some room for preferences regarding colour saturation. For instance, a shop selling fruit and vegetables might have different preferences to a sweet shop, and a branch of Laura Ashley will no doubt aim for a different look to, say, the Disney Store.
“With the TM-30 metric, the lighting industry can be more sure of itself and get the right light source for the job first time,” said Sexton.