Perhaps the last vestige of the incandescent and fluorescent era is about to disappear and we all have to adapt to the change. Measuring colour fidelity is critical for interior designers. The very appearance of every shade or hue in aroom – whether it is a textile or a painted surface depends on the illumination that shines upon it. Now the LED age has sounded the death knell for our old friend the colour rendering Index (CRI).
You might think that a new way of measurement has been devised to embrace the relatively huge spectrum of colour now possible with LED lighting – and you would be correct – partially! It’s true LED has the ability to deliver a level of colour accuracy that legacy lighting could not begin to match. However, the real motivator behind the introduction of a new metric is because of LED shortcomings.
As we have touched on before in this esteemed newsletter, the CRI method has come under increased criticism for its inability to accurately define how well LEDs render colours. Key to the issue, is a recognition that LED can have a relatively high CRI but renders red badly.
CRI is now being replaced TM-30-15 (Technical Memorandum), which is a new method of evaluating light source colour rendition as prescribed by the Illuminating Engineering Society of North America in 2015.
Whilst CRI – ( a measure developed in the 1930s) as a scale relies on just 8 colour samples contributing to the Ra value. TM-30-15 uses 99 new colour evaluation samples to measure colour fidelity Rf with an additional measure Rg for gamut area.
TM-30-15 describes a method for evaluating light source colour rendition, quantifying the fidelity (closeness to a reference) through a Fidelity Index (Rf) and gamut (increase or decrease in chroma) through a Gamut Index (Rg) of a light source. The method also generates a colour vector graphic that indicates average hue and chroma shifts, which helps with interpreting the values of Rf and Rg.
Don’t be surprised if you increasingly see references to TM-30-15 in specifications and packaging, High-end lighting manufacturers are welcoming the new measurement as a vindication of the quality of their product. The word is out that the use of the chart won’t remain exclusively within the expensive spectrum. Many lamp manufacturers of quality will be brandishing the new metric on packaging soon across a wide range of price points.
The US Government Office of Energy Efficiency & Renewable Energy is the body sponsoring the new system and they have provided a very comprehensive yet easy to follow briefing on the metric – just click on the video link below.
As a rule of thumb three useful extremes are described as follows: