Quick Lesson on Colour Rendering Index (CRI) and Correlated Colour Temperature (CCT)
Colour Rendering Index (CRI) quite simply is the measure of colour accuracy of a particular light source. Basically, how well does the light source render colour compared to a reference at the same Correlated Colour Temperature (CCT). Each light source produces a light spectrum and each light spectrum distribution renders the colour appearance of objects differently. For the last 40 years we have quantified this ability to render colour by utilizing a scale from 0-100 with 100 representing the highest level of accuracy or most like the reference light source (Daylight or Black Body). Lower CRI values indicate that some colours may appear “unnatural”.
Correlated Colour Temperature (CCT) on the other hand is the measure of light colour emitted by the light source. This measurement is based on the principle that objects will emit light if heated to a high enough temperature. As we keep heating objects they will start to glow and this glow will change from orange to yellow to white to blue as we increase the temperature. The colours produced corresponds with a temperature in kelvin (K). By comparing the colour of our light source to the heated black body we can identify what Correlated Colour Temperature it emits. Correlated Colour Temperatures of 2700K to 3000K provide light that appears “warm” while temperatures in the 4000-6000K range appear “cool”.
One common misconception about selecting a light source is that Correlated Colour Temperature will dictate how coloured objects will appear within the illuminated space. The big box stores plays this up big time with their displays. We’ve all seen the displays with the “warm- white” CFL or LED and the “Daylight” CFL or LED side by side. Psychological we look at the colour of the light emitted (CCT) from the source and we automatically think that the “warm white” will make the objects in the room appear more accurate. The reason this happens is that by fluke incandescent lamps which we’ve used for over 100 years happens to have a Colour Rendering Index of 100 and just happens to emit a “warm” 2700K. While our mind will come to this conclusion in the store, closer observation of the Lighting Facts label would indicate that the CRI may in fact be the same and that the colour of the objects they illuminate would be look the same. Conversely 2 light sources may have the same Correlated Colour Temperature but very different colour rendering ability.