Since the 1930’s the colour rendering Index or CRI has been our measure of accurate colour under artificial light. The familiar chart with a range of colour swatches has been our yardstick to judge the ability of a light source to illuminate an object in a true colour – or what you would expect to see in the presence of sunlight. Again, with the advent of LED even this trusted convention is under threat and we brace ourselves for yet another shift in the very foundations of our understanding of light. So what changes await and how do we judge the quality of the lights we are currently specifying?
Simply, the CRI measure of ultimate colour accuracy is 100. Whilst you could tolerate a CRI rating of 60 when lighting a hallway – in areas where the depiction of colour is vital, a score of at least 80 is necessary to ensure that the colours are rendered authentically. This becomes especially important when illuminating food as ‘wholesome’ or the highly processed colours in clothing and textiles as ‘vibrant’. Hence, it is easy to see why restaurant and retail interiors require high scoring CRI lamps where even 90 is desirable.
The LED / CRI Blind Spot
To further complicate things – colour accuracy of many items, including meat, fish, vegetables and fruit, can be adversely affected by the imprecise rendering of the colour red. Consult the chart and note that CRI is calculated on average values based on R1 – R8 coloured swatches. Here is where the ‘LED colour blind spot’ occurs. R9 is one of six saturated test colours not used in calculating CRI. However, R9 is important when assessing the quality of LED light. It seems that most LED sources reveal this CRI value only and as long as it has a value of over 80 Ra. In other words, LED lamps with high R9 values produce the most vivid colours that are essential for food and retail display applications. However, we cannot rely upon the traditional measurement chart to judge the level of R9 in a LED lamp.
Warning: an LED lamp can have a relatively high CRI but render red badly
So it is time for a new metric compatible with our LED world. Scientists at the US-based Illuminating Engineering Society have developed a new system commonly called TM-30 which is based on a reference set of colour samples that is more representative of objects in the real world than the pastel samples used in CRI. A Rg value will replace the existing Ra and it will accurately score LED lamps for colour ‘red’ accuracy. The new measurement has been approved in the US although it has yet to be accepted by the CIE – lightings international body.
Whilst we are waiting for this new standard to come into force – how do you choose LED lights for colour sensitive interiors? At Orange Lighting, we select from manufacturers that use in-house and external accredited laboratories that measure the colour fidelity of LED lamps. If you have an application that demands high colour accuracy – especially where produce or merchandise needs to appear authentic or vibrant, call us and we will steer you toward the right brands and products.
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giving a FREE Webinar this Friday at 2.00 pm – we will tackle all the main issues surrounding LED and colour consistency – grab your headphones and listen in! Click below to register!