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Colour Rendering Index

CRI – The Colour Rendering Index. What is it and why should you care?

CRI, the colour rendering index, Rₐ,

is an often quoted but often misunderstood specification attached to lighting. You’ll see it tossed into many a marketing word salad, sprinkled with a lux or a lumen for a bit of additional flavour, then drizzled with all sorts of claim dressing in an effort to convince you that your lights must have a Colour Rendering Index of 100. Those new-fangled LED spotties don’t have a CRI of 100 they’ll tell you, so you’re bound to ‘not see’ that 100kg red kanga that’s winding up its giant spring hoppers just off the road shoulder, aiming to join you on the front seat – via your windshield of course. If you’re lucky. Your eyes need to ‘render the colour accurately’ in order to correctly perceive what you’re seeing, apparently, and a CRI less than 100 will put you in mortal danger. Or so they say.

Salty is calling BS on the 100 CRI fallacy.

Let’s examine this a bit further, shall we? The colour rendering index is a measure of a light source’s ability to represent, or render, the colours of an object in comparison to a ‘black body’, or natural light, aka standard daylight (more about black bodies in Salty’s blog post about the good Lord Kelvin). The CRI value is a comparison of the lamp’s spectral distribution to that of the standard at the same colour temperature. The highest possible Colour Rendering Index of 100 is assigned to a light that ‘renders’ a test sample of colours in a way that is identical to daylight. In essence, a light source with a CRI of 100 reveals colours to your eyes in the same manner that sunlight would. Incandescent lamps do this, so they typically score a CRI of 100. The old Cibie halogen spotties on your old man’s 1980s HJ47 fourby probably did too. LED lights can now deliver a CRI approaching 98%, and quality LEDs are all generally above 70%. In any case, why does CRI matter? Or does it matter at all? I’m glad you asked. Grab a beverage and read on.

The standard for measuring CRI was introduced by the International Commission on Illumination, the CIE in 1965 (actually it’s the Commission Internationale de l’Eclairage, but your old mate Salty doesn’t want to sound like a posh wanker). The standard was last updated in 1974, quite a few dark nights ago. It was useful as a standard measurement back then, but lighting has progressed a bit since 1974, and thank Lucas the Lord of Darkness for that otherwise Salty would still be squinting into the night past the candle-like glow of the sealed beams on the old Series II Defender.

 Importantly, the CRI has been shown to be pretty crap at characterising the visual impression our eyes have of LED lamps, and it only measures the colour ‘faithfulness’ of ideal sources with the same colour temperature anyway – there’s ‘ol Lord Kelvin again. By now you may be starting to question the usefulness of comparing the Colour Rendering Index of a halogen light with a colour temperature of around 3000K, to that of a ‘daylight’ LED light with a colour temperature of 5700K, or thereabouts. Salty could start tossing apples and oranges at ‘ya, but you probably get the idea.

Research in the 47 or so years since 1974 has shown that the Colour Rendering Index is not a comprehensive measure of colour perception and that other aspects such as colour discrimination must be taken into account as well. The same research also showed that commercially available phosphor converted LEDs from our old mates at Osram delivered colour discrimination comparable to that of halogen lights, despite a lower Colour Rendering Index rating. Salty feels compelled to insert a big plug for the Osram Oslon® Boost HM ceramic packaged LEDs in our new DirtPRO 9 HD driving lights about now.

Some manufacturers are comparing halogen driving lights with a CRI of 100 to LED lights with Colour Rendering Index figures which are slightly less and trying to convince you that LED is inferior light… well, let’s just say that it’s not a particularly useful comparison. Objects will sometimes appear a slightly different colour when lit by lights with a different colour temperature, even if both have a perfect colour rendering index. The CRI proponents are not telling you the whole story, and the story they are telling is scientifically flawed anyway.

There are many other considerations you may want to ponder over a beer or two if you’re choosing between LED and halogen for your next driving lights. Efficiency, ingress protection level, current draw and colour temperature are among several factors that come to mind that are arguably much more important than the Colour Rendering Index, but more on all that stuff in another post.

By all means consider halogen, it has its place in the automotive lighting world, but don’t eat the word salad. It’s a bit off.

Back to the swamp.

Yours in dirt and sea,

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