Your Guide to CRI

When looking at bulbs, you've probably come across the term CRI number, but might be unsure what it means, or how it can affect the light produced.  So, to clear up some misconceptions about CRI, here's our helpful guide.

What is CRI?

CRI stands for Colour Rendering Index, and is the standard measurement for the ability of a light source to reveal the colour of various objects faithfully in comparison to an ideal or natural light source.  Numerically, the highest a bulb can achieve is a 100 CRI rating, which is supposed to give the same colour rendering of natural daylight.  This rating is often given to incandescent and halogen bulbs.

However, CRI is not the same as the apparent colour of the light itself.  This is called the Correlated Colour Temperature (CCT), and is measured in Kelvin, or K.  Warmer/ Yellower bulbs are around 3500K, whereas Cooler/ Bluer bulbs are 6500K+.  However, CCT is important to CRI, as in order to compare bulbs, they have to be the same CCT to make a fair comparison.

When?

This system was first put in place in the mid 20th Century.  Before this, both Europe and America had different ways of studying the colour rendering, and it was the International Commission on Illumination (CIE), in 1931, which created the first formal system (the CRI) that was used across all manufacturers.

How?

The basic outline of the test is comparing a palette of 8 pastel colours under the light being tested, against the same colours under an 'ideal' light.  There is then a calculation to work out this in a numerical value.

Up until 5000K, lights are compared to an incandescent bulb, but after 5000K, they are compared to a daylight source.

Why is it important?

The colour rendering of light is important because in certain industries, it is crucial to have an accurate idea of the colour of an object or person:

• Neonatal care- This is so nurses can check to make sure the baby's skin tone is healthy, and there are no problems, like jaundice
• Office Spaces- According to research, good colour rendering can make for a more happy workplace
• Retail- Good colour rendering makes the products on sale more attractive which is good for business
• Restaurants- Because no one wants to eat food which looks bland or washed out!
• Fashion- When the focus is on the clothes, and everyone is looking to copy the latest trends, knowing the exact colours can make or break the season.
• Industry- When choosing between different colour wires, being able to see what colour the wires are is necessary.  Having good colour rendering can reduce errors, making the business more effective.
• Space- Same as above, but on a much bigger scale.
• Photography and Cinematography- Colour rendering is really important  BUT because camera lenses differ from human perception of colour, a special index called the Television Lighting Consistency Index (TLCI) has been set up specifically for this

But what about LEDs?

So a CRI is a good idea, but the reality is a bit more complicated.

The Index is not recommended for white LEDs, because there is a poor correlation between rankings and the actual colour rendering, meaning that low ranked LEDs are actually perceived as giving good colour.  For example RGB-based lights has a CRI in the low 20s but renders colour well.  This might be because colour is simply a way of saying that the specific wavelength is reflected while the others are absorbed, meaning if the object is 'orange' the light doesn't have the exact wavelength needed.

Also, LEDs are perceived to increase saturation of most colours, without producing objectionable hue shifts, which means that the colour rendering is good, but just doesn't happen in the same way that the CRI measures.

Other Problems

Another problem with the scale is that something with a CRI of 80 could be an 80 overall but another one with a score of 80 might be half 70s and half 90s, producing two different colour renderings for the same level.  Therefore the only way to know is to see the bulb for yourself before you buy it.

Also, due to the differences in CCT, although incandescent bulbs have a 100 rating, 2700K temperature bulbs are too weak on the blue end of the spectrum to render the colour well.

Furthermore, the fact that pre 5000K and post 5000K have different comparison points means that the Index is unfair as the rendering under one type may be different to the other, but the rendering is only based on one.

However, although it is agreed this system has flaws, the reason it has been kept for so long is because another system cannot be universally agreed upon.

Some bulb comparisons

Taking into account the above problems, these are some really rough estimates on average CRI for these bulb types.  Many professionals state that if the difference between CRI's is less than 5 points, the difference isn't significant enough to be an issue.

Type	CRI  Rating
Incandescent	100
Tungsten Halogen	95
Fluorescents (Triphospher)	95
Quartz Metal Halide	85
LEDs	80+ (some claim a CRI of over 95)
High Pressure Sodium Lighting	25
Clear Mercury-Vapour Lamps	17
Low Pressure Sodium Lighting	-44