Cinematography Mailing List

CRI Practically Speaking

Published : 1st August 2006

I've read about CRI and understand that it's a measure of the colour quality of a light. I've never put a low CRI light next to a high CRI light to compare, however. What would I expect to see? One thing I have noticed are fluorescent bulbs that shift colour temperature. Is this a factor of the CRI or a factor of something else? Does CRI relate to ALL lights or only fluorescents?

Dan Coplan

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Dan,

For the benefit of others reading this, I'll preface by emphasising that CRI is NOT a measure of colour temperature.

>I've read about CRI and understand that it's a measure of the colour >quality of a light.

Yes, CRI = Colour Rendition Index.

>I've never put a low CRI light next to a high CRI light to compare, >however. What would I expect to see?

To the naked eye, very little difference except that 8x [lower CRI] tubes are almost exactly one stop brighter than the same colour temperature 9x [higher] CRI tubes.

However if one has a "spectroscope" [little box with a prism in it, one points at a light source and gets a rainbow display] then the peaks in fluoro sources become obvious. Every gaffer or cinematographer shooting under artificial light should have one in his/her kit!

>One thing I have noticed are fluorescent bulbs that shift colour >temperature. Is this a factor of the CRI or a factor of something else?

Possibly CRI-related but definitely operating temperature [the gas within the tube, which is related to how long the tube's been running] and ambient temperature.

> Does CRI relate to ALL lights or only fluorescents?

In theory, all light sources. However "continuous spectrum" sources like sunlight and filament bulbs have a CRI of 100 [100%].

In practice its only of interest to us for non-continuous sources as its a measure of "how close to perfect". So yes, fluorescent and arc lamps.

Cheers,

Clive Woodward
"full of bright but often trivial information"
Perth, Western Australia.

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>One thing I have noticed are fluorescent bulbs that shift colour >temperature. Is this a factor of the CRI or a factor of something else?

As Clive said :

>Possibly CRI-related but definitely operating temperature [the gas >within the tube, which is related to how long the tube's been running] >and ambient temperature.

It also has to do with the operating voltage, the age of the tube (hours of use and number of on/off cycles), the manufacturing quality of the tube, and the ballast. For instance, some "energy saving" ballasts do so by reducing the light output -- and therefore altering the CRI.

>However if one has a "spectroscope" [little box with a prism in it, one >points at a light source and gets a rainbow display] then the peaks in >fluoro sources become obvious.

And when you go to buy that special ring, a spectroscope will really impress the jeweller.

Brian Heller
IA 600 DP

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Brian and Clive have done a great job answering your questions, but I'll add 2 cents. Lamp manufacturers make a trade-off between lumen output and colour rendering. The less spectrum you try to cover, them more output you can push in a narrow band(s). That's one of the reasons why Low-Pressure Sodium lamps have extremely high lumens-per-watt...buy you wouldn't want to try to find your car in a parking lot lit by them. LPS lamps are literally monochromatic, which is why you seldom see them in general today.

Concerning CRI's, think of it as a percentage of how accurate the colours look as compared to the reference source (such as a tungsten/halogen incandescent lamp). A CRI of 95 is 95% accurate. As to whether you could see it, take a chart of hard to reproduce colours such as the Gretag/Macbeth Colour Checker and look at it under the reference light source (CRI of about 70). When you compare the chart again under a mediocre fluorescent light you will see the difference right away.

Bruce (No stock in Gretag/Macbeth that I know of) Aleksander
Lighting Director + (whatever it takes today)
ABC / Disney
Houston, Texas

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Correction/Note : The CRI of 70 refers to the MEDIOCRE light source. The reference source is always 100%. Sorry for any confusion.

Bruce Aleksander

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