>CRI Practically Speaking

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

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.

>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

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

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

Bruce Aleksander