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Re: CRI and Color Temps and why they don't correlate

Ben B. asked about CRI and Color Temp:

> As I thought I understood it, CRI
> compared a
> light's quality to Sunlight and a value of 100 most closely
> represented the
> that of the Sun.  Also, a temperature of 5,500K is considered close
> to
> natural sunlight.  If you look at the CRI and the temp. of some of
> the
> lights in the file, there is no correlation.  Some 3500K lights have
> a
> higher CRI than a 5000K light.  Apparently I have something confused.
>  Could
> someone fill me in?

Without trying to get too complicated or precise:

These are two different measurements and the correlation isn't what you
might at first think it would be.  Color Rendition Index (CRI) is a
subjective mesurement of how much colors that we humans can see will
look like the same colors we see under normal light.  CRI is sort of
like "how close is it to showing normal colors."  A bulb with an 80 CRI
"means" that colors will apear to humans about 80% like normal 9sounds
silly doesn't it? -- but at least it's something to go by so long as
you don't try to cut too fine point with it).  Two bulbs might have the
same CRI rating and be somewhat off by equal amounts on different
colors -- one is a little bluer and the other is abit redder  but both
of them are about 80% of the norm.  Or one might have a narrow spike in
one frequency that doesn't have much impact on color rendition to
humans but does effect the color Temp.

Color Temp is the color of the light when a black body is heated in a
vacumn to the given temperature.  One bulb could be heavy on the blue
spectrum, but have a big spike in the red range, while another could
have less blue and no spike in the red.  The two bulbs could them have
the same overall color temp but would render colors to humans
differently and could have different CRIs.

Hope that helps.

Scott H.

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