CRI & Temp: Can anyone simply explain the difference to me?

I did a search on CPF, but couldn't find a true definitive explanation. Maybe I'm just looking at the wrong terms. I keep seeing CRI (I know that mean Color Rendering Index) and Temp/Kelvins thrown around. At times it seems they're almost used interchangeably, but at other times seems they are unique markers.

I understand Cool/Neutral/Warm, and it seems that CRI and K's are thrown into those same discussions. How do they relate to one another?

Can anyone give me a reasonably simple explanation of the difference, and how those markers impact what I look for in a light?

Not sure if you've seen this discussion, but there's a lot of good info in here. It boils down to CRI being referenced to a particular light source that's considered "standard" for that particular color temperature.

http://www.candlepowerforums.com/vb/showthread.php?273878-CRI-of-White-LEDs

I'm no expert, I'll merely post my understanding of the issue.

Color Temperature is basically a measure of the light output, how it appears to us. White output LED's are blue LED's with a phosphor coating over the die, that when excited by the light produced by the blue LED, emit a "white" light. A 'single' layer of phosphor coating will result in a standard cold white high color temperature light. Very blue.
Manufacturers then fine tune the phosphor coating (or add additional layers, honestly I'm not quite sure which) to add further layers to convert more of the blue emitted light into a "white" light. This conversion process is nowhere near 100% efficiency, so a lower color temperature LED is likely to be less efficient.
The Lower color temp. (basically the addition of more red, green, etc) appears more natural to us, as our eyes have adapted to the incandescent sun/atmosphere combination. So the addition of more Red light (and green, etc) will make it easier for the eye to determine color, shape, etc.

Color Rendering index is a scale that's graded on how well a given light renders color, when compared to sunlight. So the better the CRI rating, the easier it is for our eyes to distinguish colors, especially very close colors.

From my understanding, the addition of more phosphor layers (and thus the conversion of a greater percentage of blue light into red, green, etc) causes several things: an increase in CRI, a decrease in color temperature, and a decrease in efficiency (remember, you have a certain amount of blue light to begin with, and each additional layer of conversion/filtering reduces this). These can't necessarily be avoided, although I'm sure there's an exception to every rule (wouldn't it be nice if there were R5, 5000*k XP-G's in large enough production numbers).

Note: Again, I'm not an expert at all of this. This is just my understanding of it all, and it may/may not help or be correct.

Color temp is how blue or yellow the light is. A high color temp (hotter) means a bluer light whereas a cooler color temp means a warmer more yellow color. A given color temperature can be arrived at in many ways which means two lights with the same color temp may not render the same appearance when pointed at various objects.

CRI compares the output spectrum of a light of given color temperature to the output curve of a an 'ideal' black body radiator at the same color temperature. The curves may have a different shape and a greater deviation means a lower CRI.

Temperature is a description used on white light, i.e. both the sun and a incan lamp has white light, but they do not look the same. The sun is a cool light with a color temperature in the 5000-6000K range and the incan is about 3000K. White both kind of light we can see all colors, i.e. red looks red, green looks green etc.. This is because both the sun and incan as a CRI of 100.
Other light sources (Like most leds) might not render colors as well, even though they look some kind of white, this type of light has a lower CRI.
I.e. with CRI=100 all colors looks correct, with lower CRI some colors might look strange.

ElectronGuru put it very well in this post:

http://www.candlepowerforums.com/vb...Malkoff-M61W&p=3631333&viewfull=1#post3631333

Correlated color temperature (CCT) is the color that a perfect black-body object would glow if heated to this temperature in Kelvins (a black-body object heated to 3000K glows a warm white).

If you used your glowing black-body object to illuminate a dark room, the colors you see on the objects around you are "true" since the black-body object has a Color Rendering Index (CRI) of 100.

When you illuminate an object with a 3000K 80CRI LED, the colors you see on that object are ~80% accurate compared to what you would see if you were using a black-body radiator.

If you have an LED with 80CRI at 2750K, then you overdrive your LED and the tint shifts to 3000K, the CRI may no longer be (and probably isn't) 80 - you have to retest it to be sure.

Some good explanations above.

Colour Temp and CRI are not the same. Temperature of light is measured in degrees Kelvin and refers to the 'warmness' or 'coolness' of white light (There's more to it but that is the basis). Daylight is 5600K, Tungsten is 3000K, Fluorescent is 4000K etc (these are just general examples)

There is a common miss conception that 'warm' or 'neutral' LEDs have a higher CRI than cool. This is not the case unless the LED's have specific phosphor layers to render and achieve a high 'CRI' specification.

Temp is simply the tint. Warm to cool. CRI is how well they produce color as compared to say, an incan light. They are not the same thing. It's just that lights with high CRI tend to be on the warm side. Something to do with manufacturing a high CRI light that's beyond my understanding. Just because a light is warm tinted doesn't mean it has a higher cri, although in general, it's usually higher cri than a cool light.

Add to that, your eyes will "adjust" the tint for you. Use a blue tinted light for a while, and it will seem white. Then turn on a warm and it will look brown at first. Use the warm for a while and it will begin to look white, and the cool light you thought was white, will be very blue looking.