Gels and Dichroic filters = Lower CRI rating?
Recently I have been researching the topic to see if gels and dichroic filters alter a lamps CRI rating. Several on other forums say they do not. However, I have recently contacted LEE FILTERS, and this is the response I got:
"CRI, which indicates the amount of the spectrum present in a light, is effected by filters that subtract specific segments of the spectrum to achieve their result, so the answer is yes."
So does this mean by adding a CTB, whether it be dichroic or gel, essentially give a tungsten/halogen lamp a CRI rating less than 100 as well all other light technologies?
From my understanding CRI is looking at the "missing" parts of the spectrum of any given light source. By adding a CTB to a tungsten, this isn't completely eliminating part of the spectrum, it is simply subtracting from the reddish part of the spectrum.
Two parts to this.
The first is the nature of the light generating technology. Are you starting with full spectrum or something less than that?
The second is that, yes, you are correct, all a filter can ever do is remove something. It can't ADD light in a particular spectral band - only remove it.
Now lets go farther for fun.
Say you have a source like an HMI that is pretty full spectrum, but you're shooting at sunset when the ambient light is shifted towards the warm part of the color spectrum. Adding CTO to the light, subtracts blue - and balances well with what's in the environment. It's not "making everything full spectrum" - it's better matching two sources into a spectrum that's already there. In post, if you think it's too warm, you can correct for that by shifting towards blue and get a pretty decent natural result that appears really nice.
Now lets take the same scene and throw up a bunch of cheap LEDs that skew green. This will have actual missing wavelengths in the spectrum of light being put out. They put out WAY more green than they should - not because the green is too much, but because they are NOT generating enough reciprocal light across the color spectrum. Slapping a magenta filter REMOVES some of the green, but it does not put back any of the missing magenta. So sometimes you don't get a clean color rendition no matter what post filtering you do. The wavelengths to make a full spectrum color shot just weren't captured.
Today, in the digital space, smart software can take all the pixels that skew green and replace that greenishness with NOT greenishness. Which is awesome. But it takes time and effort and many of us still think that coming back with really nice looking footage "in the can" is still the best way to practice our craft.
So I applaud you for asking questions and studying up on this stuff. Even with all the actual facts accessible on your phone in your pocket - thinking about how the facts work in the real world is its own form of education.
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