Re: Media Testing for maclife.de
Re: Media Testing for maclife.de
- Subject: Re: Media Testing for maclife.de
- From: Eric Chan <email@hidden>
- Date: Thu, 11 Sep 2008 06:21:45 -0700 (PDT)
> The
> documentation
> available for the DNG Profile Editor says nothing about
> allowed
> illuminant tolerances.
Well, it all depends. As I'm sure you realize, there are many, many (well, infinitely many) spectra that can map to the same correlated color temperature value. I have a standard household incandescent bulb here that measures 2850 K (pretty close to illuminant A) but also a Home Depot compact fluorescent "warm" bulb that also measures about 2850 K. They have completely different spectra and if you visually compare a ColorChecker (or any reasonable target) under the two lights, they will look very, very different. So, clearly a profile built based on one of those is going to have some difficulties with the other.
There is the concept that if you shoot a target very, very carefully, with a single light, away from all other color influences, minimizing reflections, etc. then a profile built from that capture will work well under all lighting. That is largely true as long as the spectra of the test lighting is closely related to the spectra of the training lighting (i.e., the light used to shoot the target), and it holds true for many flavors of natural daylight, with its fairly smoothly-varying spectrum.
If this was really true in general, however, we should expect nearly identical behavior when analyzing the white-balanced raw data (i.e., in camera-native linear coordinates). That is, shoot a raw file under one lighting condition, shoot the same target under another lighting condition, and the white-balanced raw values should be pretty close. Unfortunately, in general they are not. Some materials will be close, even nearly identical, but others will be very far off, with a very different balance in R/G and G/B. From this, it should be clear that a single profile (whose responsibility is to map the white-balanced camera-native RGB values to colorimetrically-meaningful values, such as XYZ or Lab D50, or RIMM) cannot work for both.
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