[argyllcms] using a digital camera as a colorimeter

Hello everybody!
I have been using ArgyllCMS to produce an RGB profile for an Epson Stylus Pro XL. I am using the printer in Linux with the gimp-print driver. I am using the "scanner as a poor man's colorimeter" technique with an IT8.7 target from Wolf Faust.

Profiles I generated from scanners have consistently produced a strong green/cyan cast on the prints.

I then used a video camera + tungsten light source and, to my surprise, got a better result, but still with a color cast. (Well, not much of a surprise, tungsten wasn't filtered to daylight, I used the white balance on the camera.)

I then used a digital camera (Kodak DX6440) under direct sunlight and got an *excellent* result, with almost no color cast. This camera doesn't have a "raw" mode (only sRGB JPEG), and there is some clipping on the primary colors, as one would expect in the sRGB space. No doubt I will do this again with a camera that gives more control and more range... I have only tested the profile with "photographic" images, I haven't tested it against very saturated color patches that may reveal errors due to the clipping.

My interpretation for those results is :
a. the light source could be a more important factor in color rendition of these devices than the spectral response of the RGB filters. The scanners I used offer the worst light source i.e. cold cathode tube, that looked very "spikey" in a handheld spectroscope.
b. the scanner seems to clip highlights when using a paper with fluorescent whitening agents. The clipping is different with a camera.

Here are my questions:
1. Please comment on this.
2. What would be the difference between using a colorimeter (eg. Color mouse CM2C) and a digital camera with a good quality light source (xenon strobes)?
3. Can I use the peak error and average error reported by the "profile" command as an indicator of the accuracy of the color measuring device?
Yours, Stephan.

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