[argyllcms] Re: using a digital camera as a colorimeter
- From: "Alastair M. Robinson" <blackfive@xxxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Fri, 07 Oct 2005 00:29:19 +0100
Hi,
Stephan Bourgeois wrote:
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.
That's very interesting.
Did you photograph the IT8 target and build a profile for the camera, or
"trust" the camera's sRGB space?
If the former, did you have the IT8 target and Argyll target in the same
shot?
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.
I'm told that using Vuescan can help to avoid the clipping problem (due
to better exposure control), which in turn helps minimise the colour cast.
I had a very interesting conversion with Stephen Tweedie a couple of
months back, and he showed me a program he'd knocked up to do some
jiggery-pokery on an Argyll .ti3 file and perform a rough FWA
correction, using just XYZ sample data.
Here are my questions:
1. Please comment on this.
See above :)
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)?
I imagine that the colourmouse would have a much wider gamut. I have
one myself, and have had good results generally.
The colourmouse, AFAICT has a fairly simple light sensor (possibly even
monochrome), and cycles between several different coloured LEDs. Using
the amount of light reflected from the patch for each colour, the driver
software figures out an XYZ value for the measured patch.
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?
No - the only way to do that is to measure some patches of colour,
convert them to the printer's colour space using absolute colorimetric
intent, print them, and compare the printed patches.
The peak and average error merely tell you well Argyll's generated
tables fit the original data in the .ti3 file. With the colourmouse,
and targets of 120 - 150 patches, I seem to get a peak error of about
1.3, and average of 0.8.
What sort of values do you see using your digital camera trick?
All the best,
--
Alastair M. Robinson
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