[argyllcms] Re: using a digital camera as a colorimeter

  • From: Stephan Bourgeois <strangelv@xxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 07 Oct 2005 08:14:44 +0100

Alastair M. Robinson wrote:

That's very interesting.
Did you photograph the IT8 target and build a profile for the camera, or "trust" the camera's sRGB space?

I used the IT8 to profile the camera. Then took a snap of the print target in a separate shot, without changing the lighting conditions. The first step is necessary to relate the camera's exposure range to CIE L values, and compensate for eventual white balance shifts.

If the former, did you have the IT8 target and Argyll target in the same shot?

see above

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 am with Linux. I am afraid the clipping is due to the scanner calibration strip. I read posts about this on the newsgroup.

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.

Hmm, this is very clever, and would explain the cost of the instrument. I thought LED spectral emission to be spikey, so a collection of them would be needed to create a smooth overlap. The sensor is likely to be a silicon photodiode.

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.

The values I get when doing "profile" on the IT8 are: Snapscan1212: peak err = 7.703943, avg err = 0.807319 DX6440: peak err = 8.134495, avg err = 1.193943

When profiling the printer:
Snapscan1212:  peak err = 12.337768, avg err = 1.783332
DX6440:        peak err = 10.115366, avg err = 1.772115

No big difference between the two, but an order of magnitude larger than when using the colourmouse. This is spiking my interest for the colourmouse.
Thanks for the comments, most interesting.

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