[Lcms-user] Inaccurate sRGB-Conversion?

[Dietmar Dreyer]

Hi, I’m using littlecms 1.14 to convert scanner images to the sRGB color space, but I encountered a problem in validating the numeric accuracy of  my custom scanner profile, which I created with a recent version of the lcms_profiler (lcms_profiler_beta3.exe). The sRGB values are not quite what they are supposed to be.   I scanned a W. Faust IT8/7 Target with an Epson GT-30000 scanner at 300 dpi without any color correction settings and used this to generate a scanner profile, where I used the accurate, relative colorimetric, 33 point preset (which seemed to work best  for me). then I used the measurement file of the profiling procedure and the custom scanner profile as input to icctrans (v.1.9) in order to compare the results with the Lab values in my target reference file.   The commandline arguments I used:  - icctrans  -i <custom profile> -o *Lab –t 1 <measurement_file>   The Lab results were quite promising, but when I extended this test with the standard sRGB output profile in order to verify the accuracy of sRGB values, I got huge deviations between the color values computed by icctrans and my own values (which I computed with several color calculators from the Lab values).   For example:   Lab-Value: (93.41,   -1.20,   -0.26) Should be: 234 237 237 Icctrans: R=217.53 G=225.72 B=222.89       Most other color values are similarly far apart from the correct sRGB value.   I started icctrans with following arguments:    - icctrans –I <custom profile> -o “sRGB Color Space Profile.icm” –t 1 <measurement_file>   Any ideas what might be the problem here? Thanks!   Best regards Dietmar Dreyer