Sorry for the delay in responding, has been a rather busy couple of days
It's still unclear exactly what you are trying to do. My closest guess is to print something that provides a visual impression of the different instrument readings of the same color. That is, as series of patches that illustrate how one instrument's color measurement would vary from another.
Doug,
Yep - I have made a set of measurements off of a set of surfaces using a few different spectrophotometers measuring each one . Initially I want to visually compare the results of in print; the measurement formats are in CIE L*ab XYZ & spectral using Basiccolor Catch's ISO flavour of a CGATS text file. To get an initial idea of similarities & differences I was thinking to make a chart of the various measurements in Photoshop from the nearest whole Lab values, print them out and compare them to each other and to the original surfaces in a D50 viewing station.
Most instruments are quite consistent measuring neutral and near neutral colors if their reference white patches haven't gotten too old. But they become increasingly divergent as saturation increases. Fortunately, the visual sensitivity to color differences also decreases as saturation increases though there is some non-linearity in hue changes depending on color angle.
Thanks, will keep this in mind
My comments about variation between individuals is that the Lab values are made from the so called "standard observer" from data measured in 1931 using a small number of people. There are significant differences between individuals and different spectra with the same Lab values using the standard observer functions can result in people seeing different colors. It's an effect that can be greater than the differences between spectrophotometers. It's also a larger effect between monitors using different backlight tech. because the light is formed from combining spectrally peaked colors.
Thanks for the cue - I think this would be the next stage where I start to compare via spectral or XYZ rather than Lab - this I'd have to do in software - with amount of of samples though (6x per surface (excluding M condition variations), I wanted to weed out the ones that were obviously perceptually different first in print.
There's a rather oblique way in Photoshop. You can enter precision RGB values in a 32 bit RGB colorspace. But to do that with Lab values you will need to convert them with something like the precision calculator at brucelinbloom's site.
Looking at BL calculator; what Adaption would be the most suitable for going Lab > RGB32 (as well as XYZ > RGB32) : Bradford?
The thing you haven't mentioned, but is critical for getting such a match, is what light source(s) are being used to view the original and the print. Are they the same ? Are they different ? How close spectrally are they to D50 ?
Note that L*a*b* values from typical graphic arts instruments assume a D50 illuminant, and that a print/proofing system only offers a colorimetric reproduction of color, not a spectral reproduction. (What's the difference ? - the former will only match under the illuminant it was computed for - default D50. The latter will match under any illuminant.)
GWGill,
Good points; so the original surface and test print would be viewed under the same "GTI D50" illumination
Plus lots of other factors - are the target colors in gamut ? Does the paper have FWA/OBE ? etc.
Was thinking of using either Epson or GMG standard proofing paper to print on - that and the printer's gamut (via the Colourst RIP) would be up against the target colours (one of the sets target colours and Xrite SG target has some very saturated colours) - I'd have to take such reproduced colours with a pinch of salt
If the illuminant is not spectrally D50, then the process for getting a good match involves measuring the illuminant, converting the target colors to colorimetric values for that illuminant, and creating a profile for the proofing system that uses that illuminant rather than D50 in computing colorimetric (i.e. L*a*b*) values. And using absolute intent.
Understood - I have had to do this quite often before for artists who have made work under Flourescent lighting and can't believe that a certain colour is what they painted when they see the resulting documentation image on screen (or in print under full spectrum & D50 lighting)
[ But this does depend on what sort of fidelity you are after. If it just has to be in the ballpark, then plugging the L*a*b* values into Photoshop may be enough. ]
For the initial viewing L*ab will give me a basic idea - I can then narrow things down and start looking at the contenders one by one on a spectral level afterwards.
Thanks all for your time on this.
Alex