Hello Barry: I tried to send a message off the list in the original thread but was returned by the server. You must be happy with your resources. Perhaps we got to wait for a better color reference target as Klaus Karcher did, but a workflow where the capture device is a medium format DSLR is the best relationship between price and quality if you (I'm in the situation) haven't access to 4x5" or greater slide sheet or don't want work with it for time consumption in the timeout for developing and verification. Other thing is that you can afford a drum scanner. You can compare "a few dozen an hour" with the production of a large format camera of a few dozen (more) a day. I can capture one hundred 35mm color slides per hour, individualy focused and exposed and can go faster with monochrome and color negative strips. With bigger photographic materials is slower. But when capture pieces of art, framing, iluminate, focusing, shot color reference chart (ColorChecker SG and Kodak Q14) at the best aperture for the lens, verify the light condition and shot, twelve captures per hour is a good result. If the piece of art is fixed to the wall takes other timing. The question here is money. Are you going to capture for general purpose without compropise or a "definitive" capture. The two are the same if we consider conservation but you must do that with the best combination of harware, software and consumables you can buy. In this sense there are rich brother and poor brother. It seems like you have a external service to do the job, and I could suggest that a "well profiled" photographer can do all the job as institution staff member as curators, archivist, restorers,... that can work closely with designers, print service and others. In the actual configuration, leaving the maths to the software, am a custodian, photographer, editor and proof printer at a (little) institution, one-man band. If it serves, having in mind that is refered to photographic conservation and distributing photographic collections, take a look at http://fotoconservacion2011.org Salud Jose Bueno

Hello Barry: I tried to send a message off the list in the original thread but was returned by the server. You must be happy with your resources. Perhaps we got to wait for a better color reference target as Klaus Karcher did, but a workflow where the capture device is a medium format DSLR is the best relationship between price and quality if you (I'm in the situation) haven't access to 4x5" or greater slide sheet or don't want work with it for time consumption in the timeout for developing and verification. Other thing is that you can afford a drum scanner. You can compare "a few dozen an hour" with the production of a large format camera of a few dozen (more) a day. I can capture one hundred 35mm color slides per hour, individualy focused and exposed and can go faster with monochrome and color negative strips. With bigger photographic materials is slower. But when capture pieces of art, framing, iluminate, focusing, shot color reference chart (ColorChecker SG and Kodak Q14) at the best aperture for the lens, verify the light condition and shot, twelve captures per hour is a good result. If the piece of art is fixed to the wall takes other timing. The question here is money. Are you going to capture for general purpose without compropise or a "definitive" capture. The two are the same if we consider conservation but you must do that with the best combination of harware, software and consumables you can buy. In this sense there are rich brother and poor brother. It seems like you have a external service to do the job, and I could suggest that a "well profiled" photographer can do all the job as institution staff member as curators, archivist, restorers,... that can work closely with designers, print service and others. In the actual configuration, leaving the maths to the software, am a custodian, photographer, editor and proof printer at a (little) institution, one-man band. If it serves, having in mind that is refered to photographic conservation and distributing photographic collections, take a look at http://fotoconservacion2011.org Salud Jose Bueno

I'm not out of the office

This message apparently did not get through to the list last week (sent: Fri, 03 Jun 2011 18:20:02 +0200). Therefore I send it once more: Wheeler, Barry wrote:

THANKS to those of you who have responded to my earlier message. I am interested in continuing the discussion directly with others who are interested. In particular, I am interested in exploring camera profiles and the set of colors used to improve the accurate of specific cameras.

[...]So I simply import a color set (as Lab values) into ColorThink and display the colors, a gamut boundry, and the ColorChecker colors. Thus, by "distant" I am referring to the visual distance between color points in the ColorThink plots. I find this presentation is immediately comprehended - and usable! - by the staff.

The method you describe is indeed useful to estimate whether or not the gamut of a certain output medium or working space profile is sufficient to cover the gamut of an original. The gamuts of working space and output mediums should enclose all possible colors of the original in order to avoid clipping. Furthermore the method can be used to estimate whether the gamut of the profiling target is sufficient to cover the gamut of your originals. As the extrapolation capabilities of LUT profiles are rather poor in general, the gamut hull of the profiling target should enclose all possible colors of the originals in order to avoid extrapolation errors. So the method you describe is valuable to evaluate two possible sources of error. IMHO the pure colorimetric distance of target and original points is not a very meaningful criterion however: For example you can print a profiling target that perfectly matches the Lab values of your original. When you create a profile with this target, you might get very good results as long as you measure the accuracy of your system with the same target (or a different target printed with the same pigments) -- but when you apply this profile to the scan of your original, there might be still huge and systematic color errors! A pure colorimetric approach can not reveal the source of this problem as the source of the problem is "buried" in the /spectral/ properties of original, target, light source and sensors. From a mathematical point of view, the tristimulus values (Lab or XYZ values that characterize the color perception of the human observer) and the RGB values returned by the camera or scanner are projections of the spectra (in an high-dimensional spectral space) onto two different 3-dimensional subspaces. Projections onto lower-dimensional subspaces always imply a loss of information. A more descriptive analogy for this kind of projections might be shadows casted onto projection screens: When you see a circular shadow on a projection screen, you can not distinguish weather e.g. a ball or a flat, round disc casted this shadow. When you use a second projection screen at a different location, the shadow of the ball will still be a circle, but the shadow of the disc might degrade to an oval or even to a thin line. The Lab values we see are just /one/ projection of the "high-dimensional truth". And the RGB values returned by the Camera or sensor are /a different/ projection. When you try to build an ICC profile that maps one of these projections to the other (and exactly this is the role of the ICC profile), you will get in trouble sooner or later: Neither of the projections contain enough information to set up an error-free mapping. Some of the mapping instruction had to be simply contradictive: The profile has no idea whether the circular shadow steams from a circle or a disc and therefore it can not map it once to a line and once to a circle. <http://digitalproof.info/colorsync-users/paramere.rlc.c3d.73.pdf> shows measurements of two gray samples. Both samples are almost perfectly neutral gray and more or less indistinguishable when viewed under D50 (i.e. they are metameric). When you calculate Lab values for both samples, you'll get almost the same results: Delta E is only 0.76. But when you capture these samples with a sensor whose red sensitivity is slightly shifted to the right (and sensors like this are quite common!), you will inevitably get quite different RGB values. When you create a profile from a target which contains only the "flat" kind of grays (that's the case with the ColorCheckerSG for example) but your original contains pigments which result in curves like the red one in my plot (curves like this can be found quite often, e.g. in dyed textiles or photographic prints) the gray patches of the color checker will be perfectly neutral in the scan, but the scans of many gray textiles or photographic prints will be way to red. There are two ways to mitigate this kind of problems: 1) the "projection screen" of the sensor should be as close as possible to the one for the human standard observer, i.e. the sensor should satisfy the Luther condition. Unfortunately there are tight constraints in terms of sensor design and even more in terms of retroactive sensor optimization and they generally involve hardware modifications. 2) the profiling target should represent the /spectral/ properties of typical originals as close as possible and the profiling software should deal with the remaining contradictions as smart as possible. In order to optimize the color fidelity of Cruse scanners I have followed both paths. With the old hardware configuration and standard profiling tools and targets, customers often complained about red shifts for example (even though e.g. color checker scans were perfectly neutral). With the modified hardware, my new target and a special profiling solution (based on Graeme Gill's excellent ArgyllCMS), there are no more /systematic/ red shifts. As the colorspace in this critical region will be locally compressed and the profiling software has (and seizes!) the chance to find the optimal compromise, some pale shades of red and green are now /a little bit/ too grayish, spectrally flat grays are /a little bit/ greenish and some grays with high IR reflectance are still /a little bit/ too red -- but the overall color error is /dramatically/ reduced. I think in order to quantify the accuracy of reproduction systems objectively, we badly need standardized, representative /spectrally/ defined test targets. And to optimize the reproduction quality of cultural heritage materials, we need either multispectral imaging devices or at least new, smarter ways to incorporate the spectral properties of these objects in traditional, tristimulus based reproduction workflows. Klaus Karcher