That is the Kodak Q-13 set, which includes the "Color Control Patches" and a gray scale. I’ve found only a very small amount of information "on the web" regarding the values you should expect for each patch, and quite a bit of controversy in the four-color printing industry regarding their use.
Timo has a page that expounds his viewpoint on the Kodak targets at
http://www.aim-dtp.net/aim/calibration/middle_gray/ Norman Koren weighs in on the Q-13 set at
http://www.normankoren.com/color_management_4.html#Q-13_colo r Another reference … this time from Cornel University … is
http://www.library.cornell.edu/preservation/kodak/kodak-htm. htm Carl Macmillian has a little bit of information on his site, developed from the Gray Scale and Color Control Patches from the Kodak Color Dataguide CAT# 156 9136. It is not exact … but might be a relatively close "starting point."
http://www.carlmcmillan.com/Test_images.htm There was a long discussion of using these targets in the Applied Color Theory mailing list (moderated by Dan Margulis … hosted by ledet.com). Some of the references and highlights are as follows.
Some of the references from that discussion were Values for Q-13 gray patches, by Iliah Borg, are at
http://www.pochtar.com/gamut_view/gamma.xls His values do not allow for the print base density of roughly 0.05, and might be a bit different than what you would get from a scanner or a camera image from the Q-13 grayscales.
He also has values for the Macbeth Color Checker at
http://www.pochtar.com/s/GMB_CC.tif (in Lab, 44kbytes)
However, Peter Fegan posted:
"…
Having put these color bars in images next to artwork, and then having the luxury of color correcting to the actual artwork under a Solux light, I can say that the color and gray bars have almost no usable value if you are truly trying to match the art. I think the biggest culprit is that the dyes in the printed color bars don’t have the same spectral characteristics as those in
the art, let alone how the film or digital camera used to record the art responds. Anyone who relies on color bars alone is bound to be disappointed.
…."
John Castronovo posted:
"…
People should realize that the value of the target isn’t that it represents the painting, but that it represents the LIGHTING. As such, when the gray scale is correct, the best AVERAGE color correction has been achieved.
Of course, there are going to be colorants in the original that will be off color, sometimes dramatically so, and those have to be handled on a case by case basis. However, we have to begin with the best overall correction, and that happens when the gray scale is correct.
…."
Dragonfly Imaging and Printing then posted:
"…
I find that once I have corrected for the lighting, I am much closer to the ‘original’. This makes color adjusting easier for the artwork. Only the grayscale seems to be of much use in this context.
…."
Then Thomas P. Kaczmarek posted:
"…
The gray scale hasn’t been corrected to begin with. What actually happens, during photography, is that each original artwork will be color corrected individually, sometimes with a radically different combination of CC filters. Some other methods used to reproduce art is using cross polarized lighting and using a separate bump flash to control contrast. In fact, by adding different CC filters to the bump flash a subtle shift in color can be introduced from the shadows to highlights, what does that do to the gray scale?
All these corrections are solely aimed at reproducing the art, and will create reference materials that do not represent the lighting or any other element of shooting the original. All I’m trying to get across is that the aim of the printer should be to correct toward the image of the original. I’ve seen too many proofs that a printer made assuming the reference materials should be correct, thus causing unnecessary rounds of corrections. If the printer makes the assumption the image is correct some of those corrections can be eliminated.
Does it make sense to even include the references in the transparency? Probably not, but everyone wants to see them even though they’re worthless in color correction. They do show the accuracy of the exposure in reproducing the delineations of the gray scale from black to white, not the intended neutrality of the lighting conditions. The Color bars are completely inaccurate for color corrections. With this said will you get a customer that likes the proof if you correct toward the gray scale, pretty sure you would, but not many. At least not the ones who want accurate reproductions and artists can be a very picky bunch. Unless told otherwise by your client, a printer must assume the intended color of the original is accurately represented by the image on the transparency, not a color bar or gray scale. Sometimes you have to place your trust in the person making the repro trans, they should be color correcting toward the original, not the reference materials.
…."
= = = = =
Some earlier discussions regarding the Kodak Q-13 targets on the ACT newslist are archived at
http://www.ledet.com/margulis/ACT_postings/SeparationIssues/ ACT-Printing-to -Graybar.htm
and
http://www.ledet.com/margulis/ACT_postings/ACT-PrintGrey.htm lThe "truth" … obviously … lies somewhere in between these several somewhat divergent opinions. As to "color correcting" the artwork you have posted, I guess a couple of things you might take into consideration are:
(1) Wherever possible, correct to the actual artwork. The color text target(s) and grayscale target(s) are only an indication of what is happening … and actually probably more an induction of the lighting than anything else.
(2) The test targets in your image file look a little dirty and/or worn. They might NOT be a very accurate indication of the labeled (absolute) colors.
I would suggest basing your first cut on the patches in the gray scale on the right side of the image. The color patches are printed using printing ink, but the grayscale is an actual black-and-white print … and therefore silver based … and probably closer to a real "gray" than the color patches. In fact, I did a quick "trial run" based on those three patches, and got something that looked relatively reasonable. Bear in mind that I do not have any kind of access to the original, and am saying "reasonable" simply based on my own *perception* of what the artwork might have looked like. And, oh yes, the blue even looked fairly clean … even though it showed a relatively high amount of green in the ‘Information’ window.
FWIW: The grayscale is organized in twenty patches. beginning at a nominal density of 0.05 and increasing by 0.1 density units per step. Use the "A" patch (step 0) as having a density of roughly 0.05, the "M" (step 7) patch as having a density of roughly 0.75 (roughly middle gray, or somewhere around 105 to 128 on the TIFF RGB 0-255 scale), and the "B" patch (step 16) to have a density of 1.65. The final step should be something like a density of 1.95.
"Bill G" wrote in message
I’m trying to correct the color in a photo of
a print downloaded from the Library of Congress.
I’ve posted a reduced version of it at:
http://www.geocities.com/tang_horse/ColorCorrect.html
The photo displays "Kodak Color Control Patches" which seem to indicate that the blue has suffered greatly in the various processes.
Is it possible to use these patches to automate or at least simplify color correction for this print with PS’s tools? I’ve tried just about everything that uses the Eyedropper, the Help file and several books. I think I’m just missing something conceptual.
Thanks,
Bill
