"Ken" wrote in message
(I said)
Yes, there are any number of them. The Gretag-Macbeth Color Checker is very
very popular – costs 90 dollars or more, and there are web pages where the
RGB and Lab numbers for the squares are published, as well as TIFF images of
the chart, using idealized numbers.
What do you mean by ‘idealized’?
By idealized, I mean they have been arrived at by an unknown process, presumably involving averaging several charts together. In fact, GM publishes XYZ values for the color patches, but it’s problematic how to convert these values to RGB – which should give you an idea of where applied color science is at right now.
The problem is, the Color Checker was
designed strictly as a visual checker, and not intended to be used for this
purpose.
Even though it wasn’t *designed* for this purpose, might it not suffice?
I suppose so, but most people would say no. You wouldn’t believe the level of complexity involved in determining whether it would work or not. As an example of the kind of discussions that go on, here are some words from Tom Lianza, a color authority at GMB, about using the chart to calibrate a camera:
1. When asked, I suggested to the Munsell folks that we DO NOT publish target information, other than what had been published in the original work done by McCamy. The color checker was never intended to be a colorimetric calibration target, so publishing standard values would imply a use for which it was not initially designed. I did point out that we needed to make a reference to sRGB because the target had been co-opted by so many media sources as a measure of camera accuracy. This would need to be done with great care because it is a very difficult problem. I don’t think there was enough time for too much discussion, so we find ourselves in the situation that we are in today.
2. The ColorChecker has a unique set of colorants which are designed to reflect (pardon the pun) a range of colors based upon naturally occurring spectra. This means that the colors themselves, will not appear constant under varying types of illumination and that in most situations, the colors will change visibly with respect to one another under gross changes of illumination.
Still, the problem is that your screen does a great job on reds, greens, and
blues, and a poor job on magenta, cyan, and yellow, so any printed target is
going to leave some corners of your display uncovered.
I don’t quite follow this, but I’ll give it more thought. I am still very new to the world of colour management so it takes a while for me to get my head around some of the concepts.
The short answer is that the colors on the chart do not cover the pure colors that your monitor is capable of generating, nor do they cover the same range of brightnesses. For these reasons, and others, using a Color Checker to set up your display will probably not work very well, though it may help.
It would be a brain-numbing task to set up the digital file but in absence of a commercial product it may be an option.
There is no reason you couldn’t make one from common objects, and I’ve done
a small amount of work in this direction. For example, a box of Crayola Crayons contains more colors than a Color Checker, and costs a dollar or two. I’ve scanned these values in, and made the values of the crayons, certain brands of paint chips, and other common objects, available in text
form here:
http://www.curvemeister.com/downloads/pins/
Thanks for sharing these data with us.
How consistent have you found Crayola crayons to be? Have you sampled a few different boxes?
My secret sources in Eaton, Pennsylvania claim that the colors are carefully controlled within a deltaE of 3. OTOH, the formulation changes from time to time. I’ve compared older crayons with my newer set and the colors are different.
BTW – Munsel, the father of color theory, was also the inventor of crayons, and sold the rights to market them in North America to Binney and Smith (the Crayola company), whose marketing weenies promptly changed the names of the colors. Originally, the colors had numeric names, something like Green 2 blue 3 for blue green, and these are still used in Europe today to specify colors rather precisely.
http://www.cis.rit.edu/fairchild/PDFs/PAP21.pdf BTW, the Lego file seems to have only instructions for the format of your pin files and no data about the blocks.
Thanks for pointing this out.
These are pin files, intended to be used with Curvemeister, but you can open
them as normal text files to get the Lab color numbers for each "pin".
The only tool I have available (that I am aware of) to produce a digital file using Lab values is Photoshop CS2. PS only accepts integer values, while your values cited are to two decimal points. How significant are the decimal points? Would rounding to the nearest interger significantly affect the results?
Yes, you can round them off. If you want RGB values, use Linebloom’s calculator to convert:
http://brucelindbloom.com/index.html?Home.html (lots of interesting stuff here, BTW)
I think if you came up with a procedure based on comparing inexpensively available target colors with a digital image, you would do the graphics word
a service. Or you could do what most people do: spend some money on an instrument, and cross your fingers.
Let’s assume one finds a way to *check* the calibration and finds that the calibration is inaccurate in a certain area. How easy/difficult is it to adjust a profile file to make the necessary corrections?
If you have the time and energy to persue this, Timo Autiokari (a nemesis to many traditionalists), has devised some very interesting charts, sort of like Adobe Gamma on steroids, that will check your disaply’s gamma for individual channels. Be careful, though, this is a little like the beginning of Dorothy’s adventure, and there are lions and tigers and bears: some of Timo’s ideas are excellent, particularly the calibration charts. Other of his ideas require independent thinking and evaluation. I guess what I’m saying is he’s a great guy, but don’t take everything he says at face value. He is also very good an answering questions, so you may fine, as I did, that he is a valuable resource.
http://www.aim-dtp.net/ As a matter of interest, how do *you* calibrate your monitors?
I use an Eye One Display 2 Pro. I have to do this because I teach an online class (free – starts Sunday the 7th) and I need to reassure people that my monitor colors are reasonable. I would emphasize that a calibration device is not that great a benefit, and many people buy them who don’t really need them, and are disappointed when their printed output still does not match the display.
—
Mike Russell
www.curvemeister.com/forum/
