Wednesday, April 21, 2010

Tolerancing color in presswork - CIE L*a*b* and DeltaE

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10 comments:

  1. Thanks for the excellent post on describing LCH and LAB.
    Are you aware of any software that will generate a grid of patches based on a starting specified value and then show patches and values within a specified DeltaE from the starting color? I think a tool like this would be helpful to communicate with colors how close or far away a color is by giving them a visual reference.

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  2. Hi Steve,

    Such a tool would be great...but I don't think it exists.

    But here are some ideas -

    Go to http://www.scribd.com/doc/17736962/DeltaE-chart
    This is a grid chart that shows a green color varied by different DeltaEs.

    You could also go to Bruce Lindbloom's excellent color website:

    http://www.brucelindbloom.com/

    Then go to-->"Calc" -->"Color Difference Calculator"

    You can enter your CIE L*a*b* values and see how their DeltaEs vary according to the different formulas.

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  3. "5) There are no CIE L*a*b* controls on a press"

    True. However, if you work using CIE L*a*b* and CMYK you'll eventually learn (Just like RGB <-> CMYK). For a starter I would recommend section E.12 (specially Figure E.11) in G7 How To Guide 2009 p.56-58.

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  4. Thanks Martin.

    I have not been able to find the "G7 How To Guide 2009" on either the idealliance or presstools websites. Do you have a URL to where it is located?

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  5. Actually I found it googling a year ago. Now, I can't find it any more, sorry no URL :-(

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  6. It is well hidden, a search on those websites turns up zero hits. It appears that it is now only available to members for a fee.

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  7. Great post Gordo.

    Concerning CIELab and delta E as a process control metric, I'd also point out that Delta E tolerances (typically 5 dE76/2.5 dH for solid primaries), usually yield a very wide window on press. Correlated to density, the equivalent of 5 delta on a solid primary could yield a window of 0.30 (or more) density units in some cases. Most pressman would scoff at the idea that +/- 0.15 density is an acceptable tolerance range. One could minimize this by first printing a density sweep from low to high density, and correlating the optimal wet density to the lowest delta E to dry target CIELab values, From there tolerances can be established based on the presses capability...which might be more in the realm of +/- 0.05 density.

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  8. I got a profile about G7 2009 version.But it`s write in chinese~
    The definition of the gray balance changed,not only according to a certain Lab value,has something to do with the paper brightness

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  9. Hey, Gordo

    I am confused this sentence you mention "The sphere, with the specified color at its center, represents the acceptable amount of difference between the specified color and other measured samples (the color output)."

    1, Does it mean no matter what color I point or specify, this color automatically locate in the dead center of tolerance sphere?
    2, Does tolerance sphere just apply in CIE L*a*b*?

    Thanks

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  10. 1 - Yes you can think of it that way. The color you specify is at the center of the sphere. If, when you print the color, it measures exactly the same as the specified color then it too is at the center of the sphere. That rarely happens in real production. The color will always be slightly lighter or darker or redder or greener or yellower or bluer. Or it might be the right darkness (L*) but the wrong hue. The sphere represents how different the color can be from the specified color and still be an acceptable match. That difference is the acceptable tolerance and is expressed as in DeltaE units.

    2 - The tolerance sphere could apply to any specification, like L*a*b*, that is defined in 3 dimensions. I.e. a specification where there are 3 variables.

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