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Thread: Spot colors: How Lab->CMYK->Lab gets calculated?

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    oxident is offline Fiery Forum Expert Contributor oxident is on a distinguished path
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    Question Spot colors: How Lab->CMYK->Lab gets calculated?

    Hi!
    I'm trying to understand how the spot editor calculates the CMYK- (and therefore the converted Lab-)values out of a spot color's original Lab value.

    To my understanding, the original Lab value is converted to DeviceCMYK using the selected output profile and then converted back to Lab in order to measure the DeltaE of this conversion.

    Therefore, no other parameters (like rendering intent, white-/blackpoint, GCR settings) are taken into account, aren't they?
    IC-306 v3.01 FS100 Pro / KM C6000, IC-308 v2.1 FS150 Pro / KM C71hc / SD-513, IC-414 / KM C754
    CWS 5.8 SP2, Windows 10 x64

  2. #2
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    You are absolutely correct.

    First, the DeviceCMYK corresponding to the spot color targeted Lab value is calculated like it would in a job using this spot color and output profile. Then, the Lab expected to be produced by this calculated DeviceCMYK is interpolated using the same output profile. The DeltaE is the difference between the targeted and the expected Lab values.

    The more a spot color is out of gamut of the DeviceCMYK space, the higher the expected DeltaE.
    Even within gamut, the accuracy of the displayed Delta-E depends on the accuracy of the profile for the print condition, which is among other things affected by the samples that were measured when the output profile was created. For instance, if a spot color would call for a pure Cyan at 33%, the estimated Lab will likely be more accurate if the measured profiling chart included samples at 30% and 35% Cyan than if surrounding patches were farther away, lets' say 25% and 50%.
    Last edited by Paul_M; 07-13-2018 at 09:19 AM.

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