Re: Finding CIELab Values from Target Chart TIF
Re: Finding CIELab Values from Target Chart TIF
- Subject: Re: Finding CIELab Values from Target Chart TIF
- From: Joseph Yates <email@hidden>
- Date: Wed, 28 Apr 2010 18:02:28 -1000
- Thread-topic: Finding CIELab Values from Target Chart TIF
Roger,
So how does PhaserMatch internally calculate what calibrations (adjustments)
to send to the printer if it doesn't have some sort of reference values to
compare & calculate differences for the acquired measurements?
How are these reference values normally described? For a CMYK printer?
The PhaserMatch screen displays the measured patch value as Lab
So I assumed that it was comparing these measured values to some sort of
reference Lab value.
I just opened the ECI2002 CMYK i1 Ref.text and you are maybe right - as it
has 4 reference numbers CMYK_C CMYK_M CMYK_Y CMYK_K
There is an associated file from PhaserMatch which Xerox said might contain
the reference values - CalAcquiredDataFile.dat
I've opened it in Word and the numbers seem to indicate
1) Patch number
2-4) Lab value of "acquired" measured patch
5-8) The last 4 values are CMYK reference numbers?
Patch 139 (last one) is 100%K in the TIF Patch Chart and in this file the
last 4 numbers are 0.0000 0.0000 0.0000 1.0000
So I assume this translates to CMYK 0/0/0/100
Not sure I understand how one uses these Reference CMYK numbers for each
patch to compare against the "measured" values for each patch which are in
CIELab?
Joseph Yates | Pacifica Island Art | Maui, Hawaii
> Joseph,
>
> May I ask what makes you think that this TIFF file as an absolute set of
> "Reference CIELab values"? I could be wrong but, to my knowledge, these
> charts never have an a-priori set of associated CIELab values.
>
> Best / Roger
>
>>
>>> Dear List
>>>
>>> I have a 140 Patch Target TIF file that I don't have access to the
>>> Reference CIELab values.
>>> The chart is used by PhaserMatch 4.0 (created by X-Rite for Xerox).
>>>
>>> Requests to both Xerox and X-Rite have not been fruitful and so I'm
>>> now wondering if their is a way to "extract" these values from the chart
>> itself.
>>>
>>> I have opened the the Chart TIF in Photoshop and can read the L*a*b*
>>> values in the Info palette - but only as single digit L*a*b* values.
>>> Is there a method to obtain more precise (minimum 3 decimal places)?
>>
>>
>> The Lab values you are seeing in photoshop are just a simulation. They are
>> calculated from the RGB / CMYK values in the tiff file using your current
>> colour settings. This is assuming that the TIFF file is not an Lab file.
>> If you wish to have more precision, use photoshop or any other image
>> viewing software to extract the RGB/CMYK values for the patches. You can
>> then use Colorsync utility or any other such tool to extract more precise
> Lab
>> values using CMYK/RGB profiles for the process you are trying to simulate.
>>
>>
>> Peter Skarpetis
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