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: Peter Skarpetis <email@hidden>
- Date: Thu, 29 Apr 2010 14:19:28 +1000
On 29/04/2010, at 2:02:28PM, Joseph Yates wrote:
> 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?
It prints the target using the cmyk values in the file. it then measures them to obtain lab values. These cmyk, lab value pairs are then used to calulate both a forward (cmyk -> lab) and a reverse (lab -> cmyk) transform for the device.
Absolute lab values for the target does not make sense as the target is in device coordinates ie. cmyk and lab. All you can do is calculate how to convert between the 2, ie. device dependent space (cmyk) and device independent space (lab).
Peter
>
> 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
>
>
> _______________________________________________
> Do not post admin requests to the list. They will be ignored.
> Colorsync-users mailing list (email@hidden)
> Help/Unsubscribe/Update your Subscription:
>
> This email sent to email@hidden
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Colorsync-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden