[argyllcms] Re: Question regarding White Point and spotread
- From: Brad Funkhouser <brad.funkhouser@xxxxxxxxxxx>
- To: "argyllcms@xxxxxxxxxxxxx" <argyllcms@xxxxxxxxxxxxx>
- Date: Sun, 02 Nov 2014 06:54:00 -0600
Have you spotread the patch 10 to 15 times in succession, to see your
measurement variability as the lamp heats up? Also move the read point around
on the patch to mitigate small differences in direct reflections caused by
texture? And even a tiny difference in instrument height above the patch will
change luminosity reading. Is your spotread setup truly identical to strip
reading of original target? Are you pressing down more, or less for different
readings, etc.
When I experimented with all these factors with i1pro, I was (wrongly)
expecting near perfection, and was surprised by the degree of variability.
- Brad
> On Nov 2, 2014, at 3:59 AM, <robert@xxxxxxxxxxxxxxxxxx> wrote:
>
> Hi Brad,
>
> Between print and profiling I left the print overnight (minimum 16 hours).
>
> After the spot test I left a couple of hours. I’ve just redone the
> measurement (so about 16 hours again) and the values are now:
> 88.01, -13.56, 87.23 (so a bit worse).
>
> BTW … this may (or may not) be relevant: the profile was made using
> i1Profiler (with 2584 patches) and not Argyll.
>
> Robert
>
> From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
> On Behalf Of Brad Funkhouser
> Sent: 02 November 2014 00:59
> To: argyllcms@xxxxxxxxxxxxx
> Subject: [argyllcms] Re: Question regarding White Point and spotread
>
>
>
> Curious... how long did the inks dry between print and measurement of the
> profiling target? And between print and measurement of the spot color test?
>
>
> Thanks.
>
>
> - Brad
>
>
>
>> On Nov 1, 2014, at 4:31 PM, <robert@xxxxxxxxxxxxxxxxxx> wrote:
>> I wonder if you would be kind enough to clarify something for me?
>>
>> I’m trying to do a spot color test from a document through to print, and
>> this is what I’m doing (using Photoshop and Argyll):
>>
>> The spot color has a Lab value of 90, -14, 87 in Photoshop.
>> I convert the document to the print destination space (relative colorimetric
>> with BPC).
>> After conversion, the spot color is RGB 240, 247, 52 (or 0.941176, 0.968627,
>> 0.203922). The Lab value from Photoshop is still 90, -14, 87, as expected.
>> xicclu (rel. col.), with the RGB colors above through the profile
>> (forwards), gives Lab 90.33650, -14.520704, 87.115660. I assume that
>> Photoshop is effectively doing the same as xicclu but is rounding the values.
>> I also tried fakeread (rel.col) which gives me exactly the same Lab values
>> as xicclu.
>> I print the image with no color management.
>> spotread gives me Lab values of 88.987, -13.637, 87.268. This is a dE-Lab of
>> about 1.6 compared to the xicclu reading.
>>
>> The dE-Lab error seems too high as I have only just calibrated the printer
>> (iPF6400) and profiled the paper (Canson Baryta, so good paper).
>>
>> colverify has an option to normalise each file’s readings to white XYZ, but
>> xicclu, fakeread and spotread have no such adjustment.
>>
>> I would have thought that the paper white would need to be taken into
>> account in comparing the spotread value to the image Lab value. Should the
>> paper white be measured and the spotread value normalised? If so, how
>> should this be done?
>>
>> I appreciate your help.
>>
>> Robert
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