[argyllcms] spec2cie

  • From: Joe Tschudi <tresorjoe@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 1 Oct 2013 15:58:12 +0200

Hello Graeme

>       CustomTest_illumread.sp seems a little strange. specplot shows a CT of 
> 8098K,
> and an L*a*b* compared to D50 of 100.000000 -12.215330 -27.322065, which is
> quite blue.

The test with illumread has been made using a Grafilite 
(http://www.grafilite.com/product_info.html) in combination with an EFI proof 
paper that contains almost no optical brigthener. So I am not sure why the 
results come out so blueish when using specplot.
But anway, if I use a third-party profiling engine to use the custom illuminant 
from spotread (not illumread) I get a CCT of 5863K which I presume is ok since 
it is what Grafilite is proposing with their budget light box. With illumread 
something's odd, I also get 8096K with the external application - maybe I made 
a mistake. But again, since I just need to measure the custom illuminant and no 
OBC is present in the paper, spotread should go well for my purpose.

But even when using the spotread result to convert the test.ti3 using spec2cie, 
I get LAB 91.12 / -4.29 / -4.91, which is far more blueish than what the source 
color looks like under a CCT 5863K light source (and the original color 
definitely looks beige, not blue)

I don't really understand this topic deeply enough, so maybe my basic 
presumption is wrong. How can I convert correctly a spectral reflectance curve 
to a custom illuminant in order to get right LAB values (under this custom 
illuminant) for further processing (printing, etc.)? In the end I want to see 
the right color under the right light presuming I already did the ICC profile 
using the custom illuminant as my white reference*

I've used an i1Pro RevE using spotread (I presume it's M0) for all the 
reflective and emissive meaurements.
I did a profile using the CustomTest_spotread.sp ~ 5863K as my white point 
using a third-party profiler successfully.
Now I have TEST.ti3 and CustomTest_spotread.sp as my source color and my 
viewing light and I need to convert TEST.ti3 to LAB in order to print the color 
(abs.col.) and view it under the custom illuminant. 
Note that when doing all this with D50 default it's almost perfect apart from 
the fact that the viewing light is slightly more blueish, so I need the custom 
illuminant to correct this behaviour slightly (that is why I am assuming that 
from 5000K to 5863K there can only be a slight change of LAB and not such a 
drastic one as I am having with spec2cie).

Thanks again for your help in advance.
Best regards,

Attachment: TEST.ti3
Description: Binary data

Attachment: CustomTest_illumread.sp
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Attachment: CustomTest_spotread.sp
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Attachment: SpectralCalculator10nm.xls
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