Mike Chaney's Tech Corner

I have some framing glass that has a bit of color to it.

Reading the print's RGB with a ColorMunki Photo I read these numbers on the print's surface:  (RGB)= 231, 196, 173.

With the same print under the glass I read these numbers:  (RGB)= 226, 195, 177.

So I take it the print would have a bit of cyan and blue added to it under glass?

So can I apply some general correction as a permanent "Glass Correction Filter" in QU for any prints that I want to put under glass to neutralize the "under the glass numbers" back to the print's RGB numbers "without the glass?"

Tia.

Mack

Hi Mack,
Yes, it's called a Print Filter which is invoked with the tick box in Job Properties. The thing about a Print Filter is that it is applied to the print only, you do not see the effect on the image on-screen.
I'm not sure how you would create the filter, trial and error I would think but you could need to use either individual saturation values, or curves or Select Colour.
One technique  could be to initially make the filter to reproduce the effect of the glass (on screen) and then make a "reverse" version of it to use the Print Filter.
Terry

What paper?

--
Met vriendelijke groet, Ernst

http://www.pigment-print.com/spectralplots/spectrumviz_1.htm
December 2012, 500+ inkjet media white spectral plots.

Epson Luster.

Would it matter though if all I am trying to do is match the current print to the "under glass" one?

Might be able to do it with the Adjust > Saturation (Uncheck the Lock colors too) somehow as Terry suggests, but the QU numbers (-100 to +100) don't follow the RGB ones (0-255) so it could take some time to match.  Maybe interpolate QU -100 to be RGB=0  and QU +100 to equal RGB=255?  Does seem a bit odd the difference using 'negative' red numbers other than starting at zero.  ???

In the numbers above (1st post), the green value is only one point different so there might not be a big density shift overall, just the red and the blue seem to be targeted for a change (i.e. Print needs to be warmed up a bit (red + yellow) for "under glass.").


Mack

Yes Mack,

It matters because papers like that have OBA content and the fluorescence diminishes when glass like that is used on it. The Colormunki also does not measure below 430 NM (but reports down to 380 NM if I recall it correctly) and is UV-cut so the quoted RGB values are already compromised. It would be wiser to use a trial and error approach by making some proof prints to get the glass covered print more or less equal to the bare print (in the display light the prints will be viewed with) and then see what the RGB value difference is and make the print filter with. It will differ for papers with and without OBA content. Use the eyes in this case. I had a similar task once when reproductions had to be made for some 1920 constructivist's artworks, every inkjet print paper shifted behind the glass and not equally.

--
Met vriendelijke groet, Ernst

http://www.pigment-print.com/spectralplots/spectrumviz_1.htm
December 2012, 500+ inkjet media white spectral plots.