Re: Is it metamerism, or constancy?
@ John Lind
you wrote "If the spectral curves of two colors are the same, they will match anywhere to anyone."
Well, that is an interesting statement, but I think we can all agree here that it is very unlikely that one might get even a small 1mm x 1mm portion of an Epson proof and a press sheet to have an identical spectral curve.
Related to SWOP -- No one has followed anything close to SWOP guidelines for 20 years !
Champion Textweb, a No. 5 coated groundwood with 70 brightness - a very yellowish stock - is not being made anymore, nor to my knowledge is anyone printing at 133 line screen - certainly not Magazine publication printers, "which was who SWOP was designed for - so SWOP is irrelevant, long live Gracol.
Of course I abadoned all hopes of the SWOP group back in 1989 when I read this;
http://findarticles.com/p/articles/mi_m3065/is_n6_v18/ai_7627311
Clearly, it was doomed to irrelevance before then - no one prints to SWOP guidelines.
@ Todd Miller;
I fail to see much anyone can add here beyond the fact that proofs are designed to simulate a press sheet UNDER A VERY SPECIFIC controlled lighting condition.
When you print a color document that is intended to simulate how the final press sheet will look like, it is important that ALL PARTIES understand that the "proof" is created with certain assumptions of the viewing condition, one very popular one being in a viewing booth where the light emits a specific color temperature, most often 5000K.
Outside of these viewing conditions, metamerism occurs, colors no longer visually match, and you might as well wear color tinted glasses.
So, to your questions ;
"The Epson's constancy is excellent, the press sheet not so good."
- jahn comments --- HA HA HA ! that is really funny - so, why not print thousands of Epson proofs and send that out ? (smile) - yes, I always like to point out how long 1 color epson proof takes to print verses how long a printing press takes to print that same image...
"We print using paper that has UV brighteners added, is this the cause of the unconstancy in the press sheet? "
- jahn comments -- YES. One of Pantones BIGGEST issues with printing the Pantone guides is paper inconsistency.
"Or is it the inkset we use at press, or both?
- jahn comments -- YES. You can also ad blankets, humidity, plate wear, temperature, ink viscosity and a long list of things that cause each press sheet to not be perfectly matched to one another - but again, consider that the marking engine in the epson is spraying droplets of water much smaller than a human blood cell at a spining target using inks and paper far different than you are printing with on the press, and you will grasp how tricky it is to make them look the same in ONE ligting condition, never mind SEVERAL lighting conditions !
"Does anyone have any suggestions for resources that we could use to research this issue?"
Jahn comment - teach eveyone that you must view proofs and press sheets in a 5000k light booth, period.
"We would like to make the press sheet more constant in different light sources."
Jahn comments - This is technically impossible, and this has been proven by countless studies. Do not waste another nano second persuing this goal.
"Would software such as Alwan with its 'super GCR' help this."
GCR has nothing to do with overcoming metamerism. When a designer builds objects with 400% density (100% of all 4 CMYK process colors) systems like Alwan can convert these into something more press friendly. Some people report ink savings, but most uses exclaim better grey balance control without compromising saturate colors too much. Again, this will not help your press match your Epsons (which is backwards philosophically) but may help sheet to sheet consistency
hope you find this information helpful !
For those who want to learn more about metamerism related to monitor and print represnetations of press simutaions, here is a couple of links to a well written blog posts;
http://jimraffel.com/2008/01/16/67-matamerism-hard-copy-vs-monitor/
http://jimraffel.com/2005/10/26/golden-nugget-16-matamerism-color-management/