[Daniel Koretz Participant Posts: 87]

Papers & Media

on: December 8, 2020 at 6:25 pm

I’ve seen a number of assertions that coated papers with OBAs have, ceteris paribus, larger gamut volumes than coated papers without. However, I’m unable to find documentation. Is there a good source on this? Mark, I found a review of papers you posted on Luminous landscape that gave the range of volumes across a variety of papers, some with OBAs and some not, but I didn’t see a table or plot summarizing that.

[Mark D Segal Silver Member Posts: 951]

Re: OBAs and gamut volume

Reply #1 on: December 8, 2020 at 6:30 pm

I haven’t seen any systematic evidence that larger gamut volumes should necessarily be attributed to optical brightening agents in particular. But that doesn’t necessarily mean anything.

[Daniel Koretz Participant Posts: 87]

Re: OBAs and gamut volume

Reply #2 on: December 8, 2020 at 9:27 pm

Thanks. If you’re not aware of anything addressing this, I doubt I’ll find anyone who is.

It occurs to me that as a statistical matter, unless some manufacturers produced otherwise identical papers with and without OBAs, it might take quite a large sample of papers to separate the apparent effect of OBAs from all of the other factors that affect gamut.

What started me on this is a comment I received from a photographer who suggested that the problem I was having with out-of-gamut reds in a flower image was at least in part because I was using a paper with no OBAs. (In fact, I was using the original Canson Baryta Photographique, but he thought I was using Breathing Color River Stone Rag.) But in your recent review, you noted that in your tests, River Stone had a gamut volume of 916,577. Unless you have changed how you measure this, that’s larger than the gamut volume of any of the 8 papers you reviewed for Luminous Landscape in 2016, some of which did have OBAs. That suggests that in this particular case, at least, OBAs per se had nothing to do with the problem.

 

 

 

[Mark D Segal Silver Member Posts: 951]

Re: OBAs and gamut volume

Reply #3 on: December 8, 2020 at 9:58 pm

Daniel, one point I should have mentioned is that in making profiles for papers with OBAs, one has a meaningful choice of measurement condition, which in profiling lingo means the selection between M0, M1 and M2 measurement conditions. These measurement conditions vary in how they handle UV florescence from OBAs. Conditions M0 and M1 both take UV into account, whereas M2 excludes UV. I have noticed in these cases that when I make profile sin each measurement condition for papers having OBAs, the gamut volume of the M2 profile is slightly less than it is for the M0 or M1 profile. This would imply therefore that including for UV does expand the gamut volume – a bit. The OBAs cause a narrow shift of response in the 360nm to ~380~390nm range of the spectrum. Because it is concentrated in this area and has a small impact on gamut volume, it is unlikely to explain very much in terms of visible differences in the overall reproduction of saturated colours in a print. The gamut volume numbers I use come analysis of profiles in ColorThink Pro. I don’t believe Chromix has changed the math for calculating these volumes over many years. The differences of gamut volume therefore relate to how the profiles are made and of course the properties of the paper and ink. Like you, I don’t think your problem with red has anything to do with OBAs.

 

[Daniel Koretz Participant Posts: 87]

Re: OBAs and gamut volume

Reply #4 on: December 9, 2020 at 10:29 am

Mark, thanks. This makes perfect sense, given that UV light causes OBAs to fluoresce.

[Andrew Rodney Participant Posts: 416]

Re: OBAs and gamut volume

Reply #5 on: December 9, 2020 at 1:47 pm

It makes no sense to me that OBA’s would have any effect on Gamut volume or size and not all plotting software is created equally too:

http://www.colorwiki.com/wiki/Color_Management_Myths_26-28#Myth_26

Author “Color Management for Photographers" & "Photoshop CC Color Management" (pluralsight.com)”

[Mark D Segal Silver Member Posts: 951]

Re: OBAs and gamut volume

Reply #6 on: December 9, 2020 at 4:43 pm

Hi Andrew,

Just to be clear, I am reporting on observations from using two applications consistently: i1Profiler and ColorThink Pro. Usually I don’t change settings on either, and I presume over many years Chromix (the author of the Color Myths Wiki) has not changed the math for calculating gamut volume. In i1Profiler, as you know, for any one paper being profiled we can save the profiling measurements in M0, M1, M2 measurement conditions all at once as a pwfx file, then generate a profile for each measurement condition of our choosing. Quite a while ago I had generated profiles for several papers with OBA content, and made profiles for both M0 and M2 measurement condition. When I did that, I noticed that the gamut volumes of the M2 profiles were slightly less than those of their M0 counterparts, as calculated with ColorThink Pro’s “Profile Inspector”. It would appear that the fluorescing could be bumping up how the spectrophotometer reads that end of the scale, but that’s a supposition. The data is not a supposition, it’s what was reported. This is no big deal, however, because the difference would not affect 99% of printed output. That was my basic point.

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