Unwanted Absorptions

J

joe pasky

Member
I read this today. It sounds counter-intuitive.

There is one highly advantageous aspect of the
so-called “unwanted absorption”: the
nonlinearity of mixing produces a gamut for
subtractive reproduction that is wider than
would be the case for ideal, non-overlapping
pigments.

Has anyone seen any data to support this argument?

Joseph J. Pasky
G7 'Expert'
Cathay America - Shenzhen China
 
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joe pasky said:
I read this today. It sounds counter-intuitive.

There is one highly advantageous aspect of the
so-called “unwanted absorption”: the
nonlinearity of mixing produces a gamut for
subtractive reproduction that is wider than
would be the case for ideal, non-overlapping
pigments.

Has anyone seen any data to support this argument?

Joseph J. Pasky
G7 'Expert'
Cathay America - Shenzhen China
Click to expand...


I am not totally sure I understand the question but if it is comparing over printed inks with non over printed inks there is a kind of an explanation that makes sense to me.

For a screen, if the dots are over printing, the second down dot might have less ink film thickness due to wet trap than if the two inks are printed as non overprinted dots, where the second down dot would have a normal amount of ink film.

With the two dots over printing, this will result in more white paper area between dots. This will mean that the Gamut would tend to increase in the L direction. The combination of the two inks screens whether overprinting or not runs from the maximun L value on the L axis, where the screen values are 0% out to the overprinting of the two solid inks where the screen values are 100%. Between those two points is a line that forms a ridge of the Gamut volume. This line would be different for the two conditions of overprinting dots or adjacent printing dots. The overprinted ridge line would have greater L values and have a slight hue shift when compared to the adjacent printed dots.

Does this make sense and does this describe your question?
 
Hmm....I don't think so.

What I assumed they were referring to is 'hue error' and 'gray value'.

I've been noodling around with the idea of wide-gamut (4-color) inks. What I've seen called 'wide-gamut ink set' has very 'clean' magenta and cyan colorants which I assume would give very low hue-error and gray-value readings.

But this published 'white paper' seems to suggest the contrary.

It just doesn't sound correct that a ink-set with a lower hue-error/gray value would reduce the gamut. I would expect the opposite.

jjp



cheers,
jjp

jjp
 
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joe pasky said:
I read this today. It sounds counter-intuitive.
There is one highly advantageous aspect of the so-called “unwanted absorption”: the nonlinearity of mixing produces a gamut for subtractive reproduction that is wider than would be the case for ideal, non-overlapping pigments.
Click to expand...

It would be nice to have a bit more of the text because this small bit seems to be confusing terms.

First regarding linearity.
All color created by halftone screening is non-linear.
We see color in presswork as a result of light being filtered by the dot of ink as well as reflected directly of the surface of the paper never having been filtered by the ink. Because we see the aggregate of the two, the effect of paper/substrate and ink need to be considered together.
Light that reflects directly off the substrate effectively contaminates the color of light that has been filtered by the ink. The result is the hue shifts color as you go from 1% dot (maximum paper color contamination) to 100% (minimum paper color contamination). This is exacerbated by the amount of optical brightners in the paper since ink is an effective blocker of fluorescence. The result is another color shift as you go from 1% dot (maximum fluorescence effect) to 100% (minimum fluorescence effect).
The non-linearity is further compounded when dots overprint because the ratio of overprinting dot areas are not consistent through the tone scale which results in a variation in trapping - at some points the dots will overprint to varying degrees while at others they will print beside each other with varying amounts of unprinted paper between them. The variation in overprinting also causes a variation in density/opacity compared to dots that print beside one another, which again affects color.
I think the only printing which would potentially be relatively linear in a color sense is collotype (continuous tone lithography).

Overprinting halftone dots does does slightly reduce gamut compared to printing dots beside one another, however both deliver a non-linear response. That being said, printing dots beside one another to try and gain the extra bit of gamut introduces other "show stopper" issues related to color shifts caused by slight misregistration - so it is not an effective strategy for increasing gamut.

If you want extra gamut, you can move to a much higher halftone screen frequency - e.g. FM screening, increase your solid ink density slightly (about 10 points), or switch to wider gamut process inks.
For comparison, here is an example of the larger 20 micron FM gamut (translucent) compared to 175 lpi GRACoL:

FMgamut.jpg


and here is the wide gamut Toyo Kaleido inks compared with GRACoL - both 175 lpi:

Gamuts-1.jpg


best gordo
 
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Great info...thanks to Eric and Gordo.

To the best of my knowledge, System Brunner is the only color monitoring and control system that measures, displays and calculates IFT moves with consideration for the the trap conditions you mentioned: halftone dots printing on bare paper at the highlights; partially overprinting at the midtone; and the majority of overprinting at the shadow.

However, this 'graying' from multiple internal reflections doesn't seem to be what the author was referring to.

The new G7 'how to...' suggests adjusting the gray balance scale, if necessary, to compensate for differences in paper color. (blue white vs. cream white). The basic approach 'automatically' accounts for MIR.

Your gamut plots seem to indicate that, as I suspected, the author is wrong in is assertion...or I am not understanding his point.

Am I correct in saying that the TOYO ink set is using very clean colors? or perhaps a Rhrodamine red? (I saw the brochures last year, and have long-since forgotten: old timer's disease)

What I'm trying to understand is how best to 'engineer' the entire workflow to take advantage of the wider gamut. It seems to me, for example, it makes no sense to have a wider gamut on press, if you compress it in photosphop by using a sRGB color space for the images.

So, what I'm thinking is that you need to shoot RAW when doing the photography or scan transparencies RGB with with a wide-gamut profile (Don Hutcheson posts one: donRGB.icc). On press you would need to use the G7 P2P target with the wide-gamut ink set to calibrate for a balanced gray-scale and correct tone curve. The thing I'm a bit unsure of is where to target the SIDs. I hear conflicting advice. TOYO says 20% higher than 'normal' density reading because their inks are more transparent. (anyone know if they are using a 'nano pigment dispersion' technique to achieve this?).

The next step would be to read the IT8 target from the 'characterization' run (after the CtP is calibrated for tone and balance). But, I'm fuzzy as to what do do with this profile.

cheers,
joe
 
My comments [GP] interwoven..
[JP] "However, this 'graying' from multiple internal reflections doesn't seem to be what the author was referring to."

[GP] Graying, or loss of chroma, does not come from "multiple internal reflections". It comes from light that reflects from the paper mixing with the light that has been filtered by the ink.

[JP] "The new G7 'how to...' suggests adjusting the gray balance scale, if necessary, to compensate for differences in paper color. (blue white vs. cream white). The basic approach 'automatically' accounts for MIR."

[GP] You can put a curve in a single color (C,M,or Y) to restore gray balance on paper that has a subtle difference in hue from neutrality. Typically this is done to the Y channel if possible since it's hard to see the tone distortion in the Yellow printer.

[JP] "Your gamut plots seem to indicate that, as I suspected, the author is wrong in is assertion...or I am not understanding his point."

[GP] The author's assertion is confusing because it's unclear whether he is comparing different printing gamuts to other printing gamuts or to CRT devices. He says "the nonlinearity of mixing produces a gamut for subtractive reproduction that is wider than would be the case for ideal, non-overlapping pigments." [GP the nonlinearity of mixing does not create a larger gamut] "The gamut of everyday printing inks, and everyday photographic dyes, extends beyond the CRT gamut in certain regions of color space." [GP that's correct, but again not related to nonlinearity]

[JP] "Am I correct in saying that the TOYO ink set is using very clean colors? or perhaps a Rhrodamine red?"

[GP] The use of Rhodamine red would not account for the extra gamut of Toyo Kaleido inks. Using Rhodamine red would just skew the gamut - and you could do this easily your self. ("Magenta" is a powerful gamut swing color. You can use anything from violet to warm red instead to compensate for CMYK gamut deficiencies - but other colors will suffer) This will be the subject of one of my personal blog posts LOL.

[JP] "It seems to me, for example, it makes no sense to have a wider gamut on press, if you compress it in photosphop by using a sRGB color space for the images."

[GP] Hopefully you wouldn't use sRGB for print :)

[JP] "So, what I'm thinking is that you need to shoot RAW when doing the photography or scan transparencies RGB with with a wide-gamut profile (Don Hutcheson posts one: donRGB.icc)."

[GP] Adobe 1998 is already way larger than TOYO Kaleido - let alone conventional CMYK. I see no reason to use donRGB.
Here's a comparison. Adobe 1998 is translucent - TOYO Kaleido is opaque.

RGB1998.jpg


[JP] "On press you would need to use the G7 P2P target with the wide-gamut ink set to calibrate for a balanced gray-scale and correct tone curve."

[GP] You can use whatever technique that achieves the correct gray balance and tonality.

[JP] "The thing I'm a bit unsure of is where to target the SIDs. I hear conflicting advice. TOYO says 20% higher than 'normal' density reading because their inks are more transparent."

[GP] That makes no sense. If their inks are more transparent you will need to run a thicker ink film to achieve the same SID value. Maybe they mean running to a higher SID value?

[JP] "The next step would be to read the IT8 target from the 'characterization' run (after the CtP is calibrated for tone and balance). But, I'm fuzzy as to what do do with this profile."

[GP] I guess you'd create a separation profile to take advantage of the wider gamut.

best, gordo
 
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joe pasky said:
Gordo: Enjoyed your blog. I 'stole' your idea for the 'Live Feed'. Nice gadget. joe
Click to expand...
Thanks. Yes, indeed, a great blogger gadget. Nice seeing how people arrive at and from where they arrive one's blog site. But it would be nice if people left a comment as well. ;-)

best, gordo
 
Quick question Gordo...Why is the Gracol 175 lpi gamut plot different when plotted against FM compared to Toyo in your first post in this thread?
 
Just some general comments on this interesting discussion.

Charles Poynton has not had much experience with printing so it might not be so good to be too critical about his printing related comments. Although, from how I understand his comments, I tend to agree but it is not clear enough to be sure. It can be in the context of what is being described that can possibly cause confusion. Also terms like wider gamut might be different from larger gamut in volumetric terms.

Charles Poynton's experience has been with other mediums and that is where his expertise lays. What he has also done is provide a collection of educational material about colour, which I have used years ago to help me understand the physical issues of colour.

Some comments.

Density of an ink or print has no direct relationship to how you see the colour. If you are talking about the density of say Magenta, you can not see what was measured to produce the density value. The density of Magenta is determined by measuring the reflected light, filtered by a green filter. You can not see green when looking at Magenta. This is the same for any other colour to.

There is no direct relationship between density with transparency. If an ink is more transparent it does not have to mean that it requires more ink film to get to the same density. The density calculated from the reflection in a specific part of the spectrum of the total reflected spectrum depends on the absorption of light in that region. It could be absorbed by a transparent ink, a semi transparent ink or an opaque paint.

The thing of interest is the shape of the spectral curve. How close is it to an ideal shape and how it changes due to changes in ink film thickness. Understanding how spectral curves change when ink film changes helps to understand how hue changes in printing.

A reflection spectral curve would be on a plot of wave lengths along the bottom ( regions of RGB) and the vertical scale would be zero for the bottom line and 1 for the top line. For a given spectral curve that represents the reflection from a printed solid ink film on paper (zeroed to the paper) we will see the whole curve move down as the ink film increases. The movement is not proportional to the bottom line but moves proportionally relative to the top line. This is very important because any final spectral curve defines the colour relative to the bottom line.

An ink will tend to change hue or not change hue as the ink film increases will be dependent on the shape of the spectral curve. A perfectly flat curve which would be a gray, would tend not to change hue as the ink film increased but a metameric gray which might have a non flat curve has the possibility to change hue.

With Magenta, where the red side of the curve tends to be higher than the blue side, this can result in a hue shift as the ink film increases. As the ink film increases, the blue side tends to drop faster than the red side and then the ratio of red to blue, relative to the bottom line changes.

With offset printing, dots don't have the same ink film thickness as the solids have and can show hue shifts for some inks. Overprinted inks will be printed thinner due to wet trapping than they would be if printed directly on paper and that can also cause a hue shift to the over printed ink. FM dots have less ink film and can show hue shifts.

It can be seen in flexo type printing where globs of ink are deposited just outside the dot area by being squeezed out from under the dot . This can cause hue shifts in the screens due to this thick ink deposit.

So offset printing is non linear not only because of the incremental dot sizes of screens but also because of differences in ink film thickness of dot structures and wet trapping affects. The printing of the CMYK channels are also not independent because the print of one can affect the print of the others.

These kinds of things are always very interesting but fortunately even though things are very complicated, it should not mean that predicting colour in printing should be. If you have a paper and a set of inks, a screen system and print to some standard densities (of your choice) then the press should print a range of colours based on the combinations of the different screens values (0% to 100%). It prints what it prints and if it prints this combination consistently, then you have a process that can reproduce what ever colours are in that discrete range made up of those combinations.

Playing around with curves, whether they be TVI or neutral density, does not change how the press prints those discrete colours in its range. You can not increase the range or gamut by using different curves. You can increase the gamut by using more saturated inks or by running to higher densities, etc. Looking at the spectral curves will help give one an idea of what is possible.

Fluorescents add to the difficulty but it potentionally can also be managed by analysing how it affects spectral characteristics.

Lots of interesting problems.
 
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meddington said:
Quick question Gordo...Why is the Gracol 175 lpi gamut plot different when plotted against FM compared to Toyo in your first post in this thread?
Click to expand...
The 175 lpi profile used to compare against the FM screen is not the official GRACoL7 profile. The profiles I used to create that graphic came from a test that I ran specifically to compare the gamuts of 175 lpi AM vs 20 micron FM. Note that the SIDs are virtually identical. A curve had been put in the plate to normalize the tones of the FM to the AM screen. We wanted the only difference in the profiles, and therefore the gamuts, to be caused by the screening. The data that was used for those profiles was not massaged to smooth them out as is the case for GRACoL7. The 175 lpi presswork was close to but not exactly GRACoL 7.
For the Toyo ink vs 175 lpi I used the official GRACoL 7 profile as I thought that would be more relevant comparison.
I could replot if you like, but I don't think it would make a difference to the conclusions.

best, gordo
my print blog here: http://qualityinprint.blogspot.com/
 
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