Pantone Variation

[sgirard]

Ok guys, forgive me if this question is somewhat vague. I'm neither a pressman nor a prepress guy. But I'm not completely ignorant either!

Anyway, we are experiencing visible variation in a job we print regularly. PMS 283 I think? It is "blue". Solids only (100%, no screens). The dry density will read the same on two sheets from two separate runs but you can see a distinct difference.

Same plates, same press units, sometimes the same ink lot, other times not (we don't mix our own).

Other than variation in the ink and paper supplied by our vendors, what should we be looking at?

Is dot gain an important factor here if we're just printing solids? Other mechanical/press issues?

Now that I think about it, maybe this doesn't really belong under color management

Thanks in advance for any suggestions.
-sean

 

[dimitri]

Density is not ideal for measuring spot colors. Density focuses on the peak wavelenghts of the cyan, magenta, and yellow process colors. A special color has a different peak wavelength, that a densitometer fails to capture. Sometimes the secondary density, which for the case of 283 would be that of the magenta filter, could provide more information.
Could you provide some numbers of cyan and magenta density readings.
Could you also provide some LAB numbers of the two samples you are comparing?
Could you also provide the LAB of the different stocks. This is a light color, and it's appearance could change based on the stock.

One likely cause on top of my head would be to have the printing unit contaminated with the previous color you were printing. Or the pigmentation of the two inks is slightly different, and even if you are putting the same amount of ink on the paper, there is a shift in the hue of the ink.

 

[gordo]

Adding to dimitri's suggestion -
PMS 283 is 93.7% transparent white, 1.6% process blue, 4.7% Reflex blue. So, in addition to being easily contaminated, small variations in the process blue and Reflex blue could cause visible color differences which can be exacerbated on press because of slight ink/water and density differences. Also, the Reflex blue component can be problematic on press. A less likely, but still possible contributer would be differences in the amount of optical brightners in the paper.

best, gordon p

my print blog here: Quality In Print current topic: The Wayback View - A visit to Sun Chemical Ink, 1951 (video post)

 

[sgirard]

Many thanks for the quick responses. Some interesting things to think about.

Dimitri - I do not have all of that specific information available, but I will get it and post back.

Also I have found out that we are not actually dealing with 283, but a custom blend based on 283. So that is sure to add to the finger pointing.

 

[Greg Imhoff]

Hi Sean: Gordo and Dmitri give good advice. Your latest leans towards formulation and so being custom made inks, ask if physical proofs (of batches) and or LAB data exist in order to see or compare one batch to the other. ie: Delta E batch report.

If you do not have a spectro but have a Densi set your "Filter value" to read ink film thickness via "K or V" output. This setting is for "Visual" thickness to a ANSI traceable value used as a tool for press operators to estimate actual press ink film laydown.
"K or V" as a Densitometer ink film thickness value is more sensitive than other Density values to help assure color match if without a spectro. The value cannot advise how your color pigments vary nor can an operator affect this on press --- that is for a Spectro at the time of color formulation.

 

[meddington]

Part of the problem I've found is that some ink venders (and printers that mix their own) are mixing ink primarily by formula and subjective visual match to a swatch book. Afterall, there are no true "official" CIELab values to aim toward. Often, if there is measurement, its comparing the sample to measurement of a physical Pantone book, and we all know that there can be differences between Pantone books.

 

[nuress]

Pantone Variation

This is a VERY common problem in a pressroom when matching PMS colors with a densitometer and not a spectrophotometer. Many colors, that look significantly different to the eye will measure very close on a densitometer for all the reasons already mentioned. There is a good densitometric solution however depending on what brand of densitometer you are using. Some older devices can only pick from C,M,Y, or K. Newer, more modern devices, can measure at exactly the precise wavelength required and this problem should then be negated.

The use of a modern densitometer in the pressroom is crucial because many press operators really understand density a lot better than they do Lab values. This is not taking anything away from them, it is just that most press operators have not been trained adequately in Lab and what the numbers mean.

 

[sgirard]

Many thanks to everyone for the insights.

I have managed to come up with some data (below).

At this point I don't have the actual ink formula, nor do we have batch drawdowns or Lab data from the ink vendor.

I'm not sure the issue is really as bad as was initially described to me (and therefore to you) although you can see some job-to-job variation.

When the Lab data for the substrates is similar (say 0 - .5 dE) then the Lab data for the printed color seems to be similar (maybe .5 - 2.5 dE). And also the visual match just seems very close.

On samples where there is a little bit of variation in the substrate (say 1.5 dE) the variation in the printed samples jumps to about 3.5 dE and these are also the sheets that seem to have more of a visual mismatch.

I have one sample here printed on a much "bluer" or "less yellow" stock (~10dE from most of the other samples) and the printed result looks quite different and measures ~5.5-6.5 dE from the other samples, which I guess isn't suprising.

Also, I am wondering if a light blue in general is going to be more subjective with respect to viewing conditions than usual (inconstancy?). Maybe this is playing into people's perception of what they are looking at.

I still would like to isolate the influences of paper, ink, and press chemistry/mechanics but I think that might be getting into printing hoodoo that is above my pay grade.

Anyway, density measured w/ an X-Rite 508 (absolute, not -paper) and Lab measured w/ an i1:

K: .28, .27, .29, .28, .28, .28, .27, .29, .29

C: .34, .34, .35, .34, .35, .35, .35, .35, .35

M: .22, .20, .22, .21, .22, .21, .20, .23, .23

Y: .14, .13, .14, .14, .14, .14, .13, .15, .15

The "blue" sample (i.e., the sample on noticeably different stock):

K: .25

C: .31

M: .19

Y: .09


L*77.5 a*-09.0 b*-18.0 / Paper: L*95.9 a*-0.5 b*2.5
L*79.8 a*-10.1 b*-15.9 / Paper: L*95.0 a*-0.7 b*1.6
L*77.6 a*-11.2 b*-18.0 / Paper: L*95.6 a*-0.4 b*2.9
L*79.7 a*-10.5 b*-15.5 / Paper: L*96.1 a*-0.5 b*2.7
L*79.7 a*-10.4 b*-15.5 / Paper: L*96.1 a*-0.5 b*2.7
L*78.8 a*-09.5 b*-15.6 / Paper: L*95.2 a*-0.7 b*1.3
L*93.6 a*-09.6 b*-16.3 / Paper: L*95.2 a*-0.7 b*1.0
L*79.4 a*-10.4 b*-15.8 / Paper: L*95.4 a*-0.7 b*1.1
L*79.7 a*-10.4 b*-16.1 / Paper: L*95.3 a*-0.7 b*1.0
L*78.0 a*-08.9 b*-16.9 / Paper: L*96.0 a*-0.6 b*2.5
L*77.7 a*-08.9 b*-17.7 / Paper: L*96.0 a*-0.4 b*2.5

The "blue" sample:
L*80.9 a*-07.0 b*-20.7 / Paper: L*93.6 a*1.5 b*-7.0

 

[dimitri]

Just look at b*: You got 9.00 Deltab* between the two stocks that forces a 4.0 Deltab* on the printed color. It's pretty hard to imagine that there is anything else than the stock!

-D

 

[Eric van der Kop]

i1 Xrite

i1 Xrite

I have been reading this artikel.

In general it is diffficult to compare colors. Please use a i1 from X-rite or somewhere else.
You can download the software called i1 Share. X-Rite: Get exactly the color you need, every time, anywhere in the world.
This tool can help you in looking at delta E's and LAB values

Kind regards
Eric

 

[Print Guru]

Sean,
To add some extra parameters to the game:
+ The more transparent white in the ink – the more it is better to mix new ink every print run. Yesterday ink with 93% white will print less smooth than a freshly mixed ink. The older mix will show tiny round spots like oil spots - they exist even on some color patches in the Panton book.
+ Paper structure will change the color as percepted by the human eye especially in uncoated paper without sizing.
+ Even the finest paper may show, from the same ink mix – small color deviation on each side of the paper.
+ Printing short or long grain also contributes to the issue on hand.
+ If paper and ink are exactly the same in both print runs then compare the print time – morning - cold rollers, at noon hotter, or afternoon – even more.
+Blankets and plates should be the same as they carry their structure to solids and may add to color deviations.
+ However - from my experience, the dominant factor in this problem is the initial colors/shade of the paper. This parameter should be checked prior printing and compensate with the spectro while mixing the ink.
I hope I could help