process control in highlights

[mglouis]

Hello.

We run violet plates for sheetfed, LPNV2, that are rated for 1-99% up to 200 LPI and maintain/confirm plate linearization, meaning file value = plate value, using a ccdot which has a resolution of .5-99.5 in 1/2 % increments is rated to be accurate +/- 1%.

Dot area across a plate holds within a 2% range as does dot area across multple plates.

CtP curves are a result of G7 + Rampage's 25 fulcrom curve requirement. 1% file value per NPDC should plate as 0.9 -1.3%. 3% file value @ 2.3 -2.9%

Highlight control, especially in regards to raster 4cp gradients that fall out to 0%, has proved especially challenging for us. An example gradient may go something like this: [c 2.35 m 2.35 y 2.35 k 2.35 (tac 9.41)] down to [c 1.18 m1.18 y1.18 k1.57 (TAC 5.10)] down to [c 0.39 m0.39 y 0.39 k0.78 (TAC 1.96) down to [0,0,0,0]. "Moderate" variation in darker tones goes undetected for the most part. In highlights, we can all agree loss of process control in this range is unsightly.

At either ctp or press or both, tone reproduction in the highlights falls apart. It does not seem unreasonable for a designer to request a gradient that zeros out smoothly but I am having difficulty finding a process that enables this to happen reliable. Pleasing results are a crap shoot.

Can anyone suggest process improvements, equipment modifications, or tips and tricks, for accurately reproducing file values in extreme highlights? If you can manage excellent highlight control, how do you do it?

Thanks,
Matt Louis

 

[KevinC@EFI]

Hi mglouis -

I'm not sure if a total re-investment (or trade-in) of your CTP system is realistic, but you've hit a fairly fundamental limit of Gaussian imaging technology - something that is likely not addressable with a violet platesetter. There are some band-aids like hybrid screening and such that help hide this limitation by increasing the size of the smallest dot, which may help depending on your needs (mixed AM/FM screens - like Kodak Maxtone and Agfa Sublima).

Violet laser systems (and some thermal as well) use a "Gaussian" laser dot - essentially where the power is distributed more in the center of the dot, and fades to the edge like a bell curve. Those "shoulders" on the dot are more sensitive to process variation than the center is - everything from plate manufacturing differences, to processing, even to humidity impact on the photo-initiatiors in the plate. The smaller the dot or the finer the screen, the larger the proportion of "shoulder to center" on each printing dot, meaning that the effect of variation is magnified.

I won't go into a Kodak SQUAREspot anatomy lesson here, but suffice it to say that we use a 10,000dpi imaging system with extremely steep shoulders to image each 2400dpi dot on plate. This reduces the ratio of shoulder-to-dot, limiting the impact of any variable in the process. From this we can ensure consistent imaging of even single pixels, allowing industrial reliability and consistency. It's like using a sharp pencil to draw each dot rather than a fat paintbrush.

I can provide you more info offline if you like, or even send you test plates to "put my money where my mouth is". Let me know if there's anything I can do to help you out.

If this highlight issue only affects a small portion of your jobs, maybe outsourcing to a tradeshop with a SQUAREspot device is an option? If not, I can put you in touch with a local Kodak rep to see if there's any way we can help in other ways.

Kevin.

 

[otherthoughts]

Matt,

General opinions for your consideration.

Yule-Nielson Vs. Murry-Davies- Look into this on your Densitometer
Usually the Yule-Nielson measurements are used when measuring plates. Since
you are interested in the physical size of the dot on plate rather than it's
appearance. On press sheets and proofs you use Murry-Davies since you are
interested in an "approximation" of the appearance.
​

I believe that the proper exposure intensity is reached on the plate when it clips both ends of the tonal range symmetrically. For example 1-99%, 2-98%, 3-97%, 4-96%, 5-95%. The resolution of the plate material itself, in conjunction with the precision of the CTP unit will determine whether you can produce an accurate 1-99%(excellent)dot-area range on the plate or the 5-95%(pitiful)dot-area range. Consider the 3-97% range and worse completely unacceptable.

The most accurate way to measure this (in my opinion) is with a plate-densitometer set to use the Yule-Nielson equation.
Calibrate the densitometer so that the non-image areas of the plate read 0% and the solid ink areas read 100%.
Then adjust the plate exposure so that both ends of the dot-range are clipped symmetrically. Create your own test file with tints from 0-5% and 95-100% stepped in .5% increments and 10% to 90% tints in between, using the CMYK color-space with no color management. I prefer to run the test without a linearization/calibration curve in effect at the RIP as well. If you find yourself with plenty of exposure latitude, adhere to the plate manufacturers recommendations as much as possible.

Once the best exposure intensity is identified and set, then apply, check and adjust your linearization curve as needed at the RIP. When you are done, both ends of the dot range should be clipping symmetrically and a 50% dot from the test file should produce a 50% dot on the plate(measured with your densitometer).

From this point on you can introduce and evaluate any digital exposure tools/files you wish to employ (Ugra/Fogra Digital Print Scale, CTP manufacturer digital files, plate manufacturer digital files). If these digital files/tools correspond nicely with the manual results you have just determined via your densitometer, I would consider these digital files/tools reliable and vetted.

Sorry for carrying on as I have, I just wanted to set some foundation for what I really wanted to say

A perfectly exposed, calibrated and developed CTP plate, hung and run on an old press in need of large doses of attention and maintenance, may only be capable of producing a tonal range from 5% to 80%(Murry-Davies). Whereas a new, well attended to and maintained press might be able to print a 2% to 98% dot using the same plate.

The deficiencies in tonal range of the old nasty press, really need to be addressed at the press. Profiling and creating a device profile for the old nasty press does not change the presses 5% to 80% tonal range. The device profile simply does the best it can within the presses limitations.

The best tonal range I measured on press before I retired was 2-98%. The worst, 6-75%.

The highlight dot is the most critical. A 2% dot becoming a 0% dot is a 100% decrease in value. A 2% dot becoming a 4% dot is a 100% increase in value.

The shadows are not so critical, a 95% dot becoming a 100% dot is a small change. A 75% dot becoming a 100% dot is still only a 25% increase in value.

An old and perhaps outdated rule of thumb is that a decent press should be able to print from a 5% to 95% tonal range.

Kevin's point regarding dots with hard or soft edges/shoulders are typically more apparent on the press than they are when evaluating the plate alone.

For what it's worth?
Otherthoughts

 

[Lukas Engqvist]

As is it comes down to maths. Your tones, are for an ideal world. The fade to zero problem is a big one, and the old ways of dealing with them was to add noise (this is actually doing a similar thing as a hybrid raster noise is generating an FM raster.) Increase the LPI (bigger dotts allow more steps)
I'm not sure what ratio of dpi to lpi you need to get two decimal places on a % of dot, but @200lpi expecting 10000 levels of grey (ie 100.00% ) means you need about 20000 dpi ?

Having said that with a hybrid raster we get very good fade outs, but I'd tweak the curves after visual on paper for the last 5%. As I understand the equipment its acurate to ±1% area, not 1% of 1% meaning that measuring 1% area on a CC dot guarantees that the dot is 0-2% area... may be me that has missunderstood.

 

[mglouis]

Wow. Homework assignments! I will return with feedback as to how this goes, and will keep an very eager eye out for updates to this thread. Lukas, you understood correctly, when measuring a 1% file value intended to plot at 1%, the ccdot guarantees 0-2% area. CCdot itself is muddying the waters so I plan to borrow newer technology to see if I can get a clearer understanding of what is plating. If a new video dot meter improves the process, I will buy it. (Yes I am taking recommendations) I have tried reading plates with a spectrophotmeter in the past by locking the highest contrast "filter" into place but I struggled with variations in emulsion and clear area, skewing the dot area readings. I'll try again.

Our presses print 2-97% cleanly and repeatable on gloss stock but the variance outside of this is what I am hoping to make more predicable. If it falls off at 0.5%, then I wish for this to be consistent. Also if one channel drops off before another then rainbowing happens and obviously I wish to prevent this. (It's a mess and I have yet to see an icc profile capture this phenomenon.) I am also curious why on our Docucolor5000 which cannot hold 1-2% dots can print these gradients smoothly, although the gradient length has been shortened. I think I am seeing a print density issue (via TVI, either natural or forced) within the highlight range. I am willing to track and trend highlight print density but upon an unknown ctp condition I am afraid I will get much benefit.

Thank you all very much. I have more ideas to work with now than I did.
Matt Louis