Hi Stephen,
I’m not really inside colour theory, but the non-linearity might be needed because ink tends to "bleed" into the white paper surrounding the dot.
That way I would imagine that 50% dot coverage (intended) actually produces a visible coverage of maybe 60% or so.
Then a non-linear offset (or bias) could prevent this.
Rob
Since I am getting older, my memory may be suffering, but if I’m right, a 50% dot means that 50% of the dot pattern is opaque, the other 50% is clear. This effectively cuts the light transmission in half. If we carry this over to photographic density, this is a density of 0.30 – that being the common log of the reciprocal of opacity (2). A photographic density of 0.30 attenuates one half of the light. Photographic density is stated in log values whereas dot percentage sticks with a linear scale.
My background is in photography, but I served a few years in color separation. I remember having problems with the same question you pose, but the above is what I remember. This probably doesn’t fully (or even partially) answer your question, but maybe it will give you a start.
You’re exactly correct, Rob. I good example of this would be the difference in printing on newsprint as opposed to film.
I can make a 50% "block" print any dot percentage I want with the use of gamma curves (what Rob is calling an offset or bias). It is very possible for me to print a block that reads 50% with a reflective densitometer. Yet, many "experts" are saying that we shouldn’t be printing 50% as a true 50% – which makes no sense to me. This is why I’m shouting out to all those color theorists out there.
Yes, but IMHO there would be no rule for it.
I like to write with a fountain pen, you know, with liquid ink, and sometimes I have paper that has a very high "bleed factor": when I turn the page, side B appears to be unusable because it all bled through.
The point is that it sometimes is hard to predict what offset one must use. Or bias. (I’m a broadcast eng, so pardon the wrong jargon).
The problem for printers like you is perhaps bigger if you judge the quality of prepress on a CRT monitor: they also bleed but inverse!
(white to colour. Rats! life aint easy).
I don’t exclude that I’m wrong, but this is my two (euro)-cents. Rob
The problem is, if you compensate for dot gain and make proofs from compensated film using an analog proofing system that does not simulate the same amount of dot gain as the actual press, then what good is it? You have lighter CMYK numbers in the film making lighter proofs.
The ideal, in the pre-computer days, was to proof un-compensated film, then when approval was given, create new film for plating that includes compensation. But nobody does that anymore…
Now the best bet is to send uncompensated data to a good digital proofer, then send compensated data to the imagesetter film or plate.
Actually, the biggest problem is the fact that not many people understand gravure – and the majority of proofing systems – analog or digital – are designed for offset/flexo. The actual dot created by an electromechanically engraved cylinder is totally different than the dot created by CTP systems or traditional plate exposure. Please be aware that I am not attempting to compensate for dot gain at all – I am mearly posing the question as to what one should really expect to print in terms of dot area – should 50% be 50% or should it be something different? If it should be something different, then why fool ourselves and call it 50%.
Ah, so it’s a philosophical discussion…
Rotogravure dots are kind of like little diamond shaped intaglio wells aren’t they?
What would a true 50% rotogravure dot be?
Does gravure actually have dots? I was under the impression that gravure is analog in that the deeper the color the heavier the ink. The "dots" are simply a mesh that separates the adjacent tones.
How does one prepare a gravure with a digital process?
One other thing. Middle gray as represented by the lightmeter for correct exposure is actually 18% gray, not 50%.
John,
interesting doc.
Isn´t rotogravure the same as copper gravure, here by cylinders instead of plates ?
"A Rotogravure print is velvety – often richly so – for the same reason that a mezzotint has this delightful quality:
it is loaded with ink, which is raised, and there are no sharp, wiry lines or edges. In certain places, under a
glass, one can distinguish the regular pattern of a halftone screen, with this difference, however, that whereas in
the ordinary halftone print (relief) the round dots of ink are seen to vary in size, but to be of uniform blackness,
those of a Rotogravure print (intaglio) are square and of uniform size but vary in blackness. In many places the
lines of the screen are entirely obliterated by the generous supply of ink. "
How can the dots be square ? How can they vary in blackness ? Perhaps the dot size is a result of the micro surface and the etching process. Variations in blackness may be
a result of a very thin ink layer which is torn when applied to the paper.
Best regards –Gernot Hoffmann
Actually, I believe there are only a few companies in the US that etch cylinders for gravure (if any at all). In gravure, a copper cylinder is etched or engraved with thousands of cells – these cells carry ink to the substrate. Practically all gravure printers in the US use equipment invented and sold by a company formerly known as Ohio Electronic Engraving(ers) or OEE for short. OEE was bought by Max Daetwyler (a major supplier of gravure equipment worldwide). Quite honestly, gravure was digital quite a bit before offset/flexo, due to OEE equipment.
Traditionally, stepped films were wrapped around the copper cylinder after coating the cylinder with photoresist. The cylinder was exposed, then etched in the unexposed areas. The cylinder is then chrome plated for durability.
In an OEE system, the copper is engraved by a diamond stylus. Voltage is passed through the engraving head, causing the stylus to "hit" the copper and carve a small cell. This stylus works at speeds of up to 8100 cells per second (latest and greatest technology) – older engravers run around 3000 cells per second. Off the top of my head, a cell is approximately 120 microns wide – depending on line screen.
What many people don’t understand, is that due to this engraving process, you can’t rotate the angle of your dot. The diamond hits the cylinder the same way and spirals around the cylinder while it engraves the image. A miore is avoided by changing the compressions and screens of each CMYK color – compressions esentially change the shape of the cell – a 30 compression cell resembles a nice square "dot" while a 60 compression cell is very elongated. A typical CMYK setup for OEE engraved cylinders would look like this: Cyan – 175/60, Magenta 175/30, Yellow 174/45, and Black ~210/30-34 (keep in mind those second numbers are compressions, not screen angle).
From a digital standpoint – 1 up files are sent to a programming station, where the files are RIPped, stepped, etc. Then that file drives the engraving head as it engraves the image into the copper cylinder.
Building a proper gamme curve to ensure a nice tone from 100%-0% is accomplished by creating an engraving tone for each of the CYMK colors. This tone is built from Max engraving voltage to Min voltage (lower the voltage, the fewer dots are created). These tones are then printed with X substrate and Y inks. Each voltage value is read with a reflective densitometer and viola – a curve is built. So now we know that X voltage will produce Y dot area. Our current programing software allows 255 voltage steps – and a voltage index of 127.5 does not produce a dot area of 50% – it’s generally around 35-40% depending on which color (different colors have different engraving specs, remember?)
So back to my original question – wouldn’t you want that final 100 step tone scale to print as linear as possible? Or is there some color theory consensus that images look better when 50% is printed at 60%, etc.
If you made it this far, I hope you enjoyed the read 😉
Also, if anyone would like to view some nice examples of printing with this process – about all major cigarette labels are printed in this fashion. If you were to purchase a soft pack of Marlboro Lights here in the US, I can almost guarantee is was made by the company I work for. Pall Mall, GPC, Basic, and Merit are a couple of brands off the top of my head that have tones on the labels – grab a label and a 10-30x loupe and enjoy.
Great read, Stephen. I thought gravure had hit the skids a long time ago.
I have an Edward Weston book printed in gravure. One of my most treasured.
The etching process is similar to the mastering process on the old vinyl recordings. There, a diamond stylus engraves the information on a copper plate coated with acetate. What is interesting is if we got the stylus set too deep and actually hit the copper, kiss the stylus good-by! You are actually etching the copper. What is the geometry of the tip? Does it wear quickly?
I expect that the curve you plot, voltage against density, is the voltage required to etch to a certain depth. So, I don’t understand what you mean by "tone" in this discussion, as tone is usually a function of incident vs reflected light. A trade term?
Also, at first you indicate that the number of dots is a function of the applied voltage, then later it’s the area of the dot as a function of voltage. Now, if the ink varies in depth, I would expect a relationship between voltage (or power) and dot depth. So we seemingly have three different parameters all a function of applied voltage.
A bit confusing to me.
As for linearity, the eye is non-linear, so I would expect a power curve and not a linear curve.
Thanks
Lawrence
As far as wear, I can’t be sure, as I’m not directly involved in the day to day operations. The geometry of the stylus ranges from 100 degrees to 130 degrees (lower the angle, the more volume of ink the cell will hold). Generally speaking, I can get the same amount of volume from a 200 line screen engraving with a 120 degree stylus as I can a 175 line screen with a 130 stylus – that’s really generalized though.
I dunno, explaining myself via writing is not my strong suit. Basically, the higher the voltage, the deeper the diamond "chisels" out copper. A cell opening of about 25 microns is about is small as you can go and still produce a "dot" on 60# C1S fine quality paper. When I say "tone" I’m referring to a vignette (from 100% – 0%) created by half tone dots – 50% dot area meaning half substrate, half ink. Lower voltage produces a smaller cell, and fewer cells in a given area – thus once the ink is on paper, one can read this pattern with a reflective densitometer at get a dot area reading.
What do you mean by "power-curve"?
Ok, I get it. Going deeper means going wider as well.
Sounds like an industry I would like. I enjoyed cutting masters. And, I dearly love the mezzotint look. My prints are now almost exclusively on a matte paper with K in the CMYK printer settings raised a couple of notches. I get a mezzo-like effect that way.
Also, the pitting of the copper sounds very similar to the pitting the laser does in CD burning. Has any laser cutting tools become available to gravure printing? It would seem that the laser would really have great control of the geometry. And, no stylus’ to replace!
The cutting stylus for records is a complex shape having both cutting and burnishing facets to it. It is actually a chisel.
A power curve is the curve produced by raising a number to a power, like y=x ^2, or y=x squared.
Oops, double post!
Sorry!
There are several laser technologies, but the technology hasn’t quite caught on in the industry. Traditionally, the copper cylinder is plated with chrome – making it extremely durable – and once the chrome is worn, it can be stripped and re-plated. The current laser engravers engrave into either a nickle or zinc (which one, I can’t seem to recall at this moment). You can’t remove chrome from these, so once it’s worn, you have to re-engrave.
Side note – gravure is still quite popular for bank notes, contracts, and official documents where you want to make forgery as difficult as possible.
Gernot had asked "How can they vary in blackness?"
Would that not be due to the depth of the cells themselves Stephen? Deeper cells hold more ink and make blacker dots? Meaning that a cell for a highlight is just a wide as a cell for a shadow, but since it is so shallow, it hardly holds any ink at all… I find this a bit hard to truly grasp.
….or maybe not, if you compare it to another intaglio process, fine art etching in zinc plates. Given an aquatint of a given coarseness (analogous to cell frequency) the longer time in the acid etches deeper wells for the ink… Yeah, that’s it. Unh Hunh.
John – most gravue printing uses opaque ink. You get gray levels from the line or cell density. Same as with black and white halftones – but gravure can hold lines, dots, odd curves, or whatever shape you want…
….hmmm, I had gotten the impression that all the cells were the same size on a given plate, but varied from plate to plate to avoid moire.
Hey, I could be wrong. Most of what I know about gravure comes from some anti-forgery folks… I’ve never actually played with it myself.
I looked it up <
http://www.pneac.org/printprocesses/gravure/moreinfo13.cfm>
And it appears that both are correct, depending on whether you are using chemical etching or electromechanical etching.
Gravure sounds like a natural for digital processes. It feels strkingly similar to CD burning.
Hey, the Cubs just won again, 3 games to 1.
Go CUBBIES! :-):-)
I know nothing of printing -but: A sylus implies a cone, thus the volume of the well increases as a function of the angle the side that determine the "sharpness" of the point. This is a power curve (a second power). So an increase in depth would make the size of the hole at the surface a larger area. The dots are larger and fewer can fit on the surface in a given area. Thus an increase in applied voltage increases the ink volume (and not liearly) and the size of the applied ink. This is a very informative discussion. Thank you.
The editorial color in National Geographic is done by rotogravure.
The stylus may not be a cone. I sure would like to know more about it.
The record cutting stylus is not a cone, whereas the playback stylus until the last few years of vinyl was a cone. Elliptical stylus’ were introduced so that the playback came closer to the cutter.
I went back to the reference above and it states that the stylus is an inverted pyramid, not a cone. Still a second order function, however.
As a matter of fact, rotogravure is more popular than it seems at first. Any printing run that’s big enough benefits from being printed in rotogravure in comparison to offset.
I do not know the US market <
http://www.gaa.org/home.htm> but in Europe, lots of magazines (German Burda and the like) are printed with rotogravure.
Here you have the proccessa <
http://www.wmich.edu/ppse/gravure/pp1.htm> reasonably well explained.
Excuse me… but no one has answered yet Stephen question, I think (not that I know the answers, just trying to brint the thread back home. I am curious 😉 )
Gustavo Sánchez
(POsted from Spain)
Hi all, back from vacation – and back to the grindstone 😉
You can’t vary ink film thickness with an electromechanically engraved cylinder – you could, traditionally, with etched cylinders -etch a specific area longer and you will get more ink volume.
Gravure has traditionally been thought of as a process for long, consistant runs of high volume products – and still is – but many of us gravure printers are working to change that to short runs as well. Gravure offers superior color consistancy in long and short runs. We can also print beautiful metallics (true metallics, not silver with toners). The cost of the cylinders themselves has always been prohibitive for short volume business – the industry is constantly improving this price with newer/faster/better technology. Over the years, the company I work for has printed well known packaging, such as Marlboro, Crisco, Campbell Soup, and Wrigley gum wrappers (buy a box of Eclipse gum – the cardboard container is printed gravure – not by us though). The folded carton you get McDonald’s French Fries in is gravure as well. Just about every package involved in the cigarette business is gravure – the soft pack, flip top box, carton – I think even the tipping on the actual cigarette is even gravure.
For more information – check out www.gaa.org
No, the diamond styli we use are not conical – they are shaped more like a poorly built pyramid – for lack of a better analogy.
To give you an idea of the "hole" created on a standard spot color cylinder, here is how the "perfectly" engraved cell would look:
Keep in mind that these cells are diamond shaped, with a thin wall of copper between them and an opening at each end called a channel.
Our standard specs for line/spot colors are 175 lpi with a 45 degree compression. The wall of copper between each cell should be 7 microns wide. We also use a 120 degree stylus for this.
The cell height would be 183 microns and the width would be 160 microns. The channel between cells would be 23 microns. This will give you a cell depth of 46 microns. All told – approximately 225,403 cubic microns per cell – these numbers are based on some funky DOS program that was written 15 years ago – I can’t attest to the accuracy 😉
I suggest you ask this question on the Print Planet Forum. These guys are the color/printing experts. Just go to printplanet.com and pick the proper E-Community. They have forums for every aspect of printing and color management.
