by John Seymour, John the Math Guy
This blog post is a second or third or maybe fourth in a series of blog posts attempting to unbewilder the bewildering array of geometries for spectrophotometers. In the zero-eth post, I introduced the idea that reflected light comes in two forms: specular and bulk. That was background for the first post on spectrophotometers, where I discussed the 0/45 and 45/0 spectros. Then in a wild frenzy of blogomania, I followed up with a special article on measuring metallic inks.
A problem unique to newsprint
Suppose your business is printing newspapers. You print on a rough, uncoated stock. You can’t get a terribly high density – everyone knows that, and accepts that a black ink with a density of 1.2D on newsprint is pretty dark. Everyone assumes it’s because the ink soaks into the paper. Actually, the larger effect is because you are seeing more of the specular component, but that’s not the point of this blog post.
Here’s a funny thing that everyone in the business of measuring the color of newspaper printing knows about: dryback. If you measure the density of ink hot off the press, and then again minutes or hours later, you will see a drop in the richness of the color. The density will drop by around 0.10D. They call this dryback.
And here’s a funny thing that few people in the business of measuring the color of newspaper printing know. If you were able to measure the density of ink on the press as it is running – not just “hot off the press”, but “hot a few milliseconds after the ink hits the paper”, you would be astounded at the amount of dryback there is. (I was astonished.)
A rich black ink might have a density of 1.10D when it just comes off the press. That same ink will dryback to maybe 1.03D. When it has just been put on the paper, the ink has a density approaching 2.00D. To put that in perspective, that number is higher than almost all printing of black ink on fancy-shmancy ultra-high quality paper.
Yes. Really. I was involved in the development of an newspaper color control system, and I have seen it myself with my own two sensors. When ink is first applied to the paper, the surface of the ink is very smooth. As a result, all of the specular light heads off at one angle, and a 0/45 spectrophotometer won’t even notice it. As the ink dries, it conforms to the rough surface of the paper, and the spectrophotometer will start seeing the specular reflection.
Dryback is troublesome for process control. But three guys who were working for Gretag came up with a solution. As the story goes, Tino Celio, Hans Ott, and Mast (I don’t recall this last guy’s first name) were sunning themselves at Malibu Beach. These guys were sipping umbrella drinks and talking about the sad state of affairs when it comes to measuring the color of ink on newsprint.Celio (I think it was him) pointed out some attractive lady, commenting that it would take an instrument with a pretty small aperture to measure the color of that bikini. I am sure most everyone reading this column can relate to the situation… pointing out a hot babe or stud to a buddy. (I assume that most everyone in divorce court can relate to pointing out a hot babe or stud to their spouse.)Ott said “What are you pointing at? All I see is glare!”. Mast. always the clever one, pointed out that Celio was wearing polarized sunglasses, and Ott was not. When the two switched glasses, Ott said “Ahhh… I see her now.” A light bulb suddenly appeared above all three heads.
I may not have gotten the story quite right. I wasn’t there at the time. Maybe it was the Riviera, since these guys were Europeans. I think they were from Switzerland or Uruguay or somewhere? Maybe they were drinking a good Bordeaux? Who knows? All I am sure of is that one of these guys got this idea.
A pair of polarizing filters can be used to eliminate practically all the specular reflection that a 0/45 spectro sees. Bear in mind that bulk reflection is randomly polarized, but specular reflections remembers the polarization of the incident light. The following diagrams explain how we can take advantage of this fact to separate the bulk from the specular.
With these filters in place in your densitomoter, you can measure the sheets right as they come off the press, an hour later, or a week later. The polarized density won’t change. A polarized densitometer is a great tool because it is immune to changes in gloss.
We have ourselves a process control tool! This tool has gained acceptance among densitometerophiles in Europe. But as good of an idea as this might be, it has never really caught on in the US. I suspect that the invention of the Atlantic Ocean has to do with this.The definition of what goes into a polarized densitometer has been enshrined in ISO 13655, and polarized densitometers are referred to by the euphonious name “the M3 condition.”
The plot above shows this wives tale is just an old wive’s tale. For any particular stock, there is a linear relationship between polarized and non-polarized density of dry ink, so any comments about linearity with ink film thickness that you can make about one holds equally true for the other.
Process control, or meeting customer requirements?
A great man once said that a polarized densitometer is a great tool because it is immune to changes in gloss. It gives you an indirect indication of the ink film thickness without confounding it with the gloss.On the other hand, that same wise man is about to say that a polarized densitometer is a lousy tool because it only gives you an indication of what the sample looks like when it is viewed under polarized light while wearing polarized sunglasses with your head tilted properly. I haven’t done any in-depth surveys, but I think that most print buyers and readers of newspapers don’t look at the newspaper under this condition.
The annoying thing about customers is that they have this nasty habit of getting annoyed when the product is not what they wanted. Go figger. If we could only get rid of our customers, life would be so much easier. The print buyer (and end user) really and truly doesn’t care if the “correct” amount of ink has been applied to the paper. If the color on the paper is not the color that they were expecting, then it’s time for them to ask for rebates or go looking for another printer.
Therein lies a dilemma which is inherent to deciding on a color measurement instrument. Are you trying to do process control, or are you trying to measure the color? Do you want your printing press to run predictably, or would you prefer to get the color that your customer wants? Process control or customer satisfaction?
Process control often leads to meeting customer requirements. If the whole process is under control, then this is indeed the case. But, for anyone who has either dealt with customer complaints from the field, or who is married, it will be obvious that the best laid plans of mice and men oft go awry.
ISO 12647, parts 2 and 3
And now for the public service announcement…ISO 12647-2 is the standard when it comes to defining print. The purpose of this standard is to serve as a set of acceptance criteria for print. It is often cited as part of a contract for a printing job. Part 2 is about commercial web offset printing and part 3 is about cold set web offset (AKA newspaper) printing.Since the inception of part 2 in 1996, this standard has been clear that quality monitoring is the thing, and not process control. (Well, at least for the solids.) Density (be it polarized or non-polarized) is not a reliable indicator of the color that you see, and should not be used as an acceptance criteria. All the colors of the solids and the solid overprints in the standard are specified in CIELAB, since CIELAB is the closest thing we have to our perception of color.The standards are clear that densitometers are a useful tool for process control, internal to the printing plant. Generally, the printer establishes the density that will get to the proper CIELAB value with any particular substrate and ink combination, and will run to that. But ISO 12647-2 and -3 make it clear that the printer and print buyer should not converse in density when it comes to setting targets and tolerances. And as I have noted here, the printer and print buyer darn well better not even think about talking polarized density. There are just some things that are better left behind closed doors.
About the Author
John Seymour holds the title principal engineer for QuadTech, where he has been doing research in printing, color theory, and imaging since 1992. John was instrumental in the development of QuadTech’s Color Control System and AccuCam. John currently holds seventeen patents and has authored thirty technical papers. He is an expert on the Committee for Graphic Arts Technologies Standards and ISO TC 130, and currently serves on the board of the Technical Association of the Graphic Arts. He writes a blog under the pen name “John the Math Guy”, which is described as “applied math and color science with a liberal sprinkling of goofy humor.”
Prior to working with QuadTech, John worked as a scientific programmer in medical imaging, satellite imagery, electron microscopy, and spectroscopy. He holds bachelor’s degrees in mathematics and in computer science from the University of Wisconsin-Madison.
John had a hobby job as a karaoke host, going under the name “John the Revelator”, and before that his hobby job was teaching remedial math at a local university. He likes to think that he is gifted at “edutainment.” John teaches a color science class for QuadTech and has traveled as far as South Africa, England, Germany, and Hong Kong on speaking engagements.
Visit John’s Blog, John, the Math Guy – Applied math and color science with a liberal sprinkling of goofy humor, at http://johnthemathguy.blogspot.com/