A Guide to reflectance measurement devices, part 2

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.

Blogging my way to insanity

Today I look at one special case of 0/45 spectros, the polarized instruments. Stay tuned for the last section of the post, which is a public service announcement about the difference between process control and customer specs.

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.

Do you realize just how much dryback we have today!!!??

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.

Wow. Really?

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.

What are we gonna do?!?!?

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.

Celio, Mast and Ott, celebrating their brilliant discovery

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.

All tiny facet of the sample surface that are tilted at 22.5o

will direct specular light to the detector

A polarizer is added just after the light --
note that the specular reflection has the same polarization

Adding an s polarizer at the detector will eliminate this specular reflection

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".

Comparison

Naturally, you're gonna ask how polarized and non-polarized densities compare to one another. Surely there is a simple conversion, right?  As is often the case when I am asked a question, I have two answers: yes and no.

The plot shows measurements of a total of forty solid black patches, all of which were fully dried. Each patch was measured by a densitometer with and without polarization. The patches range in density from very light to very heavy, according to the pressman's subjective view. Ten of the patches were printed on a matte stock, ten on low gloss stock, ten on a medium gloss, and ten on a high gloss stock. The gloss of each was eyeballed by my very carefully calibrated eyeball.

The x axis of the plot is the density as measured without a polarizer.  The y axis is the amount that the density of the patch increases when measured with a polarized densitometer.

The red arrows illustrate the conversion from non-polarized to polarized on a matte stock. For that particular stock a density of 1.10D shows a difference of about 0.28D. That is, a 1.10D non-polarized density would be read as 1.38D polarized. Similarly, the blue arrows show how the density changes for a matte stock. A nonpolarized density of 2.00D is increased by only 0.06D when the polarizer is kicked in.

There are a couple of interesting things to note from this graph. First, for any particular stock and ink, there is a very simple transform between non-polarized and polarized. The difference between one and the other falls along a nice straight line. That's good news. There is a simple transform!

Second, the actual line for correction is highly dependent on the paper stock. For a very glossy stock, the correction is minimal; for a matte stock it is greater. That's bad news. The simple transform is not universal. To make matters worse, note that the medium gloss patches show the largest change. The patches on the low gloss stock have a conversion much more similar to the patches on the glossy stock.

Third, I will put to rest an old wive's tale. Density is kinda sorta linear with ink film thickness. In truth, the relationship flattens out as you go higher in density / ink film thickness. The tale that the old wife told me is that polarized density is much more better - polarized density maintains this linearity over a wider range of density.

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.

Using a polarized densitometer to monitor color
is like using this guy as an accountant

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.

Measuring metallic inks

Cliff Crosfield of Atlas Packaging in Exeter in the UK asked a rather open-ended question of me on LinkedIn. I started to answer on LinkedIn, but then realized that I had a lot to say (go figger - like that never happens!). I decided to open the answer up to a larger audience.

“What are your thoughts on the use of spectros for measuring "metallic inks"?”

Dr. Smith turns ordinary printing inks into platinum

This is a two part question. The first is about process control and the second is about making sure the product looks right. Spoiler alert: the two parts have different answers. Spoiler alert squared: the second part doesn't have a good answer. Yet.

The topic has to do with printing with inks that contain metal flakes so as to create a metallic luster. Metallic ink is made up of flakes of metal: aluminum, copper, zinc, or silver. The metallic effect is assured when the flakes a) all lay flat, and b) cover up the entire substrate.

Flakes of metallic ink shown on left, regular ink on right

(from Rosenberg, TAGA 2001)

This blog post is about metallic inks, but some of my comments below may also apply to measurement of other sorts of processes that create a metallic luster. Many processes put ink on silvery stuff. This includes decorating (you may call it printing) beverage cans with inks that do not contain opacifers (that is, stuff that hides the metal underneath), printing CMYK inks over a silver ink (as in Color-Logic inks), and printing directly on metalized films (Mylar). Another process is laminating a printed clear film onto a metalized film, like we would see in a potato chip bag. Further afield, I am not addressing issues with metallic luster having to do with car paint, aluminum foil, or polishing up the handle of the big front door, although some of the same issues apply.

Process control

The only thing I can control on press is the amount of ink that I put on the substrate. Clearly that should be the thing that I measure. Right???!?! So, what sort of measurement best correlates with ink film thickness of a metallic ink?

Evaluation of the metallic luster of chocolate candy production

I know of three papers on this topic.

Mannig and Verdeber (of Eckart) presented a paper at TAGA in 2002 where they compared measurements of various metallic inks with the amount of ink on the substrate. Polarized and non-polarized 0/45 spectros were used for the measurements, as well as spherical instruments. Their conclusion was that measurements from polarized instruments correlated best with ink film thickness.

Mannig and Verderber - the instruments with polarization show a strong relationship

Breede and Sharma (Ryerson University) presented a paper on this subject at TAGA 2008. They compared a gloss meter (20°, 60°, and 75° incident angles) with a densitometer and a spectrophotometer (both 0/45 instruments). Their conclusion is that the gloss meter didn’t work well, but that L* or virtually any of the density channels correlates well with ink film thickness. 

The third paper was also presented at TAGA, in 2008 by Habekost and Dykopf (also of Ryerson). They compared a number of different measurement geometries: nonpolarized 0/45, polarized 0/45, spherical, and a glossmeter with three angles. They found the best correlation with the cyan channel of a polarized densitometer.

Polarized geometry seems to be the hands down favorite when it comes to measuring how much metallic ink is being put on the substrate. Why is that? I have blogged before about reflected light being a combination of bulk and specular reflectance. Of the two, we are interested in the bulk. If we want to measure the amount of ink, we want to get rid of as much of the light that reflects from the surface of the ink as possible. That suggests 0/45, and particularly polarized.

My Spectrolino, outfitted for an afternoon of measuring metallic inks at the beach

More specifically, we are interested in the percentage of the substrate that is covered with little mirrors. Mirrors have this non-intuitive property that they look black to a 0/45 or 45/0 measurement device. A first surface mirror which has been properly cleaned has a 0/45 density of over 3.0D. I recently used this property as one way to assess the design of a collection of spectrophotometers.

Why did polarization help? If you think just about the metal flakes, it shouldn't. Metals have such a high index of refraction that polarization does affect them much. Polarization helped because it reduced the effect of surface roughness of the other stiff left on the page, like varnish. 

Based on these studies and theoretical concerns, I would recommend using a polarized spectrophotometer, with either visual channel density or L*, for process control. Another recommendation based on these studies is that TAGA is a great place to look for egghead kinda papers about printing. I am not at all biased by the fact that I am the Vice President of Papers for TAGA.

Product evaluation

The second part of the question is what sort of measurement correlates best with the print buyer’s expectation from a metallic ink? 

Now, I’m not a print buyer, but I’m going to pretend for the moment to empathize with them. I’m sorry, but when the guy in the brass section gets up for his solo, it don’t mean a theen if it ain't got that sheen. I really don’t care whether the printer maintained the "correct" number of grams of ink per square meter. Metallic inks are darn expensive, and if I’m not getting the metallic luster that I want, then it isn't worth it. I’m not gonna pay big bucks to listen to Brad Paisley play jazz on a calliope even if his dog has cute earmuffs.

"I don't see what's wrong! The polarized density is well within tolerance!"

The printer may come back and say that his process is completely in control. Yesterday, a 3.2 micron thickness of ink gave him an acceptable luster. Tomorrow that should be acceptable as well. Right???!?!?

Habekost and Dykopf might beg to differ. Remember they were mentioned before? In addition to comparing measurements to ink film thickness, they also looked at visual matches. They used the corresponding metallic inks from a Pantone book as the reference, and compared sheets printed with a range of ink film thickness to these. The figure below is from their paper, with just a little bit added by me to clarify. The plot shows a lot of information, but I draw your attention to the yellow area, which represents the sheets where humans identified a good match, as contrasted against the blue area, which highlights sheets that a polarized densitometer thought were good.

Disagreement between man and machine on what constitutes an "acceptable match"

Polarized density failed miserably at predicting a visual match because it measures something other than what a human sees as being a match. Polarized density may be a good process control parameter, but in this case, it is worthless for testing conformance to the customer's real requirements. I might even say that polarized density is even worse than worthless. If a polarized densitometer were to be used to control the press, the pressman would be running to a density of about 1.6D, whereas a density of about 1.2D would have been acceptable. In addition to being a poor match, the job would have used about 30% more ink than it should have.

The thing is that so long as everything in the process is under control, process control works to provide acceptable product. In this case (the Habekost-Dykopf paper), one of the key raw materials changed. The brilliant folks at Ryerson were printing on a stock different than the stock used in the Pantone book.

Clearly we need something better than polarized density as a metric for customer acceptability. In Habekost's analysis, the spherical instrument provided the best correlation, 

Goniophotometry

It's hard for me to write a blog nowadays without mentioning goniophotometry, so, here goes...

One further technical paper is worth mentioning. This one was from Artur Rosenberg of Fogra, presented at TAGA in 2001. (Not that I am trying to push TAGA or anything.) Rosenberg used a goniospectrophotometer to measure a series of samples with varying degrees of metallic luster. His samples went from from polished metal plates to traditional inks, with a number of metallic inks in between. 

Goniospectrophotometer - the octopus of the spectrophotometer kingdom

The astute reader will recall that Rosenberg had been mentioned in my blog post about the indicatrix. I referred to this very paper.

Rosenberg pointed his goniospectrophotometer at the indicatrix in the vicinity of the gloss angle. He defined a measurement that he called “F” (standing for metallic luster) which is the product of the height and width of this specular peak – basically it is the area under the peak. In his tests, he found that this area correlates well with the perception of metallic luster.

I am still trying to get my head around this result. In Habekost's experiments, gloss meter measurements were rather poorly correlated with our perception of metallic luster. On the other hand, Rosenberg’s F is somewhat akin to the measurements that a gloss meter makes.

My own pet theory is that the width of peak is crucial for our perception of metallic luster. After all, the most metallic thing I know of, a mirror, has an indicatrix with a perfect spike at the specular angle. But, that’s further research. Maybe I’ll find a goniospectrophotometer under the Christmas tree so I can test my theories. In the mean time, I’m not sure just what to recommend to truly measure the metallic luster of metallic inks. It's pretty clear that polarized 0/45 geometry is not the choice, though.

A Guide to reflectance measurement devices, part 1

You wouldn't think it would be all that hard. You go into McSpectros, and ask the guy behind the counter to show you a reflectance measurement device. You expect the guy to ask whether you want to measure the specular or the bulk reflectance. You read my blog on specular and gloss, so you know that those are the two critical parts to look at.

When you get to the front of the line, you expect to be shown a couple different models. Maybe one will be pimped out with an LED light show synced up with Beyonce for the Millennial crowd. The deluxe model (eligible for senior discount) will have a cup holder for a Venti sized Starbucks, and will play Michael Buble as measurements are made.

Oh... are you in for a rude awakening! There is a bewildering array of choices. I started counting up the different possibilities for configurations of an instrument, and came up with 18 different types that are in use and officially blessed in the standards. Some of them are interchangeable -- measurements made with one device should match those of another, at least in theory. But  there are still 12 different types of non-interchangeable measurements that can be made. The wonderful thing about standards is that there are so many to choose from.

"I'd like a grande, two pump, d/8 spectro tea latte with SPEX, please."

In reality, the big question is "gloss or bulk". You want to measure one or the other, or both. But rather than ask that simple question a wise reflectance measurement device salesperson will ask what line of business you are in.

Are you in the graphic arts?

If you are in the graphic arts, then you got it easy. All the instrumental decisions have been made for you. Thou shalt use either a 45/0 instrument or a 0/45 instrument. A 45/0 instrument is one where the light hits the sample at 45 degrees (preferably in a cone, all around the sample) and measures the reflectance at 0 degrees, which is to say, perpendicular to the surface of the sample. A 0/45 instrument simply interchanges the illumination and detection angles. The cool thing is that a wise old fellow named Helmholtz once said that 45/0 and 0/45 are interchangeable (usually). Most everyone I know believes him, so it must be true.

Illustration of either 45/0 geometry or an upside-down umbrella

One of the rationales for picking the 45/0 or 0/45 geometry is that it emulates the way one would normally read a Victoria's Secret catalog. Perhaps this may not have been apparent to everyone, but whenever I take a sidelong glance at this catalog that has been discretely addressed to my wife, the first thing I notice is that the magazine is printed on a high quality glossy stock. If I should happen to pick it up (which rarely happens, of course) I will naturally orient the prurient magazine so as to avoid seeing the specular reflection. One could argue that this natural viewing condition is something like 45/0.

Another rationale for 45/0 is that, when you convert these reflectance measurements to density, you have a number that is almost kinda sorta proportional to ink film thickness. One of the weaknesses in the correlation between the two is that darn specular reflectance. Even though 45/0 was designed to get rid of specular, a little bit or a lot of the specular will show up, depending on the smoothness of the surface.

This leads me to the topic of next week's blog post, polarized spectrophotometers.