What measurement condition is your spectro wearing?

These days, all the fashionable spectrophotometers are sporting the new measurement condition, M1. It's all the rage from Alabama to Aukland. If your spectro hasn't adopted this new look, then, sorry. It just won't get invited to the parties with all the cool spectros.

But what is this hip new fashion trend, and why should you care? This blog post goes undercover to get the inside story about the new measurement conditions in ISO 13655. This reporter investigates the four measurement conditions (M1 through M3), but more importantly, explains why you would choose one over the other.

Summary of the measurement conditions

Way back in 2009, when all of our spectros were wearing styles appropriate to that long-lost era, the ISO Technical Committee 130 came out with a new fashion edict. Spectral measurements henceforth shall be made according to one of four types of illumination, and the one chosen shall be reported along with the data.

(Note that the word "shall" is standards-speak for "you gotta do this if you want to comply with the standard. The other key word is "should", which means "as experts, we recommend doing this, but it is not a requirement for compliance".)

Why all the fuss? The big driving force behind this is the proliferation of Fluorescent Whitening Agents (FWAs) in paper. These are substances (stilbenes, for the chemists and lingo-philes in the crowd) that absorb ultraviolet light and re-emit that energy as blue light. This makes paper look whiter - which is what everyone wants and craves.

I should make a note here. The popular media (Fox News and MSNBC) call them "OBAs", meaning "Optical Whitening Agents". While this moniker is correct - stilbene does make paper brighter in an optical kind of way - I would say that so does TiO2 and bleach. The term "OBA" fails to emphasize the key operative, which is fluorescent light.

I have already written a bit about the basic problem and the reponse to it. The following exciting articles focus on the "M1" stuff.
Layman's Guide to ISO Print Standards

ISO Comes to Inline Color

Here are the four exciting choices for measurement conditions:

 Most handheld spectros use an incandescent bulb to illuminate the sample. Remember those kinda lights? A little piece of wire called a "filament"? Some electricity going through it? And then the wire gets hot and glows. Guess what? This light source doesn't have all that much UV content. And guess what again? The amount of UV varies a lot from one instrument to the next.

M0 is based on a hypothetical incandescent light source. For an M0 measurement, the light hitting the sample should (note the word) conform to CIE standard illuminant A - which is to say, a light bulb. The word "should" is important and intentional. This little tiny loophole allows anyone to use the older spectros and remain compliant. This would be a totally dumb idea, but you could use a lightning bug with a hangover as the light source for your M0 illuminant and still be compliant. No one's gonna check. M0 is the nightclub that any spectro can enter.

If you want to find out if your local neighborhood color scientist is hip, just ask whether he or she is raving about M1. All the hip ones will say M1. The illumination for this measurement condition is based on a theoretical daylight called D50. This puppy packs a pretty good wallop of UV content, so this will excite those ol' OBAs, if you know what I mean.

Does your spectro want to strut its stuff at the M1 Bistro? The security guard is checking IDs at the door, and any spectro failing to provide the proper levels of OBA ain't gonna be ordering an avocado-tini at this joint.

OBAs love attention. The only reason they show up at parties is to be seen. But when the spectrophotometer breaks out it's M2 light source, the OBAs become as invisible as that woman who married Jimmy Fallon. An M2 light source will be almost kinda completely devoid of UV light.

How do you get into this club?  If your spectro comes knocking at the door, there will be a test, and I'm, not talking "written exam". The test is described in Annex H. To perform the test, you need to get pretty intimate with the instrument. It would make a TSA agent blush.

BTW, the TAGA 2015 conference in Albuquerque (March 22-25, 2015) will feature a micro-conference on OBAs. We had a whole bunch of papers on the subject, so the VP of papers decided to talk it up big. Here I am, talking it up big.

M2 might seriously have been considered a contender for preferred condition. It does level the playing field when it comes to UV excitation. All M2 instruments have no UV, so none of them should cause OBAs to glow. And they should all agree.

But, to make this whole opera work, the light booths really should have the same amount of UV content. And if there were no UV content in the viewing booth, then papers with OBAs would look dingy. And no one wants that.

You may think that M2 is an exclusive club, but M3 is even more exclusive. First you need to get into the M2 club even to be able to bribe the bouncer to get into M3. And you gotta be wearing sunglasses. Not just any sunglasses, but sunglasses with polarizing filters. As a result, specular highlights and fluorescence are not allowed in M3 disco.

So... if you happen to be thinking about dating a spectro, how do you decide which one is right for you?

Do you have OBAs?

I know this probably sounds personal, but the first concern is whether you have OBAs in the house. If you don't have OBAs, then (theoretically), there should be no difference between M0, M1, and M2 measurements. You could use your older instrument that doesn't offer a choice, or you could go with one of the newer instruments that offer M1.

Do I smell OBAs?

That question "do you have OBAs?" is pretty much the same as "are you measuring paper?" The thing is, paper is normally brown - the color of a grocery bag. Special processing must be done to make paper white. That could involve bleaching, or it could involve OBAs. Today, almost all paper uses at least some of the latter.

So if you happen to be measuring ink on paper, then you just gotta plunk your money down for an M1 instrument, because you will find your old M0 instrument disagreeable. It will disagree with your light booth, with M1 instruments, and even with M0 instruments from other families. The thing is that pretty much all paper for commercial printing will have OBAs added, so you have a choice as to whether to deal with a petulant teenager, so buy into the M1 craze.

I should add that switching over to the M1 instrument is only part of the change that you are facing. When you make the change, you will find that you need to make sure that your lighting booth adheres to the 2009 version of ISO 3664, so it has the same amount of UV content as the spectro. And all the data that you had previously measured, like profiles of your press and target colors, will need to be updated. I know, not a simple solution. 

That covers web offset, sheetfed, and newspaper printing. The situation in packaging is a bit more complicated, so let me describe some cases. Let's say that you are measuring color on foils or poly. I might be wrong here, but I don't think you will run into any OBAs. Foils and poly and the floodcoat use something like TiO2 to add whiteness and opacity. Floodcoats wouldn't benefit by OBAs, since they are not brown. So, like I said, I could be wrong, but I think it would be silly potatoes to add OBAs to foils.

If you are measuring kraft paper, then I think you are likely free and clear as well. If the paper is brown, then clearly no one cared enough to take the time to add OBAs to make it white. Then again, if the kraft paper includes a certain amount of recycled paper, then the OBAs might be sneaking in through the back door.

How about printing on white cardboard or card stock? Now things get uncertain. You gotta ask yourself, how did the cardboard get white?  Sometimes, cardboard is made white by applying a floodcoat of white stuff. This white stuff might be something like titanium dioxide, which is naturally white. So, it is unlikely to contain OBAs. Then again, cardboard may be white because it has been laminated with paper. If that paper is white, then you can guess that it has OBAs. Another possibility is that the white card stock might be white because the paper is white. So, it could have OBAs.

When in doubt... I would recommend having a UV light source around. Anyone who survived the sixties is familiar with these. Today you can get UV flashlights made from LEDs. Cheap and convenient, and a good way to test for the presence of OBAs.

UV LED "blacklight" flashlight from Amazon for $12.89

Do you want CIELAB values?

The second concern is whether you want CIELAB values. In my not-always-humble opinion, computing CIELAB values from a polarized instrument (that is, M3) is just plain silly. The whole point of CIELAB is to emulate how our eyes see color. Unless your product is destined to be viewed by people wearing polarized sunglasses under light that is polarized the other way, then CIELAB is probably the wrong choice.

ISO 13655:2009 agrees with me on this one. Here is a quote from Annex G:

"Notwithstanding the beneficial effects of crossed polarizers [M3] in the special cases mentioned above it needs to be noted that for most other instances in colorimetry the use of polarizers is counterproductive."

So, if you want to compute CIELAB values, then you must use M0, M1, or M2.

Are you doing process control on a cold set press? 

M3 does not mix with CIELAB, but it does mix with density. ISO 13655 has this to say about where the M3 condition might be applicable:

"It was discovered that the installation of crossed polarizing filters serves to extend the linear part of the density versus ink film thickness dependence towards higher values, and serves to greatly reduce density dry-back."

There are two points here. The first is that M3 "extends the linear relationship between density and ink film thickness." This is believed by many, but it is unfortunately incorrect. In my blog post on polarized densitometers I presented a plot showing that there is a linear relationship between polarized and unpolarized density. If polarized density is linear with ink film thickness, then unpolarized must be as well.

The second point from the ISO 13655 quote is that M3 reduces dry-back. Measurements made directly after printing on a cold set press will not change as the ink dries. This is true, and that was really the whole point of the blog post on polarized densitometers. So... I won't belabor the point here. But I will encourage you to go read the post.

And I will also reiterate a point that I have iterated a couple three times in previous blog posts. There is a difference between "process control" and "satisfying-your-customer control". The first is about making sure your process is appropriate and repeatable. The second is about making sure that the payments from your customer are appropriate and repeatable. Density and M3 are process control parameters. CIELAB is a product expectation control parameter.

So, M3 can be useful when you are measuring ink that is not quite dry, but its use should be limited to within a given plant. No interchanging of data, you hear?

Are you doing process control on materials with OBAs?

Just in case you have been just too enthralled with this page-turner of a blog post to have been keeping track...

M0 is ok if you have a legacy instrument, and you aren't really that into OBAs.
M1 is preferred, especially if you might think you have a little issue with OBAs.
M3 is acceptable, but only for process control - no CIELAB allowed.

What about M2?

OBAs are tricky little devils. They make the paper look whiter. But when you apply ink to them, a funny thing happens. The ink blocks the UV, so that the paper under the ink does not get artificially brightened. This can make things a little weird if you are a process control freak. Especially when you come upon an ink that doesn't happen to block the OBAs. The relationship between the density of the paper and the density of the solid ink gets befuddled.

That last paragraph was written yesterday. This morning, I looked at some data that I got from my good buddy, Gerry Gerlach. His data refutes the stupid statement that I made "The ink blocks the UV." Good God! What was I thinking. In his data, cyan, magenta, and black all do a pretty good job of blocking the UV. But for yellow ink (and aqueous coating on bare paper) there is a large difference in b* between the M1 and M2 measurements. In other words, the ink and coating are letting the UV light through, i.e. are transparent in the UV. I suspect that not all yellow inks do that, and certainly clear coatings may differ. 

So, M2 is a process control thing, maybe better than M1. If you are trying to establish that you are putting a consistent amount of pigment on the paper from day to day, this might be a good thing to try. But as with M3, I caution that this is not the same as making the correct color.

M2 has found another purpose in life just recently with the invention of the OBA index of a paper. It has been noted that OBAs will tend to decrease the b* of a paper. A substrate might measure a little yellowish under M2, maybe b* is +2. If you measure that same substrate under M1, the b* might go negative, maybe -3. The OBA is the difference between the M2 and M1 measurements. In this example, the OBA index would be 5.


Are you sufficiently confused?

I hope this has blog post on illumination conditions been enlightening, no pun intended. Actually, the pun was intended. But the enlightenment was also intended.

Why does Elmer's glue change color as it dries?

Some days, I watch paint dry. Other days, it's laundry. Today, I watched Elmer's glue dry. Yes, I know. Everyone is envious of my exciting life.

A funny question hit me the other day, "Why does Elmer's glue change color as it dries?" When you squirt glue out of the bottle, it starts out a milky opaque white. As it dries, it changes color. The final color is clear. I had an inkling about why this might happen, but I thought I should double check some facts before I settled on that explanation.

Wet glue (right), partly dried glue (left), and
closeup of partly dried glue

What is Elmer's glue made out of?

The first thing I wanted to know is "what is the stuff made out of"?  Everyone knows that it's made from trimmings from the hoofs of heifers when they go to the salon. Right? Naturally, I went to the Elmer's glue website to verify this.

"Elmer's Glues are chemical based. They are made or formulated from chemicals which are synthesized (created by Man). These chemicals were originally obtained or manufactured from petroleum, natural gas and other raw materials found in Nature. The exact formula and specific ingredients used in making Elmer's products are considered proprietary information, therefore, we cannot share those with you...

Elmer's does not use animals or animal parts to make glue. Our products are made from synthetic materials and are not derived from processing horses, cows or any other animals."

http://elmers.com/about/faqs/general/

So I booked a flight to Columbus, Ohio where Elmer lives. On the way to the airport, I picked up a cool spy camera, some camouflage pants, wire cutters (to cut through the barbed wire), and a couple of Snicker's Bars in case I needed to bribe a security guard. Or if I got hungry. I go all out for these blog posts, I tell you.

I suppose I could have just looked up "Elmer's glue" in Wikipedia. Then I could maybe have followed through to the link on polyvinyl acetate. But that thought just didn't occur to me until I was crouching down at the fence, dodging the searchlight from the guard's tower, and was trying to cut through the fence with nail clippers. My psychiatrist doesn't want me to have a wire cutters.

Here's what Wikipedia would have told me: Elmer's glue is an emulsion of polyvinyl acetate in water.

I don't know about you, but I was really excited when I heard this from the security guard as we chowed down on Snicker's bars. Luckily, he has a fondness for Snicker's bars, or I would be writing this from a maximum security prison. 

The clues are falling into place nicely. Now, all I need to know is the index of refraction of PVA. A little more googling, and I came up with this wonderful tidbit. The refractive index of polyvinyl acetate is 1.46 to 1.47. That's exactly what I wanted to know.

Explanation of wet glue 

Elmer's glue is primarily made of particles of polyvinyl acetate suspended (technically, emulsified) in water. It looks milky because those particles have a different index of refraction than the water they are in. (Water has an index of refraction of about 1.3.)  

Fresnel's law says that when light encounters a change in the index of refraction - as when light hits a polyvinyl acetate molecule - some light reflects. As a result, it changes direction. The same thing applies to light inside a polyvinyl acetate particle. Some light reflects. If there are enough of these encounters, none of the incident light will make it through to the underlying substrate, which is to say, the stuff will be transparent.

The actual photomicrograph shown below is an actual photomicrograph of particles of polyvinyl acetate in water. (This is wet Elmer's glue, by the way.) Imagine that you are a cute little photon (a tiny tiny particle of light) trying to make your way through this glop of wet glue. Anywhere you go, you run into a particle and you bounce. Chances are, you will bounce back in the direction that you came.

Particles of polyvinyl acetate suspended in water

(actual photomicrograph)

Now imagine that you are a person watching a zillion of these photons. What will you see?

First point... Not many photons are going to make it all the way through this glop of wet glue. That is to say, if you look at the wet glue, you won't see anything on the other side. In other words, wet glue is opaque, or at least mostly kinda.

Second point... Water is mostly kinda clear. That is to say, as photons bounce around between particles of polyvinyl acetat, they are likely to keep bouncing until they exit, rather than get absorbed. Note that this is also true of polyvinyl acetate. Note also that this holds for all wavelengths of light. In other words, wet glue is white, rather than gray or blue.

So what happens when the ink is dry?

The key thing that makes glue opaque and white is the combination of two components that have different indices of refraction. When glue dries, one of those components go away. Dried Elmer's Glue is mostly just a monolithic mass of polyvinyl acetate, so there is no opportunity  for light to bounce around. Light mostly just kinda goes right through the dried glue. And hence it's clear.

So, now you have an answer when you kindergartner asks why glue changes color. Naturally, when I explained this to my kids - years ago - they responded "Oh... of course, Dad. It's because of the Kubelka-Munk parameters."

A Guide to reflectance measurement devices, part 3

I continue my action-packed series on the plethora of devices for measuring reflectance. Today's topic is the spherical instrument. This blog post promises to be exciting because I know virtually nothing about the topic. This of course is generally the case when I write a blog, but in this case, I might actually admit to being ignorant if you get enough beer in me. Please try.

In the first part of this series, I made the comment that your choice of instrument may not be directly related to which type works the best, but may be dictated by which industry you are in. If you are in the print industry, you are likely to be using (or required to be using) a 0/45 instrument. But, if you are measuring textiles or paint...

Are you measuring either cloth or paint?

If, by chance, you told the reflectance measurement device salesperson that you were in textiles or paint (personally, I am in lady's underwear), then she would most likely point you to the integrating sphere section of the store. There you would see a few shelves of instruments that are labelled either 8/d or d/8. These are in the "spherical instruments" department.

When paint and textiles collide

An instrument that is 8/d will have illumination that hits the sample at 8 degrees, which is to say, just off the axis perpendicular to the sample. Light will be collected democratically -- without regard to race, creed, or direction of travel when leaving the sample. This certainly has a satisfying feel to it. It should appeal to the physicist in all of us to know that we are collecting pretty much all of the reflected light. So that's good.

The picture below shows how this is accomplished. Light enters through a port near the top of the sphere. When light reflects from the sample in all directions it hits the inside of the sphere. The inside of the sphere is coated with stuff that is highly reflective and very matte, so the light bounces off in all directions. And then this light hits the inside of the sphere again, and bounces yet again. Eventually, some of the light hits the detector and is measured. That sphere, by the way, is called an integrating sphere.

Illustration of an 8/d spherical instrument or a Christmas tree ornament

(from “The Measurement of Appearance” by Richard S. Hunter, John Wiley, 1975)

While it feels good to collect all the light, on the other hand, this is clearly not a measurement geometry that simulates anything in the real world. Light comes in at one angle - that part is reasonable enough - and is measured (seen) at all angles. What? Who has eyes like that??!?!

On the third hand, remember what my high school buddy, Herrmann von Helmholtz said: you can switch the illumination and viewing and see the same thing? So, 8/d will give you the same numbers as d/8. And a d/8 measurement is more or less what you get when you look at a car on a cloudy day... illuminated from all directions (well, mostly), and viewed at close to straight on (well, sometimes). The pictures above and below are from one company that bragged about both d/8 and 8/d designs while I was in high school.

A d/8 instrument or an engineer's version of an ornament

(from US Patent 4,093,991, assigned to Hunter Labs, 1977)

SPINing and SPEXing

When using a d/8 or an 8/d instrument, you have yet another decision to make: SPIN and SPEX? These stand for SPecular INcluded and SPecular EXcluded. As with a polarized versus non-polarized spectrophotometer, this is an attempt to differentiate between the bulk reflection and the specular.

The SPIN instrument is just what I described previously. The light is captured from all directions without regard to race, creed, or sign of the zodiac. A SPEX instrument is almost the same, except that a black plug is put at the specular angle (at 8 degrees opposite the illumination). This keeps the detector from ever seeing this specular light. Clever, eh?

Depending on what you are doing with the measurements, one or the other might be more better. Consult your bartender or cosmetician for further advice.

Why is a spherical instrument good for cloth?

There is an inherent problem when you try to measure textured cloth with a 45/0 instrument. The texture will block 45 degree light from getting very far inside the warps and woofs of the cloth. The detector will miss out on seeing that rich color deep down in the fabric. Under typical conditions, our eye will see that light reflected from deep inside, since we normally have light that is hitting the fabric at angles other than 45 degrees. I might add that the fabric I most enjoy viewing is not presented as a flat piece. I much prefer fabric that has some curves to it.

So, a spherical instrument has a big advantage when it comes to cloth, or carpet, or textured paint.

This man is measuring the rough surface of a lion's tongue with a 0/45 spectro.
I don't recommend this.

Why is a spherical instrument good for paint?

You want your paint formulation software to work? Don't even think about using a 0/45 spectrophotometer! A 0/45 spectro is very sensitive to the roughness of the surface that you are painting. If you mix the pigments of the paint based on measurements of one surface and then paint a surface that has a different roughness, guess what? The 0/45 spectro will see a different color. A spherical instrument is more forgiving.

That's the good part. You can paint one surface to get your paint recipe and the use that paint on another surface and measure the same color with your spherical instrument.

But the bad part is that a spherical instrument is more forgiving - probably more forgiving than the flibbertigibbet who may or may not pay you because the house paint is the wrong color. Generally speaking, measurements made with a 0/45 spectrophotometer correlate better with what we see. Note that I have italicized those words for the benefit of those people who will disagree with me.

This is a point that I find myself making over and over again... process control versus customer needs. Frankly, I am getting sick of talking about it. Some day, I'll just dedicate a whole blog post to the subject and stop ranting to my therapist.

What if you live at the intersection of textile and print?

There are some poor sods who find themselves needing to make printed stuff look like textured textiles. Which type of spectro should they use?!?!?

Textiles or graphic arts?

Some of these poor sods are printers of catalogs. The color of the dress is critical, so they measure the lady's dress. What do they measure it with? A spherical instrument, of course. Then they go print it and wind up measuring the printed catalog with a 0/45 spectro.

There are also some poor sods who get stuck having to print proofs of textile designs. Once again, we have the graphic arts world crashing into the textile world.

What to do about this? It's tough, but if the fabric has a mostly kinda sorta matte finish, then the two instruments (d/8 SPEX and 0/45) will read similarly. Just be careful when trying to critically compare numbers from one type of instrument to another.

Are you decorating cans?

For those who are not in the know, "decorating" is the official way to describe putting ink on soda and beer cans. I know, it sounds kinda froo-froo, but I didn't make up the term. Most of what I say in these blogs is made up, but this particular factoid is true.

If you are in the can decorating business, and are looking to buy a spectro, then you have to ask the follow-up question: Coke or Pepsi? One of these companies requires that cans be measured with 0/45, and the other with spherical. If I can be trusted to explain their reasoning, one company realizes that 0/45 correlates better with what we see. The other company realizes that 0/45 measurements are hard to duplicate, since positioning is critical.

One of these is measured 0/45, and the other d/8

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.