by John Seymour, John the Math Guy
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.
Layman’s Guide to ISO Print Standards
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.
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.
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.
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.
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.
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.
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.
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.
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/