Is metamerism a big issue in print? (Part 2)

This is part 2 in a series of blog posts that recap my presentation on metamerism at Color 20. If you were actually looking for the first part, the part where I set up the characters and give the back story, then you should go to the first part.

Ahhh... so you decided to tough this blog post out? Well, good on ya, mate!

Today's post is about the intersection of expanded gamut, spot color replacement, and metamerism. I suspect that this might result in another standing-room-only blog post.

Is metamerism a big deal when you are doing spot color replacement?

Expanded gamut

In kindergarten, I was taught the lie that red, yellow, and blue were the primary colors. I will never forgive my kindergarten teacher for that. That really messed me up when I found out that the real primaries for ink were cyan, magenta, and yellow. There is a big long story about that, but for now, I will just pass the explanation along to Stephen Westland and Stephen Westland and David Briggs and David Briggs. Good articles, all of them.

And then I got even more confused when people kept talking about black ink. Why do you need black ink, if you can get all the color with just CMY? (I mean, I thought the whole idea of primaries is to give you all possible colors?) Well, there are many reasons for using black, but one of them is that you can't get all the colors with just CMY. One notable color that you can't get is black.

And guess what? Even with CMY and K, you can't get all the colors. If you want more colors, you need to add more primaries. Expanded gamut printing uses color beyond CMYK, typically orange, green, and violet, to get more of the entire range of visible colors.

(Interesting fact: Generally we would call this CMYKOGV printing, but that's a really silly order for the letters. Maybe it should be CVMOYGK?)

Spot color replacement

I know that some of you are thinking that spot color replacement is what you do to get your Dalmatian ready for St. Pat's Day. Nope. Good guess though. (To get your Dalmatian ready for St. Pat's Day, I would recommend giving him a long reddish-brown, silky coat.)

My dog Spot is not looking excited about the St. Patty's Day festivities

Historically, there has been a distinction made between process colors and spot colors. Process colors are CMYK and are used in packaging for image content. Each pixel of the printed image gets some build of those four inks. Spot colors are specialty inks that are mixed to the desired color and are printed in, well, certain spots of the printed package. The spot color inks don't usually get overprinted with other inks, and are generally not used in imagery.

This is not an endorsement. They do go well with a wasabi mustard, though.

The package above uses (maybe) a total of eight inks -- cyan, magenta, yellow, and black for the image and spot colors of dark blue, light blue, black, and green. (And there is probably a white to cover the metallic (mylar) surface. White is not referred to as a spot color, but rather a flood coat. I think this is an egregious abuse of the English language.) Each of these inks gets its own print unit.

The next print run will likely require a different set of spot colors. This leads to an expense, since the old spot color inks needs to be cleaned out between print runs. There would be an economic advantage to printing those spot color with an equivalent combination of process color inks. But if we augment those process inks with a few extra colors of inks, typically orange or red, green, and violet or blue, then nearly all spot colors can be emulated with this augmented set of inks. No need to clean up after each print run!

So, there is an economic advantage, but it comes with a hidden cost: metamerism. If one package printed with CMYK+spots should land on the shelf next to one printed with CMYKOGV, then the best we can hope for is a metameric match. Perhaps there is an excellent match under D50, but can you find a store that has good D50 lighting?

That leads us to the question of the day....

Is the degree of metamerism enough to worry about?

Disclaimer #1: I suspect that many of the people who have implemented spot color replacement have gone through the exercise of evaluating the degree of metamerism for the spot colors that are important to them. I don't intend to minimize this or necessarily replace this worthwhile test. My goal here is to help set expectations in general.

Disclaimer #2: If you are bringing a new design into production, metamerism probably isn't an issue for you, beyond perhaps needing to explain to the brand owner why the color of the package didn't match the Pantone book in the designer's living room. If you are switching production of an existing pretzel pouch to spot color replacement, and expect a short period of co-mingling on the store shelves, I leave it to you to decide on the importance of the transition period and weigh the cost of that against savings.

Those who read the previous blog will recall this image of a set of metameric sextuplets, all of which are perfect matches to my version of Pantone 147C under D50/2.

In the delivery room, with D50 lighting, these appeared to be identical sextuplets

The spectra look quite different, but when it comes down to it, is there a large metameric difference?

The following table is stolen from my Color 20 presentation. It shows the CIEDE2000 color difference between Pantone 147C and the emulated version under D65/10. This should give an appreciation for the magnitude of metamerism. The spectra look a lot different, but they still match under one illuminant, and are not a bad match under another.

Not so bad? John shrugs his shoulders. I would be cautious about trying to read much into this table. There are many combinations of CMYKOGV that could yield a given color. The software that I wrote to create the matches did not put a whole lot of thought into which one of those combinations to use. I don't claim that it yielded builds similar to any commercial spot color replacement software, Mileage may vary. This package sold by weight, not volume. Contents may have settled during shipping. Blah blah blah.

Here is another set of metameric sextuplets from my database. In this one, for whatever reason, the spectra are all a reasonably good match. We see some larger disparities around 400 nm, but these are less significant to the eye.

And another set that appear to vary about as much as the first one.

Here are the color differences.

What to make of this table? Once again John shrugs. There are bigger numbers and smaller numbers. The intent here is not to focus on one specific case, but rather to look at the data in aggregate. The database has plenty of aggregate to offer, with 3,604 metameric spectra. Here is the big picture.

I love cumulative probability density functions of color difference data

How to interpret this? John shrugs his shoulders and makes woogly eyes. I have blogged before about this sort of Cumulative Probability Density Function plot, and again here. I show below one very reasonable way to look at this data. The plot can be used to determine the percentage of color differences that are below a certain point.

I arbitrarily set 2.0 ΔE₀₀ as a tolerance. This is a typical tolerance for print under D50/2. From that alone, it seems like a reasonable starting point. But, one may argue that this is a secondary requirement in the eyes of the print buyer. (I want it to match under D50/2, and could you also make sure the match isn't horrible under D65/10?) So maybe this is too wide?

Arguing the other side, there are two contributors that we want to consider. The first is the normal process variation, for which we may set a tolerance of 2.0 ΔE₀₀. The second contributor is the color difference due to metamerism. These two contributors combine in the final analysis. If we allow for 2.0 ΔE₀₀tolerance of normal process variation under D50/2, and we allow for a 2.0 ΔE₀₀ color relative change due to metamerism, then we could see something like 4.0 ΔE₀₀ color change when they happen together.

Now for the math stuff. They two sources of variation could cancel each other out. IT could be that by fluke, the sample is 2.0 ΔE₀₀ off from the target under D50/2, but matches perfectly under D65/10. Generally speaking us folks in the stats world use "sum in quadrature" to describe how tolerances stack up on each other. A 2.0 DE00 variation and a 2.0 ΔE₀₀ variation (statistically speaking) add up to sqrt ((2.0)^2 + (2.0)^2) = about 2.8 ΔE₀₀.

So, is this a big issue for spot color replacement?

Based on this analysis, I can say this: 

If you replace a traditional spot color with another set of pigments,

and you get a perfect match under D50/2,

then you have an 8 in 9 chance of having an acceptable match under D65/10.

I make the assumption here that the normal process variation is less than 2.0 ΔE₀₀, and that a 3.0 ΔE₀₀ variation under D65/10 is considered acceptable. In the next blog post in this series, I will look at other illuminants. 

Tolerance for metameric index

I had a question from Rachel after my previous post regarding reasonable tolerances for the metameric index. I pause to define metameric index in this context. The graph and table I show above fit one definition of metameric index: the color difference that you see between two perfect metamers under one illuminant when you view under a second illuminant.

From my graph, I can say that 2.0 ΔE₀₀ is a reasonable tolerance for metameric index for D65/10. Eight of nine times you can hit that. If you pay a bit of attention to metamerism when you decide on how to render an EG color, you can do better, In the next blog post in this series, I will look at other illuminants. Hint: the change from D50/2 to D65/10 is not huge...

I hope that this leads to some good argument among the folks who like to argue about standards.

Is metamerism a big issue in print? (Part 1)

In January 2020, I spoke that the Printing Industry of America Color Conference. This colorful event is sponsored every year by the PIA in gorgeous San Diego where the weather is always gorgeous. Next year, I understand that the colorful event that is always in San Diego will occur in gorgeous La Jolla, and it will be hosted by the as-yet unnamed organization that is the combination of PIA and SGIA.

The talk I delivered to a standing-room-only crowd was on metamerism. Based on the fact that one or two of them looked up from the cell phones a few times, I would say that the talk went over quite well. This series of blog posts will recap those exciting moments.

Metamerism - when objects are the same color under one light,
but differ under another 

Now, at the conference I didn't just state my position on whether I was fer metamerism or agin it... I went right up to it and measured it. I considered several practical issues and sought to determine just how big an issue metamerism really is. And since you are part of the elite group that is reading this blog post, you have the opportunity to read a summary of my presentation.

In Part 1 of this series of blog posts, I describe the metameric database that I used to quantify metamerism. The blog post you are currently reading, by the way, is Part 1. So, when I finally get done with this introduction, I will talk about the metameric database.

In Part 2, I use this database to answer one practical question: If I switch from printing spot colors with pre-formulated inks to printing them with expanded gamut builds of CMYKOGV, will metamerism deliver a sucker punch to me in the gut?

In Part 3, I look at the magnitude of metamerism that I see when I go from D50 to a variety of popular illuminants that were standardized in CIE 15.2, and have been used for years. This leads us to a surprising conclusion about how well the Color Rendering Index works. Stay tuned!

After Part 3, I move on to Part 4. In this section I swap out the standard illuminants for a plethora of white LED illuminants. (Or is it a plethorum? I dunno.) I measured a whole pile of white LEDs and answered a pressing question: do white LEDs pose a big problem for us when it comes to metamerism?

Finally, and rather unexpectedly, I present Part 5 of these series of blog posts. In this blog post I find out how serious a problem viewing booth metamerism is. What is viewing booth metamerism? Get this: the D50 in your viewing booth is merely an approximation to the D50 in your spectro. As a result, your spectro may disagree with your eye as to whether a proof and press sheet match. Should you lie awake at night worrying about this?!?!?

Metameric Encyclopedia for the Graphic Arts

I decided that I would need a database of metameric pairs in order answer the questions that I posed. Now, John the Busy Guy Who Doesn't Have Time for Frivolous Tasks would probably have been too busy to take the time for a frivolous task like creating more than a handful of metameric pairs. But, the presentation at Color 20 was given by John the Math Guy, and I am never too busy for any sort of frivolity. I heard a rumor that there might be a world record waiting to be broken, so I took it upon myself to make a collection of metameric spectra that would make an acid trip with Jim Morrison seem like a Whiter Shade of Pale.

My metameric database (on the left) compared with the competitor's (on the right)

I started with spectral measurements of a Pantone book. These are real spectral from ink formulated as they might be formulated in any printing plant in the world. Then I brought in a characterization data set from flexographic printing. For each of the Pantone colors, I searched through the flexo data to find a close CIELAB match. If I found a flexo color that was reasonably close, I mathematically adjusted it so that it was a perfect match under D50/2.

(For the non-printing-geeks out there... Characterization data is a set of measurements of printing with a zillion or so combinations of the inks in a press. On a four-color press, the characterization data typically includes about 1,600 patches. For expanded gamut printing, it might be several times that many.)

(For the geeks out there... the mathematical adjustment was done through principal components. I determined the singular value decomposition of the full flexo data set and used the first three vectors as a basis. I then found the linear combination of the basis vectors to add to the actual spectrum so that the adjusted flexo spectrum was a perfect metameric match. I discarded any spectra with values outside the range of real printing. You know, simple obvious stuff.)

(For the those concerned about social justice and stuff out there... No spectra were hurt in the filming of this blog post or the presentation. The spectra that I arrived at were realistic and perfect metameric matches.)

I mentioned flexo. A bit more detail there. This is data that I received from Liam O'Hara, who has been a friend for many years is now a colleague of mine at Clemson. The data was an expanded gamut characterization data set. So I was investigating spot color replacement with expanded gamut. But I saw the opportunity to have even more fun with the data. I broke it into two subsets; one subset had only CMYK, and the other subset had at least one of the extraquaternary inks (that is, orange, green, or violet).

Thus, for each (or I should say, most) of the Pantone colors, I had

1) The spectrum of how a flexo press with a particular CMYK inkset might render a perfect colorimetric match to that Pantone color, and

2) The spectrum of how a flexo press with a particular CMYKOGV inkset might render a perfect colorimetric match to that Pantone color, with the caveat that the build must include at least one of O, G, and V. 

Some Pantone color were out of gamut, so they did not make it into MEGA. Some were in gamut for the expanded gamut printing, so a metameric pair was added to the database. And for some of the Pantone colors, I had not just a pair of metamers, but a set of metameric triplets! How awesome is that!?!?!

Enter the Indigo 7900

Did I stop there? Of course not. I was going for the world record!

I just happened to have data from another friend, one who happens to not be a colleague of mine at Clemson, since he doesn't teach at Clemson. He actually teaches at Ryerson, so we work at different schools together. Abhay Sharma recently pitted one piece of expanded gamut software against another in a study of the capabilities of expanded gamut software. When Abhay wasn't looking, I grabbed a copy of the expanded gamut characterization data for an Indigo 7900. Please don't tell him I have that data.

And I went through the same procedure with this data so that I potentially had two additional metameric matches for each Pantone color. We're talking the birth of metameric quintuplets! (I hope everyone is as excited as I am.)

And one more!

No. I didn't stop there! I had one more database up my sleeve. But first a little background.

Let's face it. The Pantone Formulation Guide has a few problems. The first problem, which is obvious to anyone who has casually looked through the book is the haphazard ordering and numbering. Below, I show seven pages from the swatchbook. The first and the fourth start with 256 and 263 at the top. The pages with 2563 and 2567 have been shoehorned in between. These four pages are consecutive. Much later in the book, we find consecutive pages 511, 5115, and 518, which are in more or less the same color family. In between, we have a bunch of blues and greens and yellows and grays. Just for convenience, the colors in the latter set are upside-down. The darkest, richest colors are at the top instead of the bottom.

Just to be clear, I'm not blaming Pantone for this. The hodge-podge numbering system was inevitable. The books have grown through the ages and there has been an understandable unwillingness to change the numbers on existing colors. 

Another issue with the swatch book is that the pages, when fanned out, don't have a nice, smooth flow. That is aesthetically unnerving, but from a practical sense it means that the colors are not equally spaced perceptually. And it makes you wonder whether there are holes -- colors that have been missed entirely.

I have blogged previously about how a swatch book could be ordered more uniformly. Albert Munsell created such a book in the early 1900's. Much more recently, Phil Kenyon wrote some software that organizes paint company swatch books. So it can be done. (Well... you have to somehow map 3D color space into two dimensions...)

Yet another problem with the Pantone book is that the recipes in the book don't work. Paula Gurney (recently retired from Ink Systems) explained to me that this is because the formulators of the book didn't impose a standard ink film thickness for all the recipes.

And then there's the base ink reflex blue. Printers don't like reflex blue. It takes longer to dry than the other inks, and it has this property called bronzing. It takes on a coppery tone viewed at a shallow angle. It would be good to not use that as one of the base inks.

Much to their credit, Pantone tackled all these problems and introduced a very good solution in 2007. It was called the GOE System. It was beautiful. You may have noticed the use of the past-tense in both the previous sentences? Yeah. It was a great idea, so naturally it didn't take off in the market. It was discontinued in 2013.

But I managed to find a GOE book at a garage sale and chased my spectrophotometer after it. So, I have a file on my computer with spectral measurements of a GOE book. I applied the same technique to this data. This added another set of plausible metameric matches to my database.

MEGA database

I show below a set of metameric sextuplets from the database. These six spectra have exactly the same CIELAB values under D50/2. Since the spectra are all different, one could expect that the CIELAB values would not match under a different illuminant.

Identical sextuplets, under D50/2

I don't know how many of you have spent an evening with a set of metameric twins, but I gotta tell you, a night on the town with metameric sextuplets is a bucket list event!

As an aside, some of you may have been wondering if Pantone 147 (in the diagram above) is the ugliest color in the world. It's close, but not quite. That honor goes to Pantone 448. There was actual research into this. The idea was to find a color that would best "unsell" a product. The product in mind? Cigarettes, in Australia.

And speaking of world records, how many metamers are in my database? An awesome 3,604. Are you listening Guinness? The old record might have been a few dozen, so that's a record that will stand for a while. At least until I announce Metameric Encyclopedia for the Graphic Arts II.

Stay tuned for Part 2, where I look at the magnitude of metamers caused by this whole spot color replacement thing.

This is an ongoing effort of mine. Over the next few blogs, I will describe some of the uses that I have put this database to. Any other ideas? Contact me!

An Easter egg in the film "Die Another Day"

Ok... so let me make this clear from the start. I am not a film buff, nor do I claim any more than superficial expertise in film. If you want the honest truth (as opposed to my usual dishonest truth), I will likely need to look up the title of this movie again before I finish blogging about it.

Another thing to clear up at the start -- this is not my usual blog post subject matter. Normally I go on ad nauseum about color and math and physics and other boring stuff. Not today. Prepare to not learn any science.

For those who are unfamiliar with the term "Easter egg" when applied to film, this is something innocuous put in a film just as a joke. If you would like an example, I would suggest having a look at the scene in the movie Airplane that starts at 0:00:00, and ends at 1:25:00 in the sequel Airplane 2.

The topic of this blog post is an Easter egg in the James Bond movie "Die Another Day".

To set the time stamp on the movie, the Easter egg occurs between when a) Bond uses the secret code word delectados to elicit the help of some guy, and when b) Halle Barry makes a breathtaking exit from the ocean.

An egregious image of Halle Barry being viewed through Bond's binoculars

If I were a true film aficionado, I would point out the similarity of the outfit worn by Barry to that worn by Ursula Andress in her coming out of the water scene from an earlier Bond flick, Dr. No. I might also take this time to comment on the cinematography in those two scenes, comparing it to Bo Derek running on the beach in the movie 10. Blah blah blah ... Daryl Hannah in Splash ... Phoebe Cates' pool scene... blah blah A Fish Called Wanda ... blah blah ... the primeval attraction ... subliminal reference back to one million years of evolution at the water side ... Descent of Woman...

But I can't even recall the scene from A Fish where Jamie Lee Curtis came out of a pool wearing a bikini. So I will skip all that.

Getting back to the Easter egg in Die Another Day... In the unforgettable scene just before Halle Barry emerges from the water, James borrows a pair of binoculars so he can pose as a bird watcher and get a look at the island. At this point, I had to pause the film to clean up the martini that I snorted out my nose and to explain to my wife what tickled my funny bone. I'll explain it to you as well, but first I rewind the film a bit.

I remembered that while he was in the delectado guy's office, Bond had idly picked up a book to look at, just before he picked up the pair of binoculars. I went back to see what the book was. As I suspected, it was a bird guide. I had a hunch about the author of this book. I was hoping to see the name of the author of the book, but I couldn't make it out. I think they did that on purpose to hide the Easter egg for all but the most diligent of Easter egg hunters.

But since I had already guessed who the author was, I knew what to google for. Here is a better shot of the book Birds of the West Indies:

You can probably just make out the author's name: James Bond! Yes. James Bond picked up a book by James Bond.

Now for the reveal. The real James Bond was an ornithologist. Ian Fleming (who wrote the books about James Bond the spy) was an avid bird-watcher who lived in Jamaica, so he knew of the bird book. Fleming was looking for a name for his character and felt that the name James Bond had just the right sound. Short and masculine. That's why I laughed when James Bond introduced himself to Halle Barry's character as an ornithologist. And since I knew where Fleming got the name from, I knew to look back to see if the book was an inside joke hidden in the previous scene.

The real James Bond

By Jerry Freilich - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=9724104

When Ian Fleming was contacted by the wife of the ornithologist about the theft of her husband's name, Fleming gave permission for the real James Bond to use the name Ian Fleming. "Perhaps one day he will discover some particularly horrible species of bird which he would like to christen in an insulting fashion."

Thanks to my buddy Mike for recommending the movie.