Wednesday, May 13, 2020

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.



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!

6 comments:

  1. andreas(at)coraye.comMay 14, 2020 at 1:18 AM

    Hello John,
    This post is promising a very interesting series of post. If you want I can give you a licence of our software which can help you to go further with more accuracy as by measuring a Pantone book.
    Please contact me for more information.
    Best,
    Andreas

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  2. John awesome blog! An evening with sextuplets wow, that would last me months of different viewing consolations! Than you!

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  3. john, always enjoy learning from you. Is the database public? cheers old friend! Terry

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    1. There are some perhaps minor legal issues, since this database has a lot to do with Pantone. I don't want to step on their intellectual property.

      I do have a second database that is large and based on gridpoints in L*a*b*... If you would like to make use of it, please contact me directly john (at) johnthemathguy (dot) com.

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  4. Thanks John. Great insides again in this subject. As an addition to your excellent blog it is perhaps good to remark that if lightsource metamerism exists also observer metamerism is bound to exist. Strong metameric color pairs give funny discussions then sometimes. I come from the Car Refinishing industry where metamerism is so important. Even with 50 standard mixing colors it is hard to achieve metameric free colors. This because not only light source metamerism is there but also geometric / angular metamerism is important. By the way optimizing metameric phenomena in color matching is also may be worth a blog or article. Thanks again.

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    Replies
    1. Thanks Roel,

      I have not, yet, investigated observer metamerism, but that is definitely on my list of things to look at!

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