## Tuesday, July 18, 2017

### How big is a deltaE?

Every once in a while, someone in the audience calls on me to ask a question that I know the answer to. That just happened to me, and I am soooo happy!

This is the second blog post in a series about the actual meaning of color differences. I blogged previously about the origin of the DE, in particular, the DE00. I came to the preposterous notion that "the size of a DE color difference is based on the Munsell Color System, which is all about uniform spacing of colors. 1.0 DE00 is one of 76 perceptually equal steps between pure black and pure white."

Today, I want to correct that silly idea. Contrary to what certain bloggers have said previously, the color difference DE00  is a unit of measure which is used for industrial color-difference evaluation. This color difference equation is officially defined in the document Improvement to Industrial Colour Difference Evaluation (CIE 142-2001). The summary of this document starts out with  the sentence "Recommended practice for industrial colour-difference evaluation is presented." (The italics are mine, added strictly to heighten the excitement.)

The question that sparked this series of blog posts

Here is the question that I got from my good buddy, Larry Goldberg. As you will see from his question, he is eloquent, piquant, and just a tad irreverent; three qualities I appreciate in a friend. He works for a little company called Beta Industries, where you can get microscopes and print measurement devices and a variety of other devices for the print industry.

Here is his question:

I'm looking for The Idiot's Guide to delta E, converting scientifically rigorous results to Foolproof Rule o'Thumbs.  Or the more linguistically acceptable Rules of Thumb. Or until the new, improved Fool is released.

Something in simple tabular form such as;

delta E 2000  |  Rule O'Thumb
======================
1                         Just Noticeable Difference, depending on who's asking and how much they drank for lunch
2                         Winner Winner Chicken Dinner!
3                         Good commercial color match, Kwitcher belly-achin', looks good to me!
4                         It'll be perfect when you add a little more snap to the magenta.
5                         Whaddya expect on this crappy paper?
6                       TWICE as good as commercial color match, no?  No. The buyer needs another dinner and round of drinks.
7 - 10                Roses are red, violets are blue, the grass is green the sky is blue.  Run it, they'll be wrapping the fish in it tomorrow.
>10                     This is a lot better than when they printed black and white...

If you have a dissertation, or a link, or a suggestion, it would be greatly appreciated.

Boy, have I got a technical paper for you!

The Committee for Graphic Arts Technologies Standards (CGATS) has been feverishly working on a standard that ties numerical color difference to lexical color difference. The title of the standard is "Graphic technology — Printing Tolerance and Conformity Assessment". Those of us in the biz affectionately know it as TR 016. The technical report is free, by the way. Just click on the link in the sentence before the sentence before this one.

This technical report is all about making that critical decision about whether a printed product has conformed to tolerances for color. Yes, this is the stuff of which contracts are made. Come to think of it, citing this document in a contract could simplify an agreement between print buyer and printer. I wonder if the folks on the CGATS committee ever thought about that? I'll hafta mention it during the next meeting I attend.

TR 016 defines four levels of acceptance, with explanations for when these levels are appropriate. Each of the levels has an associated tolerance for color difference (in DE00), and states that 95% of the production samples must have a color difference less than the number. In other words, most of the measurements must be within this tolerance.

Here are the levels:

Level 1 - "the most color critical applications, e.g., proofing" - 2.0 DE00
Level 2 - "color critical applications, e.g., commercial printing" - 3.0 DE00
Level 3 - "utility process color printing" - 4.5 DE00
Level 4 - "pleasing color" - 6.0 DE00

So, there you have it. The size of a DE color difference is all about tolerances in the industry. For print work, 2.0 DE00 is considered pretty darn good, and 6.0 DE00 is merely "pleasing". Within printing, it is expected that the ink will be kept to a higher tolerance than the print using that ink, and the printing of a proof must also be tighter than the printing of the final product. Kinda makes sense. The variation in the color of the stuff coming out of the print shop can't be any better than the variation of what goes in.

Other industries may have tighter or looser tolerances for color. Please add to the comments below if you know about color tolerances in other industries!

Pondering

Consider this: The tolerances for color are based on a scheme for equal steps of color, which is where the DE came from. Somehow it seems a bit odd to put those two together. But one big benefit of this scheme is that it is based on our perception of color. That's a good thing, since our perception is certainly not linear with reflectance. Another big benefit is that our perception of the size of a DE00 is largely independent of the color that you are looking at. That is, we don't need different tolerances for different colors or directions of color change.

On the other hand, I would argue that our acceptance of a difference in color between two samples is not necessarily the same as our perception of the gradations of color, especially when those two samples are not side-by-side. We are much more discriminating when we see two bags of potato chips sitting next to each other on the shelf.

A color that is slightly off will sit on the shelf until expiration date

And when the colors are not side-by-side? I would argue (without much data to support this) that our brain is much more forgiving of color changes that are strictly changes in lightness or chroma than they our of changes in hue. I would also argue (again without a grain of evidence) that we tolerate differences much better if the whole image has that same sort of shift. And once again without anything to support this, I claim that the brain is much more forgiving of colors in busy images with lots of fine detail.

Comparison of a health food drink ad in a glossy magazine and on newsprint
(Images courtesy of JMG Design Services)

There is a committee in the CIE (TC8-16) that is currently working on trying to quantify Consistent Colour Appearance -- what is it that makes our brain accept the two images above as being "kinda the same", versus if those two images were shifted in hue? There are some bright and knowledgeable minds working on this committee. And then there is one dim-witted slacker who just sits around and writes self-important blog posts all day.

But on the third hand, industrial tolerances with DE00 are amenable to measurement with existing technology. What good is a unit of measure if you can't find a ruler?

That's all for part 2 of this trilogy of blog posts. Stay tuned for part 3, where I revisit the phrase just noticeable difference, and admit that the first two parts of this series were nothing but lies!

1. Hello John,
When I try to open the CGATS link, I get an error message on a black screen:
"Blocked Plug-in"
Vots up?

1. The link worked on my computer, perhaps because the NPES computer knows I am a special person. Try this link to buy a free copy: http://npes.personifycloud.com/personifyebusiness/Store/ProductDetails.aspx?productId=157515

2. Hi John. This is in line with the conventional wisdom of "application specific" color difference tolerance. A while back I did a job regarding retail packaging in which the product and the competition were never viewed side-by-side. (They are both retail products, but shared exactly zero retailers in common.) The time course of color matching for this specific color - kind of a dark teal - was dE00 of ~ 4. This was was effectively a tolerance around the product color that defined, after 15 minutes or more, the region that was indistinguishable from the product color.

There is ample literature on the perception of color over time. It took a while to come up with a plausible and defensible strategy (these were attorneys, of course) but in the end I think I achieved both.

3. I've seen several references in my quest for color smarts that state we see hue differences first, chroma differences second and lightness differences last. So, I think you're good to go on that one.

4. Hi John,
My studies show that your "Level 4 "pleasing color" difference, DE00 of 6" is called a different color between standard and sample by most people and not an acceptable color difference throughout most of the gamut of color space. Further, isn't close tolerance color difference considered less than 5.0 units?

5. This comment has been removed by the author.

6. One topic you haven't covered with DeltaE (with reference to the picture of chips on a grocery store shelf) is that it is defined in terms of a specific observer and illuminant. If the illuminant (or possibly observer) is not the same as used for measuring DeltaE and making decisions about acceptability then the DeltaE doesn't correspond to perceived color difference.