## Wednesday, July 3, 2013

### The color of a bunch of dots, part 3

In this series of blogs on tone value increase, we have been considering two parallel questions: How to predict the color of a halftone, and what to measure about a halftone in order to do process control. In part 3 of the series - the one you are reading right now - I want to consider the process control issue.

For those who can't take the time to read my entire long blog post today (maybe because they have to tend to their sick chameleon?) I will give a spoiler. The Murray-Davies equation, which we use today to compute TVI, has some issues when we try to extend it past CMYK inks printed on a web offset press with standard (AM) screening measured with a densitometer.

TVI depends on wavelength

The graph below might be a bit of a shocker. Then again, maybe not? I guess it depends. The graph is simple enough. I took the spectra of a solid cyan, a 50% cyan, and a solid. I used the Murray-Davies equation that we all know and love to compute the TVI. Rather than using the red channel of a densitometer, I did the computation separately at each wavelength.

Will the real TVI please stand up?

Maybe it's a shocker that the TVI at 630 nm is a healthy 15.7%, and at 500 nm, it's an anemic 2.6%? I think this very clearly shows that TVI is not a direct measure of the dot size. How could the dots be so much bigger if you look at them through a blue green filter instead of a red filter? It's almost like the tone ramp (0%, 10%, 20%, ... 100%) changes in hue... [1]

This is just an oddity, though, right?  It is like the kitten born with two faces, only this kitten is a halftone patch with two different TVI values. Or rather, a halftone with a whole bunch of different TVI values. Still, so long as you make sure that you have the correct setting on your densitometer, you won't have to worry about all the other faces. And process control will work, right?

But, sometimes you don't have density information, particularly in color management. It seems that a lot of data sets have been collected with only colorimetric data; no spectra and no density data. [2] How can you compute density without the spectral data? Can you compute TVI from the XYZ values?

According to ISO 10128, Appendix A, you can compute magenta and black TVI from the Y value and yellow TVI from the Z. To compute the cyan TVI, you need to do a bit more math, but it's not that bad.  There is a formula. Another brilliant researcher presented a paper at TAGA (2008), where he offered up a more complicated formula that works even more better. Brilliant as this researcher is, he stole this formula from another brilliant guy, Steve Viggiano.

So... this is basically just a confusion, right? There isn't really anything here to upset anyone's sense of inner tranquility, right? Keep reading.

TVI does not uniquely define the color of a halftone

Speaking of standards, the ISO standards for print (the ISO 12647 series) defines colorimetric aim points for the four solids and overprints of those solids. But for halftones?  It specifies TVI aim points. This would lead one to believe that, once you get the solid correct, all you need to do is make sure that you TVI is correct. Then you will have the correct color, right? [3]

And if the TVI is not correct, the standard gives you the impression that it is easily corrected in prepress by applying a plate curve. If your 50% has a TVI of 66%, and it should be a 68%, then all you need do is add 2% in a plate curve, right?

Not so fast, boopie.

The graph below is an a*b* plot (a plot looking at color space from above) of cyan, magenta, and yellow tone ramps, along with various other combinations. The blue lines represent the color of tone scales of conventional screening. The red lines represent stochastic screening. It is apparent that certain single color tone scales (cyan and magenta) have a hue shift of several deltaE between the two types of screening.

Comparison of conventional and stochastic screening [4]

When a plate curve is applied, the color is moved along the appropriate curve in color space. (I call this curve the trajectory.) A plate curve cannot make your halftone jump from one trajectory to another. Maybe everyone knows this already, but it bears saying. You cannot use a plate curve to match color between conventional and stochastic screening.

If other forms of printing were to be included, the hue shift in the midtones would be more dramatic. Gravure and ink let printing are still farther from the graph above of conventional printing. To put this another way, hitting the correct TVI does not guarantee the correct color if there is a difference in printing modality. The solids might be spot on and the TVI might be absolutely correct, but the actual color of the 50% tone will not be correct.

TVI of spot colors

What happens if I want to compute the TVI of a spot color (Pantone or PMS)? I just have to pick the right density filter, right? Short answer: For a minority of inks, let's say a quarter of them, this actually works well. But, TVI really sucks at quantifying some spot colors. For others, it's better - somewhere around "lousy".  These are harsh words, but I have data to back them up.

I looked at a set of tone ramps of 394 spot colors. I computed the TVI for each of the tones using the Murray-Davies formula at the wavelength where the spectrum of the solid had the lowest reflectance. Then I sat back and watched the fireworks. Of the 394 inks, almost all of them (353) had a TVI (at 50%) of greater than what we would call "typical", 18%. A total of 70 of the inks had astronomical TVI values of over 35%. Most press operators I know would call that an extreme case of plugging. But it wasn't  plugging at all. The TVI was horribly large, but the color of the halftone looked just fine. Plenty of contrast in the shadows.

The graph below is the spectra of the tone scale of one of the inks that had a TVI of about 35%. Note that, around 600 nm where the reflectance is the very least, the solid, 90%, 80%, and 70% have nearly identical reflectance values. Anywhere in the red part of the spectrum, this looks like an ink with severe plugging.
The spectra of a tone scale of one blue spot color

But have a look at what happens at 450 nm. In the blue part of the spectrum, there is a very clear separation between the shadow tones. In this region, all appears to be right with the world. Despite what TVI tells us, there is contrast in the shadows. [5]
Conclusions

The use of TVI as a process control device has been proven to work for CMYK inks on a web offset press using conventional screening. But...

a) TVI is dependent on the wavelength chosen. If you are dealing with CMYK inks and conventional screening and web offset printing, then so long as you are careful about selecting the correct filter on the densitometer, this isn't a problem. The only time a problem pops up is when TVI is computed from colorimetric data.

b) When we switch from conventional web offset printing to something else, particularly when there is a difference in the crispness of the dot, having the same color for the solids and the same value for the TVI does not guarantee that the color of the tones will be the same.

c) When you look at inks other than CMYK, the TVI value sometimes is a useful parameter for process control, but it often is completely misleading.

In the next installment of this series on TVI, I will explain a slightly different equation than the Murray-Davies equation and the Yule-Neilsen equation. In the final installment, I will look at some practical solutions to the issue of process control for halftones of spot colors.

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[1] This isn't just a cyan thing. The same issue can be seen with magenta ink, and to a lesser degree, with black and yellow.

[2] Congress is acting swiftly to ban the disposal of unwanted spectral data, or at least trying to act swiftly. Right now, it is stuck in the quagmire of partisanship with Democrats tying the bill to efforts to save the rain-forests in southeast Kansas, and Republicans demanding that magenta density be removed since the word "magenta" does not appear in the Bible.

[3] Astute readers will note the proliferation of the word "right" added to the end of  a sentence for emphasis.   Some readers might assume that this is a technique used to draw attention to things we might assume but which are not correct, right?

[4] The image is from a presentation by Dr. Bestmann to the ISO technical committee 130 in September of 2011.

[5] This is a drastically shortened version of an article that appeared in IDEAlliance Bulletin magazine, spring of 2012 issue: Measuring TVI of a spot color. If you are interested in obtaining your own subscription to this magazine, have a look at the IDEAlliance website.