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Printing Industry Exchange (printindustry.com) is pleased to have Steven Waxman writing and managing the Printing Industry Blog. As a printing consultant, Steven teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.

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Custom Printing: A Few Random Thoughts on Halftones

With the Coronavirus threat upon us, I have had extra time recently, so I have taken this time to brush up on my knowledge of commercial printing. I thought I’d start by reviewing my textbooks on color prepress and custom printing.

In this light, I chose the subject of halftones. I thought my findings might be of interest to you.

Why Use Halftones?

Offset printing is a binary process. Either you print ink in a particular place on your press sheet, or you print nothing. You can’t print a continuous tone image (either color or black and white) where black tones transition into gray tones and finally into white.

So how do you get images to look right on a commercial printing press?

Long before computer prepress and scanning devices existed, it was discovered that if you photographed a continuous tone photo through a glass sheet covered with a matrix of “cells” (like small, separate windows), this process would yield an image composed of dots. Depending on the lightness or darkness of the area represented, you could have a light area comprising many small dots or a dark area comprising many large dots.

Due to the regular position of the little “windows” on the halftone screen (the same number of lines of halftone dots, all lined up across and up and down on the screen), there was a constant and finite number of halftone dots. They were just larger or smaller in size, depending on whether they were in dark or light areas.

The number of halftone dots per linear inch was called the screen ruling. For reproducing images on thin, porous paper like newsprint, commercial printing suppliers would use coarse screens (85 lines per inch, for example). For coated paper, printers would use 133lpi or 150lpi screens (or even much higher rulings).

As the printing and prepress processes were computerized in the ’80s and ’90s, all of the above information pretty much continued to be true (albeit on a digital rather than analog level).

On the plus side, the human eye is very forgiving, and from a normal reading distance, the dots in the finer halftone screens (133lpi or 150lpi) cannot be seen easily, whereas the dots of the coarser halftone screens (85lpi) printed in newspapers are usually visible. Unfortunately, trying to print finer line screens on newsprint yields muddy images, since the absorbent nature of newsprint makes the halftone dots spread. Ink goes into the paper fibers and travels. Images look horrible. So you really do need the coarse (lower-number) halftone screens for cheaper paper.

Fortunately, coated printing paper allows the ink to dry on the surface of the press sheet (rather than seeping into the paper fibers), so the halftone dots can be smaller without spreading (they’re called “hard halftone dots”). But these harder-surfaced, coated press sheets are more expensive than lower-grade papers like newsprint, so high quality photos, in which you can barely see (or not see at all) the halftone dots, usually wind up in higher-end publications.

To keep you apprised of the printer’s lingo, this growth of printing dots is called “dot gain.” One way commercial printing vendors compensate for this is to print less ink by intentionally lightening the halftone screens (by slightly reducing the halftone dot sizes) so the printed output will be correct after the inevitable dot gain.

What About Color Images?

Back in the day, printers separated 4-color images into four distinct negatives, one for each of the commercial printing process colors: cyan, magenta, yellow, and black. Then the printers produced printing plates from these negatives. All of this was an analog, photographic process. Now, all of this is achieved digitally with scanners and computers, but the concept is the same. An image is scanned and then separated into the four process colors. These are then printed not to film but directly to custom printing plates, one for each color.

But here’s the trick. Each of these four separated images really is just one halftone, just like the halftone described above that is used for black and white images. The difference is that the four halftones for color printing are rotated at slightly different angles to one another. For example, one option is 15 degrees (cyan), 45 degrees (black), 75 degrees (magenta), and 90 degrees (yellow). Yellow dots are less visible, so they can be set closer to the angles of the other screens (15 degrees in this case). The best angle for viewing is 45 degrees (so it’s used here for the black plate).

Because of this distribution of the four halftone angles (just the halftone angles, not the printing plates themselves), once the image is printed, you will see a circular “rosette” pattern in the images if you use a 12-power printer’s loupe to magnify areas of the photos. These rosettes are the visual indication that the halftone screens have been rotated. And they appear larger or smaller depending on the size of the halftone dots in a particular area.

Another way to say this is that the rosettes are more visible in areas printed with more ink (dark areas) and less visible in areas that take less ink (lighter areas).

Dot Shape

These halftone dots can be a number of shapes, including round, square, and elliptical. Round dots are the norm (from very small to very large depending on the density of ink in a particular area). Elliptical dots, on the other hand, are very good for skin tones because as they grow in size (from small at the quarter-tones to larger at the three-quarter tones), the longer dimensions of the elliptical dots eventually touch each other, and this makes for smoother gradations in flesh tones.

(Square dots are good for detail work with lots of straight lines and angles.)

I’ll say this again because it’s very complex. But the overall concept is what’s important. The way elliptical dots touch at the long ends as they get larger (to allow for increased ink distribution) makes for smoother transitions in the tones of a human face. If you’re printing high-end glamour photos for Chanel, this is useful information indeed.

Amplitude-Modulated vs. Frequency-Modulated Screening

All of what I have been explaining is called “amplitude-modulated screening.” This just means the dots are all on a fixed grid (i.e., the same number of rows and columns of halftone dots in a 150lpi screen, for instance). They are just larger or smaller depending on the ink density.

In contrast, you can now use “frequency-modulated screening” or even a hybrid that blends both AM and FM screening. Frequency-modulated halftone screens are made up of equal-sized dots (minuscule ones). They’re all the same size, but where there’s a lot of color, there are a lot of small dots.

If you look closely at the output of an inkjet printer, you’ll see exactly this kind of pattern. In contrast, if you look at the output of a laser printer, you’ll see some version of amplitude-modulated halftone screening. Laser printers may not use the exact same screening angles as used for offset commercial printing. They may not even produce the same rosette patterns. But they are based on similar mathematical algorithms as those used for offset printing.

TAC, UCR, and GCR

First, here’s the wording referenced by the acronyms above:

TAC=Total Area Coverage
UCR=Undercolor Removal
GCR=Grey Component Replacement

TAC means total area coverage. If you print 100 percent coverage of all four colors (cyan, magenta, yellow, and black) in one place on a press sheet (a high-density neutral area of a photograph), you could conceivably be printing 400 percent (100 percent x 4 colors) total ink coverage. Unfortunately, too much ink makes the offset lithographic process break down, and the paper also can’t handle that much ink (it gets too wet and comes apart). The ideal overall target might be 270 percent to 300 percent for all four colors, depending on your printer, the press, the paper, etc.

UCR means Undercolor Removal. This and GCR or Grey Component Replacement are ways to replace the cyan, magenta, and yellow in a photographic image with black. When you do this successfully, you achieve two things:

  1. You can use less ink. This reduces materials costs and ensures that there’s never too much ink anywhere on a press sheet (see above for the results of printing too much ink).
  2. When you create neutrals, or even gray areas, with cyan, magenta, and yellow, any misregister of the printing plates will cause color shifts and color casts. Replacing some of these hues with a percentage of black will minimize these color shifts significantly.

What You Can Learn From All of This

If your head hurts from all of this information, I apologize. That said, it doesn’t hurt to begin to understand the nuances of prepress and presswork. It will help you understand why you prepare design files for press the way you do. It will also help you make good choices regarding printing paper.

There are a lot of good textbooks out there that describe these technologies and their challenges. However, it’s even easier to find little booklets produced by custom printing suppliers and paper companies that give you this information in a condensed form (only what you need). The more you know, the better able you will be to discuss your commercial printing needs with both your sales rep and the pressmen actually printing your job.

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