By Frank Romano

Vignettes
Blends, vignettes and gradients (all essentially the same thing — one color blending into another color) are a frequent headache in toner-based printing.

The ease of creating these effects in illustration and layout applications has led to a huge increase in their use, and examples of bad blends can be seen in many publications.

They look great on the screen — but so does everything else. Digital presses are especially susceptible to printing banded artwork because of their resolution.

Use tint values with a fairly wide range (e.g. from 15% to 85%) because narrow tint ranges force the use of larger bands of tint value; or make the gradient smaller.

If you have a vignette as a raster image, you can sometimes reduce banding by opening it in Photoshop and applying the "Add Noise" filter moderately. This is the most common solution.

1. Learn how to mathematically determine whether a given gradient will reproduce without banding. Download this Excel spreadsheet at the end of this article to do it all for you automatically.

2. Determine the number of tints (or levels of gray) an output device can create (N) by dividing the device's output resolution by the device's screen ruling, and squaring the result.

3. Use this value to determine the actual number of tints available in a given gradient range (T) by multiplying N by the decimal difference of the tint values at each end of the gradient (e.g. in a gradient from 20% to 75% of a single color, the decimal difference is 0.55).

If the size of each band of tint is 0.05"/0.12cm or less, the banding will not be noticeable. To find the tint band size, divide the distance between the start and end points of the gradient by T.

At 600 dpi and 85-line screen, you get 49 levels of gray (N=600/85 squared). If you went from 20% to 75% (N (49) x (.75 - .20)), the decimal difference is 26.95 (T). If the length of the band is 5 inches, you divide it by T and you get 0.18—banding will be noticeable.

Recommendation: Sorry for the formulas, but they do work. If you shoot for .05" or less, you will be in great shape. Digital printers use a second-generation stochastic (random) screening approach, which could lessen the need for these calculations. But always get a sample to be safe.

Toner Coverage
In general, total toner coverage of any given area on the sheet should not exceed 320% (but some might require less). What this means is that the combination of overprinted process colors should not exceed 320% out of a possible 400%.

Technically it's possible to print 100% of each of the four process colors, but the result will be a thick, raised, shiny glob of yucky black.

Recommendations:
1. With this limit in mind, go to your Photoshop separation setup (File>Color>CMYK Setup) and create a new instance of CMYK Setup settings that uses a 320% total "ink" limit.

While you're in there, you can reduce the amount of dot gain in the default settings to 10% or so. You can probably even go lower than that, but you should perform some tests with separated images to make sure that the particular printer you'll be using doesn't exhibit any optical gain.

2. If you use the default SWOP settings in Photoshop's CMYK Setup, your images will be less intense than they could be, but unfortunately most designers and even some printers never use anything other than the default settings. Take the time to try different values, contact the printer — or even the manufacturer — to determine the optimal values.

Dot Gain
Dot gain (sometimes called Tonal Value Increase) is a real issue on lithographic presses and results from a number of factors — chiefly ink spreading on the paper, dot bloat in conventional plate imaging, and the simple mechanical smashing of wet ink into paper with several tons of pressure.

Toner-based printers do not have these problems, of course, but dry toner can scatter, and liquid toner can spread slightly on the drum, before it is transferred to the paper, so you may see minor dot gain on digital jobs.

Recommendation: Ask your printer if you should make any adjustments for dot gain and total coverage if you perform your own conversions from RGB to CMYK in Photoshop. Keep the total coverage value in mind also when creating process colors in page layout and illustration programs.

Paper and Paper Grain
Paper grain direction is an inherent result of the papermaking process. As the paper pulp is conveyed along the wire belt in a papermaking machine, most of the paper fibers naturally align themselves in the direction of the belt, resulting in grain.

Folding: It's easy to see why paper grain direction is important by taking an ordinary index or business card and folding it in half one way, then the other.

The fold against the grain will be lumpy and ragged; the one with the grain will be smooth. It's always the printer's goal to fold jobs with the grain, but sometimes it's not always possible because cutting a job to fold with the grain could result in excessive waste or the job simply can't be run any other way.

Recommendation: If a job must be folded against the grain, make sure that the paper is scored first, to prevent a lumpy edge.

Sheets printed on a dry-toner machine could also crack at the fold; scoring helps to prevent this, though the best way to prevent cracking is to design pieces so that there is low print coverage in the fold.

Paper Swell
Another potential problem with paper grain has to do with paper's amazing affinity for moisture. As ambient humidity increases, paper fibers will swell as much as four to five times in width (against the grain) as in length (with the grain).

Books bound against the grain in a dry environment and then subjected to normal levels of humidity will have pages that resist turning, that exhibit waviness on the free edges and buckling or distortion on the bound edge—the book looks badly bound because it is badly bound.

This is a significant issue with dry-toner presses because the intense heat from the toner fusing process drives most of the moisture out of the paper, a problem shared with the heatset web offset litho process.

Paper is hydrophilic ("water-loving") and will immediately absorb moisture if it has been heated and dried out.

Heatset web presses use chill rollers to reduce the paper temperature and encourage rehydration, and some use additional rehydration units to replenish the moisture lost in the drying process.

It's also possible to rehydrate paper naturally by letting it sit awhile in an area of sufficient humidity; but once wavy, if dried-out paper has been dried out for more than 72 hours, the damage becomes permanent.

Recommendation: Choose a printer whose press is located in a temperature and humidity-controlled environment.

If you understand how to deal with these three areas, you will design great work for digital printing.

Frank Romano is the author of 39 books and numerous articles, co-author of the International Paper Pocket Pal and chairman of RIT School of Printing Management and Sciences.

Reprinted with permission from the Digital Printing Council, PIA/GATF, U.S.A. For information about the Digital Printing Council, please go to www.gain.net.

11/7/07

Excel file (XLS 9 KB)