Screen Printing Articles Industry Alert Hot Shots Photo Gallery Message Boards Visit Our Advertisers: 3M Commercial Graphics CADlink Technology Clarke Systems Estimate Software International Sign Assoc. JetUSA Matrix Payment Systems SGIA Specialty Graphics Imaging Assoc Supply 55, Inc.

Up until now, we've been talking about round dots centered within their individual squares or cells of a standard halftone grid. But halftone dots come in a variety of different shapes and there's even a popular form of halftoning method that discards the traditional halftone grid entirely.

Dot Shapes
First let's consider dot shape. Over the years halftones have sported square dots, elliptical dots and even diamond-shaped dots. Most of these alternative shapes were developed to solve problems different print technologies encountered when attempting to print halftones.

Screenprinters, for example, generally prefer to use elliptically-shaped dots in their halftone positives because elliptical dots tend to minimize the effects of dot gain. If you read the previous articles in this series, you are aware that dot gain is a particular problem for screenprint technology. Not only do the inks used in the process have a general tendency to spread out more on the substrate, but screenprinting ink deposits are the heaviest of any print technology.

Dot gain occurs when printed halftone dots are larger than the same dots in the original artwork. In a traditional halftone, the size of the dots creates the shades or tones that make up the image, so larger dots mean darker shades. Some shades disappear entirely. Darker grays, for example, tend to turn into solid blacks.

In the midtone regions, dot gain causes tonal jump, eliminating intermediate shades and creating harsh transitions from tone to tone. Halftones made up of round dots are particularly susceptible to tonal jump, because when round dots expand, they expand equally on all sides, causing large dots to link up with the dots in neighboring cells in a sudden and dramatic fashion. This results in a very noticeable tonal jump

Elliptical Dots
When ellipse-shaped dots link up, they do so in two different stages. The long axes link up first, and the short sides join up a bit later. This doesn't entirely eliminate tonal jump but at least it's broken down into two less noticeable stages. More importantly, it takes place several shades away from the highly noticeable 50% mark.

As long as dots are arranged in the traditional grid pattern at least some tonal jump will occur. But what if we just get rid of the halftone grid? And, while we're at it, why don't we abandon the idea of creating tones by the size of dots as well?

Stochastic Halftones
Welcome to the world of stochastic screening. The word stochastic means random, or containing a random element. Stochastic screening refers to an image created by dots that have been apparently placed at random rather than arranged in an orderly grid. Another obvious difference is that all of the dots are the same size. Stochastic halftones create the different tones simply by printing different numbers of dots in a given area.

The stochastic screening process holds a lot of appeal for screenprinters. If dots are all the same size, dot gain has less of an effect on the image. Dot loss, the tendency of the smallest dots in traditional halftones to fall right out of the mesh, also ceases to be a problem, because if the mesh can hold one dot, it can hold them all.

Because the dots in stochastic halftones are not arranged in a grid pattern, this new approach promised to solve at least some moiré problems. Moiré is the appearance of unwanted interference patterns that sometimes form when one pattern is superimposed on another. Screenprinting suffers considerably from moiré because the mesh in the screens forms a pattern all by itself. By getting rid of the grid, stochastic halftones eliminated at least one pattern from the mix.

AM or FM
Despite the promise of stochastic screening, traditional halftones continue to be more widely used. To distinguish the two methods from one another, graphic artists borrowed some terminology from radio. They called the traditional (the clustered-dot halftone system) AM or Amplitude Modulation halftoning, because it varies the size or amplitude of the dots to create the various shades in an image. Stochastic halftones were called Frequency Modulated or FM halftones, because images are created by varying the number or frequency of the dots. It was also called dispersed-dot screening because the dots were scattered rather than being arranged in orderly rows.

Despite the randomness that the name stochastic implies, the dots in stochastic halftones are carefully placed according to mathematical rules called algorithms. Fortunately, you don't need to be a mathematical genius to use any contemporary halftoning system, because computer software does all the calculating for you -- and at blinding speed.

Making Halftone Positives
Computers and graphic arts software like Adobe PhotoShop have put the ability to generate halftones on every screenprinter’s desktop. There is, of course, a considerable learning curve involved in becoming familiar with the software, but once you've become reasonably competent, these graphic arts programs give you hundreds of ways of manipulating photographic images, including the ability to create halftones.

You probably won't encounter any real barriers until it comes time to output your work. The computer printer sitting on the corner of your desk probably isn't up to the job of printing halftone positives. Few computer printers within the price range of most screenprinters can create output dense enough to do the job.

When it comes to halftone positives, you really have three basic choices. Do the work entirely in-house using your own computer setup, including your printer. Contract out the entire job. Use an outside supplier to do the artwork and generate the positive. Do your own computer work, but send the file out to a service bureau for outputting on their imagesetter. Before we explore these options further, let's take a moment to remind ourselves exactly what we need in a film positive.

A positive used for exposing a screen must be right-reading, emulsion-up. This means that when the positive is placed on a table with the image oriented exactly as the printed image will appear on the substrate, the emulsion will be on the film closest to you. When screenprinting positives are exposed the emulsion on the positive should be in direct contact with the emulsion on the print side of the screen.

Emulsion-to-emulsion contact prevents light from leaking in and creating saw-toothed edges in the stencil. Of course sharp, crisp edges are important in any screenprinting stencil, but in a halftone stencil, with its thousands of tiny but essential details, contact between emulsion and emulsion is ten times more important.

DMAX vs DMIN
The second essential requirement for halftone positives is high relative density. Black areas should be really black, a maximum density (Dmax) of reading close to 4.0 or above, according to a transmission densitometer reading. Although expensive, a transmission densitometer can be a handy tool, and it can also be used to check the halftone's tonal range.

The clear areas of your positive, where the film itself shows through, need to be as transparent as possible. The best films readily admit UV light without offering the slightest hindrance. This means a minimum density (Dmin) not above 0.1, which automatically disqualifies vellum and some frosted polyester films.

If you can't afford a transmission densitometer, you have may have to resign yourself to checking your positives over with a pocket microscope. This is always a good idea. However, opacity to UV light is difficult to estimate when examining a positive under ordinary light.

Sources of halftone artwork
Since the decline of the graphic arts camera, imagesetters provide the best positives available. It stands to reason that a 2400 DPI imagesetter can produce a better positive than a 600 DPI laser printer. However, few average-sized shops can afford imagesetters. Fortunately, imagesetter-quality positives are as close as your local service bureau and available from them for a modest fee. Contracting out the printing of your computer files may well be the best solution.

When you make your positives in-house, you alone are responsible for seeing that they're ready when you want to shoot your screens. However, once you cede control to an outside source, getting the positives right and getting them on time really depends on how well you and your supplier understand one another.

Keeping the communication channels open is certainly important. But it may be even more important to see that they're open in the first place. When talking to your supplier, make sure you've got your own terminology straight. Know the difference between DPI (Dots Per Inch), a measurement of resolution which can be applied to almost any digital image and LPI (Lines Per Inch), a measurement of frequency which applies only to AM halftones. Never use the terms interchangeably.

SPI and PPI are other terms used in references to digital images. The are both measurements of resolution. SPI or samples per inch applies to scanner output and PPI or pixels per inch is often used to describe resolution on a computer screen.

It's also important to put your specs down in writing, even if you have to fax or e-mail your work order. Make sure the delivery date is very clear. Remember, it's going to take time to get the positives back to you, and still more time if reshoots are necessary. Of course, it's very important to get a reliable supplier in the first place. One of the best ways of eliminating confusion is to deal with a supplier who is used to working with other screenprinters. It helps if you don't have to explain why your positives need to be right-reading, emulsion-up.

When the positives arrive at your shop, check them over immediately with your pocket microscope. You want to spot any problems right away. And, it may seem silly, but check to be sure that the emulsion is on the correct side of the film.

Sending out computer files
When you send out your computer files, you not only have to make sure that you and the service bureau understand one another but that your computers do too. When you send files on some kind of physical storage medium whether its a CD ROM, a zip drive, a SyQuest cartridge or even a portable hard drive, your supplier's computer has to have a drive that accepts that media or a compatible connector.

The service bureau's computer has to accept the physical storage media on which the files are conveyed, it also has to be able to read the files themselves. Most shared files are sent in either TIF or EPS format, but every supplier has his or her preference.

When it comes to communication, nothing helps more than a common language. Fortunately, most computers in the graphic arts world speak PostScript. Adobe PostScript is a page description language, a way of summing up all the complex information that goes into the make-up of a printed page. Basically, the graphic arts program running on your computer writes a program in PostScript and sends that to another program called a RIP (Raster Image Processor) that converts the page into dots that the printer can actually output.

Adobe PostScript has dominated the graphic arts field since the mid-1980s. The current version is PostScript 3. PostScript was designed to be device independent language, meaning that, at least theoretically, you could output the same unaltered file on your desktop PostScript laser printer or the imagesetter at your service bureau.

Technology sometimes makes it easy to forget that low-tech steps like labeling the disks you send to the service bureau, or making sure you send along a work order that specifies such essentials as the size you want your finished positive to be. Needless to say, such basics are also important.

Creating halftone positives is certainly much more demanding than creating positives for ordinary line art. The same high standards apply throughout the stages that follow. Printing halftones is an advanced technique, simply because it allows so little room for error. Next time, we're going to take you through the stages of the process from coating and exposing halftone screens to pulling prints, and we'll continue to point out the standards that must be met in order to achieve success.