Digital inkjet proofing systems seem to have everything a newspaper could want: speed for fast imposition proofing, color accuracy for contract color, and low cost for owning and operating.

The only problem is that most use screening technologies which are not really screens at all. At least not as we know them in the offset world, i.e. lines per inch. Instead they use techniques known as dithering, error diffusion or frequency modulation. The benefit of these processes is that they produce a smooth continuous-tone quality and work well with color management to produce accurate color. The problem is that they mask the many imperfections of the four-color process.

Detecting moiré

One of the most notorious of the “process” imperfections is moiré. This happens when the four-color screen angles misalign. The result is that unpleasant wavy pattern we all know so well. This is called a screen or color moiré. Then we have what is known as subject moiré. This happens when a busy pattern in an image, such as a checkered fabric, interferes with or complicates the screen angles.

For advertising proofs, you usually want the best color match. But for overall quality control, you should be proofing with the same file that will image the printing plate.

Luckily, inkjet technology has come a long way. At a resolution of 720 dots per inch and using screening software, an inkjet proofer is capable of reproducing a halftone screen at 85 or 100 lpi.

The twist in the plot

Inkjet inks have a much wider color gamut than offset inks. So the system’s color management software re-maps the inkjet gamut to match the offset gamut, blending the colors accordingly. As a result, it adds dots that are not normally present in the CMYK rosette. This is especially true with six-color proofers. While six colors provide an even wider spectrum for matching special colors and different presses, they add more dots to achieve the right blend.

Simulating the dot

Screening technologies developed specifically for inkjet systems can mimic the rosette pattern sufficiently to detect both color and subject moiré. Under a loupe however you will not see an exact rosette as it will print. But it is a compromise worth making. Not only will you be able to detect moiré but you will have a proof that provides the look and feel of the final printed newspaper.

Dot-for-dot

Although dot simulation will cover most requirements for halftone proofs, there may be times when exact reproduction of individual dots is necessary. Dot-for-dot halftone proofs reproduce the screening structure exactly as they will later appear in the print version. The rosette patterns are identical, and so all types of moiré effect can be accurately predicted. The individual dots on the proof have the same shape, size and color, so you can see when dots are too small to appear on the printing press.

Dot-for-dot proofs, however, place very high demands on the proofing system. To start with, the system must be capable of resolutions of at least 1,440 dpi. It must also be able to handle multi-density inks, and to produce ink drops of not more than 5 picolitres. Furthermore, the raster image processor must generate the same screen rulings and angles, and interpolate the data in exactly the same way as the platesetter RIP.

For newspaper printing dot simulation is certainly adequate. But it is good to know that as technology continues to improve, we will be able to shorten the route to total quality control and higher quality reproduction.

Rosemarie Monaco is the chief executive officer of Group M Inc., a marketing communications and consulting firm specializing in the graphic arts. Send comments and questions to rmonaco@groupm.org.