Stochastic Screening - HDS
HDS is Harlequin's Patent-Pending stochastic, or 'FM' screening technology. HDS creates reproductions with increased image detail over conventional screening methods and offers complete freedom from both cross screen and object moirés.
Conventional screening methods place dots in a grid pattern to reproduce a continuous tone image. With FM screening, tone densities are varied by placing different numbers of small dots within a given area rather than by changing the sizes of a fixed number of dots as in conventional AM screening.
Tonal characteristics are simulated by enlarging or reducing the dot size in a particular area of an image. With FM screening, the dots are dispersed in a less structured manner. Varying shades are created by the irregular frequency, or dispersal, of the screening dots.
Dots seem to disappear, resulting in exceptional clarity and detail, especially effective in printing images of jewelry, fabrics, fine automobiles and skin tones. FM screening also eliminates moire patterning, color shifts and rosettes which can form in conventional halftone images.
HDS offers a random approach that dramatically improves the print-ability, stability, quality, and predictability within the printing process.
HDS uses a random component on top of a very fine and distributed structure to produce a screen that has the benefits of both purely stochastic, or random screens, and conventional screens. Individual spots do not randomly 'cluster' into structures with odd sizes and shapes thus reducing the graininess over other screening methods.
Though there is no regular and periodic structure, as with a conventional halftone screen, there is a certain 'regularity' of form and scale. Different coarseness and device resolution combinations can be specified to support a wide variety of output device technologies.
If you examine HDS tints closely you will notice that, though no conventional halftone pattern exists, at any given level of tone the tints have a certain 'homogeneous' structure, and that this structure changes smoothly up and down the gray scale.
Plates produced using HDS are reproduced easily using a wide variety of image-setting equipment and printing processes. Exceptional results have been obtained with HDS printing on newsprint with web press equipment, on high-quality coated paper with commercial sheet fed presses, and using direct output methods such as ink-jet and laser printers.
Five versions of HDS -
The HDS Stochastic Screening option includes five choices of dot grain. Each are used for different applications. Some enlarged tint blocks of HDS Super Fine, Fine, Medium, Coarse and Super Coarse are shown above.
Frequency Modulated Screens do not have the same lines per inch (lpi) characteristic as conventional screening. As a general rule of thumb, working at 1524 dots per inch (dpi) with a relatively small dot size (less than 20 um), then HDS Coarse has a dot gain on plate only slightly higher than a conventional 150 lpi screen in the mid tones, and holds up better than the conventional screen in the highlights and the shadows.
Medium FM Screening has a similar correspondence to a 200 lpi screen at 1524 dpi. Finally, Fine FM Screening has characteristics similar to 300 lpi screen platesetter resolutions. The integration of Frequency Modulated Screens means better image detail and freedom from moiré. It is a good option for many applications.