Hybrid Cross-Modulated Screening (HXM)
Harlequin HXM screening is a combination of AM and FM screening which allows higher screen rulings than is provided by AM screens alone. Available from Global Graphics as a plugin upgrade to the Harlequin RIP, HXM is a solution that addresses the issue of printing on lower quality papers while at the same time meeting higher customer expectations. HXM screening is a solution that addresses both these challenges.
Higher screen rulings can be used without major retooling, producing visually higher quality printing. The range of screens offered within the HXM screen set can handle both matte and gloss stock.
Its use of hybrid screening allows the enlargement or reduction of images without affecting quality or detail. In fact, it can significantly improve the print quality with smoother flesh tones, flat tints and gradient transitions. This is achieved by controlling the dot size in the highlight and shadow, ensuring it is does not disappear in the light areas or merge in the dark areas.
A common artifact of conventional fixed screening (AM) is moiré. This can be reduced by careful manipulation of screen angles, adjusting the input resolution and increasing the output lines per inch (LPI). Other screening types can eliminate moiré but have additional printing requirements and therefore may not be suitable in some applications.
The challenge with conventional AM screening is to be able to print the smallest dots possible to enhance the detail, maintain highlight detail and to provide smooth gradients where present. HXM screen have been designed to solve the printing challenges that come from printing at higher screen rulings.
FM screening places dots of varying size in a pseudo-random way to produce the required tints. Advantages of this include increased apparent sharpness and the removal of cross screen and object moiré. This is achieved because the dots are dispersed instead of clustered as they are in a traditional periodic screen.
Because of the variable dots, FM screening does have some limitations. These include printing blanket hardness, TVI, ink tack needed adjustment, not being printable for long runs and substrate quality which can cause piling. However, FM does make ink saving a possibility, and if process control is tightly maintained, has been shown to print purer colors.
HXM hybrid screening
Hybrid screening is designed to improve print quality by increasing the output LPI for any given resolution. For example, if a printer is currently imaging plates at 2400 DPI with output at 150 LPI and is able to hold a 1% dot when printing on the required substrate, using HXM screens it would be possible to print at 200-250 LPI without any special pre-press or on-press requirements. The TVI difference between the current printing setup and the HXM screens would need to be adjusted to optimize the printing.
The areas that suffer most when trying to increase the LPI for a given printing condition (typically the substrate) are the loss of dots in the highlights and merging of dots in the shadow, resulting in loss of detail. Using current computer-to-plate imaging systems at 2400 DPI, it has been shown that a 0.5% dot can be produced on some plating material.
However, only a very controlled printing system on the finest sheet fed presses with premium gloss paper stock can this 0.5% dot actually print over the course of a reasonable press run. This is where HXM comes in. HXM recognizes the issue and is, therefore, designed to carefully control the dot size and placement in the 0-8% and 92-100% range within the tonal scale.
Shown below are sample images of traditional screening using a Euclidean Dot and the HXM Screens using the FM to Traditional Dot. Notice how the dots differ in placement and size in this comparison as seen in the highlight end of the tone scale.
HXM Generic screen sets include those for three specific industry segments: Newspaper, Commercial and Flexographic printing. The recommended screens for Newspaper printing are the traditional dot shape screens at 1200 DPI at 210 LPI and 240 LPI. Commercial printing should again use the traditional dot screen at 2400 DPI at 133 LPI and 150. The working set for Flexo printing comprises entire round dot offerings at 1200 DPI at 75, 85, 95, 105 and 120 LPI along with 2400 DPI at 133 and 150 LPI. The HXM Generic screen sets are not exhaustive, but a reference set and bespoke sets are available by arrangement.