AM screens

AM screens or HPS (Harlequin Precision Screening) provide more accurate screening and reduces moiré without unduly limiting the choice of screen angle and frequency.

HPS allows you to select any screen frequency and to use the usual CMYK screen angles of 0°, 15°, 45°, and 75° (plus multiples of 90°) .

To reduce moiré patterning, HPS uses an adaptive screening technique that can adjust each halftone dot so that it is placed within one half pixel of its ideal location.

In some cases, you can override a set of angles requested in a job. This is especially useful if the job has requested a set of angles that optimize the output quality for a particular output device, such as a laser printer, but that may diminish the quality on other devices, such as a plate setter. You may want consistency. For example, on all pages of a single publication when the jobs come from different sources. Enforcing settings in the RIP is the simplest way of getting such consistency.

A good choice of angles for general use with color separations in offset litho work is a set in which the colors are separated by 30°, for example: 15°, 75°, 0°, and 45° respectively for CMYK (and related sets using these angles plus or minus multiples of 90°). For use with elliptical dots, a separation of 60° is recommended, leading to angles of 15°, 75°, 0°, and 135° for CMYK.

Using a Euclidean dot shape produces better saturated grays at gray values above 50%, especially at finer (higher) screen frequencies. Euclidean strategies increase the fill of halftone cells from the corners, instead of the centers, when the gray value exceeds 50%. That is, when the gray value is less than 50%, the dots are black, the background is white, and the dot size increases as the gray value increases; when the gray value reaches 50%, the dots become white, the background becomes black, and dot size decreases as the gray value increases.

Manipulating the dot shape generated can greatly influence the amount of dot gain in an image. CLOUDFLOW RIP offers a variety of dot shapes, and your choice between them might depend on the output device, resolution as well as media being used. Standard AM Dot shapes included with the RIP are:

Cosine Dot
Cross
Diamond
Double
Double Dot
Ellipse
Ellipse A
Ellipse B
Ellipse B2
Elliptical 1
Elliptical 2
Elliptical P
Elliptical Q1
Elliptical Q2
Euclidian
Inverted Double
Inverted Double Dot
Inverted Ellipse A
Inverted Ellipse B
Inverted Ellipse B2
Inverted Round
Line
Line 90
Line X
Line Y
Rhomboid
Round
Square
Square 1
Square 2
HCS^*

^* A special note on HCS

Harlequin Chain Screening uses an elongated dot shape to create better reproductions of certain images such as computer-generated graphics. Instead of the conventional symmetrical dot shape, HCS uses a long elliptical dot to produce a chain-like structure. The elliptical dot creates particularly smooth flat tints and vignettes even when working at relatively low screen rulings.

Why is the Yellow screen ruling higher than the other process colors Cyan, Magenta and Black

This is all due to the limited screen angles that are available for simulating halftones. RIP technologies typically use screen angles in 30° increments for CMK, which results in a nice triangular pattern. Yellow is in between, in a 15°. In order to match the pattern of the other separations, its ruling needs to be bigger:

More specifically, to match the pattern of the other separations, the Yellow screen ruling should be (2/sqrt(3)) * the ruling of CMK, which is around 15,4%. Taking into account a fault tolerance explains the 14% difference in screen rulings.