Pre-Press-Color halftone & Screen angles

In reproducing colour pictures that were originally continuous tone, printing

requires us to overlay the CMYK images as halftones. If the halftones are

AM (amplitude modulated), they will be formed from regular patterns of

fixed frequency. The need to rotate the angles of the halftone patterns comes

from the fact that printing cannot precisely place down one halftone dot on

top of a previously printed other-coloured dot. Commercial-quality colour

printing typically has 60 dots per cm in both the horizontal and vertical

direction. Slight inaccuracy in placing four identical patterns on top of each

other will result in an unpleasant moiré or screen clash. By rotating the

screen patterns at 30° from each other it is possible to reduce the frequency

of the moiré to a level which is below the visual threshold – it becomes too

small to be obtrusive. That reduced moiré is the pattern which is commonly

called the printing rosette.

As the halftone patterns are crossed lines at 90°, we only have 90° in

which to rotate the screens before we return to the

starting position. If we only printed with three

colours we would be OK, since there are three 30°

angles in 90°. But we print with 4 colours, and there

are not four 30s in 90, so we have to compromise.

The cyan, magenta and black are printed at 30°

apart, but the yellow, which has a low visual contrast

(it is difficult to see against the white paper) is

rotated to only 15° difference between the cyan and

the magenta.

The angle relationship between the four colours,

then, is usually fixed at 30:30:30:15. However, the

colours in this relationship may be switched. Most

usually this is done to overcome clash between the

yellow and another colour in a particularly important

picture or tint area.

Some more explanation:

When screens of cyan, magenta, and black are overlaid at their respective angles (105º, 75º, 45º) they form a moiré pattern called a "rosette." If the printer is required to use a fairly coarse AM/XM halftone screen (e.g. 85-150 lpi (newspaper & magazine work) ), then, depending on the image color content, the rosette pattern can become visible enough to be objectionable.

One way to reduce the visibility of the rosette structure is to move to a finer AM/XM screen which makes the rosette smaller and hence less visible. However, if that is not possible, then changing the separation method might be a viable option.

The majority of RGB to CMYK image conversions use "GCR" as the method (it is the default separation technique in Adobe PhotoShop). This ensures that wherever C, M, and Y inks are used black will be introduced.

To reduce, and even eliminate most rosettes, a better strategy is to use the UCR separation method on problematic images. UCR separations unlike GCR separations, primarily introduce black only in neutral and near neutral color areas. Since very little, if any, black is introduced in C and M screen tint areas – no rosettes are actually formed in those areas and hence no rosettes are visible. The result is smoother, less grainy appearing color.

While the UCR separation technique can reduce or even eliminate rosettes, there is a downside in that there will be a slight increase in ink usage as well as a slight reduction in color stability through the pressrun. That is why it should be used only for images with problematic colors - primarily dark blues and purples as well as dark skin colors/areas.

Rosettes:

Halftone dots are built inside halftone cells. Those cells have to fit together seamlessly. In order to rotate the screen, you have to rotate the cell – and there are only certain frequency/angle combinations at a given resolution where this seamless tiling is possible. The result is that at screen angles other than zero and 45 degrees, like cyan and magenta, the angles are not exactly as requested. As a result, the rosette can drift from being clear-centered to being dot-centered.

A well designed halftone screen will usually be able to maintain a clear-centered rosette across the largest diagonal plate that will be used. A less well designed screen may see "rosette drift" occurring over a distance of a few inches.

Rosette drift can also be caused by slight press misregistration caused by issues such as back sheet flare, web growth, or "waggle" (lateral sheet movement in the press). In this case rosette drift is not localized but occurs in the entire press sheet area.

Rosette basics

Printing depends on halftoning to simulate shades of gray, color, and image detail. In four color process printing, four halftones – one for each of the cyan, magenta, yellow, and black inks are overlaid to produce the image. Unfortunately, overlapping two or more halftone grids can create an objectionable pattern called a "moiré" which, interestingly is the basis of the rosette.

The greater the difference in angle between overlapping grids, the smaller the resulting moiré and the less apparent it is.

Once the grid has been rotated to 90 degrees, the moiré pattern is at its smallest and at a sufficient viewing distance seems to disappear.

Because a halftone screen is a quadratic grid (e.g. 90 degrees appears the same as 0 degrees, 135 degrees is the same as 45 degrees) the largest angle difference possible between two screens is 45 degrees, while the largest angle offset between three screens is 30 degrees (90/3=30). As a result, the defacto standard in four color printing has the three most visible process colors 30 degrees apart (C at 105 degrees, M at 75, and K at 45). Since Yellow is the least visible color it is angled at zero degrees – just 15 degrees from cyan. To further reduce moiré, the yellow screen is usually run at a higher frequency – typically about 108% of the other process colors.

The two kinds of rosettes

When screens of cyan, magenta, and black are overlaid at their respective angles (105, 75, 45) they form a moiré pattern called a "rosette."

Note that the yellow screen is not included since, because of its higher frequency, it does not form part of the rosette.This type of rosette is called a "dot-centered" or "closed-centered" rosette because each of the patterns has a dot in its center.

The second type of rosette is called a "clear-centered" or "open -centered" rosette. It is created by shifting one of the process colors one half row of dots from the other two colors.

In general, dot-centered rosettes:

• show a less visible pattern than clear centered ones

• have individual dots that land on top of one another - reducing chroma/gamut slightly

• produce color slightly differently than clear-centered rosettes

• tend to lose shadow detail

• with slight misregistration cause significant color shift

• are more popular with low screen frequencies - 100 lpi and lower

In general, clear-centered rosettes:

• show a more visible pattern than dot centered ones

• look slightly lighter due to more paper showing between dots

• produce color slightly differently than dot-centered rosettes

• tend to preserve shadow detail better

• resist color shifts better when slight misregistration occurs

• are more popular with high screen frequencies - 150 lpi and higher