Screening / Halftoning - Part 3

Attributes of AM (conventional) screening

There are four attributes of a conventional screen which must be understood

if halftoning is to be commissioned or approved. They are:

❑ dot percentage,

❑ dot shape,

❑ screen ruling,

❑ screen angle.

Dot percentage

The term ‘dot percentage’ is the means by which a fixed tonal value can

be described. In a given area, such as one of the sections of the scale , if the whole area is taken to be 100%, the dot percentage

describes the proportion of the square that is covered by black image. If,

as you would find in the highlight end of the scale, only a small part of

the square is covered by the halftone pattern, the dot percentage value for

the square will be low, perhaps five or ten per cent. Conversely, at the

shadow end of the scale the percentage coverage will be far higher, perhaps

80 or 90%. If the paper is unprinted it will have zero coverage; if

there is complete coverage the halftone value is 100%.

The dot percentage always refers to the image coverage on either the

film or the printed result. A magnifier with an enlargement of about 10

is an essential tool for checking dot percentages on film. However, if individual

dots need to be studied for shape and formation then a dot microscope

with a minimum magnification of 30 should be used. So, 10 for

dot area coverage (per cent) or 30 for individual dots.

By using a densitometer, a measuring device that calculates how much

light is absorbed by a given area of film or image, it is possible to translate

light absorption into a figure which can be expressed as an equivalent area

of halftone dot. The densitometer does not actually measure the area of a

halftone image but expresses darkness, calculated using either the Murray

Davies or the Yule Neilson equations, in terms of halftone area coverage.

It is the principal method by which halftone printing is controlled.

Dot shape

The overall shape of a halftone dot determines some of its visual and

printing characteristics. There are three main dot shapes in common use

for printing – square, elliptical and round. The reason for choosing one

shape, as opposed to another, is a combination of considerations including

the purpose of the picture, the printing process and the substrate (paper,

metal, film etc.).

Square dots are considered the most suitable for general purpose work

in that they provide a compromise between rendering fine, sharp detail

and smooth tonal transitions. However, they do suffer from the problem

that at a 50% value all four corners of a square dot link, simultaneously,

to all the four dots surrounding it. This sudden link is visible as a step in

what should be a smooth tone change.

Elliptical dots are more able to represent smoothly changing values in

the mid-tones than are square dots, because their links to the surrounding

dots do not happen in a single tone level. Across the long axis of the

ellipse the dots will join at about 30%, but the short axis will not link until

the coverage is up to 70%. The trade-off is that elliptical dots are more

troublesome to control in difficult printing conditions and can produce

visible ‘chains’ through the printed image. This is why another name for

an elliptical dot is a chain dot.

The most stable of the main dot shapes, particularly in relation to dot

gain, are the round ones. Round dots would be the natural choice for

newspaper printing because of the inherently high dot gain associated

with coldset web offset printing on newsprint. But there is a trade-off in

that it is difficult to keep detail open above 75% dot area coverage.

Screen ruling

The term ‘screen ruling’ refers to the number of halftone dots per linear

measurement of the pattern. The ruling can be expressed either as lines

per centimetre (lpc) or lines per inch (lpi). The term is a throwback to the

days when screens were made from ruled glass plates, and referred to the

number of cells per linear centimetre of the screen. A 60 lpc screen ruling

would produce 60 dot centres per linear centimetre of the image.

The screen ruling of any particular halftone can be determined by counting

the dots over a measured length, using a microscope or a tester made

specifically for the purpose. The most common type of screen ruling tester

is a small piece of film, on which is reproduced a line pattern and a scale.

By rotating the tester in contact with the halftone and noting the interference

patterns generated, the ruling frequency can be read directly from

the scale.

The decision about what screen ruling is the most appropriate for any

given job will be based upon the printing process and the materials used.

Simply stated, the finer the screen ruling, the more lines per centimetre

and the finer the detail which may be reproduced on a high-quality paper

surface. There is, however, a trade-off. The finer the screen ruling, the

more sensitive the image to dot-gain, the greater the likelihood of significant

tone changes occurring when the job is printed and the more difficult

the job to control on the press.

In any situation where the risk of the halftone dots spreading is

increased, due either to the process, as in the case of flexography, or to the

materials used, as in the case of web offset newspaper printing, a coarser

screen ruling will be chosen to minimise the effect of the dot change on

the detail and tone range of the printed image.

Screen angle

The purpose of using a halftone pattern

is to simulate various levels of grey with a system which is only capable

of reproducing one tone – usually solid black. The pattern of the screen

itself should not be readily visible to the viewer. Early in the development

of the use of halftone screens, it was recognised that the pattern of the

halftone was less noticeable and there was an improvement in the perception

of the detail of the picture when the screen pattern was angled at

45°. It is for this reason that any single ink printing, regardless of colour,

which is relying on amplitude modulated halftones for the representation

of tones, should be reproduced with the pattern at 45°.