Screen Frequency

Screen Frequency must be fine enough to convey the sharpness and detail of the original. At screen rulings of 150 lpi the dots are almost invisible to the unaided eye. At 200 lpi they are invisible.

High screen rulings (>200 lpi) are employed to:

1. provide extra sharpness and crispness to an image.

2. reproduce fine detail such as type.

3. bring out the underlying grain of an original.

• Screen rulings of 600 lpi or more have been achieved and it is impossible to detect the screen pattern without magnification. Generally increasing the ruling beyond 250-300 lpi has no visible effect.

• The screen frequency used should be tailored to the targeted printing process. Newspapers employ 80-100 lpi while a brochure printed on coated paper with a sheetfed offset press may employ 200 lpi.

• Specifying higher screen rulings may make the printing process more challenging since a finer degree of control is required.

Screen Angles

A halftone screen consists of a pattern of dots with well defined rows and columns. Rotating the pattern by an angle yields a different pattern with exactly the same screen ruling.

• For a single color, an angle of 45° for the rows (or columns for square patterns) is the least noticeable.

• For multiple colors the screens for the different colors must be rotated with respect to one another to avoid objectionable moiré patterns.

• The ideal angle between two colors is 30°. In four-color printing the angles have been traditionally chosen as 15° C, 75° M, 0° Y and 45° K.

• The pattern generated by this process is called a rosette.

• With digital halftones, not all angles can be achieved and still maintain the shape of the parent cell.

• Thus, other slightly different angles may be specified or techniques such as supercell screening may be employed. For AGFA 9800 imagesetter, the rulings and angles for a nominal 150 lpi are C 154.6@14.93° , M 154.6@75.07° ,Y 150@0° and K 154.28 @45°.

Dot Shapes - Conventional photographic halftones are square at a 50% and become circular as the dot becomes larger. At 50% the dots join up with adjacent dots creating a jump in tonal value.

• In the midtones and graduated fills these can cause noticeable artifacts. A posterization effect is obtained in fleshtones.

• Such problems can be dealt with by using elliptically shaped dots or diamond shaped dots. These work better for images with much midtone detail.