PRINTING TECHNOLOGY

A moiré pattern is an artifact that occurs in the print reproduction process when any two, or more, repeating patterns overlap each other. Moiré in the print reproduction process is similar to the distortion effect on television. The most common types of moiré encountered in the print production process. Scanning/sampling moiré These artifacts a re caused by the frequency/angle of the scanner sensor (flat bed and drum scanners, or digital camera sensor) harmonically beating with a pattern in the object being scanned. In this case the artwork ends up having the moiré embedded in it and is part of the image. Subject moiré These artifacts are caused when the halftone screen that is being used to reproduce the image on press harmonically beats with a pattern in the image being reproduced.This artifact is sometimes referred to as "screening moiré" since it is the halftone screen that is causing the problem. Screening moiré Screening moiré, which is a term tha...

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%. ...

There are two ways that this halftone screening equivalency is usually measured. One is equivalency of detail rendering - the ability of the screening to render image detail. The other is lithographic equivalency - how they perform on press lithographically. Note that in both cases, because the respective screening technology is so different, equ ivalency can only be an approximation. Equivalency of detail rendering Since halftone dots form the printed image - more dots per linear inch translates into more detail that can be rendered. With an AM screen the detail rendering ability is specified in lpi (or lpc) - i.e. halftone dots per inch (e.g. 175 lpi or 60 lpc). Since an FM screen has no "lines per inch" determining the equivalent detail rendering equivalency is usually done by drawing a line through the FM screen and counting how many dots are intersected (crossed) in a distance of one inch. For example the distance measured is 1/16th of an inch. In that 1/16th of an i...

Inks used in color printing presses are semi-transparent and can be printed on top of each other to produce different hues. For example, green results from printing yellow and cyan inks on top of each other. However, a printing press cannot vary the amount of ink applied to particular picture areas except through "screening," a process that represe nts lighter shades as tiny dots, rather than solid areas, of ink. This is analogous to mixing white paint into a color to lighten it, except the white is the paper itself. In process color printing, the screened image, or halftone for each ink color is printed in succession. The screen grids are set at different angles, and the dots therefore create tiny rosettes, which, through a kind of optical illusion, appear to form a continuous-tone image. we can view the halftoning, which enables printed images, by examining a printed picture under magnification. Traditionally, halftone screens were generated by inked lines on two sheets of...

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 plac e 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 hav...

Additive colour When the process of creating colour uses sources of light – such as the tiny points of phosphor on the surface of a domestic television tube, a computer screen or the projected light from a video projector – this is called additive colour. The colour is created by adding together different amounts of the additive primaries: red, gr een and blue (RGB). If all three of these are present, their combined effect will be white. If none of the light sources are available, the result will be black (the absence of light). All the other tones and hues that the device is able to present will be created by the precise control of the individual (and relative) brightnesses of the RGB sources, because mixing two primary colours creates a secondary colour: red plus blue makes magenta, red plus green makes yellow and blue plus green makes cyan. A colour monitor reproduces colours by presenting red, green and blue light from individual points that are so small that the eye cannot disti...