Dot gain
The technical term dot gain , abbreviated as TWZ, describes the effect that halftone dots of the print template (e.g. a digital image file or a film) appear larger on the printed sheet due to the process, i.e. the print image becomes darker than intended in the template. If you look more closely, halftone dots are never transferred in exactly the same way with every technical transfer, but rather deformed in some method-typical way. Deformations, enlargements, but also reductions occur.
Choice of expression
In practice, different expressions are used for the same thing: dot gain, dot gain, dot gain, dot gain, dot broadening, print gain. Dot gain is used in the standard offset process and in the relevant specialist literature (H. Kipphan, Druck und Medientechnik, H. Teschner, Druck- und Medientechnik and Riedl, Neumann, Teubner, Technologie des Offsetdrucks). The leading German-speaking specialist institutes, FOGRA in Germany and UGRA in Switzerland, only use "dot gain".
Practical meaning
The change in the halftone dots between the original (graphic film, slide or data record) and the finished print is technically relevant. Every reproducible duplication process ( reproduction ) can be characterized by measurement technology and described by so-called characteristic curves . When preparing the image data, the changes can be compensated for in advance using the tonal value correction . The aim is to match the visual effect of the grids exactly as in the original, not to obtain the most beautiful reproduction possible. Dot gain can be positive or negative. Photographic processes can e.g. B. lead to a reduction of the points (overexposure). Many technical processes, especially in the transfer of the grids by printing processes, tend to have a magnifying effect. They are most involved in the change and require the most corrective action. Hence the expression "dot gain". Tone values are calculated from measurements of the optical density and given in% of the optical effect of a full tone area. The tone value increase is determined from the difference between the tone value of the original (data, reprofilm) and that of the print, i.e. it records all changes in the screen effect that occur in a processing chain (see also print screen ). Its amount is strongly dependent on the initial tone value, since it starts small in the course, goes through a maximum in the middle range and becomes small again towards the 100% value. Meaningful information must always be z. B. "9% TWZ at 25% halftone value". Tone value increases can go up to more than 25% with regularly executed prints, so they are not negligible when planning raster print objects. With modern grid techniques , not only the changes in area become important, but also the deformations, because many advantages are achieved through special grid shapes (keyword point closure ). There is still no possibility of a targeted advance compensation by prepress.
causes
Offset printing
Reproductive technology
The preparation of the template (e.g. slide ) or the data for the printing form ( letterpress, offset, gravure, screen printing ) or for the printing itself ( NIP printing techniques ) are the tasks of prepress. Already in the photomechanical processes (copying over re-films) it was possible to make targeted changes. In digital processing, there are diverse and precisely calculable ways of influencing.
Classic offset
In planographic printing with dampening solution , there is constant competition between the color (as an emulgate of 10 to 30% dampening solution in color) and the free dampening solution on the plate. The printing areas on the plate are typically over-colored over their edges: This fluctuates over the course of the run and depends on the machine construction, color, dampening solution and substrate.
Color transfer
In a printing machine, screen dots are transferred between two cylinders by squeezing between z. B. slightly enlarged printing form and substrate. The indirect printing processes have particular disadvantages here because they have an additional nip (pinch point). The impact of crushing processes depends heavily on the contact pressure of the two cylinders involved and the compressibility (softness) of their surfaces. The TWZ is lower when printing on coated paper than on natural paper (uncoated). But the color also has an important influence.
Furthermore, errors in the so-called processing come into play here, in which a raster point is elongated by different circumferences of the printing plate and blanket cylinders. An increased TWZ can also be simulated by duplicating .
Optical processes
Optical processes such as catching light usually reinforce the effect of a grid surface. They are recorded by our perception in the same way as by measuring devices, so they are physically present and not imagined. Since the light trap starts at the point edges, it is dependent on the grid frequency, among other things. With all fine grids it is therefore stronger than with coarse grids.
Waterless offset
In waterless offset , there is no over-coloring of the halftone dots, or only so little that it has not yet been detected. This predestines this type of printing for particularly fine screen structures, i.e. high screen frequencies and fine, non-periodic screens. The other reasons for changes in the tone value and the dot shapes are retained as in classic offset.
High pressure
Depending on the support conditions such as type of machine, settings, color quality and printing the expanded high pressure the tone values with its hollow character . The same applies to letterpress and flexographic printing. With dry offset (letterset, indirect letterpress printing) an additional nip is added.
Gravure
In gravure printing , the webs are partially or completely flooded in the middle raster areas. The expression "area coverage" is therefore completely useless here and only the tone value is used.
screen printing
The through-printing (screen printing) can also never depict the free areas of the stencil on the substrate with great precision.
Pressure characteristics
A diagram with the tonal values of the original (film, data) on the abscissa and the tonal values of the finished print on the ordinate should have a diagonal as a measurement curve from 0 to 100% if the reproduction were completely unchanged. If, on the other hand, you get higher values from the print than from the original, as it actually behaves, then the measuring points lie on a bulging curve. This is the usual representation of a pressure characteristic. In the standardization of offset printing, an area has been defined based on extensive experience that can be complied with according to the current state of technology with the equipment in practice. The dot gain is not treated as an error here, but as a process characteristic. Any pressure with a characteristic curve within these limits is therefore in accordance with the standard. The printer must adhere to these specifications for his contribution, because prepress companies set up their data accordingly, ie "subtract" the expected TWZ beforehand ( tone value correction ). Deviations from this can only be made in a special, bilateral agreement between a single prepress company and a designated offset printer without endangering the end result.
Individual evidence
- ↑ Deutscher Drucker 30/2006, p. 26, screen dot change , ISSN 0012-1096
- ↑ Deutscher Drucker 30/2006, p. 28, paper and light trap , ISSN 0012-1096
literature
- Helmut Kipphan: Handbook of the print media . 1st edition. Springer-Verlag, Berlin Heidelberg 2000, ISBN 3-540-66941-8 , pp. 369 .
- Helmut Teschner: Print and media technology . Dr.-Ing Paul Christiani GmbH & Co. KG, Konstanz 2010, ISBN 978-3-86522-629-7 , p. 431 .