Dot gain

The technical term dot gain, abbreviated as TVI, referred to the effect of grid points of the artwork appear larger (eg a digital image file or a movie ) on the printed sheet due to the process, the image that is darker than those in the template. More specifically, grid points are never transferred matter for each technical transfer but deformed in any way typical procedure. This occurs deformations, enlargements, as well as reductions.

Expression of choice

In practice, different expressions for the same thing be used: dot gain, dot gain, dot gain, dot gain, dot gain, dot gain. Dot gain is the standard offset and in the relevant literature (H. Kipphan, Printing and Media Technology, H. Teschner, printing and media technology and Riedl, Neumann, Teubner, technology of offset printing ) used. The leading German technical institutes, FOGRA in Germany and UGRA in Switzerland, only use " dot gain ". The practice should also agree on this term now.

Practical significance

Technically relevant is the change of the grid points between the template (graphic film, slide or record) and the finished print. Each reproducible replication process (reproduction) can be measured technically characterized and described by so-called characteristic curves. In preparing the image data, the changes on the Levels can be anticipated compensated. The goal is to make the visual effect of the grid exactly as in the original, not to get a nice reproduction possible. Dot gain can be positive or negative. Photographic processes can for example lead to a reduction of ( overexposed). Many technical processes, especially in the transmission of the raster by printing methods, but tend to have a magnifying effect. You are most involved in the change and most require corrective action. Therefore, the term " dot gain ". Tonal values ​​are calculated from the optical density measurements and expressed in % of the optical effect of a solid area. Dot gain is determined from the difference between the tone of the original ( data, process film ) and the pressure, so summarily captured all the changes in the grid effects that occur in a processing chain (see a print height). It is strongly dependent on the Ausgangstonwert in magnitude, since it starts small in the course, passes through a maximum in the central region and the 100% value back becomes small again. Meaningful information must therefore always be, for example, " 9% TVI at 25 % halftone value ". Dot gain can go quite up over 25 % on regularly exported printing, and are therefore not negligible in the planning of screen printing objects. With modern scanning techniques are not only the areal changes important, but also the deformations because many benefits are achieved through special grid forms (keyword point circuit). There is no possibility of a targeted advance - compensation by the prepress.

Causes

Offset

Reproduction technology

The preparation of the submission (eg slide) or data to the printing form ( letterpress printing, offset, gravure, screen printing) or to pressure yourself (NIP printing techniques ) are the prepress tasks. Already in the photomechanical process (copy on repro films) could be selectively altered. In digital processing, there are many and exactly calculable possibilities of influence.

Classic offset

In lithographic printing with fountain solution on the plate takes a constant competition of the color ( as emulgate 10-30 % dampening solution in the ink ), with the free dampening solution. The image areas on the plate are typically over-dyed on their edges. This varies in the course of circulation and depends sensitively on machinery, ink, fountain solution and printing material from.

Color transfer

In a printing machine grid points are in transit between two cylinders by Quetschvorgänge between eg printing form and the substrate, somewhat enlarged. Specific disadvantages have here the indirect printing process, because they have an extra nip ( pinch point ). The effect of Quetschvorgängen strongly depends on the contact pressure of the two participating cylinder and the compressibility ( softness) of their surfaces. Thus, the TVI is less than on uncoated paper ( uncoated) when printing on coated paper. But the color has an important influence.

In addition, here are still errors in the so-called settlement to support, in which a dot is drawn by different sizes of printing plate and blanket cylinders in the length. Also, by doubling, increased TVI be faked.

Optical processes

Optical processes such as the light gathering enhance generally the action of a grid area. You are covered by our perception as well as of measuring instruments, and are therefore physically present and not conceited. Since the light -catching tackles the point edges, it is dependent on the grid frequency. For all fine screens so he is also stronger than that of coarse grids.

Waterless Offset

In waterless offset does not occur or only such a small over-coloring of the grid points that they could not be detected so far. The ideal variant for this pressure particularly fine grid structures, ie high screen frequencies and fine, non-periodic grid. The other reasons for changes in tonal value and the dot shapes remain as obtained in the classical offset.

High pressure

Depending on the support conditions such as type of machine, settings, color quality and substrate of the high pressure increases the tonal values ​​with its hollow character. This is similar to letterpress and flexo printing. When dry offset ( Letterset, indirect high pressure) comes back an additional Nip it.

Gravure

In gravure printing, the bars are partly or completely flooded already in middle grid areas. Here, therefore, the term " coverage " completely useless and only the tonal value in use.

Screen printing

The stencil printing ( screen printing ) can also never quite precisely depict the vacant posts of the template on the substrate.

Pressure characteristics

A graph of the tonal values ​​of the original ( movie, data) on the abscissa and the tonal values ​​of the final pressure on the ordinate should have a diagonal as a curve from 0 to 100 % when the reproduction would be completely unchanged. Is obtained from pressure on the other hand, higher values ​​than the original, as it actually behaves, then the data points lie on a bulging curve. This is the common representation of a pressure curve. In the standardization of offset printing has been defined from extensive ranges each experience, an area which can be met by the current state of the art equipment with the practice. Dot gain is not treated here as an error, but as the process characteristics. Each pressure whose characteristic lies within these limits, so is the standard. The printer must comply with these requirements for his contribution because prepress companies set up their data then, the expected TVI So before " pull " ( Levels ). Thereof may be waived only in a special bilateral agreement between a single prepress company and also a certain offset printer, without compromising the final result.