Contact grid
The contact screen enables halftone images to be resolved into printable halftone dots . It is available in gray and magenta raster , with the magenta raster producing better results. Previously, a halftone image could only be rasterized with the help of the glass engraving screen invented by Georg Meisenbach . The main difference between the two processes is that the engraved glass grid is located in the beam path of the repro camera , which means that there must be a distance between the grid and the light-sensitive material, the film, while the contact grid is in direct contact with the film and is exposed under vacuum.
| Four CMYK printing colors and their angles. | |||
|---|---|---|---|
The advantages of the contact grid compared to the glass engraved grid are firstly greater sharpness and detail reproduction of the grid images, secondly better differentiation of the tonal values and thirdly easier contrast control. Glass engraved grids create a diffraction of light behind each grid window , which causes a loss of sharpness and reduces the detail. With contact grids, this diffraction does not occur due to the direct contact between the grid and the film.
The contrast of the raster images can be controlled with color filters during exposure. A yellow filter, in conjunction with the magenta color of the contact screen, reduces the contrast of the positive screen, while a magenta filter increases the contrast. This allows halftone recordings to be screened in a targeted manner depending on their density range.
The rasterization with a contact grid takes place in a repro camera or a contact device under vacuum . The unexposed film is positioned with its layer side on the layer side of the contact grid, in front of which the halftone recording is positioned and sucked in by means of a vacuum. With color sets, the corresponding screen angles must be taken into account, which is done with the help of markings. The exposure takes place under the previously calculated dosage of white, yellow and red light.
In 1972, four-color prints were shown for the first time at the drupa trade fair , which had been created with electronically rasterized color separations on the Chromagraph DC 300 drum scanner by Rudolf Hell . In the years that followed, the electronic rasterization of the artwork displaced both the glass engraving and the contact raster from prepress .
Individual evidence
- ↑ a b c d Basics: Contact grid ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , accessed January 16, 2010
- ↑ Basics: Raster techniques ( page no longer accessible , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , accessed January 16, 2010
Web links
literature
- Helmut Kipphan (Hrsg.): Handbuch der Printmedien. Springer-Verlag, November 2000. ISBN 3-540-66941-8
- Michael Limburg: The digital Gutenberg. Springer-Verlag, November 1996. ISBN 3-540-61204-1