Photolithography (printing technology)
With photolithography or photolithography , a method is known, with the photographic images of a to -replicating template in the tone values corrected on the lithographic stone transferred and prepared for printing. After lithography had been superseded by offset printing in the 1950s , only the misleading job title photolithographer remained , although this job no longer had anything to do with a lithographic stone . The later correct job title was artwork preparer - specializing in offset printing .
history
After the invention of photography and the glass engraving grid , there were attempts to use the new technology in lithography as well . The French Niépce copied photographic negatives onto the litho stone in 1822. He knew that a light-sensitive layer of asphalt loses its solubility if it is exposed to sunlight for a few hours. The exposed stone was developed with ether and a positive asphalt image remained, which took on color and could be transferred to paper.
However, these lithographs made with the help of photography were no longer original lithographs, but rather poor reproductions, i.e. copies of an original that hardly had any halftones. At that time there was no way to resolve the photographic image into printable halftones . Georg Meisenbach is considered to be the inventor of the grid , who developed the high-precision glass engraving grid in 1881 and was thus able to break down halftones into printable halftones for the first time using a photographic method. Due to the differentiated tonal value reproduction, this technology enabled printed reproduction in six or four colors instead of twelve or more and was thus far more economical than conventional chromolithography.
Basically, there are three process stages in photolithography, namely firstly the photographic recording and the creation of the color separations , secondly the manual correction by the photolithographer and thirdly the transfer of the recording to the stone by the stone copy and the preparation for printing the lithographic stone.
Repro photography
At the beginning of the 20th century, Karl Schumacher and the Klimsch & Co company developed a reproduction camera or repro camera with which two-dimensional templates could be reproduced exactly. This camera was significantly larger than the traditional studio or handheld cameras and could fill a large space. James Clerk Maxwell had already developed the filter technology for color separations in 1861 and Meisenbach followed suit with the invention of the glass engraving grid in 1881. This created the prerequisites for the use of photography in repro technology.
A horizontal repro camera was mounted on a vibrating tripod, which caught every vibration of the building. It consisted of a template holder and a lighting device with two or four carbon arc lamps. The standard, in which the interchangeable lens was located, was connected by a flexible bellows to the rear part of the camera, which contained the grid system and the cassette with the light-sensitive material. After the exposure, the cassette was sealed light-tight and the exposed photographic material (glass plate or film) was developed in the darkroom. Often the room was designed so that the back of the camera was directly in the darkroom .
The repro photographer uses color filters to create color separations from a colored original . A four-color printing requires each one color separation for yellow, red, blue and black. The repro specialist calls these colors yellow or yellow, magenta , cyan and depth or black . During exposure, a color filter is placed upstream in the lens that corresponds to the complementary color of the color separation, i.e. a violet filter for yellow, a green filter for red and an orange filter for blue. Black, which is only used to increase the contrast, is recorded without a filter.
In order to be able to print the color separations, a division into raster points is necessary. This rasterization also takes place in the reproduction camera by placing a raster disk in front of the photographic plate or film to be exposed.
In four-color printing , in which several rasters are printed on top of each other to display a color image, one tries to avoid the moiré effect by an angular distance of 30 ° per color separation. Yellow is excluded from this because the resulting moiré is barely visible. Common screen angles for four-color printing are according to DIN 16 547: yellow = 0 °, cyan = 75 °, black = 135 °, magenta = 15 ° or yellow = 0 °, cyan = 15 °, black = 45 °, magenta = 75 °
Manual corrections
After the photos had dried, the work of the photolithographer began because the results from the repro photography were incorrect and required manual retouching . His workplace consisted of a light table that sloped slightly from the back to the front. Several lamps with neutral light that was as uniform as possible shone through a translucent but opaque sheet of milk glass. The photolithographer assessed the color separations on this glass plate in order to then carry out the necessary corrections. He had certain chemicals and tools available for this.
The chemicals included Farmerscher attenuators to lighten tone values, as well as mercury amplifiers and Keilitz paint to darken tone values. Red chalk was used to cover opaque areas. In addition, the photolithographer used peelable or washable masking varnish with which he could protect parts of the picture that could not be changed. He now had the opportunity to process the color separation as a whole in a flat bowl with attenuator or amplifier. He made partial retouching with a brush. However, all corrections were made based on his personal judgment and experience. The goal was to come as close as possible to the template or the original in the print result. Good, experienced specialists usually achieved this result straight away. Other work utensils of the photolithographer were raccoon hair brushes of various strengths, rulers, drawing pens, scrapers and, last but not least, the thread counter .
Negative stone copy
The finished retouched negatives served as templates for the stone copy. A prepared stone was made photosensitive with an egg white chromate solution. This consisted of a solution of distilled water , dry protein, ammonia and ammonium dichromate , with which the stone was poured, distributed evenly in a centrifuge and dried. The photolithographer then placed the retouched negative layer upon layer on the stone and weighed it down with a glass plate. The areas outside the negative were given a cover made of black paper. The exposure to carbon arc light was carried out in a stone copier , as a result of which the exposed areas were hardened. The stone was then rolled in with black printing ink and the copy was developed with a cotton ball in a shallow basin filled with water. The unexposed areas came loose and a positive reversed color separation appeared on the stone. This could now be edited manually again before the stone was prepared for printing.
Positive stone copy
In a second method, a screened slide was used instead of the photographic negative. This method had the advantage that the positive image could be processed again by the photolithographer. The exposure process was the same as for the negative copy, but now the non-printing parts of the image were hardened, while the unexposed areas remained soft and water-soluble and were prepared for printing after development. Lithographers and lithographers call this chemical process etching . The fat-friendly printing parts, i.e. the drawing, should be reinforced in their properties and the non-printing parts of the stone should remain fat-repellent and water-absorbent. The etch consists of a mixture of nitric acid , gum arabic and water, which is applied to the stone surface with a sponge and takes effect. Nothing is removed or etched away by the etching, but only optimizes the printing properties of the stone. The process can be repeated several times and is considered complete when the first test prints have been made without any changes.
Individual evidence
- ↑ a b c Photolithography, accessed on June 29, 2009 ( Memento of the original from July 3, 2009 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ A b Walter Domen: The lithography: history, art, technology. Dumont paperback books, Cologne 1982, ISBN 3-7701-1431-0 , pp. 238-240.
- ↑ a b c d e f Development of repro technology, accessed on July 3, 2009 ( MS Word ; 64 kB)
See also
literature
- Walter Domen: The lithography: history, art, technology. Dumont paperback books, Cologne 1982, ISBN 3-7701-1431-0 .
- Henry Cliffe: Lithography Today, Technology and Design. Ravensburg 1968.
- Helmut Kipphan: Handbook of the print media. 1st edition. Springer Verlag, Heidelberg 2000, ISBN 3-540-66941-8 .
- Jürgen Zeidler: Lithography and stone printing in trade and art, technology and history. Christophorus-Verlag, Stuttgart 2008, ISBN 978-3-419-53486-1 .
- Jürgen Zeidler: lithography and stone printing. Ravensberger Buchverlag, 1994, ISBN 3-473-48381-8 .
