Bedienungsanleitung Nikon LS3510 (Seite 68 von 87) (Englisch)

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7-4 Scanning for Reproduction

Software Reference for Scanners

developed with a tanning agent so the gelatin becomes hard and attracts water. The

unexposed areas wash off and the remaining metal surface does not attract water.

The plate is wrapped around a printing cylinder and it dips into a water bath. The

dark areas of the plate (metal) do not attract water, but the light, gelatin coated

areas become water-saturated. Next, the plate comes into contact with an ink

cylinder covered with the particular process color for that image layer — cyan for

the red sep, magenta for the green etc. The ink only sticks to the parts of the plate

that are not wet, since the oil-based ink repels water. The image is then transferred

from the plate to a “blanket” intermediate cylinder, and then to the paper. Hence

the name Offset . Lithography comes from the Greek word ‘lithos’ for stone. In the

early days, lithography was performed with stone plates. This process is

essentially different from all other printing processes which rely on a relief, or

three dimensional image on the plate to carry ink to the paper.

In newspaper printing the letterpress method is still common. It uses a

printing plate exposed to light and then etched so that the dots are raised up and

away from the plate surface. The dots are variably sized, as in offset printing, to

allow shading by dithering so that continuous tones are simulated with greater and

smaller dot percentages on white paper space. These raised letterpress dots are

inked from an inking cylinder and they then transfer their ink dots to paper. The

resolution in this system is relatively low (up to 100 DPI reliably) and it does not

allow for anything lighter than a 5% dot (not paper white) since these areas have to

support the paper from collapsing against the plate.

Another popular method is called rotogravure and is familiar to most Sunday

Times Magazine readers. In this process the cylinder is engraved so that the

exposed areas form small ‘wells’ for each dot. The wells vary in depth and so they

hold a different quantity of ink for each dot depending on how much they were

exposed and engraved or etched. The preparation for such a plate is very expensive

and there is no 100% reliable method of proofing(other than on press) as in the

offset Matchprint® or Chromalin® methods. For this reason, the process is used

solely for extremely long runs (over 1,000,000) where the plate is much more wear

resistant than offset plates. One of the nicest attributes of gravure is the heavy,

saturated look to the image. Using gravure plates, you can put more ink on paper

in a single pass at high speed, than you can with offset. This leads to essentially

the highest visual dynamic range (for luminance and saturation) achievable on

high-brightness paper. Another important attribute is that the process is very

insensitive to paper stock surface. This means that you can print on very poor

quality paper and achieve very good results as compared to offsets’ need for very

high quality stock surface for good results.

Perhaps the only disadvantage to gravure is that line art must be reproduced

with the same cell structure, so that type may only be printed at a maximum

resolution of 175 DPI (compared to the line art generated at 2,000-3,000 DPI on

laser image setters) so small type sizes are difficult to print legibly. Gravure can be

printed in a continuous tone method because of the variable depth “wells”, but

offset can only be printed as a halftone, unless a very slow and expensive process

called collotype is used. Collotype is the ink printers equivalent to the

photographers “dye transfer” process,oor the Techni-color® motion picture

process. Again, tanning is employed on a gelatin based matrix to produce a 3D-

relief image. This matrix holds the dye or pigment which is subsequently

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Nikon LS3510 Seite 68

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