Prepress (Production of Films and Plates)

• Gray balance. Printing the standard colors cyan,

magenta, and yellow together does not produce a

neutral gray but a brown tint. To counteract this,

the positive cyan color separation is produced with a

5–10% higher area coverage than the magenta and

yellow separation.

• Masking the original. Color transparencies usually

have a wide density range, for instance 2.80 optical

density units, which is detrimental to the reproduction

process. If a slightly blurred negative (mask) is

produced from the transparent positive on very

slow reacting panchromatic continuous-tone film

that has a density range of 1.00, the combination of

transparency (original) and mask will now have a

density range of only 1.80 (2.80 – 1.00 = 1.80) density

units, which is closer to the range reproducible in

the print. Additional color filters used during the

production of the mask allow color values to be corrected

at the same time (e.g., correction of desaturation

when the color of the filter is the complimentary

color of the color separation being corrected).

• Masking to correct color separations. An auxiliary

recording (mask) is produced photographically for

each color separation and has the opposite errors to

those in the separation. The color separation and

mask are copied together so that the errors cancel

themselves out.

• Alteration of dot size by controlled over-exposure (dry dot etching). When copying halftone screened color separation films in the contact copier, an enlarge-

ment of the dots can be achieved by over-exposure

caused by light scattering effects in the light sensitive

coating. There is a systematic relationship between

the degree of increase in dot size and the exposure

time.When copying from a negative to a positive,

over-exposure will give a positive correction.Overexposure

will give a negative correction when copying

from a positive to a negative (fig. 3.1-55).

• Reducing or intensifying halftone dots on the film. The

aim is (also known as “halftone etching”) to reduce

the size of the halftone dots without “dry dot etching”.

The developed and dried film is treated with an

aqueous reducing solution that gradually converts

the black silver of the image to a water soluble silver

salt, which is then washed away. This process occurs

mainly outwards from the edges of the blackened

areas of the film coating.Halftone dots with a flat

blur edge react particularly well to the reducer.Halftone

dots are intensified on the film by dyeing the

coating with a dye that attaches itself only to the

blackened areas of the coating, or by increasing the

quantity of black silver in the image as a result of a

physico-chemical process.

• Undercolor removal (UCR; see sec. 1.4.2). Electronic

color separation devices allow gray values in the

colored image to be produced by the black color

separation alone, instead of by overprinting the

three process colors. Tertiary colors are made up of

black and only two of the process colors. The advantage

of this system of color separation correction

is that the gray of the image is reproduced correctly,

even if there are deviations in color of the

process colors, which would show up immediately

as tints in a gray produced from the three colors.

Proof (see sec. 3.1.6)

• Simulation of the dot gain of the printed job. Analog

color proofing processes start with the films of the

separated colors. In order to produce a proof that

corresponds to the expected printed result, the dot

gain produced by the printing process must be simulated.

For example, in the toner-based proofing

process (see fig. 3.1-43) dot gain may be controlled

within certain limits by varying the exposure time

and toner supply.

• Simulation of the substrate. The optical properties

of the substrate have a large influence on the printed

result. These may be approximated in the proofing

process by the use of various carrier materials

available or by laminating translucent films with

different diffusion factors due to different surface

roughnesses.

Дата добавления: 2015-11-16