Archive for the ‘Lithocam’ Category

As an alternative to dot gain the relative print contrast Krel.(%) is often determined, particularly to check the screen in the three-quarter tone.

A print should have a contrast as high as possible. This means that the solids should have a high ink density, but the screen should still print open (optimum halftone value difference). When the inking is increased and the ink density of the dots rises, the contrast is increased.

However, the increase in ink feed is only practicable up to a certain
limit. Above that limit the dots tend to exhibit gain and, especially in three-quarter tone, to fill in. This reduces the portion of paper white, and the contrast decreases again.

If there is no measuring device available with a direct contrast display, the relative print contrast can be calculated or determined on the basis of the FOGRA PMS.

If the contrast value deteriorates during a production run in spite of constant ink value in solid DV, this may be a sign that the blankets need washing.

If the solid density is correct, the contrast value can be used to assess various factors which influence the print result such as rolling and printing pressure, blankets and underlays, dampening, printing inks and additives.

Since the contrast value, unlike the dot gain, depends to a large extent on the solid density it is not suitable as a variable for standardisation. This is why in the recent past its importance has decreased significantly.

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ISO 12647-2 LAB values for different paper types

Like all complex systems, digital plate setters, also known as computer to plate systems, require suitable control aids in order to monitor output quality during daily production and to be able to guarantee produc tion standards. The complexity of computer to plate systems is apparent in the large number of components involved in the process.

The combina tion of digital data from various different application programs, with various different RIP and output device parameters, different types of plate and development conditions for plates as well as printing requirements for the transfer of tonal values place high demands on the control of the work as it progresses.

After extensive test exposures and practical investigations a plate control strip that is suitable for use in produc tion was developed. It is a further development of the Ugra/FOGRA-PostScript Control Strip. This is the standard tool for PostScript capable output devices and continues to be important as a control aid for fi lm exposure and for digital work flow.

The Ugra/FOGRA Digital Plate Control

Wedge is arranged as a total of 6

functional groups or control panels:

• Information panel

• Resolution panel

• Geometrical diagnostics panels

• Chequered panels

• Visual reference steps (VRS)

• Progress wedge

The 11 visual reference steps are a new feature. In this group a chequered panel is surrounded by a reference panel, and the dot percentage increases in 5 % steps from 35 % to 85 %. Under theoretically ideal conditions and if the transfer characteristics are linear then when the two panels have a dot percentage of 50 % they should blend with each other, i.e. the impression of brightness and the measurable tonal value should give a dot percentage of 50 % in both areas.

Due to the plate type, exposure calibration, developer and transfer characteristics this is hardly ever actually achieved under operational conditions, and shifts upwards or downwards occur. The VRS panels that are important for day to day production are those that enable the optimum setting to be chosen and output results to be achieved. Deviations from the appearance of the VRS panels that is ideal for the production process can be identifi ed by a visual check. Other panels provide information about resolution as well as the progress wedge, with which the tonal value transfer can be checked.

In order to eliminate production dependent differences in plate materials, zero points are located between the rows of halftone panels. Consequently the position for the densitometric measurement of zero (substrate only) and the dot percentage lie beside each other.

The resolution panel contains two semicircular panels. In the first panel positive lines radiate out from a point and in the second the lines are negative.

The thickness of the rays corresponds to the theoretical resolution of the output device. The geometrical diagnostics panels also contain lines that are oriented to the particular resolution setting of the output device. The chequered panels are below the geometrical diagnostics panel.

Labels are positioned over each of the panels and the sides of the squares are one, two and four units long.

Ihara LIthocam Plate reader – world class equipment

 

Internal drum imagesetters are used for both typesetting and repro. They are available on the market as both filmsetters and platesetters. The material to be exposed is held in position inside a partially open hollow cylinder. The laser is then moved along its exact center.

On some units, only the deflection unit is moved. The laser beam is focused onto the material using a lens and deflected onto the film via a fast-rotating prism.

The image lines and the feed are effected by moving the optical system. The material is not moved during the exposure process.

The rotating deflection unit is a small component and can rotate at high speed. This means that production can be very quick using a single laser beam. Although the optical paths are significantly longer than on external drum imagesetters, on the whole, it is easier to buffer vibration since only small masses are being moved. The optical system as a whole is kept significantly more simple.

This type of imagesetter enables maximum quality in the repro sector at very high speeds and at a moderate price. It has established itself on the market as a filmsetter and platesetter.

Ihara Lithocam plate reader to measure plate dots