For the evaluation of the quality of offset printing plates appropriate measuring control strips have been developed and new types of measuring devices are also created.
Unlike measuring control strips, which were used for control of the quality of the process of making printing plates by conventional methods, CtP technology provides the ability this measuring control strips to be in digital form and to be directly transferred to the printing form. On this way we can avoid inter-process steps that in conventional process led to the loss of quality of tonal values during processing and manufacturing of printing plates. For this reason, the digital metering control strips provide more opportunities for control of the process of developing of the printing plate.Using them can be controlled the intensity of laser operation, the output resolution of the device, fidelity of reproduction of raster tonal values, correctness of developing substances…
With the technological development of the CtP also have been developed and measuring control strips which are used for control.
Among the first appeared Ugra / FOGRA Digital Plate Wedge. As the first original solution for controlling the quality of developing of printing plates she served as a standard for the design of other measuring control strips for CtP devices, such as Agfa DigiControl ,
GATF Digital Plate Control Target, System Brunner.
Measurement and control of the printing form is very important to check whether the plate is properly lighted and developed. Manufacturers are trying to maximally standardize their measuring devices in order to solve several problems such as high contrast, the size of the raster point and determining the texture on the plates, while a single plates may vary In contrast, tonal surface, the way of screening and their physical characteristics.
To control the tonal values in the raster on printing plates are used measuring devices called Plate Readers.(from www.sidleipzig.de)
These devices, in contrast to the densitometer, which operate on the principle of measurement of reflected light, digitally recorded detail of raster structure with high resolution. This digital image is analyzed using software that is part of the measuring device. On the basis of geometric analysis, this software provides data on the percentage of coverage of the plate with raster dot, on the corner and line screens of conventional raster, dot diameter, and magnified view of raster images.
High-quality and modern equipment for measuring the printing plates are always trying to find the best contrast between printing and non printing surfaces independently of the fields of tonal values that are examined.
A complementary light source which may be in addition to standard RGB is white, UV and IR. This allows for measuring devices to choose the best contrast depending on the type of printing plate.
Addition of UV and IR light makes it possible to see materials such as a blurred latent image on the board or bright tones of yellow colour on prints.
To enable accurate measurement of percentage of dot coverage, especially in problem areas of light and dark tones, it is necessary to mention the type of printing plate of the device and method of halftoning, so the measurement algorithm could compensate for this specific characteristics.