Printing plate measurement

The printing plate measurement is a test method for quality assurance in the filmless prepress of the offset printing process . After initial makeshift methods, this test method is now implemented using the electro-optical method of digital image analysis .

A printing plate measuring device from Techkon that works on the principle of digital image analysis

Before using digital image analysis, densitometers - here a device from GretagMachbeth - were also used to measure printing plates.

The print rasters exposed and developed on a printing plate are analyzed from the point of view of the tonal value behavior in the edition print. As a result, characteristic curves are created that represent the complete actual range of the halftone value from 0% to 100%. Purpose of the characteristics is the calibration of the printing plate exposer on with it configured RIP - software , to achieve printing results with any time repeatable quality. This calibration contributes to standardized conditions in prepress and is therefore one of the prerequisites for the successful use of color management in the context of implementing ISO 12647-2 or the offset printing process standard .

Procedural history

As long as transparent master films were used for the photomechanical transfer of the text and image information, i.e. the color separations, to the printing plate, transmitted light densitometers were sufficient to determine the tone values. The elimination of film copies when the printing industry switched to computer to plate in the 1990s suddenly required new measuring methods. In the absence of suitable measuring principles and devices, incident light densitometers were initially used for this. However, the sometimes insufficient light-dark contrast on the differently colored printing plate types was the reason that the densitometers could not deliver sufficiently reliable measurement results.

The only alternative to the densitometric integral measurement of the gray levels on the printing plate was image analysis, more precisely the automatic measurement of the geometric shape of the raster elements. It was not until the end of 1998 that the originally very complex laboratory measurement method of planimetric evaluation of video images was implemented in hand-held measuring devices. This success was tied to image sensors , which only became available with the emerging digital camera technology.

The device class of the first generation was called Dotmeter (Centurfax ccDot) or Plate Reader (ACME) because they could “measure” or “read” the “dot” (raster point) on the printing plate. However, they still had the drawback of only working with a grayscale image sensor, which meant that they were even inferior to a color densitometer, at least in terms of light-dark contrast. The Techkon Digital Microscope DMS 910 solved this problem as early as the beginning of 2000 using a CCD matrix sensor such as can be found in color video cameras .

The second generation of devices from the mid-2010s is characterized by two decisive improvements that lead to a higher measuring accuracy of the method.

• Instead of an incandescent lamp, which contributes less to the formation of contrast in the blue and green spectral range than in the red range, light-emitting diodes are now used to illuminate the sample. The LEDs used emit particularly broadband, which means that a white light that is spectrally distributed as continuously as possible is created by generating white mixed light with as little gaps as possible between the spectral components red, green and blue. This is the only way to reliably depict the light-dark contrasts of all plate colors.
• With further progress in digital camera technology, matrix sensors are becoming available that allow a significantly higher optical resolution of the digital camera module without incurring higher component costs if CCDs would still be used for this. It is the active pixel sensor manufactured in CMOS technology . In addition to the increased readout speed, it is no longer inferior to the CCD in terms of its broad spectral sensitivity and can therefore cope with all contrast problems. In addition, it is less prone to blooming than the CCD - an undesirable overexposure that can occasionally occur due to reflection, especially when measuring on the metallic printing plate surfaces, although the geometry of the measuring optics is gloss-free (45 ° between light source and sample and 0 ° between sample and sensor).

In addition, data is no longer transferred to a connected PC via a serial interface , but via USB . Although this has no influence on the measurement accuracy or the speed of analysis, it does make the automatic device recognition that is generally expected today easier.

With the SpectroPlate, the German manufacturer Techkon is also the only provider of the second generation of devices that uses a color camera module for image analysis. The American manufacturer X-Rite only uses a color camera for real-time display when positioning the device, while the sensor of the grayscale camera may have to be supported by inking-related look-up tables for printing plate makes that appear low-contrast. Regardless of this, such lookup tables are useful if measurements have to be carried out on so-called chemical-free printing plates; Because these plates are not subjected to any chemical development process and therefore also no coloring, the subject to be measured is extremely low in contrast, close to a photographic latent image .

In an exceptional case, incident light densitometers still have their justification for printing plate measurements, which is why even some makes of the following device class of spectral densitometers support this function. This exceptional case consists in measuring on printing plates whose carrier material consists of polyester or even paper and which are therefore used as an inexpensive alternative to aluminum printing plates - of course only in the medium quality segment and in smaller printing press formats. The good densitometric measurability is based on the photographic blackening of the silver halide copying layer with which the printing foils are provided due to the process, because black grids on a white background meet the high contrast requirements of the densitometer. In this case, the printer does not need to purchase an additional printing plate measuring device.

Process description

System components

A system for pressure plate measurement always includes a pressure plate measurement device and PC software for calculating the characteristic curve. The characteristics can only be passed on to the RIP via the software. In practice, the characteristic software, the RIP software and the job management of the plate imaging system (imagesetter) are often installed on one and the same computer; but at least there must be a network. The characteristic curve values ​​are processed in a suitable manner as correction values for the illustration of the individual tonal value levels on the printing plate.

The actual printing plate gauge consists of the following basic physical components:

• a housing that can be securely positioned with just one hand and that is ergonomically easy to operate, for example with a large release button;
• a graphics-capable display attached to the top of the housing, which 1. serves as a positioning aid with the live preview image, 2. as a display for the function menu, the selected functions and settings, 3. as a display for the analyzed image and the measured values;
• a processor module that ensures the device functions in conjunction with the installed graphic display user interface and the image analysis software;
• a gloss-free sample illumination, which generates a high light-dark contrast in coordination with the spectral sensitivity of the image sensor;
• a high-quality, high-resolution microscope optics, which offers an optimal magnification for a specific purpose;
• a video camera image sensor, which 1. delivers the live preview image, 2. when the measurement button is triggered, the instantaneous light-dark information projected by the optics “freezes” and forwards it to the processor for immediate image analysis;
• a memory that accepts the analysis images and saves them together with the calculated geometric values ​​(“measured values”);
• a USB port for data transfer to the PC;
• an accumulator that supplies the device with direct current;
• a storage console that recharges the measuring device's accumulator with the help of an electrical mains connection.

Depending on the manufacturer, there are differences between the components. An American supplier uses two separate image sensors: a color image sensor as a positioning aid and a grayscale image sensor for the measurement. A German manufacturer also enables the device to be used as a digital microscope , whereby the frozen color images are passed on as high-resolution digital photos to another image memory, bypassing the image analysis.

The printing plate measuring device is completed by two virtual components:

• the firmware, which ensures all operating, storage and background functions;
• the image analysis software, which converts the frozen camera image into a gray value matrix.

The PC software that can access the measuring device usually has the following functions:

• bidirectional data exchange with the measuring device - 1. towards the measuring device: installation, activation, update and upgrade of firmware and image analysis functions; 2. from the measuring device: import of the preview images and the measurement data;
• Representation of the measured values ​​as a table or graphically as a characteristic diagram;
• Management of tables and characteristics in relation to individual "job data" (production and material parameters, printing plate type and batch, date, RIP, imagesetter and target printing machine according to compensation requirements);
• Table export to transfer the data to the RIP software in a standard format (for example in Microsoft Excel or unformatted text).

Handling the measuring device

For most plate manufacturers, imaging the printing plate with UV, visible or IR radiation is followed by a chemical development process, which also includes the dark coloring of the later printing plate sections. In this way, a sufficiently high light-dark contrast is created between the printing and non-printing plate parts - in most cases this is the remaining layer portions and the exposed aluminum oxide - which enables measurements. Measurements are made on a suitable image object, usually a tone value wedge whose screen tone values ​​are known from the digital database (see section Application ).

Measuring device for printing plate measurement

The measuring device is placed on the printing plate by means of a visual positioning aid

• (on devices of the first generation) in the form of an easily visible perforated panel,
• With the help of the graphics-capable liquid crystal display on the top of the device (current generation), where the moving image from the integrated video camera module is displayed in real time,
• alternatively via a PC monitor if the measuring device is connected to a PC, where the same preview image is displayed in real time in a software window (first and current generation).

As soon as the measuring device has been placed on the desired measuring point, the measurement can be started. The measurement function is triggered either by pressing a button on the device housing or by clicking the mouse in the user interface of the PC software. Depending on the manufacturer, the measurement process takes between one and three seconds. The measuring device can then be moved to the subsequent measuring point, for example to the adjacent field of a tone value step wedge.

Users expect from modern plate measuring devices that they can also be used independently; This means that all measured values ​​are temporarily stored and only read out from the memory after the device is connected to a PC. As ergonomically pleasant as this way of working may be, because no connection cable to the PC interferes with the measurement, it is both cumbersome and error-prone. The assignment of the measurement series of a plate set (printing plates for the four process printing inks and, if necessary, special colors) to the respective "job data" can be easily performed on the PC monitor with the help of clear software, so that any repetitions of individual measurement series can be avoided. In this respect, a measurement in which the device is online with the PC software is always safer and saves time.

Contrast and threshold definition

The light-dark contrast between the non-printing and printing areas of the plate is not discreet . This means that the light-dark transitions - i.e. the edges of the exposed and developed grid, line and text elements - are more or less gradual. The less gradual, i.e. the steeper the transition from the layer surface to the exposed aluminum oxide surface, the more sharply the respective element is represented on the printing plate. Exactly this reproduction with the sharpest possible edges is aimed for in plate imaging, which the various imaging technologies are not able to achieve in a uniformly high quality.

The edge sharpness is influenced

• on the wavelength of the laser beam and thus its refraction behavior in optical systems, i.e. its focusability ;
• the distance of the laser module from the printing plate surface - depending on the imagesetter design "inner drum", "outer drum" or "flat bed" in the millimeter or decimeter range - whereby the influence of the focusing quality of the laser beam is more or less great;
• in connection with the wavelength of the sensitivity of the plate copy layer, which is spectrally matched to it, and thus of the ability of the layer material to be able to form the steepest possible edge transitions after the imaging, photochemical development and physical conditioning processes;
• on the roughness of the exposed aluminum oxide, the microporous surface of which allows the layer edge to run more or less strongly.

The illumination of the plate surface at an angle of 45 ° causes the formation of shadows on larger unevenness when measuring incident light at an angle of 0 ° - especially in the aluminum oxide, i.e. in the non-printing areas. As a result, the measuring device perceives the dark shadows in the micropores in addition to the printing, darkly colored layer areas. Irrespective of this, dust, scratches and other disruptive factors can cast disturbing shadows.

Therefore, the technical requirement must be placed on the measuring device that it differentiates the intensity of the shadows from the gray value of the colored layer during the image analysis. This error suppression must lead to the fact that only the layer parts are recognized as printing elements, i.e. This means that all disruptive gray values ​​are not mapped in the gray value matrix. Technically, this requirement is solved in that the measuring device automatically sets a contrast limit, which is referred to as a threshold value . This threshold value is the brightness level at which all gray values ​​are discretely distinguished in the analysis image: All lighter gray values ​​are converted into white signals, all darker gray levels into black signals.

Preview and analysis image from a PC software during the printing plate measurement

In the case of low-contrast or critical plate colorations, the software is supported in finding the correct threshold value by the lookup tables (LUTs) mentioned above, in which the brightness level at which a distinction is made between “white” and “black” is specified numerically. Color sensors, which LUTs do not actually need, provide additional security and show their strengths especially when measurements have to be carried out on printing plates that have to be developed without chemicals and with minimal contrast.

Geometric image analysis

After only black (printing) and white (non-printing) pixels are now present in the prepared gray value matrix, the next analysis step can be processed. The coordinates of each individual black and white image point are used to identify the course of edges through to geometric figures - the areas enclosed by the edges - from all black and white transitions:

• positive screen elements between 0 and 50 percent tone value or negative screen elements between 50 and 100 percent tone value and vice versa
• positive screen elements between 50 and 100 percent tone value or negative screen elements between 0 and 50 percent tone value.

If dark objects that are not involved in the printing process - i.e. shadows, spots or scratches - are displayed in black as supposedly printing elements, the image analysis software should be able to recognize these objects as errors and not include them in further analysis to flow in. Such errors can be recognized automatically according to certain mathematical criteria, namely if the respective object

• the homogeneity of a white surface interferes, namely from a defined, in any case very high white percentage value (conversely, this also applies to the homogeneity of black surfaces if white scratches appear in them);
• is well below the average size of neighboring grid elements;
• has a contour that is atypical for both periodic and non-periodic grid element shapes.

The software recognizes the desired geometric grid parameters from the corrected contour and surface structure. In addition, the most important parameter can now also be calculated: the area coverage . It can be equated with the screen tone value previously measured densitometrically . While the luminous flux conditions are analyzed in a densitometer, the process of digital planimetration is used for image analysis .

application

Preview and analysis images on the display of a printing plate measuring device

Grid parameters

The geometric analysis identified

• the screening model, d. H. the modulation method, so
  • an amplitude-modulated grid (AM), also called periodic or autotypical grid, whose elements are regularly shaped and whose center points are distributed at equal intervals and in a preferred direction;
  • a frequency-modulated grid (FM), also called non-periodic or stochastic grid, whose elements are irregularly shaped and whose centers are randomly distributed at unequal intervals and without a preferred direction (the 1st and 2nd generation of the FM grid differ that the influence of the Random number generator was reduced in order to specifically avoid disturbing clusters of pixels and elements);
  • a "cross-modulated" screen (XM), also known as a hybrid screen because it has features of both AM and FM screens;
• the geometric paths, d. H. the distances from edges to edges as well as heights, widths and diameters of grid elements, that is
  • the screen ruling for AM and XM screens,
  • the grid element sizes for FM and XM grids;
• the preferred direction of the grid elements in AM and XM grids, d. H. the screen angle that was generated in the RIP for the relevant color separation.

The degree of area coverage F, i.e. H. the printing area on the measured printing plate is determined planimetrically by multiplying the covered area (black) by 100 percent and set in relation to the sum of the total area - the covered (black, S) plus uncovered (white, W) areas. For this, the black pixels (S) must be counted; the total area (S + W) is known from the start, since it corresponds to the total number of pixels of the matrix sensor.

${\displaystyle F={{S} \over {S+W}}\cdot 100\,\%}$

Characteristic curves of tonal value transfer

PC software for creating, managing and exporting characteristic curves

Individual measured values ​​have no meaning with regard to the aim of the printing plate measurement. Only one characteristic curve can describe the behavior of the plate imaging system over the entire tone value scale from 0 to 100%. There are two types of characteristic curves:

• the "transfer characteristic" - measured actual values ​​(y-axis) over the setpoints (x-axis);
• the "tone value increase characteristic" - target / actual difference values ​​(y-axis) over the target values ​​(x-axis).

The value triples (target value; actual value; target-actual difference value) are transferred to the RIP software and processed there in a suitable manner in order to implement one of the two types of characteristic curve. This brings about a flat-rate linear output behavior (actual values ​​= target value) of the imagesetter by correcting the characteristic tone value transfer behavior of the imagesetter (target values ​​+ differential values).

Ultimately, the goal is to ensure standardized and reproducible conditions in printing plate imaging. This is the prerequisite for the dot gain values ​​that

• as part of the process standard offset printing as “characterization data for standardized printing” for the various offset printing scenarios (“production conditions”)
• or that have proven themselves in operation,

can be used reliably. They complement the color management in that when the color separation files are output, the color reproduction properties of the process chain (in the form of ICC profiles ) can be automatically based on an identical tone value transfer behavior of the platesetter using the characterization data. Due to the fact that nowadays dot gain data is taken into account in the ICC profile, it would be counterproductive to include a correction curve for the typical dot gain of the offset printing process in the RIP software in addition to linearization. Until the introduction of standardized characterization data in color management, superimposing a tone value gain correction curve over the linearization curve was an option that was often practiced.

This process reproducibility can therefore only be guaranteed by regularly performed printing plate measurements. In order to be able to record the series of measurements on the printing plate, suitable control means are required. which, in addition to numerous test elements, also offers a gray level wedge. This wedge consists of screen tone value measurement fields, which are in the target values ​​0 (completely screen free), 1%, 2%, 3%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70% , 80%, 90%, 95%, 97%, 98%, 99% and 100% ("full tone") can be imaged on the plate. Each of these fields is recorded one after the other with the plate measuring device. The result is a characteristic. Software functions that can provide a characteristic curve made up of mean values from several series of measurements provide the necessary statistical security .

Digital microphotography

Recordings made when using a plate measuring device with a color camera sensor as a digital microscope

Since the sensors used are matrix sensors from digital camera technology, it made sense for a German manufacturer of plate measuring devices and spectral densitometers to implement the camera function as an additional benefit of the printing plate measuring device. In connection with the microscope optics, the device delivers digital microscope images. This option can be used in many quality assurance tasks in the print shop:

• Assessment of the resolution of printing plates and imaging systems;
• Checking the register quality on printed screens ;
• Assessment of the homogeneity of paper surfaces as part of the incoming material inspection.

The images can be archived as evidence together with the test protocol data and offer the printer special security of evidence in the event of complaints.

Today's technical solutions

In the main area of ​​application, plate measuring devices are now offered as handheld devices for individual measurements . Usually connected to a PC, the device is placed on the offset printing plate where the screen step wedge has been imaged to record a characteristic curve . Each measuring field is measured individually by positioning the device under visual inspection and triggering the actual measuring process by pressing a button on the device or by clicking the mouse on the PC user interface.

The integration of a plate measuring head in highly automated lines for printing plate imaging has a different purpose : permanent process and quality control . Several test elements (targets) arranged next to one another are imaged on the printing plate. Using a frozen camera image taken from a short distance, contrast, completeness of details, resolution and freedom from distortion are checked. An image analysis as described above is dispensed with because some problems can only be recognized by finer gray value gradations in the video image. A different image analysis is carried out anyway: the actual image is compared with a target image. This type of analysis provides information about the correct execution of all functions in laser imaging and chemical development. Deviations from a reference image show whether laser diodes have failed or the concentration of developer chemicals has dropped. The system then gives an alarm and marks the detected deviation on a monitor.

Implementation of industry standards

Scroll menu in the device display with the list of lookup tables

The Fogra Measuring Bar [FMB] is a sample printing plate that is precisely illustrated with raster step wedges. It is considered a recognized comparison standard for pressure plate measurement technology. The wedges have been prepared in both AM and FM screening. When measuring the wedge fields with a plate measuring device, very specific degrees of area coverage must be displayed. These reference values ​​were determined exactly with a laboratory measuring device at Fogra Forschungsgesellschaft Druck eV in Munich and recorded in an enclosed report. Fogra, for its part, proves the reference status of the laboratory measuring device through comparative measurements carried out at the Federal Institute for Materials Research and Testing in Berlin.

The latest generation of printing plate measuring devices available on the German market can be calibrated internally to the FMB reference values ​​by adding a special look-up table. This calibration is permanently stored in the device; time-consuming recalibrations are not required. When the measuring devices are started up for the first time, all that needs to be done is a white calibration on the supplied white standard; the device is now ready to measure. With this activation of the FMB reference, all measurements can be compared with one another - regardless of the type of measuring device.

Solutions for other printing processes

Printing forms are now also produced without a film using other printing processes. Thus there is also the need to measure on the printing form. But unlike offset printing, in which the printing and non-printing elements lie in one plane, the printing forms used in other printing processes have three-dimensional elements:

• raised printing elements (relief) in flexographic printing ;
• Deep structures (cells) in gravure printing - as well as on screen rollers in flexographic printing, in coating modules and short inking units in waterless offset printing ;
• permeable fabrics (stencils) in screen printing .

Measuring devices that are capable of 3D image analysis are now based on measuring microscopes . In addition to measuring the two-dimensional structures, the relief and cell depths are “sounded out” with the help of focusing . (Measurements in screen printing are not necessary because the structure with the selected screen fabric cannot be influenced.) The measurements can be quite error-prone because subjective settings have to be made. Thus, the dome-shaped profile of gravure cells can only be assessed with great difficulty in terms of the most interesting parameter, the ink emptying behavior. And transparent flexographic printing plates also irritate the measuring device due to the appearance of light diffraction on steep edges and the lack of clues for focusing the image.

Web links

• Manual printing plate measuring device and analysis software

Individual evidence

1. ↑ DMS910 Digital MicroScope Fills Calibration Needs Of Latest Platesetters , accessed January 18, 2016.
2. ^ Forschungsgesellschaft Druck eV : Characterization data for standardized printing , accessed on January 18, 2016.
3. ^ Forschungsgesellschaft Druck eV : Fogra-Digital-Plattenkeil , accessed on January 18, 2016.
4. ↑ Ifra Special Report 2.32: Automated process control of CTP production. A feasibility study (PDF; 598 kB), accessed on January 18, 2016.
5. ↑ NELA PQM +: Fully automatic printing plate quality measurement  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , accessed January 18, 2016.  
6. ^ Forschungsgesellschaft Druck eV : Fogra Measuring Bar [FMB] , accessed on January 18, 2016.