The use of a printing screen is a prepress process within printing technology . The glass engraving screen , which was invented by Georg Meisenbach in Munich in 1881 , made it possible for the first time to screen halftone images with the help of photography .
For technical reasons, only a certain, limited selection of colors can be used in printing machines, which are then only printed in pure form. Most printing processes can therefore not display halftones, apart from the depth-variable or surface-depth-variable gravure printing or the NIP process. The problem is that either a dot of color can or cannot be printed at one point. It is usually not possible to control whether little or a lot of color is printed on a pixel. As a solution, texts, images and graphics - provided that they should not consist exclusively of the pure printing inks - are rasterized to represent different shades of gray or color tones. In order to be able to represent mixtures between the printing inks or different brightnesses, very fine printing dots of the colors are printed next to one another in order to achieve the desired resulting color impression.
During rasterization or rasterization, image data are converted into specified print data. Binary information is calculated from halftone templates: "1 = print / 0 = do not print". The impression of hue values and gray levels is achieved by a suitable arrangement of grid points.
Up until the 1980s, raster effects were created using glass engraved grids or special screen film foils, but since then special imagesetters or laser imagesetters have been used that work with specially developed software. These are called the Raster Image Processor (RIP for short). During their screening process, generated data - for example in PostScript or PDF files and font files - are converted into printable pixel data. Depending on the technical requirements, it is possible or necessary to use different types of screening.
General information on the printing grid
A halftone screen dot is composed on an area of, for example, 16 × 16 imagesetter pixels ( screen cell). The more imagesetter pixels there are on this 16-square, the darker the area appears. However, if there are only a few imagesetter points in the square, the point appears bright from a distance of the viewer.
Grid point, grid cell and grid width
The grid width is the distance between the individual grid cells. From this, the screen frequency is calculated as the reciprocal of the screen width. The number of grid cells per unit distance is in "lines per centimeter" or "lines per inch " ( lines per inch stated lpi).
Usual values in offset printing are 54 to 120 lines / cm (about 135 to 266 lpi) and in newspaper printing 48 to 60 lines / cm. As already written above, it is customary to build up a raster cell from 16 × 16 = 256 points of the imagesetter, i. In other words , during exposure , the individual raster points are composed of several laser spots (points of the exposure unit). With an imagesetter with 2540 dpi or 1000 dpcm this results in 1000/16 = 62.5 ⇒ approx. 60 grid.
Since there are technical limits to the size of the screen dots that can be reproduced in printing , the lowest tonal values in particular cannot be reproduced in the usual screen rulings. Another problem is the unintentional connection (merging) of adjacent halftone dots ( dot closure ), which can lead to a sudden increase in the tonal value density where a uniform increase is actually desired.
Periodic grids are arranged at a certain angle. When combining several color separations, the various screens can generate moiré effects due to unfavorable screen angles .
In four-color printing, in which several screen angles have to be printed on top of each other to display a color image, one tries to control this effect by different angles of the color separations. Common screen angles for four-color offset printing are according to DIN 16 547 on the one hand
- Yellow = 0 °, cyan = 75 °, black = 135 °, magenta = 15 ° or
- Yellow = 0 °, cyan = 15 °, black = 45 °, magenta = 75 °
According to the text and workbook Basics of Print and Digital Media , the screen angle is:
- Yellow = 0 °, cyan = 75 °, magenta = 45 °, black = 15 °
All grids from the 100 AM grid and finer, as well as the FM grids, are referred to as fine grids. This generic term makes it easier for print customers to order a photo-realistic print without having to deal with the advantages and disadvantages of the individual processes. The usual requirements are:
- the grid points are not visible to the viewer with the naked eye,
- PSO conformity,
- no object moiré ,
- no rosette formation,
- smooth technical grid,
- no additional costs.
Basically, a distinction can be made between amplitude-modulated (autotypical raster) and frequency-modulated (stochastic raster) methods. With amplitude-modulated screening, the size is varied to generate different tonal values, with frequency-modulated screening, the number of screen dots per screen cell is varied.
Amplitude modulated grids
Periodic procedure, autotypical : The area is divided into a fixed number of grid cells (e.g. 60 grid: 60 × 60 cells per cm²). The variation of brightness and color impression mainly takes place via the size of a point in this cell (the amplitude ). With the size of the points, their shape also changes, e.g. B. from a positive circle point over the barrel shape to a square ("cross position", tone value 50%), above this the shape is reversed as negative points until a complete coverage ( full tone) is achieved. Thanks to elliptical-chain-shaped instead of circular-square grid structures, the sudden dot closure at 50% is distributed over two tone values that are apart (for example 40% and 60%), so that the tone value jumps and breaks are attenuated at dot closure. Another possibility of damping is the detachment of individual pixels from the raster point in order to place them elsewhere ( dithering ). For this purpose, the area around the grid point must also be included in the calculation (super cell) .
The figure on the right shows two examples with different dot shapes. The halftone image above is divided into squares, the size of which is based on the gray value. The screen angle, which is the direction of the smallest spaces between the squares relative to the reading direction, is 0 ° in the upper part of the image. A screen angle of 45 ° generally delivers more appealing results, see the drawing below.
In order to be able to generate printed images from four or more colors, the individual color separations are arranged at different grid angles to one another (see above). As a result, the grid points work together to form a so-called rosette pattern, which can have a disruptive effect on coarse grids and leads to reinforcement when overlaid with image motifs that are also periodically structured (e.g. in textile fabrics). This shows up in a disturbing interference pattern (moiré). For this reason, AM screens are replaced by frequency-modulated screens for problematic subjects.
Frequency modulated grids
- Non-periodic procedure
The area is divided into points of equal area. The variation of brightness and color impression takes place via the number of points in the area (the frequency ). In order to avoid pattern formation (moirés), the points are arranged stochastically , see sub-image below right. This raster technique also enables a greater fineness of the resolution and thus more precise correction of the tone value increase, whereby the color space in offset printing can be expanded by around seven percent. In addition, the amount of paint can be reduced with the same color effect, which has advantages in the drying process and in color consumption.
- Frequency-modulated 1st order grid
This technology only became established in practice with the introduction of computer-to-plate technology, i.e. the direct imaging of the digital database on the offset printing plate. Previously, due to the fineness of the screen dots (between 10 and 30 micrometers), a reliable transfer from the film to the printing plate was not possible. In addition, the RIP computers were not yet able to cope with the increased computing effort in an economical time frame. 1st order FM grids are characterized by an absolutely chaotic, i.e. 100 percent stochastic (random) distribution of the grid elements. This can lead to unwanted fluctuations (accumulations or failures) that can disturb the homogeneity of the image.
- Frequency-modulated 2nd order grid
This newer generation of FM screens uses more order in the screen structure. Here, the arrangement of the grid elements is no longer done by a random generator, rather the structure is comparable to the distribution of the silver grain in photographic images. Due to their structural proximity to photography, these FM screens are increasingly used to implement photographic halftones in print.
Hybrid screen: These screen processes combine methods from AM and FM screening. They have their origins in the flexographic printing process , whose photopolymer relief printing forms free-standing AM screen dots in the light tonal values (lights) cannot be made as small as desired without breaking away. For this reason, a certain grid point size is not fallen below and the number of grid points in the lights is reduced (thinned out). This method represents a stepless transition from AM screening (mainly in the mid-tones) to FM screening (in highlights and shadows) and has meanwhile also been adapted for offset printing.
Another method consists in placing screen dots of the same size and shape (which is based on the shape of a 50% AM screen dot) in stochastically determined positions and with increasing frequency in all tonal value areas. The brightness gradations then result from the thinning to the complete overlap. In offset printing practice, solutions can be found that move between these two extreme methods, so that the shape of the halftone dot is also changed in a targeted manner.
The error diffusion , a mixture of the latter method and dithering comes in inkjet printers are used. It is not possible for you to create pressure points with different circle diameters; Due to the technology, they are only able to set tiny, but always the same pressure points. Therefore, they do not arrange different sized printing dots in an orderly grid, but distributing equally sized printing dots according to the random principle on the medium to be printed. Note: Some ink jets are able to print ink drops of different sizes and thus print dots with different circle diameters. For example, thanks to piezo technology, Epson printers can output 2–8 picoliter ink droplets through the nozzle. The piezo crystal controls the amount of ink fluid that flows into the pressure chamber. The larger the amount of ink, the larger the ink drop.
Because of the topography of the printing form , the gravure printing process requires some compromises in the screen technology. Basically, a gravure form (consisting of a copper cylinder with a chrome-plated surface) has so-called cells or cells as grid elements. They are filled with thin paint and empty in direct contact with the paper. In order to form a liquid-tight cell, a completely closed wall (webs) must be built up. For this reason, the font is also screened in gravure, which would be a "malpractice" in all other printing processes. The modulation, ie the control of the amount of ink, takes place via the variation of the cell volume. There are three classic methods to choose from:
- area-variable volume change (amplitude-modulated, autotypical)
If the cell depth remains the same, only the cell-to-cell ratio changes, ie with wider cells the cells become narrower. Because of the capillarity in the lights, this method does not make industrial sense.
- depth-variable volume change (amplitude-modulated, autotypical)
If the ridge-to-cell ratio remains the same, only the cell depth changes. This is associated with a lowering of the U-shaped bowl bottom ( dome shape ) when steep side walls are formed, which can change the emptying behavior of the bowls (residual color volume remains).
- Depth of area variable volume change (amplitude-modulated, semi-autotypical)
Both the cell depth and the bridge-to-cell ratio change: in the lights, narrow, flat cells, in the depths, wide, deep cells. Due to the printing form exposure with laser and subsequent etching (think process) or, above all, laser engraving (method by Datwyler and Hell), it is now possible to replace mechanical engraving (stylus engraving), which is limited to amplitude-modulated structures. This means that three new modulation methods are also available:
- Edge-optimized, surface-depth-variable volume change (amplitude-modulated, semi-autotypical)
The edges of pictures, lines and writing, which have been dissolved into cells, are given straightened bars that follow the structure of the motif.
- frequency-modulated surface-depth-variable volume change
Almost any FM screen can be engraved.
- cross-modulated area-depth-variable volume change
stepless transition to cell thinning in the highlights and shadows.
In practice, the last three methods are combined.
Generation of the grid
Grids can be generated with photographic processes (analog) or electronically (digital). Similar processes are distance rasterization as well as rasterization with the help of a contact raster in the repro camera or in the contact device. However, both methods are rarely found today. The electronic raster generation takes place with a so-called raster image processor (RIP). The data is usually transferred to the RIP as a Postscript file.
The Postscript printing screen
State of the art in the 2010s
The most widely used printing screen is the Postscript screen. It is used by almost all printing machines as well as by laser and inkjet printers. In this case, color gradations of individual colors are created by dividing the color into a grid of lines and lines of individual points that are at right angles to one another . The desired brightness of the color is determined by the diameter of the individual, mostly circular or elliptical printing points: the lighter the individual color is to be printed, the smaller the diameter of the printing point concerned. This is based on the photographic grid. With the usual four-color printing , the grids of the individual colors are printed on top of each other at an angular offset of 30 °.
Although this type of screen is called a postscript screen , postscript allows other screening algorithms to be implemented.
The page to be rasterized is broken down into points that correspond to the smallest possible point size of the output device. For example, on a 600 dpi laser printer, such a dot is 1/600 of an inch. For the screening of halftones, rectangular areas of dots are combined into grid cells , the size of which results from the selected grid width. Each cell corresponds to a grid point. The shape of the halftone dots, in the screening ( halftoning ) by spot functions ( Spot Functions ) or threshold-fields ( Threshold Arrays determined). Dot functions are small PostScript procedures (note: PostScript is a page description language ) that rank the dot positions within a grid cell in a brightness ranking. Threshold value fields contain a separate threshold value for each position in the grid cell. If the brightness value of the page is lighter, the pressure point becomes white, if it is darker, it becomes black. Both raster techniques decide for each print point based on the color value of the page at this point whether a print point will be black or white.
This method of calculation leads to an important consequence for the prepress stage : Even at the edge of a raster cell of a light image area, individual print points can be black if the page has a very dark color right here. For images that contain fine information with hard contrast, such as B. scanned line drawings, it can make sense, regardless of the color model of the image, to output it in a very high resolution, up to full print resolution (e.g. 1270 dpi). For comparison: With normal photographic motifs, it is sufficient if the image resolution corresponds to half the screen width, i.e. around 300 ppi with a screen width of 60 lpcm (= 153 lpi).
- Clemens Hartmann, Arne Schneider: grid block. Widths, angles and effects . Hermann Schmidt, Mainz 2008. ISBN 978-3-87439-765-0 .
- Illustration of the differences between the 1st and 2nd order of frequency-modulated screens, model Kodak Staccato (PDF; 514 kB)
- Adobe Systems Incorporated: PostScript Language Reference Third Edition (PDF; 7.4 MB) ( Addison-Wesley Publishing Company ), Chapter 7.4 (PDF; 7.41 MB)