DE69714197T2 - Visible control strip for imaging media - Google Patents

Description

1. FIELD OF INVENTION

The present invention relates to devices and methods for exposing substrates as well as the substrates themselves. In particular, the present invention relates to methods and devices for exposing and developing imageable media including photographic film and printing plates. The present invention also includes devices and methods for printing and the printed substrate. In particular, the present invention relates to a visual control strip suitable for quality control of exposure devices and exposed media.

GENERAL STATE OF THE ART

Various types of control strips are known. As described in US-A-4,852,485, analog control strips are used, which consist of a piece of structured film that could be attached to a lithographic film before it is brought into contact with a printing plate.

CH-A-0 681 929 describes a "test wedge" or control strip, stored as a digital quantity on a storage medium such as a diskette or in a computer. The control strip consists of a multitude of control fields. Each control field contains a pattern, e.g. a star target, fonts, a series of lines which may contain elements such as checkerboard squares, lines or dots.

EP-A-0 518 559 describes a method and apparatus for producing a control strip. A digital representation of the control strip is printed to form a visible analogue representation of the control strip. The control strip can be printed at the same time as a colour main image to be reproduced. The control strip consists of control patches and the elements of the control patches can be defined by the user.

PostScript is a programming language developed by Adobe Systems Inc., California, USA, for defining page, label, color and graphics parameters of images to be output on a raster imaging device such as a printer, imagesetter or platesetter. PostScript is described in the "PostScript Language Reference Manual", second edition, Addison-Wesley, 1990 (hereinafter referred to as the "Adobe Reference"). PostScript files can be included in the main image file as an encapsulated PostScript file (EPS file) as described in Appendix H, pages 709-736 in the Adobe Reference.

The general techniques of colour reproduction, e.g. printing, photographic films, display devices, are described in "The Reproduction of Colour in Photography, Printing and Television" by Dr. W. G. Hunt, Pountain Press, UK, 1987 (hereinafter referred to as the "Kodak Reference"). From this book, Kodak's "Three-Aim Point" process for making printing plates is known, which produces results with uniform tonal reproduction and color balance despite Image Spot Size Deviation Variables (ISSDV) inherent in any image production or reproduction system. One such variable or parameter is the development time for photographic film. Photographic so-called "hard dot" films, such as Kodak Imageset 2000 IHN and Imageset 2000 ILD, are very sensitive to the development time. If this time is short, small spots form. If this time is longer, larger spots or spots with a larger area form. In multiple systems, such as direct thermal systems, no development is required. In direct thermal systems, the size or area of each spot can be influenced by the amount of energy applied to a specific microdot. In such a system, an ISSDV is therefore the amount of thermal energy applied locally.

Standard "templates" are provided in the form of three (grey) subfields with neutral density: Template "A" represents a reproducible minimum density at an average transparency or reflection printing, subfield "B" is a similar subfield with maximum density and subfield "M" is a similar subfield with medium density. When processing the photographic material, the subfields are arranged near the working image and processed with it. From experience, standard values and tolerances have been determined for the densities produced by subfields A, B and M on typical masks and separation negatives.

From DE-A-195 07 665 a control strip for visual inspection is known, which consists of two adjacent elongated fields. The first field has large elements, the size of which is essentially independent of the lighting fluctuations. The target density of the large elements is position-dependent. The second field has fine elements with essentially the same tone value. The effective tone value depends strongly on the lighting conditions. Although it is possible to assess important lighting fluctuations using this method, the arrangement of the various fields makes it quite difficult to accurately assess small fluctuations in process parameters and to obtain a quantitative measure for correcting the effect.

TASKS OF THE INVENTION

An object of the present invention is to provide a method and apparatus for controlling the marking of an imageable medium and/or for controlling the exposure device to produce images with the correct density.

Another object of the present invention is to simplify the calibration or routine control of the image quality of exposure devices and exposed media.

In particular, it is an object of the present invention to provide a means for assessing and controlling the exposure of exposure devices and exposed media.

Yet another object of the present invention is to reduce waste of exposed materials in the setting/proofing phase of the reproduction of color and black and white images.

Yet another object of the present invention is to improve the reliability of calibration or routine control of image quality of exposure devices and exposed media and to make such control less operator sensitive.

A further object of the present invention is to optimize the size of a visual control strip while simultaneously maintaining its functionality as a quality control instrument.

BRIEF DESCRIPTION OF THE INVENTION

The above objects are realized by the specific features of claim 1. Specific features of preferred embodiments are set out in the dependent claims.

The visual control strip according to the present invention is preferably described as a set of PostScript commands, but the invention is not limited thereto. This provides the freedom to integrate the strip into many printing applications, such as computer-to-plate and computer-to-film applications. This strip can even be used in the final stage of making a printing plate or film.

In a preferred embodiment, the visual control strip according to the present invention is arranged on the imageable medium at a location that is functionally irrelevant in terms of image. For example, it can be arranged at a location on a printing plate that remains without printing ink during the subsequent printing process.

The present invention also includes a set of visual control strips, wherein the target values of the ISSDV sensitive Control fields (ISSDV = Image Spot Size Deviation Variables) of the respective strips differ from each other. It is advantageous to use three strips with target values for the sensitive fields of 25%, 50% and 75% respectively.

The present invention also includes an exposed medium and a method for evaluating exposure quality.

Further objects, advantages and embodiments of the present invention will become apparent from the following description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic block diagram of a digital exposure process according to the present invention.

Fig. 2A to 2C show the effect of laser intensity on spot size in a digital imagesetter.

Fig. 2D illustrates a spot or cluster of spots on an exposed medium.

Fig. 3A to 3E show the formation of halftone dots in a typical amplitude-modulated halftone scanning process.

Fig. 4A shows a typical conventional frequency-modulated grayscale with randomized patches.

Fig. 4B shows the sequence of patch filling in the grid cell of a conventional quasi-random rasterization method.

Figures 5A to 5C show visual control strips after exposure on an imageable medium according to the present invention with different dot percentages for the ISSDV sensitive part 39.

Fig. 6 shows a set of visual control strips according to the present invention.

Fig. 7 shows another visual control strip according to the present invention.

Figures 8A to 8C show further visual control strips.

Fig. 9 shows a checkerboard field according to the present invention.

Figures 10A and 10B show pixel line arrays according to the present invention.

Figures 11A and 11B show grids according to the present invention.

Fig. 12 shows a schematic plan view of an offset printing plate according to the present invention.

Fig. 13 shows a comparative control strip in which insensitive fields are placed next to each other and next to a sensitive field.

Fig. 14 shows a control strip in which insensitive fields are not adjacent to each other, but are completely embedded in a sensitive field.

Definitions

"Useful image" according to the present invention refers to the image to be recorded on an imageable medium, excluding any other images that may be used to control the exposure process.

A "microdot" according to the present invention describes the smallest addressable spatial unit, i.e. the smallest addressable part of a substrate or medium that can be addressed by an exposure device in order to cause a density change in that unit. A microdot can have any suitable shape, such as square, round-elliptical, etc.

In this description, reference is often made to "density". In the context of the visual inspection material, "optical density" is meant. This can be the optical density for the transmission or reflection of light. The light can be white light or monochrome or monochromatic light. The "light" can also be UV light or infrared light. However, "density" can also mean "lithographically effective" or "ineffective". This means the absorption or repulsion of printing ink.

A "spot" according to the present invention is the smallest actual image that can be produced by an exposure device on an imageable medium. A spot is the result of the "reproduction" or "marking" of a microdot on the medium. In a photographic system, a spot is marked by exposing a portion of the photographic substrate with a suitable amount of light is illuminated. A spot produced on a photographic substrate is a developed spot. Depending on the nature of the photographic material, a black or white spot, or more generally a spot with a high or low optical density (for reflection or for transmission of light) and/or an ink-receptive or ink-repellent spot is formed. The spot produced on a printing substrate is a printed spot. A spot may have any suitable shape, such as square, round or elliptical. Typically, the centre of a spot has the same location as the centre of a microdot. Each microdot can usually contain one spot. A spot can be larger than its corresponding microdot, that is, it can fully cover it, together with parts of neighbouring microdots. A spot can be smaller than its corresponding microdot, that is, it can cover only part of the microdot. A spot can also partially overlap the corresponding microdot and neighbouring microdots. This is the case, for example, when the microdots are square and the spots are circular, the area of the spot being equal to the area of the microdot and the center of the spot coinciding with the center of the microdot. A microdot may be left "empty," i.e. no spot is placed in the microdot. Whereas the microdot is a fictitious area, a spot has a low or high density with respect to its neighborhood, or it is (results in) a "lithographically effective or ineffective (change)" for a lithographic printing plate, i.e. the spot accepts or repels ink. A spot may also have a specific "UV density" with respect to its neighborhood. In fact, in contact systems using UV light sources, the "UV density" (and not the traditional "visual density") of the spots on the contact film is important. In a positive-working system, a "light spot" is formed when the corresponding location has been irradiated. In a negative working system, a "black spot" is formed when the corresponding spot has been irradiated. Light and dark can be replaced by "lithographically effective" and "lithographically ineffective" respectively.

A "dot" according to the present invention is merely a dot or a cluster of dots on the exposed medium. One particular type of dot is the "halftone dot", which is a dot containing a varying number of spots. As the halftone dots are resized, "halftones" are reproduced, i.e. shades of gray lying between white and black.

A halftone dot is formed by only one or a cluster of spots or by a cluster of missing spots. If the spots are black and the density of the area is low, black spots are grouped into small halftone dots or sparse halftone dots on a "white" background. If there are black spots and a high density area, the black background is formed by black spots. The halftone dots are the small or sparse white areas. In this case, the halftone dots are formed by clusters of "non-spots".

The size of the halftone dots and/or the distance between adjacent halftone dots depends on the grayscale value for the area in which those halftone dots appear. In dot size modulation halftone screening techniques, which use a periodic grid of halftone dots, the size of the halftone dots depends on the grayscale value required. In stochastic halftone screening or frequency modulated halftone screening, which varies the distance between halftone dots rather than their size, the distance between halftone dots depends on the grayscale value required.

Since each microdot can usually contain at most one spot, it is clear that a halftone dot can also be defined as a cluster of microdots.

"Image Spot Size Deviation Variables" (ISSDV) according to the present invention refers to those exposure process variables that affect the size of the spots of the image formed on the exposed medium and cause the spot size to deviate from the desired size. It is understood that the ISSDV also affect spot sizes, since each spot can consist of one or more spots. However, the influence of the ISSDV on spot size can depend on the spot design. and can differ considerably from the impact on a single spot.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with reference to certain specific embodiments, but the present invention is not limited thereto but only by the claims. In particular, the present invention will be conveniently described with reference to the Adobe PostScript programming language and to the drawings, but the invention is not limited thereto.

General methods for exposing suitable media may be either analogue, e.g. contact film, or digital. One type of digital method and apparatus is described in EP-A-0 518 559, but the present invention is not limited thereto. Figure 1 is a schematic block diagram of such a system 1. A computer 2 or similar device is used to produce a digital representation of an image, optionally using a scanner 3 for scanning in an image or a store 4 of pre-recorded images, optionally accessible via a network 5 such as the Internet. Digital representations of images may be produced by graphics software such as Quark XPress , Adobe PhotoShop , Adobe Illustrator , Aldus Pagemaker , Corel Draw or the like. The digital representation is preferably stored as an output file 6 in graphics software and a programming language independent of the output device, such as PostScript. The present invention is not so limited. The output file 6 is optionally transmitted via a LAN or network 7 to a raster exposure device 10 such as an imagesetter. In a raster exposure device, the image is produced line by line, ie in a raster. Either in the raster exposure device 10 or elsewhere, an interpreter 8 is provided for converting (raster image processing) the output file 6 into a raster data file 9 specific to the exposure device, which can be processed by the raster exposure device 10 via scan conversion. It It is understood that the scanning exposure device 10 may further include separate or integrated devices necessary for the development of the imageable medium, e.g. the processor may include developing and fixing sections for the photographic film or printing plate. The output of the scanning exposure device 10 is an exposed medium 11, e.g. a photographic film or a printing plate. The exposure system 1 may be a computer-to-plate system.

An example of a suitable raster imaging device 10 may be a Creo PlateMaster controller connected to a Creo 3244 platesetter with plate conveyor, plate processor and plate stacker, all supplied by Creo Products Inc. Burnaby, B. C., Canada. The interpreter 8 may include a Creo Allegro RIP Station supplied by the same company, which is compatible with PostScript Level 2. Suitable imageable media include N90A printing plates supplied by Agfa-Gevaert AG, Wiesbaden, Germany, or Lithostar LAP-0 printing plates supplied by Agfa-Gevaert, N. V., Mortsel, Belgium. The printing plates may be based on a thin metal foil such as electrochemically grained and anodized aluminum (the most common plate thicknesses are 6 mil, 8 mil and 12 mil, i.e. 0.15 mm, 0.20 mm and 0.30 mm respectively) or may have a polymer base such as polyester. For color printing, a set of color-separated printing plates is usually provided, e.g. cyan, yellow, magenta or cyan, yellow, magenta and black. According to the present invention, the same control strip can be used regardless of the color to be used with the printing plate.

Another suitable raster exposure device 10 can be a printer such as an inkjet printer, a thermal transfer printer or an electrostatic printer. Examples of these are: a Designjet 750C from Hewlett-Packard Corp., USA; a Summachrome exposure system from Summagraphics Inc., USA; or a Chromapress system from Agfa-Gevaert, N.V., Mortsel, Belgium.

A suitable raster exposure device 10 can also be an imager for photographic film, such as the SelectSet Avantra 44 from Bayer Inc., Agfa Division, Wilmington, USA.

Exposure devices 10 can be calibrated at regular intervals according to the relevant manufacturer's instructions. Such complex procedures are not suitable for routine production control of exposure quality. The visual control strip according to the present invention has been designed to provide a fast, direct and reliable routine control of exposure quality that can be carried out by untrained personnel. In the case of printing plates, the control according to the present invention can preferably be carried out before the actual printing using the plate starts and using the minimum amount of space on the exposed substrate.

Continuous tone images cannot be easily represented or reproduced by digital devices. An image is usually "rasterized," i.e., converted into an array of dots of varying size or spatial frequency. These dots are so small that the human eye sees them not as individual dots, but as areas of varying tonal values. A typical conventional digital raster exposure device 10 records an image on the exposure medium 11 according to a Cartesian array of elements of the image. In accordance with the present invention, a microdot or pixel is the smallest addressable spatial unit on the exposure medium 11 of the Cartesian addressing system of the exposure device 10. For a printer, imagesetter or platesetter, this is the basic spatial unit that forms all other graphic structures, such as lines or areas of color. In output devices, microdots are also referred to as device pixels or RELs (recorder elements). If the digital raster exposure device 10 is an imagesetter, then it produces an image on the medium 11 that, when developed, consists of an array of black, white, or colored spots. If the exposure medium 11 forms a lithographic printing plate, then the developed spot on the exposure medium 11 either accepts or rejects ink. When the printing plate is used to print the final image, then that final image consists of printed spots, each printed spot corresponding to a developed spot on the medium 11.

In a negative working imageable medium 11 such as computer to film, the developed spot on the exposed medium 11, representing a microdot, has a high optical density when illuminated with light, i.e. typically black spots to white light, as is well known for so-called "negatives" produced by a conventional camera. When contact films are produced by the use of UV (ultra-violet) light, the density is a density to spectral UV light. In a negative working computer to plate system, the illuminated zones usually become receptive to the printing ink after optional development and fixation.

In positive working material, the optical density of the spot is low when illuminated with light. In a positive working computer-to-plate system, the illuminated areas are usually repellant to printing ink.

A typical dimension for a microdot on a 400 dpi (dots per inch) printer is 63.5 µm and 7 µm on a 3600 dpi imagesetter. This means that at a resolution of 3600 dpi, the imagesetter 10 addresses approximately 20,000 microdots per mm² of the imaging medium. The present invention is not limited to any particular type of imageable substrate. The spots can be produced by any suitable means, e.g., by heat on heat-sensitive substrates, by UV or visible or infrared light on photosensitive substrates, by the application of powders, liquids, inks, pigments or other substances to an appropriate substrate.

For example, both thermal mode and photomode systems are known in the art. In photomode materials, the image-forming reaction is initiated directly by photons with a specific wavelength. In thermal mode materials, the image-forming reaction is initiated by heat. This heat can be applied directly, as in thermal direct printing systems. Alternatively, the heat is applied indirectly via the conversion of photons to heat, e.g. via infrared-absorbing dyes. This can be achieved by exposure units with high-power infrared laser sources, e.g. from 830 to 1064 manometers.

The exposure device is typically capable of forming a spot on each microdot. The actual size of a spot on a microdot on the exposed medium 11 may vary from the specified size of the microdot depending on the settings of the exposure device 10 and the nature of the exposure medium. For example, if the exposure medium 11 is a photographic film, the actual size of the developed spot may depend not only on the exposure or development time, but also on the sensitivity and properties of the exposure medium 11. For convenience, the invention will be described below with reference to an exposure medium 11 on which black spots are produced, but the invention is not limited thereto. The present invention will be particularly described with reference to an exposure medium 11 for use in an exposure device 10.

As shown schematically in Figures 2A to 2C, the light intensity of the laser beam used in the imagesetter 10 varies across its diameter. The intensity of the laser beam decreases toward the outer parts of the laser beam. The light intensity distribution is typically Gaussian across a circular beam. Each photographic film or plate has a minimum light intensity or threshold required to produce a spot or image. In Figures 2A to 2C, the developed spot 14 produced by the laser beam of an imagesetter 10 on an imageable medium 11 is shown in black, but the invention is not so limited and the actual developed spot may be in the complementary color (negative working material) or in the same color (positive working material) as the color of the illuminating light. The stain can also be either lithographically effective or ineffective, e.g. it can accept or repel ink.

If the energy of the laser is varied while keeping the laser beam diameter constant, the actual spot size 14 on the exposed medium after development changes. With less beam energy, the spot is smaller: compare Fig. 2B with Fig. 2A; with more energy the spot becomes larger: compare Fig. 2C with Fig. 2A. The size of the actual spot 14 on the developed medium and hence the greyscale value of any part of the printed image produced therein depends on the exposure parameters, e.g. the sensitivity of the photographic material, the exposure intensity and duration of the laser beam and the development process used.

If the exposure device 10 is a printer, then the size of the printed spot may vary from the desired microdot size and may depend on the type of inks used and the characteristics of the printing substrate. For example, the size of the printed spot may depend on the dot gain. Dot gain is related to the expansion of the ink when a dot is printed on a printing substrate. If this expansion is greater than expected, then the effect of the resulting oversized printed dots is to produce an image with a greater image density than would be expected from the size of the developed dots on the printing substrate. The main factors affecting dot gain are the thickness of the ink layer on the printing substrate, the physical properties of the ink such as its viscosity, and the type of substrate surface, e.g., whether it is glossy or matte.

Summarizing the above, there are a large number of factors that can affect the appearance, particularly the color depth, gray level or density of the final image on the imageable medium 11. According to the present invention, the set of variables that cause the actual spot size produced on the imaged medium 11 to deviate from the specified spot size are referred to as the Image Spot Size Deviation Variables (ISSDV). It is to be understood that the term Image Spot Size Deviation Variables according to the present invention includes the above-mentioned factors that may occur due to changes in the exposure conditions in an imagesetter, changes in the printing substrate or ink in a printer, or the various exposure conditions, photographic substrates and development processes used in photographic reproduction, as well as any other variables that may affect the affect the image density of a final or intermediate image, causing variations in image density or spot size.

In a conventional halftone scanning process, the developed or printed spots 14 are grouped into halftone dots. A halftone dot is provided by the presence or absence of an array or cluster of developed or printed spots contained within a halftone cell. The halftone cells may themselves be part of a larger organizational unit called a "supercell." It is sufficient to describe a simple halftone dot with reference to Figures 3A through 3E. Figure 3A shows a Cartesian array 16 of microdots representing a portion of an image to be recorded on an imageable medium 11. If the exposure device 10 is an imagesetter, it is preprogrammed to illuminate or leave unexposed each of the individual square elements 12 (i.e., to produce spots 14) in the array 16 according to the requirements of the image to be produced. The imagesetter traverses the array 16 line by line or column by column. The traversing direction is known as the fast scan direction and the direction perpendicular to it is known as the cross scan direction. If the imagesetter is a printer, then it is designed to print or leave a printed patch 14 in each of the square elements 12 depending on the image to be printed. The invention will be described below with reference to an imagesetter, but the invention is not so limited. Similar principles apply if the imagesetter 10 is a printer or other digital imagesetter.

Each 8 x 8 matrix of microdots 12 shown in Figs. 3B to 3E is organized in the form of a halftone cell 13. The portion of the original image represented by a given halftone cell 13 has a certain spatially integrated grayscale value. In order to achieve the required grayscale value in the final print, the relevant microdots 12 of the corresponding cell 13 on the imageable medium 11 are illuminated with the laser light so as to produce the correct number of spots 14 to produce the correct grayscale value, e.g. a light tone such as in Fig. 3C. shown, a darker tone such as in Fig. 3D or an almost black one as in Fig. 3E. The "dot percentage" is given by the ratio of:

- the area of the microdots 14 to be illuminated to form spots

- the complete area of grid cell 13.

A medium 11 illuminated at a given dot percentage produces, after development, an image having a certain grayscale value, which may also be represented by a grayscale percentage value between 0% (white) and 100% (black). If the exposed medium 11 is a printing plate, the printing plate prints a grayscale value that is related to the dot percentage, but varies in absolute grayscale value depending on the printing technique used and the printing conditions. Printing may be done by lithography, gravure, flexography and screen printing. The spots 14 in the halftone cells 13 of Figs. 3C to 3E are grouped together to form a halftone dot 15. The apparent change in the size of halftone dots 15 is achieved by the formation of clusters of spots 14 of fixed size, the size of the clusters increasing with increasing grayscale value. The size of the halftone dot 15 is therefore spatially modulated, i.e. dot 15 is "amplitude modulated" (AM). This type of screening is called autotypical screening if adjacent halftone dots 15 are arranged linearly with a halftone angle and the centers of the halftone dots 15 are spaced apart by a fixed distance. Typical AM screening methods are the Agfa Balanced Screening (ABS) technology disclosed in US-A-5,155,599 by Agfa-Gevaert, N. V., Mortsel, Belgium, and HQS Screening and RT Screening licensed by Linotype-Hell AG, Germany, to Adobe Systems Inc., USA.

If ABS is used with a screen angle of 45º, an imaginary line connecting two corresponding points in two adjacent cells is at an angle of 45º to the vertical axis of the cell, which angle is known as the screen angle. If the cell size is 11 x 11 = 121 microdots, then the distance in the output device space between two such imaginary lines is given by 11 = 15.6 microdots. The frequency of such lines expressed in lines per inch is called the screen ruling. The screen ruling depends on the resolution of the output device 10. For an output device 10 with a microdot resolution of 2400 dpi, the screen ruling achieved with such cells is 2400/15.6 = 154 lines per inch. By varying the number of microdots in each screen cell 13, the screen ruling can be changed.

An alternative screening method is the stochastic or frequency modulated (FM) method for representing shades of gray by binary output systems. In this method, it is the number of fixed-size screen dots in a given area that determines the gray value, i.e., the spatial frequency of the screen dots determines the gray value. The distribution of dots is random or quasi-random, as shown in Fig. 4A, and they are not organized into touching clusters, except by chance when the gray value approaches medium gray to black values. The number of fixed-size dots in a given area can determine the gray value, where each dot can contain several patches. An example of a frequency modulated screening method is the CristalRaster technology from Agfa-Gevaert, N.V., Mortsel, Belgium.

A suitable FM halftone method according to the present invention may be quasi-random. For example, as suggested by BE Bayer in the article "An Optimum method for two-level rendition of continuous tone pictures", Proc. IEEE, Int. Communication Conference, Vol. 26, pp. 11-15, 1973, the sequence of filling the arrays of microdots forming halftone cell 13 is regular, but designed to achieve the same effect as a random distribution of spots, i.e. no regularly growing clusters are formed. Such a sequence of filling halftone dot 15 is shown in Fig. 4B, which represents an 8x4 cell 13 with 32 microdots. The numbers refer to the filling sequence of halftone cell 13. If no element of the 8x4 array is black, then halftone cell 13 is completely white. If all 32 elements of grid cell 13 are black, then the result is completely black. The intermediate tones are created by making the relevant number of elements black. There is no growth of dots of regular size with increasing grey tone value. Instead, the clusters of spots remain small and separated from each other and as the grey tone value increases, their number rather than their size increases. In mixed-mode screening techniques, such as those disclosed in EP-A-0 740 457, the dot size may be fixed for low densities and the mean distance between dots may be variable to increase the density or grey tone value, whereas the dot size may be increased to further increase the grey tone value.

FM screening methods are more sensitive to Image Spot Size Deviation Variables (ISSDV) than AM screening methods because a change in laser intensity in the imagesetter 10 has less effect on the nearby clusters of spots in the halftone dots of an AM screening. Where a significant number of spots are grouped together, the overlap created by oversized spots in the center of the cluster does not change the outer contours of the cluster nor the grayscale value. Only the spots at the periphery contribute to the enlargement of the cluster and therefore to the change in image density. If the dimensions of a spot were to change in size by 20%, the area of the spot would change by 44% (1.2 x 1.2 = 1.44). The change in the diameter of a cluster of 20 x 20 spots would be caused only by the peripheral spots. The change in diameter would therefore represent the same quantitative amount as for a single spot. The resulting change in area would be only 2% (20.2 · 20.2 = 408.4 = 1.02 · original area). A 20 · 20 spot cluster is therefore much less sensitive to the Image Spot Size Deviation Variables (ISSDV). The area of a 4 · 4 cluster would change by 10% (4.2 · 4.2 = 17.64 = 1.1 · original area). A 4 · 4 cluster is therefore more sensitive to the Image Spot Size Deviation Variables (ISSDV) than a 20 · 20 cluster.

FM screening preferentially uses only individual patches of fixed size or small clusters of patches, so that a change in the The size of the spots on the exposed medium 11 has a significant effect on the grayscale value of the final image. It is particularly important for FM screening processes to be able to set up the imagesetter 10 correctly and also to be able to monitor the quality of the imagesetter, the performance of the photographic film or the printing plate regularly, precisely and easily. The visual control strip according to the present invention achieves this goal.

A visual control strip 20 according to the present invention is shown schematically in Figures 5A to 5C at various exposure levels when the visual control strip has been formed on an exposed medium 11. The strip 20 may be imaged by the imagesetter 10 onto a photographic printing plate or photographic film. Alternatively, the strip 20 may be a strip recorded on photographic film and imaged onto the medium 11 by contact exposure as is conventional in the manufacture of lithographic plates. The visual control strip 20 comprises a plurality of control fields 30 to 38 which are relatively insensitive to the Image Spot Size Deviation Variables (ISSDV) and a background field 39 which is relatively sensitive to the Image Spot Size Deviation Variables (ISSDV). Preferably, an alphanumeric field 40 is also provided above or below the control fields 30 to 39. The ISSDV insensitive control patches 30-38 are arranged in such a way that visual comparison with the ISSDV sensitive background patch 39 is facilitated. This advantage is achieved by improving the contact between the sensitive and insensitive zones. One embodiment is shown in Fig. 5A, 5B and 5C. There, the insensitive patches 30-38 are completely surrounded by the sensitive patch 39. In Fig. 6, the same configuration is used. In Fig. 7, the insensitive patches 30-38 have the shape of a circular segment, whereas the sensitive patch 39 completely surrounds the insensitive patches. According to Fig. 8A, the circular sensitive patches 39 are surrounded by the insensitive patches and have graduated dot percentage values. Fig. 8B is a comparative example which is very similar to the prior art control strip according to DE-A-195 07 665. Fig. 8C also shows a comparison example in which the sensitive fields 39 are surrounded in pairs by insensitive fields 30-38. The The external dimensions of the digital control strip 20 (defined by the bounding box 41) are typically 12 mm or more, preferably 10 mm or less in width. Each ISSDV-insensitive control patch 30 to 38 has a different grayscale value. The grayscale value of each control patch 30 to 38 is also, within wide limits, substantially independent of, or at least only slightly dependent on, the Image Spot Size Deviation Variables. This independence can be achieved by a correct choice of the elements forming the patches 30 to 38; for example, they can be certain types of coarse checkerboard patterns or patterns produced by an AM screening process with a small screen ruling.

On the other hand, the background 39 is provided by a field whose gray tone value is more sensitive to the Image Spot Size Deviation Variables (ISSDV). For example, the ISSDV sensitive background field 39 can be a field created using a stochastic or frequency modulated screening method and built up by small screen dots, each screen dot being formed by one spot or a few spots. However, the field 39 can also contain a fine checkerboard pattern. According to the present invention, the ISSDV sensitivity of the background 39 is normally set (set value) to at least the ISSDV sensitivity of the screening method used for the useful image to be placed on the medium 11. Thus, if the useful image is to be produced by Agfa CristalRaster technology, the sensitive field 39 would preferably be a 50% screen field of this type. If, on the other hand, the Agfa Balanced Screening method is used, then the preset sensitive patch 39 could be, for example, a 4 x 4 checkerboard patch or a 50% ABS-type screen, the screen ruling of which is equal to the screen ruling used for the useful image.

Those skilled in the art will recognize that the ratio of the sensitivities of the ISSDV-insensitive control fields 30-38 and the ISSDV-sensitive background field 39 is relevant for the correct functioning of the visual control strip 20 according to the present invention. Provided that the sensitive and insensitive fields behave in a uniform manner with respect to the Image Spot Size Deviation Variables, then absolute values of the sensitivity are less relevant. For small deviations in spot size, the change in the area "A" of a spot is proportional to its circumference "P" (see Fig. 2D). The ratio of

- Sensitivities of the ISSDV-insensitive field 35 and

- the ISSDV-sensitive background 39 is thus approximately given by the ratio:

- the total perimeter of the elements or grid points (grouped patches) that form a unit area (e.g. 1 mm2 or within a grid cell or a supercell) of an ISSDV-insensitive control field, such as 34;

and

- the total extent of the elements or grid points (grouped patches) forming a unit area (e.g. again 1 mm² or within a grid cell or supercell of the same size) of the ISSDV sensitive field such as background 39.

For example, if field 34 is designed with a dot percentage of 50% and contains a 4 x 4 spot checkerboard pattern and field 39 has single (1 x 1) spots, then the area of the spots in background 39, when measured in the same area, is four times larger than the area of the 4 x 4 checkerboard pattern. Thus, the ratio of sensitivities is 0.25. According to the present invention, this ratio of sensitivity is preferably less than 0.35, more preferably less than 0.25, and most preferably less than 0.125. Highly sensitive fields can be formed by:

- a dot size modulated periodic halftone screen with a high screen ruling;

- a frequency-modulated halftone screen that uses small halftone dots consisting of one or just a few spots or microdots;

- a checkerboard pattern of small square patterns, each square pattern consisting of 1, 2 x 2, 3 x 3 etc. spots or halftone dots; or

- a line pattern, where the thickness of each line corresponds to one or more microdots.

Low sensitivity fields can be formed by:

- a dot size modulated periodic halftone screen with a small screen ruling;

- a frequency-modulated halftone screen that uses large halftone dots consisting of many spots or microdots;

- a checkerboard pattern with large square patterns, where each square pattern consists of, for example, 16 x 16 spots or grid dots; or

- a line pattern, where the thickness of each line corresponds to many microdots.

In fields with low sensitivity, the elements can be of such a size that the eye still integrates the microdensities into uniform densities.

The ISSDV insensitive control patches 30 to 38 may have regularly spaced dot percentages of gray tone values, e.g., 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, and 70%, associated with the numerical reference values -4 to +4 in the alphanumeric patch 40. Patch 39 may be designed to have a target gray tone value of 50%. Thus, as shown in Fig. 5A, with the imagesetter 1.0 perfectly calibrated to the film 11, the patch 34 labeled "0" under ideal conditions is indistinguishable from the background 39. Figs. 5B and 5C show the situation where the exposure of the media is not properly adjusted. In Fig. 5B, the background 39 has been made so much darker than a tone value of 50% that the field 36 designated "+2" is indistinguishable from the background 39 instead of the field 34 designated "0". In Fig. 5C, the background field 39 is lighter than a gray tone value of 50% so that the field 32 designated "-2" is indistinguishable from the background 39. To increase the accuracy of the control strip 20, more ISSDV-insensitive control fields 30 to 38 with a correspondingly smaller gray tone value difference between adjacent fields can be provided.

In a preferred embodiment, the grayscale values of the ISSDV-insensitive control fields 30 to 38 do not simply represent a linear or regular scale of grayscale, but rather they are linked to the differentiated, well-defined sizes of the actual spots on the exposed medium after development which are responsible for producing the various shades of grey. For example, the numerical values of the alphanumeric field 40 may be directly related to the effect of a specific change in the size of the spot. For example, the field 32 having the numerical value "-2" may have a shade of grey equal to the shade of grey produced on the field 39 if the actual spot size on the exposed medium after development is 2 microns smaller than the spot size required to produce the shade of grey of the field 34 designated by the number "0". Similarly, "+4" corresponding to the ISSDV insensitive patch 38 may indicate that patch 38 has a grayscale value equal to the grayscale value produced on patch 39 when the spot size on the media after development is 4 microns larger than the spot required to produce the grayscale value of patch 34 designated by the number "0". By associating the numeric patch 40 with the effect of the patch size change, adjustments to the imagesetter 10 may be more easily made. In a preferred embodiment, inputs to the imagesetter 10 with the number in patch 40 corresponding to the control patch 30-38 which cannot be distinguished from the background patch 39 may be processed by appropriate logic circuitry in the imagesetter 10 and result in an automated exposure adjustment.

The present invention is not limited to a gray tone target value of 50% for the background field 39, and other values may be chosen. It is particularly advantageous to use a sequence of strips 21 to 23 as shown in Fig. 6, wherein the background field 39 of each strip 21-23 may have a different gray tone target value, e.g. strip 21 at 25%, strip 22 at 50% and strip 30 at 75%. The ISSDV insensitive fields 30 to 38 are determined for each strip 21 to 23 such that each field 34 of the relevant strip 21-23 designated by the number "0" has the respective gray tone target value; ie for strip 21 the respective field 34 designated by the number "0" has a gray tone value of 25%, for strip 22 it has the value 50% and for strip 23 it has the value 75%. Through this set of strips, the user can The exposure of the exposure unit 10 can be set and controlled more easily for non-linear systems, for example when recording on photographic film using PM screening methods. It can also be used to select other exposure criteria. For example, the exposure setting can be selected which leads to a visual match between the field 34 designated with the number "0" and the background field 39 on the strip 21 (target = 25%) instead of on the strip 22 (target = 50%).

According to the present invention, the ISSDV insensitive fields 30-38 and the sensitive field 39 are not limited to a linear array as shown in Figures 5 and 6. The fields may be arranged in any suitable two-dimensional array. For example, the ISSDV insensitive fields 30-38 may be arranged radially on an ISSDV sensitive background 39 as shown in Figure 7. Because of the familiarity of operators with analog clock faces, such an arrangement may be readily used when formed into twelve evenly spaced radial fields around 360°, thereby forming a "gray tone clock". Such a clock may have particularly small dimensions, e.g., the dimension of a small wrist watch such as 15 mm x 15 mm. In such cases, the numeric field 40 can be omitted or eliminated, since the operator can read the "hours" 1 to 12 of the "clock" without the aid of the numbers. The operator does not need to be able to read the (often very small) Arabic numbers of the "hours".

The present invention is not limited to foreground patches 30-38 (Figs. 5A-5C) being the ISSDV insensitive patches. A comparative embodiment is shown in Fig. 8A. The control strip 24 comprises a series of background patches 30-38 having different grayscale values, the pattern of each patch 30-38 being substantially insensitive to the Image Spot Size Deviation Variables (ISSDV). Patches 39 sensitive to the Image Spot Size Deviation Variables are each arranged as a foreground patch in one of the background patches 30-38. Each patch 39 has a target grayscale value, e.g., 25%, 50%, or 75%, or the like, as previously described. Alternatively, the ISSDV sensitive patches 30-38 and insensitive are.

Fields 39 are arranged one above the other as shown in a comparative example of Fig. 8B or alternately with one another as shown in Fig. 8c. The insensitive fields can assume discrete or continuously varying values (as shown in Fig. 8A, 8B and 8C).

The ISSDV sensitive field 39 (Figs. 5 to 7) can be a checkerboard field, a pixel line field or a grid field. The ISSDV insensitive fields 30-38 of the control strip 20 according to the present invention can also each be a checkerboard field, a pixel line field or a grid field, but it is preferred if the grid field is not produced by an FM screening process with small grid points consisting of one or only a few spots.

These three basic types of fields are shown in Figures 9 to 11 in accordance with the present invention. Figure 9 shows a checkerboard, Figures 10A and B show pixel line fields, and Figures 11A and 11B show raster fields. Each of these fields contains one or more microdots. Within the fields, dots, lines or fills consist of an array of microdots. Each dot, line or fill represents an element. Multiple elements form a pattern. The elements may form a pattern with a repeating pattern cell arranged like tiles to fill the area of the relevant control field 30-39. Alternatively, the control field 30-39 may contain a series of lines or larger dots arranged in a predetermined pattern. A larger dot preferably comprises an integer number of microdots or spots.

When the visual control strip 20-24 according to the present invention is a digital control strip, it is preferably scalable. Scalability refers to the ability of the digital control strip to transform, i.e., change size to a different physical size, e.g. smaller or larger in one or two dimensions. According to the present invention, the strip is designed to maintain the integrity of the pattern in the fields. This means that the scaling does not refer to the number of spots in an element of a pattern nor to the relative location of the spots. Thus, if the external dimensions of a field are changed differently in two orthogonal directions, the field is deformed, but the elements in the field are not deformed. The undeformed elements fill the deformed field - if the field has become smaller, the number of elements in the field is reduced. This property of the digital stripe according to the present invention can be achieved by defining the elements of a field in device space and the size of the fields themselves in user space. Device space is the internal coordinate system used by the raster exposure device 10 to scan convert the raster data file 9, and it is usually expressed or "measured" in "pixel" units. User space is the internal coordinate system used to create the output file 6 in the device-independent language such as PostScript, and it is usually expressed in metric units such as 1/72 of an inch (see Adobe Reference, p. 151) or millimeters. A current transform matrix (CTM) can be used to transform output file 6 into raster data file 9 (see Adobe Reference, 4.3.2 Transformations, pages 152-154). This matrix converts the data in output file 6 into data in raster data file 9, taking into account any difference in resolution between the coordinate systems of user space (the device-independent language) and device space (raster exposure device 10). Thus, a distance of X units in user space defined in output file 6 is converted by the CTM into the corresponding number of pixels Y in device space, resulting in the same distance in device space as represented by X units in user space. By using CTM, the distance produced by exposure device 10 is thus independent of the resolution of exposure device 10.

On the other hand, data in output file 6 that is defined in the facility space is not affected by the CTM. Thus, X distance units of the facility space defined in output file 6 result in X distance units in the facility space. The actual size, in metric units, of elements defined in the facility space is facility dependent - the size depends on the number of dpi (microdots per inch) of the imaging unit 10. For example, the setup space distance X printed by a 300 dpi printer would be 10 times larger than by a 3000 dpi printer. It is generally not recommended to specify data in setup space because its appearance is setup dependent and thus could appear distorted relative to user space.

Further explanations of terms such as user space, furnishing space, pattern, pattern cell, tile, fill, CTM, scan conversion can be found in the Adobe reference, which has already been included here by reference.

The microdot spacing and element spacing of a control panel 30-39 is preferably defined in device space, whereas the dimensions of a control panel according to the present invention can be defined in user space. The control panels 30-39 are preferably scalable and their size can be defined by the user. A control panel 30-39 is filled with as many elements as required depending on its size, with the elements being clipped at the boundaries of the control panel. Since the pattern elements are defined in device space, their actual size on the exposed substrate depends on the resolution of the exposure device. On the other hand, the size of the panel itself is set by the user.

A control field 30-39 according to the present invention can be generated in PostScript as follows with reference to Figures 10A and B:

«

/PaintType2

/PatternType1

/TilingType 2

/Bbox[0 0 8 1]

/XStep X_StepL

/YStep 1

/PaintProc {

8 1 true [1 0 0 1 0 0]

{< 80> }

imagemask

» }

matrix

makepattern

/OnePixelLinesVer exch def

This fragment of a program expression defines a pattern "OnePixelLinesVer" which represents 8 x 1 pixels in a matrix of 8 x 8 pixels in device space. The pattern consists of a vertical line 47 (Fig. 10A or B) with a thickness of one microdot or pixel in device space. The pattern is defined in an 8 x 8 matrix so that the line width can be modified to have a thickness of up to 8 pixels in other arrays. The repeat distance X_StepL between two arrays is defined in user space not listed above such that this repeat distance is independent of the output array. The complete array is defined in user space as follows:

0 0 X_fieldY_field 1.0/PixelLinesVer setpattern rectfill

which creates a field of the required size that is filled by the pattern OnePixelLinesVer and clipped at the boundaries of the field. Note also that the field can be scaled in user space without changing or deforming the pattern of the OnePixelLinesVer element defined in device space. As shown in Figures 10A and B, the pixel line 47 can be a black line surrounded by white, or vice versa.

If pixel line arrays are used, their performance may depend on whether the lines are parallel or perpendicular to the fast scan direction in the imagesetter 10. To provide an indication of differences between the fast scan direction and the cross scan direction, it is advantageous to use pixel lines that are orthogonal to each other, i.e. to have a portion of the control array with vertical lines and a portion with horizontal lines.

To create a checkerboard pattern, the above script can be modified to create alternating XxX black and white squares in the 8 x 8 matrix 46 (Fig. 9). In this case, the distance X_StepL is not defined. Instead, the 8 x 8 matrix is specified as the pattern cell and tiled in array 30-39 using the rectfill command. This creates a pattern of black and white, for example 4 x 4 device pixel squares, the size of which is device dependent and cannot be scaled (see Fig. 9). The dimensions of array (30-39) are specified using scalable dimensions in user space.

Fig. 9 shows a checkerboard field according to the present invention. The checkerboard may consist of a repeating element or a repeating pattern cell 46 of X x X black device pixels combined with Y x Y white or transparent pixels. If X = Y, then the result is a theoretical dot percentage of 50% and it should produce a grayscale value of 50%.

Figures 11A and B show schematic representations of a raster field. Each raster cell contains a raster point 48. This results in a regular array of points 48 in lines at the raster angle. The grayscale value is determined by the size of the raster point 48. As shown in Figure 11B, a raster field can be formed by white points on a black background.

The ISSDV sensitive patches 39 can be generated using uniform gray tone patches with stochastic or FM screening techniques such as the Agfa CristalRaster technology from Agfa-Gevaert, N.V., Mortsel, Belgium, or other types of ISSDV sensitive screening techniques such as the Bayer halftone screening technique mentioned above. Alternatively, checkerboard, pixel line or halftone patches characterized by a relatively large perimeter "P" per unit area "A" can be used (see Fig. 2D).

To assist in setting up the imagesetter 10, a correspondence table can be prepared as shown in Table 1 below. The values are for an output device 10 with a resolution of 2,400 dpi and a sensitive control patch 39 with a checkerboard pattern of 4 x 4. The dot gain "x" in Table 1 refers to the increase (+x) or decrease (-x) in the radius of the spot above the specified value for patch "0" of the control strip 20 of Fig. 5A, ie for a theoretical dot percentage of 50%.

A dot percentage of 50% according to Table 1 below is realized by defining a square supercell with two square grid points. The length and width or size of the supercell is 8 microdots. The size of each grid point is 4 microdots. The first microdot is located in the top left corner of the supercell. The second microdot is located in the bottom right corner of the supercell. As such, both microdots touch at a corner point that is exactly in the center of the supercell.

It is assumed that, ideally, each spot fills exactly one microdot. At a resolution of 2400 microdots per inch (2400 dpi), the size s of the ideally square spot is equal to the size of a microdot, i.e. 25.4 mm/2400 = 10.58 µm. The size of an ideally square halftone dot in a checkerboard pattern formed by 4 x 4 ideal spots or halftone dots is 4 x 10.58 µm = 42.33 µm. This size corresponds to a halftone dot with no dot gain or dot loss, or a dot gain of 0 µm, as shown in the middle row of Table 1 below. The area of such a halftone dot is (42.33 µm)2 = 1792.11 µm². Two such square halftone dots are arranged in a supercell consisting of 8 x 8 microdots. The size of the supercell is 8·10.58 um = 84.66 um. The area A of such a supercell is A = (84.66 um)2 = 7,168.44 um. The dot percentage of two such raster points arranged in such a supercell is: 100%·2·1,792.11/A = 50.00%. This percentage also represents a dot gain of 0 um in Table 1 below. If the dot gain is -1 um, which actually means a dot loss of 1 um, this means that each side of the ideal square raster point is shifted by 1 um towards the center of the raster point. This means that the size of a such a non-ideal halftone dot with a dot loss of 1 um is effectively 42.33 um - 2 um = 40.33 um. The area of both square halftone dots in the supercell is then (40.33 um)2 = 1,626.77 um². The dot percentage is then reduced to 100%*2 · 1,626.77/A = 45.39%. This percentage is also shown in Table 1 below for a dot gain of -1 um. The theoretical dot percentages associated with the other dot loss values can be calculated in the same way according to the following equation:

y = 100% 2 (d 0 + 2x) 2 /A

In the above equation,

- y is the theoretical grey tone value as a percentage;

- d0 is the size of the ideal grid point in µm;

- x is the dot gain of each individual spot and thus the shift of each side of the ideal raster point to the center of the raster point for negative values of x; and

- A is the area of the grid cell or supercell that contains the two grid points.

The above equation does not apply to positive values of x or dot growth. However, it is clear to the person skilled in the art that dot growth of a black halftone dot can be judged by the dot loss thus caused by the adjacent white halftone dot to which the equation applies. This explains why the dot percentage found for a dot increase of +1 µm, ie 54.61%, is complementary to the dot percentage found for a dot loss of 1 µm, ie 45.39%. Table 1

Table 1 can be used if the ISSDV sensitive patch has a 4 x 4 checkerboard pattern. The dot percentages of the ISSDV insensitive patches are preferably set to the grayscale values that correspond to the theoretical values of column "y" caused by the -5 to +5 micron dot gain of the base patch. If the visual control strip at "-2" (corresponding to a -2 micron dot gain of the base patch) shows a match between the ISSDV insensitive and sensitive patches, then the image produced by the imagesetter 10 with a dot percentage of 50% has an effective grayscale value of about 41% instead of 50%. On the basis of this converted value of the achieved grayscale value, the necessary modifications can be made to the imagesetter 10. The screen ruling of the sensitive patches is preferably equal to the screen ruling of the useful image data. The grid width of the insensitive fields is usually smaller.

However, Table 1 can also be used to control the degree of over- or under-exposure. If under-exposure is to be used intentionally, the degree of under-exposure can be selected from the table and the exposure unit 10 can be adjusted accordingly.

The visual control strips 20-24 (Figs. 5, 6, 7) according to the present invention can be used in the following way. If the visual control strip is a digital control strip, then the digital representation of the strip 20-24 is integrated into a digital representation of a normal page in the computer 2, for example as an EPS file. This file can be imaged directly onto a printing plate. The control strip is preferably located in an image-wise and functionally irrelevant part of the page layout. For example: if the control strip is used to check the quality of the exposure unit used to expose an offset printing plate without mounting the plate on the printing press and making a proof print, then the control strip can be arranged in an area of the plate which lies outside the zone to be inked. An exposed offset printing plate 90 is shown schematically in Fig. 12 and comprises a substrate 91, e.g. aluminum or polyester, on which after the exposure in a raster exposure device and possibly a subsequent development step, an image has been formed. Printing press locating or registration holes 92 may be provided. Within the boundaries of the locating holes 92, an inkable area 93 is defined. The normal pages or graphic images have been exposed onto the plate 90 within this area 93. There are many arrangements for securing the printing plate 90 to the plate cylinder, for example as described in US-A-4,643,093, to give just one example (plate mounts on web offset printing machines). When mounted on the offset printing machines, the inkable area 93 is exposed to the ink rollers. Outside the inkable area 93 there is a peripheral area 94 which is not inked and serves no image-related purpose. This region has the mechanical function of locating and securing the plate to the printing machine, but has no function with respect to the reproduction of the image itself, ie no image-wise functionality. Preferably, one or more of the control strips 20-24 according to the present invention are disposed in this region 94. It is particularly preferred if the control strip 20-24 of the present invention is disposed in the plate portion of zone 94 which is received in the plate locking or clamping device of the printing plate cylinder.

Since the control strip 20-24 according to the present invention is preferably scalable, it can be mounted in the available space in the ink-free zone 94. Since the field elements are preferably not scalable, they are still suitable for quality control regardless of the size of the fields, provided that they are each larger than a minimum size of preferably 2 mm.

In a preferred embodiment, the type of field, e.g. checkerboard, pixel line, grid field, the type of screening method and the screen angle for both the ISSDV-sensitive and ISSDV-insensitive fields, as well as the target value(s) of the sensitive fields are set according to preset values. However, the operator can also set any of these variables from a menu to tailor the Visual Control Strip 20-24 to the needs of a specific job.

When the visual control strip 20-40 according to the present invention is an analog strip for use in photomechanical screening or contact illumination, the strip can be used in the following manner. The visual control strip 20-24 consists of a piece of film which can be included in the page layout film. Again, this piece of film including the visual control strip 20-24 is preferably arranged on a part of the layout film which lies outside the useful printable and inkable area of the printing plate produced from the layout film.

Furthermore, the visual control strip 20-25 according to the present invention has been described with reference to a plurality of relatively ISSDV insensitive patches and a single relatively ISSDV sensitive patch, but the invention is not so limited. The visual control strip according to the present invention also includes a single relatively ISSDV insensitive patch 39 and a plurality of relatively ISSDV sensitive patches 30-38 (the visual control strip would still appear as in Figures 5-7, but the sensitive patches would be insensitive and vice versa). The relatively sensitive patches 30-38 could each have different grayscale target values, e.g. E.g. 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, and the insensitive field could have a grayscale value of 25% or 50% or 75%.

The simplest form of the visual control strip 20-25 according to the present invention is a single relatively ISSDV insensitive patch adjacent to or surrounded by a single relatively ISSDV sensitive patch, or vice versa. Such a visual control strip can be used to identify when the predetermined grayscale value of the relatively ISSDV insensitive patch is equal to the target grayscale value of the relatively ISSDV sensitive patch.

Fig. 13 shows a comparative example of a control strip in which a single, relatively ISSDV-sensitive field 51 is placed next to several relatively ISSDV insensitive fields 52. The sensitive field 51 represents a 50% gray tone and is screened according to Agfa CristalRaster technology, a type of frequency modulated screening. In reality, the size of a frequency modulated screen dot is 21 µm according to the test described below in connection with Fig. 13. The insensitive fields 52 have a dot percentage of 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70% and 75%. They are screened according to Agfa Balanced Screening (ABS) technology as disclosed in US-A-5,155,599. The screen ruling of the ABS screen is 120 lpi (lines per inch). Because of the shorter outline in terms of the area of each halftone dot in ABS screening at 120 lpi, the fields 52 are less sensitive to overexposure and underexposure.

Fig. 14 shows another test strip and a sensitive field 53 with the same structure as the sensitive field 51 and insensitive fields 54 with the same structure and the same dot percentages as the insensitive fields 52. However, according to Fig. 14, the insensitive fields 54 are completely and separately embedded in the sensitive field 53, whereas in Fig. 13 the insensitive fields 52 are arranged next to each other and also placed next to, i.e. below, the sensitive field 51. A series of exposures were made with both control strips according to Fig. 13 and Fig. 14. Log H of subsequent exposures increased by 0.05 from exposure to exposure. Two types of offset printing plates were exposed:

1. Lithostar LAP-0 0.30 mm, digitally exposed with a Stinger system at 2,400 dpi (dots per inch)

2. Setprint SET-HN-J 0.20 mm, digitally exposed by an Agfa SelectSet Avantra 25 system at 2,400 dpi.

The exposed offset plates were presented to five test persons to identify the control strips with a correct exposure. All five test persons preferred this evaluation on the control strip according to Fig. 14, in which the insensitive fields 54 are completely surrounded by the sensitive field 53. The evaluation of the correct exposure on the strip according to Fig. 14 is more convenient, faster and more precise than the evaluation on the strip according to Fig. 13. The circular Areas 54 generally disappear into the background 53 if there is a correspondence of the circular area 54 with the background 53. In Fig. 13, a boundary line between the fields 51 and 52 is perceived even if the insensitive field 52 corresponds to the sensitive field 51 due to an optical illusion. The evaluation of the correct exposure of a control strip according to Fig. 14 is accordingly more uniform and less subjective than an evaluation of the exposure of a control strip according to Fig. 13.

Claims (12)

1. Visual control strip (20) for reproduction on an imageable medium (11), such as a printing plate (90), a photographic film or a printing substrate, by a reproduction process, comprising: - a plurality of first control fields (30-38), each having a first point percentage which differs from that of another first control field; - at least one second control field (39) arranged immediately adjacent to the first control fields and having at least one uniform second dot percentage value substantially equal to one of the first dot percentage values of the first control fields (30-38); wherein the first control fields (30-38) are less sensitive to Image Spot Size Deviation Variables (ISSDV) of the reproduction process influencing the spot size of the reproduced control strip (20) than the second control fields (39) which are more sensitive to Image Spot Size Deviation Variables (ISSDV); characterized in that the first control fields (30-38) are completely surrounded by one of the second control fields (39). 2. Visual control strip (20) according to claim 1, wherein at least one of the first, less sensitive (30-38) or the second, more sensitive control fields (39) contains a raster, pixel line or checkerboard field. 3. Visual control strip (20) according to one of the preceding claims, wherein the ISSDV sensitivity of the first, less sensitive control field (30-38) is preferably less than a third, particularly preferably less than a quarter and most particularly preferably less than an eighth of the ISSDV sensitivity of the second, more sensitive control field (39). 4. Visual control strip (20) according to one of claims 2 to 3, wherein the dot percentage values of the plurality of first, less sensitive control fields (30-38) lead to a continuously increasing Sequence of spot sizes for the second, more sensitive control field (39), wherein the sequence between the spot sizes preferably has a constant spot size interval. 5. A visual control strip (20) according to any one of the preceding claims, wherein the visual control strip (20) is a digital control strip and each first (30-38) or second control field (39) contains at least one element, each such element being non-scalable and each such control field being scalable independently of the elements. 6. Visual control strip (20) according to claim 5, wherein: - the representation of the digital control strip is stored in a language independent of the output device; - the elements are defined in the output device space; and - the fields are defined in the user space. 7. A plurality of visual control strips (21, 22, 23) according to any one of the preceding claims, wherein: the second, more sensitive control field (39) of at least one visual control strip (21, 22, 23) has a second dot percentage value that is different from the second dot percentage value of any other second control field (39) of any other visual control strip (21, 22, 23). 8. Exposed medium (11) with at least one visual control strip according to one of the preceding claims, which is preferably arranged in an image-wise irrelevant part of the exposed medium (11). 9. Exposed medium (11) according to claim 8, wherein the exposed medium (11) is a printing plate (90) for a printing machine and the image-irrelevant part can be arranged in the locking device of the printing machine. 10. Method for assessing the exposure quality of an exposure system using the visual control strip (20) according to one of claims 1 to 8. 11. The method of claim 10, wherein the exposure system is: - a system for producing exposed printing plates; or - a computer-to-plate system. 12. Device for producing a visual control strip (20) on an imageable medium (11), such as a printing plate, a photographic film or a printing substrate, the device comprising: - a means for generating several first control fields (30-38): - each with a first point percentage value that is different from that of another first control field; - relatively less sensitive to Image Spot Size Deviation Variables (ISSDV); and - means for generating at least a second control field (39): - relatively more sensitive to the Image Spot Size Deviation Variables (ISSDV); - located immediately next to the first ISSDV-insensitive control fields (30-38); and - with a second point percentage value that is substantially equal to one of the first point percentage values; characterized in that it comprises means for generating the first control fields (30-38) which are completely surrounded by the second control field.