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Title:
METHOD AND SYSTEM FOR EXPOSING AN AREA ACCORDING TO DIGITAL IMAGE DATA
Document Type and Number:
WIPO Patent Application WO/2006/000217
Kind Code:
A2
Abstract:
Disclosed is a method of exposing an area according to digital image data representing an image, the digital image data comprising a plurality of image pixels, each pixel corresponding to a pixel value indicative of whether an exposure of said pixel is required. The method comprises dividing the image into a number of sub-images; generating a sequence of said sub-images; and exposing a sequence of sub-areas of said area corresponding to the sequence of sub-images. The dividing step comprises identifying respective groups of pixels that require an exposure, wherein the pixels of each group are located in a predetermined proximity from each other; and for each identified group of pixels, generating a corresponding sub-image to comprise that group of pixels.

Inventors:
Dybdahl, Niels (Tjørnevænget 25, Skovlunde, DK-2740, DK)
Application Number:
PCT/DK2005/000410
Publication Date:
January 05, 2006
Filing Date:
June 20, 2005
Export Citation:
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Assignee:
Esko-graphics, A/s (Industriparken 35-37, Ballerup, DK-2750, DK)
Dybdahl, Niels (Tjørnevænget 25, Skovlunde, DK-2740, DK)
International Classes:
H04N1/00; G06K15/02
Foreign References:
EP0746800A11996-12-11
US5485285A1996-01-16
US5689294A1997-11-18
EP0905965A21999-03-31
Attorney, Agent or Firm:
Zacco, Denmark A/s (Hans Bekkevolds Allé 7, Hellerup, DK-2900, DK)
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Claims:
CLAIMS:
1. A method of exposing an area according to digital image data representing an image, the digital image data comprising a plurality of image pixels, each pixel corresponding to a pixel value indicative of whether an exposure of said pixel is required, the method comprising dividing the image into a number of subimages; generating a sequence of said subimages; and exposing a sequence of subareas of said area corresponding to the sequence of subimages; characterised in that the dividing comprises identifying respective groups of pixels that require an exposure, wherein the pixels of each group are located in a predetermined proximity from each other; for each identified group of pixels, generating a corresponding sub image to comprise that group of pixels.
2. A method according to claim 1 , wherein the predetermined proximity is determined by an exposure model of an exposure head used to expose the area.
3. A method according to claim 1 or 2, wherein the exposing comprises scanning across the area along a predetermined scan direction as to expose a strip having a predetermined lateral strip width with respect to the scan direction.
4. A method according to claim 3, wherein generating each of the sub images comprises generating the subimage to have a lateral dimension no larger than the lateral strip width.
5. A method according to claim 3 or 4, wherein the pixels are arranged in lines along the scan direction, and wherein identifying comprises processing each line of pixels to identify linear groups of pixels in that line; wherein all pixels within a linear group of pixels have mutual distances smaller than a predetermined distance; and wherein pixels of different linear groups have distances larger than the predetermined distance; and combining linear groups from different lines of pixels to respective groups of pixels.
6. A method according to claim 5, wherein the predetermined distance is determined by exposure model of an exposure head used for exposing the area.
7. A method according to any one of claims 1 through 6, wherein the dividing comprises dividing the image into subimages according to a first division algorithm resulting in a first sequence of subimages; and dividing the image into subimages according to a second division algorithm resulting in a second sequence of subimages; and estimating exposure times for the respective first and second sequences based on an exposure model of an exposure head used for exposing the area; selecting the sequence of subimages corresponding to the smaller exposure time.
8. An exposure system for exposing an area according to digital image data representing an image, the exposure system comprising an exposure head movable relative to an area to be exposed, and a control unit adapted to control movement of the exposure head relative to the area and to provide image data to the exposure head to be imaged; wherein the control unit is configured to divide the image into a number of subimages; generate a sequence of said subimages; and control the exposure head to expose a sequence of subareas of said area corresponding to the sequence of subimages; characterised in that the control unit is adapted to identify respective groups of pixels that require an exposure, wherein the pixels of each group are located in a predetermined proximity from each other; and for each identified group of pixels, to generate a corresponding sub image to comprise that group of pixels.
Description:
Method and system for exposing an area according to digital image data

This invention relates to a method and a system for exposing an area according to digital image data representing an image.

Known exposure systems typically comprise an arrangement adapted for exposing a light sensitive material, e.g. a printing plate, a material adapted for rapid prototyping, a film, etc. The exposing is performed for the purpose of obtaining certain changes of properties of the exposed material, e.g. resulting in the establishment of an image on the illuminated material or a certain structure.

In order to obtain the desired exposure, the light is modulated according to image data, typically in the form of a two-dimensional array of pixels, where each pixel carries information as to whether that pixel is to be exposed. In e.g. grey-scale images, each pixel further carries information about the degree of exposure required for that pixel.

One prior art method of modulating light transmitted to an illumination surface applies a so-called spatial light modulator. Examples of such modulators include a digital micro-mirror device (DMD), a liquid crystal display (LCD), etc. The spatial light modulator is adapted for modulating an incoming light beam into a number of individually modulated light beams.

A problem of the prior art is that the obtainable modulating speed is somewhat limited due to the nature of the applied light modulators combined with the required energy and illumination speed for some applications.

Known types of exposure systems include the so-called flat-bed systems that expose a surface by moving an exposure head with a spatial light modulator across the surface to be exposed. The spatial light modulator modulates the light according to image data that represents a sub-image of the image.

One known type of such devices comprises a light source, a light modulator with light modulating cells arranged in rows and columns. The light modulator is imaged onto a light-sensitive material and a relative movement between the light modulator and the light-sensitive material is produced, perpendicular to the rows. Furthermore, a data pattern is scrolled through the various columns of the light modulator at a rate that ensures that the image of any data pattern is held essentially stationary relative to the light-sensitive material during movement. Typically, a surface is exposed by scanning successive longitudinal strips across the surface.

International application WO 03/087915 discloses a system of this type with a movable exposure head that comprises two spatial light modulators, thereby facilitating a scanning by means of two modulators at one time. Even though this prior art system provides improved exposure efficiency, it involves an additional light modulator and two optical systems, i.e. an increased complexity of the system including additional hardware components.

European patent EP 0 746 800 discloses an exposure and modulation device operating according to a step-and-repeat process. In this system, the original image is broken down electronically into partial images which are displayed successively on a liquid crystal screen and reproduced on the area to be exposed such that the images recombine to form a total image of the original on the area. According to this prior art system, the image content of each partial image is checked by an analyzer circuit or software to determine whether it contains image information, and the partial image is imaged on the area as a function of the results of this check. Hence, only partial images that require exposure are actually exposed, thereby avoiding unnecessary exposure steps.

It is a problem to provide further improved exposure efficiency without the need for an additional exposure head.

The above and other problems are solved by a method of exposing an area according to digital image data representing an image, the digital image data comprising a plurality of image pixels, each pixel corresponding to a pixel value indicative of whether an exposure of said pixel is required, the method comprising - dividing the image into a number of sub-images; - generating a sequence of said sub-images; and - exposing a sequence of sub-areas of said area corresponding to the sequence of sub-images; wherein the dividing comprises - identifying respective groups of pixels that require an exposure, wherein the pixels of each group are located in a predetermined proximity from each other; - for each identified group of pixels, generating a corresponding sub- image to comprise that group of pixels.

In particular, by generating the sub-images as to comprise a group/cluster of pixels to be exposed, it is ensured that each part-image includes image information and the sub-images are generated as to match the image content, thereby providing an improved division into sub-images. In particular, the location of the sub-images within the image is optimised as to reduce the total required exposure time.

It is a further advantage that the method automatically ensures that the sub- images comprise image content. No sub-images are generated for areas above a certain size that do not require exposure. Hence, the combination of the generated sub-images does not necessarily correspond to the original image area, but it covers the image areas that require exposure. It is an advantage that a subsequent check of each partial image to determine whether it contains image information is not necessary, thereby further increasing the efficiency of the method.

Preferably, the predetermined proximity is determined by an exposure model of an exposure head used to expose the area. In some embodiments, the exposure model provides functionality for calculating the required time for exposing a given area for a given set of exposure parameters, e.g. for a given exposure path, a given division of the area in exposure strips, etc. Hence, the proximity is determined by the estimated exposure time, i.e. directly by the parameter to be optimised, thereby avoiding possible sub- optimisations caused by properties of the exposure system.

When the exposing comprises scanning across the area along a predetermined scan direction as to expose a strip having a predetermined lateral strip width with respect to the scan direction, a particularly efficient exposure method is provided.

Preferably, generating each of the sub-images comprises generating the sub- image to have a lateral dimension no larger than the lateral strip width. It is an advantage that the process allows the generation of strips of different widths, thereby providing an improved adaptation of the sub-images, i.e. their positions and sizes, to the actual image content.

In another preferred embodiment, the pixels are arranged in lines along the scan direction, and wherein identifying comprises - processing each line of pixels to identify linear groups of pixels in that line; wherein all pixels within a linear group of pixels have mutual distances smaller than a predetermined distance; and wherein pixels of different linear groups have distances larger than the predetermined distance; and - combining linear groups from different lines of pixels to respective groups of pixels. Consequently, the pixel clustering/grouping is performed in two stages: Firstly, pixels within one line/row along the primary scanning direction are grouped into line segments. Subsequently, the line segments of different lines are combined to sub-images. This two stage process has proven particularly efficient, in particular since it generates sub-images particularly suitable for a scanning/scrolling-based exposure method.

In yet another preferred embodiment, the dividing comprises - dividing the image into sub-images according to a first division algorithm resulting in a first sequence of sub-images; and - dividing the image into sub-images according to a second division algorithm resulting in a second sequence of sub-images; and - estimating exposure times for the respective first and second sequences based on an exposure model of an exposure head used for exposing the area; - selecting the sequence of sub-images corresponding to the smaller exposure time. Consequently, a computationally very efficient method is provided which results in a reduced exposure time without the need for a computationally very expensive exhaustive search for an optimal solution. Preferably, at least one algorithm is selected as to have a worst case behaviour that is at least as good as the un-optimised exposure of the entire area. It is understood that more than two alternative algorithm may be employed.

Further preferred embodiments are disclosed in the dependant claims. The present invention can be implemented in different ways including the method described above and in the following, and a system, each yielding one or more of the benefits and advantages described in connection with the first-mentioned method, and each having one or more preferred embodiments corresponding to the preferred embodiments disclosed in connection with the first-mentioned method.

In particular, the invention further relates to an exposure system for exposing an area according to digital image data representing an image, the exposure system comprising an exposure head movable relative to an area to be exposed, and a control unit adapted to control movement of the exposure head relative to the area and to provide image data to the exposure head to be imaged; wherein the control unit is configured to - divide the image into a number of sub-images; - generate a sequence of said sub-images; and - control the exposure head to expose a sequence of sub-areas of said area corresponding to the sequence of sub-images; wherein the control unit is adapted to - identify respective groups of pixels that require an exposure, wherein the pixels of each group are located in a predetermined proximity from each other; and - for each identified group of pixels, to generate a corresponding sub- image to comprise that group of pixels.

The control unit may be a suitably programmed microprocessor, a suitably programmed computer, a dedicated control circuit, or any other suitable processing means. In particular, the term processing means also comprises general- or special-purpose programmable microprocessors, Digital Signal Processors (DSP), Application Specific Integrated Circuits (ASIC), Programmable Logic Arrays (PLA), Field Programmable Gate Arrays (FPGA), special purpose electronic circuits, etc., or a combination thereof. The above and other aspects of the invention will be apparent and elucidated from the embodiments described in the following with reference to the drawing in which:

fig. 1 schematically shows an example of a flatbed printer;

fig. 2 shows an example of a small portion of a ripped image at different stages of the optimisation process;

fig. 3 shows an example of generated sub-images of a first algorithm for an image including six characters.

fig. 4 shows another exemplary image with exposure strips generated according to the first algorithm;

fig. 5 shows the same exemplary image but with exposure strips generated according to a second algorithm;

figs. 6a-d show the exposure area positioned at four possible starting points for the exposure of a strip;

fig. 7 illustrates an example of a further optimisation step when defining exposure strips;

fig. 8 illustrates an example of an exposure model.

Fig. 1a schematically shows a perspective view of a flatbed printer. The flatbed printer of fig. 1 exposes a printing surface 119 of a printing plate 106 by scanning the printing surface with an exposure head 100 in longitudinal, transversally extending strips. Fig. 1 b shows a schematic top view of the exposure head 100 and the printing surface 119 and illustrates the movement of the exposure head across the surface for an exemplary sequence of strips 121, 122, 123, and 124. The strips extend in the longitudinal direction (y-direction), also referred to as the primary scanning direction. After the exposure of one strip, the exposure head is moved to a starting position of another strip as indicated by the dotted arrows 125. This movement may involve a movement in the longitudinal direction (y-direction) and/or a movement along the transversal direction (x-direction), or a combination thereof. In fig. 1b, a scanning pattern involving fours strips is indicated. Preferably, the system allows a scanning in both the positive and negative y-direction as indicated by the arrows 126. It is understood that the scanning speed along the strips (i.e. along the arrows 126) may be different from the speed at which the exposure head can be moved from an end position to the next start position (i.e. along arrows 125). However, at each start and end of a scan, the exposure head has to be accelerated or slowed down.

The printer has an optical exposure head generally designated 100 that houses an optical system for directing a light beam towards the printing surface along an optical axis 104. The light beam is modulated according to digital data representing the image/pattern to be printed. In preferred embodiments, the light beam is modulated by a spatial light modulator, thereby allowing simultaneous exposure of a certain exposure area. An embodiment of an exposure head and a preferred modulation method that allows exposure of longitudinal strips by means of a scanning/scrolling operation are disclosed in WO 03/087915.

To this end, the exposure head 100 is movably mounted to a portal shaped support structure 101 such that the exposure head can be moved in a longitudinal direction 109 (the direction of the y-axis of the coordinate system 107) across the printing surface. The support structure is further movably mounted on guides or threaded shafts 102 in a transversal direction 118, i.e. the direction along the x-axis of the coordinate system 107. Hence, after a longitudinal scan, the exposure head is moved in the transversal and/or longitudinal direction allowing the exposure head to expose a new strip in the next longitudinal scan. The longitudinal movement of the exposure head along the support structure and the transverse movement of the support structure are caused by suitable motors (not shown) such as step motors, or by another drive mechanism.

It is understood that alternative means for providing a relative motion of the exposure head and the printing surface may be provided. For example, the support plate may be moved rather than the exposure head.

The scanning operation of the exposure head 100 and the modulation of the light beam are controlled by a control unit 114 via respective control signal connections 115, and 116, respectively. The control unit 114 may be a suitably programmed microprocessor, a suitably programmed computer, a dedicated control circuit, or any other suitable processing means.

In one embodiment, the control unit 114 comprises a raster image processing unit (RIP) and an exposure head controller. The RIP receives the original image and generates a sequence of scan strips to be exposed. The RIP sends the generated scan strips including their start positions and the corresponding image data to an exposure head controller which controls the exposure head to expose the corresponding strip of the printing plate.

In the following an embodiment of the process performed by the RIP will be described with reference to figs. 2 and 6.

The process described in the following provides improved exposure efficiency for an exposure system with an exposure head using a light modulator such as a DMD or an LCD for exposing printing plates in strips along the Y-axis. The exposure head can also be moved relative to the printing plate in the X-axis allowing the whole printing plate to be exposed. An example of such a system is a flatbed XY platemaker, e.g. the system described in connection with fig. 1.

When a printing plate is to be exposed, the whole area does not necessarily need exposure. Usually there are large areas that should not be exposed, because they do not include image information. Typically the image content is represented by image pixels arranged in rows and columns, each pixel has an associated value indicating whether or not that pixel is to be exposed (binary pixels) or even to what extend the pixel is to be exposed (e.g. for grey-scale images). The exposure can be optimized as described herein by selecting the right strips to be exposed and the right sequence of these strips.

The following describes the steps of an embodiment of an optimization method:

Step 1 : The optimization process receives a model of the exposure unit/process. The model of the exposing unit provides the following information about the exposing unit: - The maximum height of one exposure strip (i.e. its lateral dimension) - The time needed for exposing an area specified by its height and width - The time needed for moving from one position to a second position defined by the respective absolute positions. - The movement speed during exposure of a strip.

This model of the exposure unit takes into account the time needed for accelerating the exposure head up to the nominal speed and the fact that it is possible to simultaneously move both the exposure head in the Y-direction and the printing plate in the X-direction. It is understood that different exposure systems may correspond to different exposure models including additional and/or alternative parameters. For example, if an exposure head allows an efficient exposure of adjacent strips, this feature is preferably covered by the exposure model.

Step 2: A Raster Image Processor (RIP) generates the image to be exposed in the form of a matrix of binary values.

Fig. 2a shows an example of a small portion of a ripped image. The image portion 200 is shown enlarged in order to make the individual pixels visible. Depending on the resolution of the image, in a typical image a single pixel may have size of a few micrometers, e.g. 10μm. The image 200 includes several dark areas which require exposure, while the white areas do not require exposure. For example, the dark area 201 comprises two pixels while area 202 comprises a larger number of pixels that require exposure.

Step 3: The optimisation process reads this matrix and, optionally, down- samples it, e.g. by a factor of 8 or 64, in order to reduce the amount of data. The threshold is set so that the process can determine whether there is any data to be exposed within 8x8 or 64x64 pixels.

Fig. 2b shows the image 200 of fig. 2a after the down-sampling. Area 201 has been reduced to one pixel of the down-sampled image, while area 202 comprises four pixels.

Step 4: As the exposure head can expose long sequences of data in the Y- direction, the down-sampled data is joined to lines of data along the Y-axis: Pixels to be exposed that are immediately next to each other are joined. Pixels to be exposed, whose mutual distance is so short that it will be most efficient to expose both pixels in one exposure strip, are joined as well. How long this distance is allowed to be, is determined using the model of the exposure unit described in Step 1 :

For some exposure systems, the speed at which the exposure head can be moved in the scanning direction during exposure is considerably smaller than the speed at which the exposure head can be moved without exposure. However, the exposure head also needs time to change between the two speeds and to initiate a new exposure. Hence, in some embodiments, the maximum distance between two pixels that are still joined depends on the above parameters. For some exposure systems, this distance is relatively large due to the time needed to accelerate and slow down the exposure head.

Alternatively or additionally, the maximum distance is determined by other parameters of the exposure model: When two pixels are located on the same x-coordinate but with a long distance in y-direction between them, it may be more advantageous to expose them in different exposure strips that are displaced in x-direction rather than as part of the same (longer) exposure strip. Hence, the increase in exposure time when not joining two pixels corresponds to the time needed to move the exposure head in x-direction, i.e. the movement time for a distance equal to the width of an exposure strip.

For example, for an exemplary model the predetermined maximum distance may be several centimetres, e.g. 106 mm, i.e. all pixels on the same X- coordinate with distances below 106 mm will be joined.

The joined lines are from that point on treated as areas containing image data to be exposed. Fig. 2c shows an example of pixels 201 , 204, and 205 in one line that are joined to a linear group indicated by the box 203.

Step 5: The exposure head can expose a much wider area than the lines found above. For example, if the down-sampled pixels correspond to originally 8x8 pixels of 10 μm each, each of the lines of the down-sampled image is 0.08 mm wide. However, a typical exposure head may be able to expose a strip with a strip width of several mm, e.g. 11 mm, in one exposure strip. So the image area lines are preferably joined to larger image areas. This is not a trivial task to do. Actually it is computationally very intensive to compute the most efficient joining of the image line areas. In order to reduce the computational cost of the method, two fast algorithms are used, none of which necessarily ensures that the best possible joining is found. The algorithm achieving the lowest total time is used.

The first algorithm can achieve advanced results by creating overlapping image areas. However its worst case behaviour is not good. It might generate a set of image areas that would take longer to expose than if image areas were created over the whole plate area in a top-bottom manner.

The second algorithm is simpler, but defaults to the same result as if exposure strips were created over the whole plate area in a top-bottom manner.

Both algorithms join image areas to areas that can fit into one exposure strip, whose maximum width is determined by the exposure head. But the two algorithms have different limitations on how the exposure strips can be placed.

Step 6: The first algorithm can join any image areas that have overlapping Y- coordinates as long as they can fit into the same exposure strip. The resulting image areas might overlap.

Fig. 3 shows an image including six characters A, B, C, D, E, and F that are joined into two different exposure strips 301 and 302 that are overlapping. The first algorithm runs in two passes: First it tries to join all image areas that overlap more than a predetermined constant, e.g. 50% of the length of the longer line segment, in the Y-direction in order to primarily join image areas that have similar extents. For example, starting with a line segment ranging from ximin to ximax, any line segments that overlap more than 50 % in the y- direction and ranging from x2min to x2max and for which x2max-x1 min<Δxmaχ and x1 maχ-x2min<ΔXmax are grouped together. Here, Δxmax is the maximum width of an exposure strip. The process then continues with the next line segment. This process is repeated until no more areas can be joined.

In the second pass, the process joins image areas that overlap with any amount in the Y-direction and that fit into the same exposure strip. Hence, line segments that have not been joined in the first pass, because they do not overlap with another line segment with at least 50%, are joined in a second pass if they fit into an exposure strip.

Fig. 4 shows an exemplary image with exposure strips 31 , 32, 43, and 44. It is interesting to note that the exposure strip 44 is considerably shorter and narrower than the exposure strip 43 immediately below.

Step 7: The second algorithm sorts all the image areas according to their X- coordinates and places the first exposure strip at the image area with the lowest coordinate. All other image areas that are within the X-coordinates of that exposure strip are joined into that exposure strip or other exposure strips that are placed at exactly the same X-coordinates. This prevents any exposure strips from overlapping with other exposure strips which again ensures a worst case result that is at least as good as if exposure strips were created over the whole plate area in a top-bottom manner.

Fig. 5 shows an example of exposure strips 31 , 32, and 33 generated by this second algorithm. It is interesting to notice the difference to the previous algorithm: The first exposure strip designated 31 has the maximum height and covers the whole width.

Both algorithms are used because it is infeasible to predict which delivers the lowest total exposure time.

Step 8: The resulting exposure strips from both algorithms are examined, to determine if some of the exposure strips can be joined to lower the total exposure time, e.g. by comparing the respective exposure time determined according to the exposure model discussed in connection with step 1.

Step 9: When the exposure is started, the exposure head is positioned at the top of the printing plate (x=0). When the exposure is completed, the printing plate is moved out again, so that the exposure head is at the top of the printing plate again. This is represented by adding empty image areas representing the starting and ending point of the exposure head to the start and end of the list of exposure areas.

Step 10: The exposure sequence of the exposure strips is determined. This is done in two passes. The first sorts the exposure strips according to their position, so that exposure strips immediately above each other (i.e. adjacent in x-direction) are placed next to each other.

The second pass uses a "hill-climbing" algorithm, where one exposure strip is moved to a different position and then the total exposure and movement time is calculated according to the above exposure model. If the time is lower than the previous time, then the exposure strip is left at its new position.

The calculation of the total exposure and movement time is somewhat complicated as the exposure strips do not have a simple starting and ending point. The exposure head moves equally well from the left side or the right side, and the height of an exposure strip is often lower than the maximum height determined by the exposure head, so there is some freedom to select the vertical starting position for the exposure head, as is illustrated in fig. 6.

In fig. 6, the area covered by the exposure head is designated 600. Figs. 6a- d show the exposure area positioned at four possible starting points for the exposure of a strip 601. In the example of fig. 6, the maximum height that can be exposed in one strip is designated 602. This height is larger than the height 603 of the strip 601.

To make calculations simpler, the process assumes that there are four possible starting points for an exposure strip. In fig. 6, these staring points are designated P1 , P2, P3, and P4. It is understood that positions between e.g. starting points P1 and P2 are possible as well. However, in this embodiment of the invention, these possibilities are not considered in order to simplify the process.

To determine which starting point to use, the previous and the following exposure strip of the sequence are taken into account. The ending point of the previous exposure strip has already been determined by determining its starting point. The four potential starting points for the current exposure strip and the four potential starting points of the following exposure strips are taken into account to determine which starting point to use. The combination of starting points for the current and the following exposure strips that results in the lowest movement time from the previous exposure strip to the current and from the current to the following exposure strip is selected, even though the starting point of the following strip might be changed when the successor strip of the following strip is taken into account. Step 11 : The total time for movement and exposure is calculated for the final sequences of exposure strips from the two algorithms of steps 6 and 7, respectively. The sequence with the lowest total time is used as best sequence.

Step 12: As shown above, two exposure strips might overlap and cover the same area. To prevent an area from being exposed twice, the image information from the overlapping area is removed from the first exposure strip and only included in the second.

Step 13: Each exposure strip is stored as a separate sub image inside one common TIFF file, so that the TIFF file contains multiple sub images. Standard TIFF tags are used for the positions and a special tag is used for direction information. The TIFF file is stored in a spooling system, so that the exposure optimization can be done in parallel with exposure of already optimized images. It is understood that other file formats may be used as well.

Step 14: The TIFF file is sent to the exposure system and the exposure system exposes each sub image as an exposure strip.

It is noted that the above process may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer.

In the above, an example of a method for defining exposure strips and for sorting the defined exposure strips has been described. It is understood that, alternatively or additionally to the above steps, other optimisation steps may be performed. Fig. 7 illustrates an example of a further optimisation step when defining exposure strips. Fig. 7a illustrates two overlapping areas 701 and 702, each of which may comprise a plurality of lines. In the example of fig. 7, it is assumed that the two areas 701 and 702 cannot be joined into a single large area, even though the entire length (in y-direction) of area 702 falls within the length of strip 701 , because the combined area would have a width larger than the maximum width of an exposure strip.

Hence, in a preferred embodiment of the process, the process determines if two areas overlap (the overlap area is designated 703 in fig. 7a) and if the overlap extends in y-direction over the entire length of one of the areas (area 702 in the example of fig. 702). If the above conditions are fulfilled, the process removes the overlap area from that area (i.e. from area 702 in fig. 7), resulting in a reduced area with a smaller width. The result of this step is shown in fig. 7b, where the reduced area is designated 702'. Consequently, in subsequent steps, additional lines may be added to the now reduced area 702', thereby providing a more efficient definition of exposure strips.

Fig. 8 illustrates an example of an exposure model. Fig. 8 illustrates the exposure model by means of an example of a movement pattern of an exposure head 100 across a printing surface 119, where strips 821 , 822, and 823 are exposed and wherein the exposure head 100 is moved between exposure strips along arrows 833 and 834.

This example of an implementation of the exposing unit model is based on an assumption that there are linear relations between the input parameters and the output parameters. In particular:

The maximum height (xmaχ) of one exposure strip is set to a fixed value, as illustrated by exposure strip 821 in fig. 8. The time needed for exposing an area is calculated based on a constant exposure speed SE. Furthermore, a constant initial start-up time TEs is added to take into account a delay/penalty for each starting-up of an exposure step.

If the height (the extent of the area in x-direction) of the area to be exposed is larger than the maximum height xmax, then the needed number of exposure strips is calculated, and the required exposure time is determined as a sum of the individual exposing times for the individual strips. A single exposure startup time is added in this case. However, at the end of each exposure strip, the exposure head has to change its direction and a turn time constant Tt is added for each needed turn, as illustrated by the turn 833 between strips 821 and 822 in fig. 8.

The time needed for moving the exposure head) along arrow 834 from one position 831 to a second point 832 (e.g. between two exposures, i.e. from the end of one strip 822 to the start of a subsequent strip 823) is calculated by assuming constant speeds SMY and SMX for each of the two motors responsible for the y- and x-axis movements, respectively, and by adding a constant movement startup time TMs- Here it is assumed that the motors can work in parallel and the time needed is determined by the axis where the exposure unit has to be moved over the longest distance.

For example, in a particular example, the values used for the above parameters were: - Maximum height (xmaχ) was set to 11 mm. - The exposure speed (SE) was set to 100 mm/s. - The exposure startup time (TEs) was set to 1 s. - The turn time (Tt) was set to 2.2 s. - The movement speed (SMX=SMY) was set to 200 mm/s. - The movement startup time (TMs) was set to 1 s.

It is understood however, that the above values are merely examples, and that they depend on the specific exposure system used. It is further understood that other examples of exposure models may include additional and/or alternative parameters. Consequently, more advanced models may be generated to increase the accuracy of the exposure model, e.g. by including non-linear dependencies of the model results from the input parameters and/or the like.

For example, some exposure may be able to take advantage of scanning strips of equal width as it would not have to set up parameters for each strip. The electronics of other exposures may be able to set up the parameters for each strip without performance loss, so there is no need to generate areas consisting of multiple strips all with the same width. Such differences may, for example, be controlled by an additional penalty parameter (e.g. an additional delay time for exposing strips of different widths).

In some examples, some or all of the parameters may depend on other parameters. For example, the exposure speed may depend upon the printing resolution and/or the plate type of the printing plate. The latter takes account of the fact that different plate types typically need different amount of light and the exposure speed is typically adjusted accordingly. As another example, the startup times may in some embodiments be calculated from actual acceleration tables used for the stepper motors and from the needed printing speed. Similarly, the exposure startup time may depend upon the exposure speed, so in some embodiments the exposure startup time may be calculated depending upon the plate type.

It is also possible to feed back calibration data from the actual exposing unit and use them for calculating the model of exposing unit. Factors like the brightness of the light source used in the exposure unit and dust have influence upon the exposure speed, and the actual data could be used for the optimization calculations. Hence, for a given specified exposure area (or areas) and exposure path, the exposure model provides an estimate of the exposure time for the selected path.