WO2006032272A1

WO2006032272A1 - Method of evaluating the exposure level of a light sensitive material

Info

Publication number: WO2006032272A1
Application number: PCT/DK2005/000581
Authority: WO
Inventors: Bent Clausen; Torben Verner Jensen
Original assignee: Esko-Graphics A/S
Priority date: 2004-09-22
Filing date: 2005-09-14
Publication date: 2006-03-30

Abstract

A method of evaluatíng the exposure level of a light sensitive material based on a plurality of predetermined continuous-tone grey values. The method comprises providing a digital representation of each of said plurality of predetermíned continuous-tone grey values, and imaging each of the digital representations on respective areas of the light sensitive material resulting in a plurality of exposed areas, each area being exposed according to an image of one of the digital representations.

Description

Method of evaluating the exposure level of a light sensitive material

The present invention relates to a method of evaluating the exposure level of a light sensitive material.

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. It is typically desirable to determine, evaluate, or control a desired exposure level for a given light sensitive material.

It is known to evaluate the exposure level of offset print plates by means of a so-called UGRA plate control wedge. The UGRA plate control wedge is an elongated strip of film made from two different films, a high resolution line film and a continuous-tone film. The continuous-tone film is stripped into a window of the line film with an adhesive tape, and it comprises 13 continuous-tone steps. These plate control wedges are widely used in order to determine correct exposure levels for print plates, e.g. correct exposure times. To this end, the film strip with the continuous-tone plate control wedge has to be positioned on a print plate and exposed. From the exposed image of the continuous-tone steps, the exposure of the print plate can be verified.

However, it is a problem that the conventional plate control wedges are expensive and their use is time consuming, as they have to be physically placed on top of the print plate to be exposed for each test exposure. Attempts have been made to provide automated devices for the automatic placement of the film strips on the print plates. Even though such automated placement devices reduce the manual work involved in the exposure control, these devices are mechanically and electronically complex and the placement process remains time-consuming and error prone.

The above and other problems are solved by a method of evaluating the exposure level of a light sensitive material based on a plurality of predetermined continuous-tone grey values; the method comprising

- providing a digital representation of each of said plurality of predetermined continuous-tone grey values;

- imaging each of the digital representations on respective areas of the light sensitive material resulting in a plurality of exposed areas, each area being exposed according to an image of one of the digital representations.

Consequently, a low cost, easy-to-use exposure control is provided without the need for physically placing a film strip on the print plate. It has furthermore turned out that a surprisingly accurate exposure level control can be achieved by a digital representation of different predetermined continuous-tone grey values. According to embodiments of the invention, a number of predetermined areas of a print plate are exposed according to respective digital representations of a plurality of predetermined continuous- tone grey values. Each area represents a certain density, preferably corresponding to one of the density values of the continuous-tone wedge of a conventional UGRA plate control wedge. In particular, each area may be composed of a binary raster pattern, a random bit pattern such as a frequency modulated digital representation e.g. generated by an error diffusion process, or the like, such that each area has a bit density corresponding to the desired density for that area.

According to a preferred embodiment of the invention, during exposure of the light sensitive material according to the digital representation of a continuous- tone value, the image of the digital representation is blurred as to effectively provide a substantially uniform, analogue exposure level for each area rather than an exposure level that varies from pixel location to pixel location between two or more levels. Consequently, a more reliable exposure level control is provided.

In known exposure systems, 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 binary pixels/dots, where each pixel carries information as to whether that pixel is to be exposed or not. One method of reproducing multi-level digital images with binary pixel/dot patterns of varying size is referred to as printing with amplitude modulated dots whose sizes vary with the required density in the corresponding region of the original image. Another method that is convenient for digital representations with fixed pixel/dot size involves frequency modulation wherein the spatial density of identical pixels/dots is varied to reproduce a corresponding grey/colour density. Different methods for generating such digital representations in general and frequency modulated pixel patterns in particular are known as such in the art. One method for generating frequency modulated pixel patterns is referred to as error diffusion or error propagation, e.g. as described in EP 0 201 674.

Preferably, the pixel resolution of the pixel pattern for the test areas is chosen as high as possible for the given exposure system, preferably corresponding to 2400 dpi or higher, thereby providing a particularly uniform effective grey value of the blurred image.

When the blurring of the images of the digital representations results in respective blurred images that have a resolution lower than the pixel resolution of the pixel pattern of the corresponding digital representation, a particular uniform exposure of each of the areas is achieved that corresponds to an effective continuous-tone grey value. Hence, when the resolution of the blurred image is low enough to not resolve the individual pixels/dots of the digital representation, the area is exposed with an effective average density corresponding to the desired continuous-tone grey value.

When the respective areas are spaced apart from each other by at least a predetermined distance, and when the blurring of the images of the respective digital representations results in respective blurred images with a resolution high enough to resolve said predetermined distance, the blurring of one area does not influence the exposure - and thus the resulting grey value - of the margins of the neighbouring areas, thereby providing a particularly accurate exposure control.

In one embodiment, blurring the images of the respective digital representations causes the areas to be exposed by respective relative light energy levels; wherein the relative light energy level of each area has a spatial variation over said area around a mean light energy level; and wherein the blurring is selected to be strong enough to cause the variation of the light energy level of each area to be smaller than a predetermined fraction of the difference between the average light energy level of said area and the average light energy levels of each of the other areas. Consequently, when the blurring is sufficiently strong to cause the spatial variation of the effective light energy level within each area to be sufficiently smaller than the differences between the average light energy levels exposing the neighbouring areas, a reliable determination of the exposure level based on the test areas is possible.

In one embodiment, the blurring is provided optically, in particular by causing the exposure system to image the pixel pattern onto the light-sensitive material slightly out of focus, thereby providing a particularly simple and effective blurring mechanism that is easy to control and results in a well- controlled and uniform blurring. There are a number of different exposure systems known in the art. In so- called computer-to-plate (CTP) systems a digital representation of the image data is imaged directly on the print plate. One type of CTP systems is referred to as internal drum systems where the print plate is positioned on the inner surface of a drum. In another type of CTP systems, the so-called flat¬ bed systems, the print plate is positioned on a flat surface and an exposure head is positioned in a predetermined distance from the surface and scans across the print plate. In general, all such CTP systems include an optical system for focussing a light beam that carries the image information on the print plate in order to ensure a sharp reproduction of the exposed image. Consequently, by providing an optical blurring by means of a selective defocusing during the imaging of the test areas, existing adjustment means for controlling the focal length may be reused, thereby avoiding the need for expensive constructional changes.

One prior art method of modulating light transmitted to an illumination surface according to image data 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.

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

In one embodiment of a flat bed exposure system the exposure head comprises a light source, a light modulator and optics, and wherein the exposure head is suspended above a base plate and is moved across a print plate that is placed on the base plate. In particular, the exposure head floats on a fluid film that is generated between the exposure head and the print plate. By controlling the flow/pressure of the fluid, the elevation of the exposure head can be controlled. For a certain elevation of the exposure head, the light-sensitive surface of the print plate is positioned in the focal plane of the optical system, thereby causing trie pixel pattern generated by the spatial light modulator to be imaged onto the print plate.

Hence, according to an embodiment of the invention, By slightly changing, e.g. increasing, the height of the exposure head above the print plate (e.g. by 2-3 mm), the projected image is effectively blurred. Hence, in this embodiment, a blurring is achieved by increasing the elevation of the print head over the print plate during exposure. It is understood that the defocusing may be provided by other means, e.g. by adjusting one or more optical elements of the optical system.

In an internal drum exposure system, the optical defocusing can be achieved by selectively defocusing the scan beam when exposing the test images of the digital representation of the continuous-tone grey values.

It is understood that the defocusing may be controlled automatically or by means of a manual adjustment of the optical system. A particular convenient system is provided when the defocusing is performed automatically, e.g. under the control of a control unit controlling the exposure system.

It is further understood that an optical blurring may be performed by other suitable means, e.g. by changing the beam path of the imaging beam, e.g. by means of one or more prisms, mirrors, beam splitters, or other optical elements. Alternatively the light beam be filtered by a suitable filter, e.g. a blurring or diffusing filter

In alternative embodiments, the blurring is achieved electronically. As described above, in a system using a spatial light modulator, 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 of the exposure head. When the scrolling rate is changed, or the scrolling direction is reversed, the projected data pattern is not held stationary, thereby effectively providing a blurring of the projected data pattern.

In order to determine a correct exposure level, a print plate may be exposed with the above described digital representations at different exposure levels, e.g. by moving the exposure head across the print plate at different speeds. A desired exposure level may subsequently be determined by processing the exposed print plate and by comparing the grey/colour densities of the exposed areas.

Preferably, the plurality of predetermined continuous-tone grey values comprises a sequence of continuous-tone grey values with a uniform ratio of consecutive grey values of said sequence. In one embodiment, the digital representations are arranged in a test image comprising a sequence of areas, where the pixel patterns of adjacent areas have spatial densities that cause a difference in exposure by a factor of V2 « 1.4, thereby allowing a comparison with traditional plastic strip wedges that have the same factor between adjacent areas. However, other density steps are possible as well, for example a factor of 2, or any other suitable factor.

The areas may have any shape. Convenient shapes are square and rectangular areas, e.g. arranged as a linear sequence of such areas.

In some embodiments, the method further comprises exposing a test area of the light sensitive material with a test image, the test image comprising the plurality of digital representations and additional information; wherein exposing the test area comprises

- exposing the test area with the plurality of blurred images by means of an exposure process;

- exposing the test area with the additional information by means of the exposure process; and

- causing the exposure process to selectively blur the generated image when exposing the test area with the plurality of digital representations.

Hence, the test area may provide both the reproduced continuous-tone grey values and additional information that requires a high-resolution / sharp reproduction, e.g. additional test areas comprising dot patterns, line patterns, or the like. Further examples of additional information include text information, e.g. a labelling of the individual areas representing different grey values, and frames or other delimiters around the individual areas, in order to more clearly delimit the individual areas from each other.

In some embodiments, the method further comprises

- exposing an image area of the light sensitive material with a predetermined image by means of an exposure process; - exposing a test area of the light sensitive material with a test image by means of the exposure process, the test image comprising the plurality of digital representations; and

Hence, the test area with the blurred, e.g. out-of-focus, images of the digital representations of the respective grey values may be printed in a predetermined position on every print plate, e.g. along a predetermined edge of the print plate, in addition to the actual image data that is to be reproduced on that print plate. Consequently, the test area provides a reliable and convenient means for routine quality control of the printing process.

The present invention relates to different aspects including the method described above and in the following, corresponding devices, and computer programs, each yielding one or more of the benefits and advantages described in connection with the above-mentioned method, and each having one or more embodiments corresponding to the embodiments described in connection with the above-mentioned method.

In particular, the invention further relates to an exposure control system for evaluating the exposure level of a light sensitive material based on a plurality of predetermined continuous-tone grey values; the system comprising

- means for providing a digital representation of each of said plurality of predetermined continuous-tone grey values, each digital representation comprising a pattern of a plurality of pixels;

- means for causing an exposure system to image each of the generated digital representations on respective areas of the light sensitive material resulting in a plurality of exposed areas, each area being exposed according to an image of one of the digital representations. It is noted that the exposure control system and the method described above and in the following may be controlled by software carried out on a data processing device or other processing means caused by the execution of program code means such as computer-executable instructions. Here and in the following, the term processing means comprises any circuit and/or device suitably adapted to perform the above functions. In particular, the above term 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.

Hence, according to another aspect, a computer program comprises program code means adapted to cause a data processing device to control the exposure control of an exposure control system as described herein, when said computer program is run on the data processing device.

For example, the program code means may be loaded in a memory, such as a RAM (Random Access Memory), from a storage medium or from another computer via a computer network. Alternatively, the described features may be implemented by hardwired circuitry instead of software or in combination with software.

The above and other aspects will be apparent and elucidated from the embodiments described in the following with reference to the drawing in which:

Fig. 1 illustrates bit patterns of a wedge image and magnified segments of such a binary wedge image.

Fig. 2 illustrates the generation of a wedge image from a blurred bit pattern and a frame and text overlay. Fig. 3 illustrates the effect of the blurring of the bit pattern.

Fig. 4 illustrates an embodiment of an exposure system implementing an exposure level control method described herein.

Fig. 5 schematically shows a perspective view of a flatbed printer with a fluid film generation unit.

Fig. 6 shows a perspective view of a fluid film generation unit and schematically illustrates the control and gas supply circuits of the fluid film generation unit.

Fig. 1a illustrates the bit patterns of a wedge image, generally designated 100. The wedge image comprises five areas 101 , 102, 103, 104, 105, each representing a different continuous-tone grey value. Each area is filled with a frequency modulated pixel pattern with respective spatial densities, such that area 101 is filled 100% with pixels, area 102 is filled to 71 % by pixels, area 103 is filled to 50% by pixels, area 104 is filled to 35.5 % by pixels and area 105 is filled to 25% by pixels. Hence, the spatial density of pixels between adjacent areas differs by a ratio of V2 » 1.4.

The pixel patterns of each of the areas 101 , 102, 103, 104, and 105 of the digital wedge image 100 may be generated by any suitable method for reproducing multi-level digital images with binary pixel/dot patterns, e.g. by generating a frequency modulated pixel pattern of fixed sized pixels wherein the spatial density of identical pixels/dots is selected to reproduce a corresponding grey/colour density. One method for generating frequency modulated pixel patterns is referred to as error diffusion or error propagation, e.g. as described in EP 0 201 674.

Fig. 1b shows magnified segments of each of the areas, i.e. segment 111 illustrates a magnified segment of area 101 , segment 112 illustrates a magnified segment of area 102, segment 113 illustrates a magnified segment of area 103, segment 114 illustrates a magnified segment of area 104, and segment 115 illustrates a magnified segment of area 105. As can be seen in fig. 1b, each area comprises binary pixels that can assume two values "black" and "white". The ratio of the number of black pixels relative to the number of white pixels, i.e. the spatial density of black pixels per unit area, increases from right to left, i.e. area 105 has the lowest spatial density of black pixels while area 101 has the highest spatial density of black pixels.

Fig. 2 illustrates the generation of a wedge image from a blurred bit pattern and a frame/text overlay.

Fig. 2a illustrates a binary wedge image as described in connection with fig. 1. Fig. 2b shows the binary wedge image of fig. 2a after blurring. Hence, fig. 2b corresponds to the resulting exposure distribution of the light sensitive material. As can be seen from a comparison of figs. 2a and 2b, the blurred binary wedge image of fig. 2b, generally designated 200, has been blurred such that the resolution is lower than the pixel resolution of the binary wedge image 100 of fig. 2a. Consequently, in the blurred image 200 the pixel structure of the original image has become substantially undistinguishable, and each of the areas 201, 202, 203, 204, and 205 is effectively uniformly exposed by an effective continuous-tone grey value corresponding to the spatial density of pixels in the corresponding area 101 , 102, 103, 104, or 105 of the original image 100. In some embodiments, a test area of the light sensitive material is exposed with further image information in addition to the blurred binary wedge image as described above.

Fig. 2c illustrates an embodiment of such further image information, in particular a wedge text/frame overlay image, generally designated 210. The overlay image 210 comprises a number of frames 211 , 212, 213, 214, and 215 corresponding to the areas of the binary wedge image 100. The overlay image further comprises text information, e.g. an enumeration or another designation of the different areas. In the example of fig. 2, the text information is exemplified by numerals "0", "1", "2", "3", and "4".

In particular, a test area may be exposed twice, once with the blurred binary wedge image and once with the further image information. In one embodiment, the test area may be exposed by scanning it with the exposing light beam twice. During one scan, the optical system is defocused, and the image is exposed according to the binary wedge image, thereby generating a blurred binary wedge image on the light sensitive material. In a second scan, the test area is exposed with the further image information, while the optical system is focussed on the surface of the light sensitive material.

Fig. 2d shows an example of a combined wedge and overlay image generally designated 220, e.g. resulting from a two-stage exposure with the blurred binary wedge image 201 and the overlay image 21.1. The resulting combined image 220 thus appears as a sequence of areas 211 , 212, 213, 214, and 215, each exposed with an effective continuous-tone grey value and surrounded by a sharp, well-defined frame and enumerated with the corresponding text information. Hence by combining the blurred binary wedge image and an overlay image with text and/or frame and/or other image information, a total test image is achieved with effective continuous- tone grey values and additional information without decreasing the imaging quality of the additional information due to the blurring of the wedge image.

Furthermore, the frames of the overlay image 211 separates the individual areas 211 , 212, 213, 214, and 215 from each other by a well-defined d istance corresponding to the thickness of the frame lines 226. When the thickness of the lines 226 is larger than the blurring of the pixels of image 100 in the blurred image 200, the areas of the combined image appear with a uniform grey level over the entire area of each of the individual areas 211 , 212, 213, 214, and 215.

Fig. 2e schematically shows an example of an imaged and processed printing plate that was exposed according to a combined image as shown in fig. 2d and with a certain selected exposure level.

In this example, the plate is completely black for the areas 231 and 232, corresponding to the two densest effective grey values of areas 201 and 202 of the blurred image 200. Areas 234 and 235 corresponding to the two least dense effective grey values of areas 204 and 205 of the blurred image 200 are completely white, while area 233 corresponding to the a medium effective grey values of area 203 of the blurred image 200 has been partially exposed. Hence from this exposed test image, it may be determined whether the exposure a desired exposure level can now be determined whether the selected exposure level was correct. Since the effective grey values of the different areas of the wedge differ by a constant factor, the amount of any necessary correction to the exposure level may likewise be determined from location of the transition between completely white and completely dark areas.

It is understood that this determination may be performed manually by an operator or automatically by the exposure control system. For example, the exposure control system may include a scanner that scans the test image of the exposed and processed print plate to obtain a digital representation of the test image on the processed print plate. An image processing module may then determine between which of the areas 231 , 232, 233, 234, and 235, the transition between white and black occurs. For example, this may be performed by detecting the frame edges around the areas to determine the location of the individual areas and by subsequently determining the ratio of black in each of the individual areas 231 , 232, 233, 234, 235, for example by counting the number of black pixels in each area.

It is noted that the distinctiveness of the transition between a completely black area and a completely white area may depend on the properties of the printing plate, in particular the gradation of the light sensitive material. Some printing plates may not have a strictly binary behaviour but may reproduce some grey values. In such cases, an automated determination of the location of the transition may include a measurement of the grey levels and a weighting of the pixel count with the respective grey levels.

It is understood that even though five areas are shown in figs. 1 and 2, other embodiments may include a different number of areas for reproducing more or fewer grey values. In particular, in one embodiment, the wedge includes 13 areas with pixel densities 100%, 71%, 50%, 35.5%, 25%, 18%, 12.5%, 8.8%, 6.3%, 4.4%, 3.1%, 2.2%, 1.6%. Hence, the spatial density of pixels between adjacent areas differs by a ratio of V2 « 1.4.

Fig. 3 illustrates the effect of the blurring of the bit pattern. In particular, fig. 3a illustrates the binary values of the wedge image 100 and the blurred wedge image 200 along the lines marked A-B, respectively. In particular, line 300 shows the binary values along the line A-B in area 104 of the digital wedge image 100, and line 301 shows the spatial distribution of the light energy level that expose the different locations along the line A-B in the corresponding area 204 of the blurred wedge image 200.

In the digital wedge image 100, the grey levels vary from pixel to pixel between two discrete values "0" and "1", where the value "1" corresponds to a black pixel and the value "0" to a white pixel, i.e. line 300 is a step function between two discrete values and with a step width corresponding to the pixel size of the pixels of the image 100. In the blurred image 200, on the other hand the grey level varies continuously and to a considerable lesser extent than in the image 100. Correspondingly, line 301 is continuously varying and with only small fluctuations around an effective grey level designated by dotted line 302.

Accordingly, when a light sensitive medium is exposed with a certain exposure level according to the un-blurred image 100, each pixel will be rendered black or white depending on whether the amount of light energy reaching that pixel exceeds a certain threshold. For example, if for a certain material and exposure level, the threshold corresponds to a level indicated by arrow 303 in fig. 3a, those pixels that exceed that level will be completely darkened while the remaining pixels remain effectively unexposed, thereby resulting in a non-uniform exposure and a difficult determination of the correct exposure level.

For the blurred image on the other hand, the light energy level 301 is above threshold 303 over the entire area, thereby resulting in a uniform exposure.

Only if the threshold lies in the narrow range of fluctuation of the line 301 , the resulting exposure may be non-uniform. It is understood that the amount of the fluctuation of the grey values in the blurred image depend on the amount of blurring performed. Hence, by providing a sufficiently strong blurring the fluctuation may be minimized. In one embodiment, the blurring is selected such that the variation of the light energy level within a given area is smaller than the difference in the average light energy level for different areas (i.e. the light energy integrated over the respective areas). For example, the variation within an area may be determined as a variance of the distribution, and the blurring may be controlled such that the variance is smaller than a predetermined fraction of the difference between the average light energy levels of adjacent areas. For example, the fraction may be selected in the interval between 1/2 and 1/10, such as 1 /3 and 1/5, e.g. 1/4.

Fig. 3b illustrates the light energy level 301 of the blurred image at a border between two areas, in particular along line C-D in fig. 2b. The difference between the average light energy levels 304 and 302 in areas 203 and 204, respectively is denoted D, while the variation of the respective light energy levels is denoted Vi and V₂, respectively. Hence, the above condition corresponds to the ratios vi/D and v₂/D being smaller than a predetermined threshold value.

Fig. 4 illustrates a schematic block diagram of an embodiment of an exposure system implementing an exposure level control method described herein.

The system comprises a control unit 403 that feeds image information 409 about the image to be printed to an exposure unit 404. The exposure unit generates a light beam 408 representing the received image information. The light beam is imaged by an optical system 405 on the surface of a light sensitive material 406. The control unit 403 may be a suitably programmed computer, a dedicated control circuit, or any other suitable processing means.

As mentioned above, there are a number of known types of exposure systems, including external drum systems, internal drum systems, flatbed systems, etc. Common to the above systems is that they expose a surface by successively illuminating parts of the surface according to partial image information of the total image 409 to be printed.

The received image data 409 is stored in digital form in an image memory 401 , e.g. as an array of pixel values. Hence, the control unit 403 can retrieve the pixel information for selected parts of the image from image memory 401. It is understood that, in other embodiments, the control unit may receive the image data directly and in real time from an external data processing system without storing the entire image data in a local memory.

In external and internal drum systems the exposure of a printing surface is performed by scanning the printing surface 406 with a laser beam 408. The laser beam is generated by the exposure unit 404 and modulated according to the pixel information of the image to be printed, thereby imaging the desired image on the print surface. In internal drum systems, the print surface 406 is deposited on the inner surface of a cylindrical drum, and the optical system 405 directs the modulated laser beam into the drum along its longitudinal axis. The optical system of an internal drum system further comprises a spinner, e.g. a rotating prism, that redirects the modulated laser beam 408 towards the inner surface of the drum as to scan the inner surface. The optical system 405 further comprises a lens system for focussing the laser beam 408 on the surface 406. By longitudinal displacing the spinner, consecutive scan lines on the print plate can be scanned. The control unit 403 controls the modulation of the laser beam by the exposure unit according to the image data and the optical system 405, in particular the rotation and longitudinal movement of the spinner.

In flatbed systems, the exposure unit 404 comprises an exposure head that is moved relative to the light sensitive material -406. In some prior art systems, the light beam 408 is modulated by a so-called spatial light modulator and the modulated beam is imaged on trie surface by the optical system 405. 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. Hence, such an exposure head is capable of illuminating a partial image comprising a plurality of pixels at a time. Accordingly, in these embodiments, the control unit 403 controls the light modulator of the exposure unit 403 and the movements of the exposure head relative to the surface of the material 406.

In some embodiments, the exposure head is moved stepwise across the surface as to expose a plurality of partial images in a step-and -repeat process. In other embodiments, the light modulator is imaged onto a light- sensitive material and a continuous 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. One embodiment of such a flat bed printer will be described below.

Still referring to fig. 4, the exposure system further comprises a memory 402 for storing digital wedge image data and, optionally, a memory 407 for storing an overlay image to be imaged superimposed with the wedge image.

In one embodiment, the digital wedge image is stored in memory 4O2 as a number of two-dimensional pixel patterns each representing a respective continuous-tone grey value as described in connection with fig. 1. Optionally, the memory 402 further comprises position information, e.g. x,y-coordinates of a suitable coordinate system, indicating a position of an area 410 on the printing surface 408 where the wedge image is to be printed. The pixel patterns may be generated off-line either by the control unit 403 or by an external data processing system, e.g. a suitably programmed computer executing a suitable image processing software package such as the Photoshop software package by Adobe Systems Inc. The generated pixel patterns are stored in memory 402, thereby allowing retrieval of the same pattern for each exposure level test without the need for a real time image generation. Alternatively, the bit patterns may be generated in real-time, thereby avoiding the need for a memory 402. In yet a further embodiment, the wedge image may be incorporated in the received image data 409 by an external data processing system, thereby avoiding the need for a separate storage of the wedge image. In such an embodiment, the position information indicating the location of the wedge image in the received image needs to be known by the control unit, in order to suitably control the optical system to blur/defocus the image when imaging the wedge image.

Hence, in some embodiments, the wedge-/overlay- image may be stored in a separate memory or at least separate memory section, while in other embodiments the wedge- and overlay- images may be generated and included in the image data in real time.

During the printing process, the control unit controls the imaging of the image data 409 as well as the imaging of the wedge image 402 and the overlay image 407. In particular, the control unit controls the exposure unit to expose a predetermined area, exemplified by area 410 in fig. 4, with the wedge image 402 and the overlay image 407. It is understood that the test area 410 may be positioned anywhere on the print surface 406. Many printing plates are provided with alignment holes along one edge of the printing plate for use during subsequent mounting of the printing plate for reproduction. The margin comprising the holes is often used for printing information which is not intended as part of the final document to be produced based on the printing plate, e.g. information intended for the printing operator, bar codes, or the like. In one embodiment, the test area 410 is located along this edge of the printing plate as to not reduce the actual printing surface available for the actual image information to be printed.

In one embodiment, the control unit 403 controls the exposure unit to expose area 410 in two passes. In a first pass the area 410 is exposed according to the wedge image 402, and in a second pass, the area 410 is exposed according to the overlay image 407. It is understood that the two passes can be performed in any order. The control unit 403 controls the optical system 405 to defocus the imaging beam 408 before imaging the area 410 with the wedge image to cause the area 410 to be exposed with a blurred wedge image. After imaging the area 410 with the wedge image 402 the control unit 403 controls the optical system 405 to re-focus the imaging beam 408, unless the imaging of the wedge image was the last step of the exposure process of printing plate 406.

The defocusing and re-focussing of the imaging beam may be performed by any suitable mechanism, e.g. by changing the distance between the exposure unit and/or the optical system and the printing surface, by displacing one or more optical elements of the optical system, and/or the like.

In one embodiment, where the position of the area 410 is the same for each print plate, the selective defocusing/blurring may be achieved by a separate optical element, e.g. a prism, mirror, glass plate, diffuser plate, a lens, or the like which is mounted in a predetermined position relative to the print plate such that the light beam 408 is fed through that element only when the exposure unit is positioned as to expose the area 410.

In the following, an embodiment of a flatbed system is described with a particularly convenient mechanism for selectively defocusing the imaging when exposing the test area 410. Fig. 5 schematically shows a perspective view of a flatbed printer with a fluid film generation unit. The flatbed printer of fig. 5 exposes a printing surface 519 of a printing plate 506 by scanning the printing surface in longitudinal successive, transversally extending strips illustrated by the dotted line 510. The printer has an optical print head generally designated 500 that houses an optical system for directing a light beam towards the printing surface along an optical axis 504. The light beam is modulated according to digital data representing the image/pattern to be printed. During normal operation, the print head is moved across the surface at a constant distance between the print head and the printing surface, to ensure a uniform printing quality and, in particular, a uniform focus of the optical system.

The print head 500 is movably mounted to a portal shaped support structure 501 such that the print head can be moved in a longitudinal direction 509 across the printing surface. The support structure is further movably mounted on guides or threaded shafts 502 in a transversal direction 518. Hence, after a longitudinal scan, the print head is moved in the transversal direction allowing the print head to expose a new stripe in the next longitudinal scan. The amount of transversal displacement 511 corresponds to the width of the longitudinal scan lines 510. The longitudinal movement of the print head along the support structure and transverse movement of the support structure are caused by suitable motors (not shown) such as step motors. It is understood that alternative means for providing a relative motion of the print head and the printing surface may be provided. For example, the support plate may be moved rather than the print head.

The printing plate 506 is placed on the base plate 505 and preferably protected against displacement, e.g. by a glass plate or by affixing it to the base plate, e.g. by vacuum suction. Typically, the printing plate is built up as a layered structure comprising a base and a radiation-sensitive printing layer. For example, a typical printing plate used in the graphical industry has an aluminium base with a radiation-sensitive emulsion.

In many applications it is desirable that the entire printing surface is scanned i.e. without any margins. Consequently, the longitudinal scans 510 may extend all the way to the transverse edges 508 of the print plate, or even beyond the printing surface, thereby allowing the printing head to be decelerated and accelerated while the scanning beam is outside the area of the printing surface.

The print head 500 is mounted to the support structure 501 by a suspension arrangement formed by parallel leaf springs 503 that provide a parallelogram linkage between the print head and the support structure, thereby allowing the print head to be vertically displaced, i.e. in the direction of the optical axis. In alternative embodiments, other suspension arrangements may be used, e.g. vertical shafts, rails, or other guides. A fluid film generation unit 512 is mounted at the bottom of the print head 500 providing a film of pressurised air between the print head and the printing surface 506 on which the print head floats. Hence, the print head glides contact-free across the printing surface. The fluid film generation unit 512 allows maintaining a constant distance between the print head and the printing surface 506, even in the presence of irregularities of the printing plate.

The fluid film generation unit 512 is movably connected to the print head 500 via guides 513 that allow a longitudinal movement of the fluid film generation unit 512 relative to the print head. This relative movement allows the fluid film to be maintained even when the print head approaches or even crosses the transverse edges 508 of the printing surface.

The scanning operation of the print head 500, the modulation of the light beam, and the operation of the fluid film generation unit 512 are controlled by a control unit 514 via respective control signal connections 514, 516, and 517, respectively. In one embodiment, the control unit has stored therein information about the position and dimensions of the printing plate on the base plate. In some embodiments, this information may include pre-set default values and/or values entered by an operator. Alternatively or additionally, the information may be automatically detected and/or verified, e.g. by detecting the edges of the printing plate by suitable sensors, such as one or more distance sensors and/or one or more reflectivity sensors, and /or the like.

Hence, in this embodiment, the print head 500 floats at a predetermined distance above the print surface. The distance depends on the air flow provided by the fluid film generation unit, in particular by the pressure of the pressurised air. Under normal operation, the air flow is maintained at a level such that the light beam is focussed on the printing surface. When the pressure is increased, the print head may be slightly elevated, e.g. by 1- 5mm, in particular 2-3 mm, thereby causing the light beam not to be focussed on the print surface any longer. Hence, the de-focussing and refocusing of the light beam before and after the imaging of the wedge image may conveniently be performed by controlling the pressure in the fluid film under the print head between two different levels.

Fig. 6 shows a perspective view of a fluid film generation unit and schematically illustrates the control and gas supply circuits of the fluid film generation unit. The fluid film generation unit, generally designated 512, comprises an elongated aperture 628 through which the optical axis 504 of the print head projects downwardly towards the printing surface. The bottom surface of the fluid film generation unit 512 comprises respective gliding surfaces 622 along the longitudinal sides of the aperture 628, Each gliding surface is provided with holes with downwardly directed openings/orifices in the respective gliding surface. The holes of the gliding surfaces of each side of the aperture are in fluid connection with a respective one of air supply channels 626 and 627. Hence, when the fluid film generation unit 512 is mounted under the print head as shown in fig. 5, and when pressurised air flows through the openings in both gliding surfaces 622, a fluid film is built up on both sides of the optical axis, both in the longitudinal direction and in the transverse direction.

The gliding surfaces 622 are formed as protrusions projecting out of the bottom surface of the fluid film generation unit 112. Consequently, the gliding surfaces are the parts of the bottom surface of the fluid film generation unit that are most proximate to the printing surface, thereby determining the minimum distance between the fluid film generation unit and the printing surface. Furthermore, the gliding surfaces are located in the immediate proximity of the elongated aperture 628.

The fluid film generated on each side of the aperture 628 is individually controllable via the air flow supplied through channels 626 and 627, respectively, thereby allowing a control of the position of the fluid film that supports the print head. For this purpose, each of the gas supply channels 626 and 627 is connected to a corresponding gas supply line 640 and 641 , respectively. Pressurised air is generated by an air supply unit 654 that generates dried, pressurised air at a predetermined pressure, such as 2-3 bar, e.g. 2.5 bar. Preferably, the air pressure is adjustable to allow an adjustment of the floating height. The air is fed via line 649 and a buffer tank 644 into lines 640 and 641. Each of the supply lines 640 and 641 is provided with a valve 642 and 643, respectively. The operation of the valves 642 and 643 is controlled by the control unit 514 via control signals 646 and 647, respectively. Hence, the air flow to the two gliding surfaces is controlled separately by opening and closing of the corresponding valve 642 or 643. The buffer tank 644 improves the stability of the pressure in supply lines 640 and 641 , in particular when one of the valves is operated. In an alternative embodiment, instead of the two separate valves 642 and 643, a two-way valve is provided between the buffer tank and the two supply lines 640 and 641 for switching the gas flow between the two gas lines. In some embodiments, at least one of the gas lines 640 and 641 comprises a throttle valve allowing a relative adjustment of the pressure under the gliding surfaces 221 and 222. In further alternative embodiments, each gliding surface is supplied from a separate pneumatic system.

The control unit 614 further controls a step motor 620 via control signal 648, thereby allowing the control of the movement of the fluid film generation unit 512 along the rails 513 via a belt 624 that is connected to one of a set of the sliders 623.

The control of the method and device described herein can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed microprocessor. In the device claims enumerating several means, several of these means can be embodied by one and the same item of hardware, e.g. a suitably programmed microprocessor, one or more digital signal processor, or the like. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

Claims

Claims:

1. A method of evaluating the exposure level of a light sensitive material based on a plurality of predetermined continuous-tone grey values; the method comprising

2. A method according to claim 1 , wherein the imaging comprises blurring the images of the respective digital representations.

3. A method accord ing to claim 2, wherein each digital representation comprises a pixel pattern of a plurality of pixels, the pixel pattern having a predetermined pixel resolution; and wherein blurring the images of the respective digital representations results in respective blurred images having a resolution lower than the pixel resolution of the corresponding digital representation.

4. A method accord ing to claim 2 or 3, wherein the respective areas are spaced apart from each other by at least a predetermined distance; and wherein blurring the images of the respective digital representations results in respective blurred images having a resolution high enough to resolve said predetermined distance.

5. A method according to any one of claims 2 through 4, wherein blurring the images of the respective digital representations causes the areas to be exposed by respective relative light energy levels; wherein the relative light energy level of each area has a spatial variation over said area around a mean light energy level; and wherein the blurring is selected to be strong enough to cause the variation of the light energy level of each area to be smaller than a predetermined fraction of the difference between the average light energy level of said area and the average light energy levels of each of the other areas.

6. A method according to any one of claims 2 through 5, wherein blurring comprises defocusing the images of the digital representations.

7. A method according to any one of claims 1 through 6, wherein the plurality of predetermined continuous-tone grey values comprises a sequence of continuous-tone grey values with a uniform ratio of consecutive grey values of said sequence.

8. A method according to claim 7, wherein said ratio is substantially equal to the square root of two.

9. A method according to any one of claims 1 through 8, wherein the areas are rectangles arranged as a sequence of rectangles.

10. A method according to any one of claims 1 through 9, wherein providing the digital representations comprises providing respective patterns of pixels each pattern having a respective predetermined spatial density.

11. A method according to any one of claims 1 through 10, further comprising processing the exposed light sensitive material; and determining a desired exposure level from the exposed areas of the processed light sensitive material.

12. A method according to any one of claims 2 through 11 , comprising exposing a test area of the light sensitive material with a test image, the test image comprising the plurality of digital representations and additional information; wherein exposing the test area comprises - exposing the test are with the plurality of blurred images by means of an exposure process;

13. A method according to any one of claims 2 through 11 , comprising

- exposing an image area of the light sensitive material with a predetermined image by means of an exposure process;

- exposing a test area of the light sensitive material with a test image by means of the exposure process, the test image comprising the plurality of digital representations; and

14. A method according to any one of claims 1 through 13, wherein the digital representation comprises a pixel pattern of a plurality of binary pixels.

15. An exposure control system for evaluating the exposure level of a light sensitive material based on a plurality of predetermined continuous-tone grey values; the system comprising

- means for providing a digital representation of each of said plurality of predetermined continuous-tone grey values, each digital representation comprising a pattern of a plurality of pixels; - means for causing an exposure system to image each of the generated digital representations on respective areas of the light sensitive material resulting in a plurality of exposed areas, each area being exposed according to an image of one of the digital representations.

16. An exposure control system according to claim 15, further comprising means for causing the exposure system to generate a blurred the image of the respective digital representations on the light sensitive material.