Time modulated stochastic screening

Abstract: A method for generating a screened reproduction of a multiple tone image comprising the steps of frequency modulation screening (S1) the multiple tone image to obtain screened data representing tones of the multiple tone image in terms of halftone dots, reproducing the halftone dots on an imaging element by means of a scanwise exposure (S3), characterised in that the scanwise exposure for rendering a halftone dot is time modulated (S2). In one embodiment the dot exposure time varies in accordance with the tone to be reproduced. The method reduces dot gain and improves gradation characteristics.

Description: FIELD OF THE INVENTION The present invention relates to a method for generating a screened reproduction of a contone image by means of a frequency modulation of the original, and more particulary to a method for making a lithographic printing plate according to the silver salt diffusion transfer process. BACKGROUND OF THE INVENTION A number of reproduction methods are only capable of reproducing a small number of image tones. For example, offset printing or electrophotographic printing methods are only capable of printing two tone values i.e. deposit ink or toner or not. In order to reproduce images having continuous tones, a halftoning or screening technique is used. A halftoning technique converts a density value into a geometric distribution of binary dots that can be printed. The eye is not capable of seeing the individual halftone dots, and only sees the corresponding "spatially integrated" density value. Two main classes of halftoning techniques have been described for use in the graphic arts field. These two techniques are known as "amplitude modulation screening or autotypical screening" (abbreviated as AM) and "frequency modulation screening or stochastic screening" (abbreviated as FM). Reference is made to Figs. 1 and 2, wherein Fig. 1 illustrates an arrangement of microdots (in a bitmap) as used in amplitude modulation; and wherein Fig. 2 illustrates an arrangement of microdots (in a bitmap) as used in frequency modulation. For a clear understanding, most of the relevant technical terms used in the present application are explained in a separate chaptre at the beginning of the detailed description (see later on). According to amplitude modulation screening, the halftone dots, that together give the impression of a particular tone, are arranged on a fixed geometric grid. By varying the size of the halftone dots the different tones of an image can be simulated. Consequently this AM-technique can also be called "dot size modulation screening". According to frequency modulation screening, the distance between the halftone dots is modulated rather than their size. This FM-technique, although well known in the field of low resolution plain paper printers, has not obtained much attention for offset printing and other high end printing methods, probably because of the disadvantages to be discussed further on. DE3934867 describes an optical scanner wherein the scanwise exposure for reproducing a halftone dot is time modulated. Both classes of halftone techniques are used in combination with a digital film recorder. A typical digital film recorder consists of a scanning laser beam exposing a photosensitive material at high resolution. The photosensitive material can be a photographic film, from which later on a printing plate is prepared by means of photo-mechanical techniques. As the present invention relates more specifically to a method for preparing a lithographic printing plate, in particular to a method for preparing a lithographic printing plate comprising the steps of informationwise exposing an imaging element and thereafter processing the exposed imaging element by a diffusion transfer process, some additional background is given hereinafter. Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink (called "oleophilic" areas) whereas other areas will not accept ink (called "oleophobic" areas). The oleophilic areas form the printing areas while the oleophobic areas form the background areas. Two basic types of lithographic printing plates are known. According to a first type, socalled "wet" printing plates, either water or an aqueous dampening liquid and ink are applied to the plate surface that includes hydrophilic and hydrophobic areas. The hydrophilic areas are soaked with water or the dampening liquid and are thereby rendered oleophobic while the hydrophobic areas will accept the ink. A second type of lithographic printing plate operate without the use of a dampening liquid and is called "driographic" printing plate. This type of printing plate comprises highly ink repellant areas and oleophilic areas. Lithographic printing plates can be prepared using a photosensitive lithographic printing plate precursor, referred to herein as an "imaging element". Such an imaging element is exposed in accordance with the image data and is generally developed thereafter so that a differentiation results in ink accepting properties between the exposed and unexposed areas. Silver salt diffusion transfer processes are known and have been described, for example, in United States Patent US 2,352,042 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde - The Focal Press - London and New York (1972). From the above it will be clear that lithographic printing is only capable of reproducing two tone values because the areas will either accept ink or not. Thus lithographic printing is a socalled "binary" process. As mentioned hereabove, in order to reproduce originals having continuously changing tone values by such processes, halftone screening techniques are applied. Yet the rendering of small dots still presents an important problem as is explained hereinafter. Laser imagesetters and "direct to plate" recorders expose halftone images on graphic arts film and plates by means of laser beam scanning and modulation. The faithful rendition of halftone levels, represented by binary bitmap images, is difficult to achieve because the image is distorted by the gaussian intensity distribution of the laser beam (Fig. 3 sketches a three-dimensional distribution of a Gaussian laser beam) and the sensitometric characteristics of the film and plate material. This distortion changes the rendition of the halftone levels, small dots (positive or negative) in lowtones or highlights and small dots in hightones or shadows may be rendered too small (over- and underexposure) and halftone dots tend to print unevenly or not at all. Generally, a black dot in a white area is called "a highlight", whereas a white dot in a dark area is called "a shadow". Such distortion in size during printing is often called "recorder gain", comprising either a "dot gain" or a "dot loss". In this context, dot gain is the increase in the size of the halftone dots during printing compared to their size on a film or plate. For example, a 50 % relative dot area on the film may print a dot of 70 %. In this case, the dot gain is said to be 20 % (cfr. "Application of a dynamic measurement for the investigation of the causes of dot gain in web offset lithography", Pobboravsky et al., TAGA 1989 Proceedings, Rochester, page 482). If Fig. 4 illustrates an actual "plate-press curve", one can see that a reproduction afflicted by dot gain is too dark anywhere except for the extreme highlight and shadow tones. Furthermore, the tonal contrast has become too large in the midtones and too small in the shadows; detail contrast has been lost. The distortion will be most noticeable in small dots, wherein not only the edges but also the density of the dot will not be rendered optimally. At the other end of the tone scale small unexposed areas will become fogged by the influence of light beams exposing the surrounding area. This means that the faithful rendition of small dots is extremely difficult. As the highlight tones in FM screening relate to a dispersion of separate halftone dots, the experience that during printing, a FM screen in the lower tonal steps often has a dot loss, can be explained. A probable reason is that the sum of the lengths of the sides of all printing FM dots (cfr. Fig. 2) in the lower tone values is greater than the circumference of an AM dot (cfr. Fig. 1) of the same percentage value or coverage. Thus a mechanical ink film spreading as a cause of this dot loss must be counteracted. Apart of the just mentioned problem of and probable reason for dot gain and dot loss, many halftoning techniques also share the disadvantage of a socalled poor "photomechanical behavior or physical reconstruction function". These terms mean that the photomechanical properties of an imaging element on which a halftone pattern is recorded depend to a great extent on the interaction between the layout of that halftone dot pattern and the imaging characteristics of the device on which it is rendered, also referred to as the physical reconstruction function of the rendering device. A thorough explanation of said photo-mechanical behavior or physical reconstruction function has been described in EP-A-94.202.329.2 (in the name of Agfa-Gevaert). From same said application, some illustrative drawings (cfr. Figs. 14.1 to 14.4) are taken over, in order to initiate some extra hinsight into the problem of recorder gain. Herein, Figs. 14.1 and 14.2 illustrate dot gain occurring with dots lined up along the main scan and sub scan direction respectively. Figs 14.3 and 14.4 illustrate dot gain resulting from replication of microdots with that of a single halftone dot having the same total area. As to a possible solution for this problem, some different approaches might be undertaken. First, an overall compensation by applying a higher or lower laser power is not acceptable because overexposure makes the small highlight dots larger but fills up the small shadow dots. Underexposure reverses this effect, opening up the shadows but reducing the highlights. Both over- and underexposure will reduce the number of rendered halftone levels considerably. Second, theoretically, the best results can be obtained using a laser beam with an optimum spot size for each scan resolution in combination with a graphic arts film or plate characterized by a high gradient and steep toe. This requires imagesetters with tight manufacturing tolerances and films with special (socalled "explosive") development techniques. Another method for reducing the problems created by recorder gain in FM halftoning is "pixel replication", i.e., the combination of "microdots" (or "rels") rendered at the fundamental recoder pitch into effectively larger halftone dots by replication. The fact that the total gain in dot size is smaller when the halftone dots are rendered using a matrix of smaller microdots is illustrated in the already mentioned Figs. 14.1 and 14.2. Pixel replication has been used commercially in the "COBRA" raster image processor of Miles Inc., Agfa Division. As indicated in Figs. 14.3 and 14.4 a reduction of dot gain is achieved. Further improvement (as e.g. relative to graininess in the midtones) has been described in the already mentioned application EP-A-94.202.329.2 (which is incorporated herein by reference), but the reproducability of (dispersed) highlights is not yet ideal, especially not if applied for preparing lithographic printing plates. Still another approach has been disclosed very recently under the title "Wavelets as a tool for the construction af a halftone screen", by T. Mitsa and P. Brathwaite, University of Iowa, USA (presented at the IS&T - SPIE Symposium on Electronic Imaging, february 1995). Although this disclosure indicates to lead to a reduced graininess in the output halftone image, the problem of insufficient reproducebility of (dispersed) highlights still remains unsolved. In general, it may be stated than in FM-screening principally a compromis has to be taken between two conflicting interests: either a reduced graininess (especially problematic in the midtones), either a reproducability of extreme tones (especially problematic in dispersed highlights and shadows). It is a big advantage of the present invention to bring a surprisingly good compromise, in particular a good reproducability of extreme tones and also a reduced graininess in the midtones. OBJECTS OF THE INVENTION It is an object of the present invention to provide a method for generating a screened reproduction of a contone image with improved reproduction characteristics, also called gradation characteristics, especially in the highlight and shadow tones of an image. It is also an object of the present invention to provide a method for making a lithographic printing plate according to the DTR-process from a printing plate precursor by means of frequency modulation screening an original with improved printing properties e.g. a larger development latitude, an extended tone scale in print, and an extended usable lifetime of said printing plate. Further objects of the present invention will become clear from the description hereinafter. SUMMARY OF THE INVENTION According to the present invention there is provided a method for generating a screened reproduction of a multiple tone image as set out in claims 1 and 2. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates an arrangement of microdots as used in amplitude modulation; Fig. 2 illustrates an arrangement of microdots as used in frequency modulation screening; Fig. 3 is a three-dimensional distribution of a Gaussian laser beam; Fig. 4 is a socalled plate-press curve; Fig. 5 is a schematic representation of a circuit for implementing a halftoning method suitable for use in the present invention; Fig. 6 schematically shows a flat bed type scanning device for use in a method according to the present invention; Fig. 7 schematically shows an internal drum type scanning device for use in a method according to the present invention; Fig. 8 is a flowchart illustrating basic steps of a first embodiment according to the present invention; Fig. 9 is a flowchart illustrating basic steps of a second embodiment according to the present invention; Fig. 10 is a flowchart illustrating basic steps of a third embodiment according to the present invention; Fig. 11 is a chart illustrating principally activating pulses in the case of pulse width activation; Fig. 12 is a chart illustrating principally activation pulses in case of duty cycle activation; Fig. 13 is a chart illustrating principally activation pulses in case of duty cycle activation with a decreasing period and a constant duty cycle; Figs. 14.1 and 14.2 illustrate dot gain occurring with dots lined up along the main scan and sub scan direction respectively; Figs. 14.3 and 14.4 illustrate dot gain resulting from replication of microdots with that of a single halftone dot having the same total area; Fig. 15 gives a three-dimensional representation af a halftone dot measured with a high precision microscopic densitometer; Figs. 16.1 to 16.3 illustrate the results of comparative experiments on different timings for same halftone dots. DETAILED DESCRIPTION OF THE INVENTION The description given hereinbelow mainly comprises four chapters, namely (i) terms and definitions used in the present application, (ii) preferred embodiments of time modulated stochastic screening, (iii) preferred embodiments for making a lithographic printing plate, and (iv) further applications of the present invention. Explanation of terms and definitions used in the present description As an aid to understanding the discussion to follow, the meaning of some specific terms applying to the specification and to the claims are explained. An "original" is any (hardcopy or softcopy) representation containing information representative for density values (e.g. density, transmission, opacity ...) of an image. The term original also includes a socalled "synthetic image", e.g. composed by a computer program. Each original is composed of a number of picture elements, shortly called "pixels". The number of pixels depends on the spatial resolutions in mainscan or fast scan direction X and in subscan or slowscan direction Y. A "contone (or continuous tone) image" or a "multiple tone image" is a representation by digital data, either explicit (previously generated) or implicit (generated on the fly) of a contone original. A contone image comprises a matrix of elements; each element can take C different values of contone levels, wherein the number C of possible values must be greater than two ( C > 2; e.g. C = 256). A "halftone image" is a representation by digital data of a contone original. A halftone image comprises a matrix of elements, wherein the number of possible values H of said elements is lower than the corresponding number of contone levels C (so H < C). A "micro dot" or "elementary dot" or "recorder element" (generally abbreviated as "rel") is the smallest spatially addressable unit on a recording device. A rel can have any form, such as rectangular or hexagonal, or circular, or square. In connection with photographic materials which were subjected to a process comprising a frequency modulation screening of an original, a frequency modulated "halftone dot" is the smallest image unit that is rendered on said photographic material after exposing and processing said material. The size of a halftone dot can be equal to the size of a rel or may comprise several rels. A halftone dot can have any form, but usually its shape is square or rectangular. A "halftone neigbourhood" (of a halftone image) comprises a fixed number of rels in a predetermined arrangement. A halftone neigbourhood can have any form, but in a preferred embodiment its shape is square or rectangular. The "size of a halftone neigbourhood" is meant as being the length of a side when said halftone neigbourhood is a square; the length when said halftone neigbourhood is a rectangle; the diameter when said halftone neigbourhood is a circle; the longest axis when said halftone neigbourhood is an ellipse and the square root of the surface area when said halftone neigbourhood has another form. By the term "neigbouring halftone dot" is meant each halftone dot in the neighbourhood of a specific halftone dot within a distance equivalent to the size of a halftone neigbourhood. The "main dimension" of a frequency modulated halftone dot is the side when said dot is a square; the length when said dot is a rectangle; the diameter when said dot is a circle; the longest axis when said dot is an ellipse and the square root of the surface area when said dot has another form. A frequency modulated halftone dot is obtained by a mechanical, an optical, and/or an electrical manipulation of the recording beam of the recording device. Although a halftone dot can consist of precisely one rel, preferably said halftone dot consists of a [m * n] matrix of rels, wherein m represents 2 or a greater integer and n represents 1 or a greater integer. Although m and n can have different values they are preferably equal, yielding a halftone dot that is essentially a square, or optionally a rectangle if the mainscan-addressability X differs from the subscan-addressability Y. A tone value on a tone scale or grey scale relates to a degree or percentage of occupance or "coverage", sometimes also called "percentage value" or "occupied capacity" (e.g. a coverage of 50%; see also Fig. 4). Preferred embodiments of time modulated stochastic screening Before explaining in full detail figure 5, which shows a circuit to perform a frequency modulation screening usable for the present invention, first some basic information has to be given. Generally, if an image is scanned, the image information is converted into tone or grey values, normally between 0 (e.g. solid-tone) and 255 (e.g. white). For the resulting halftone film, however, there are only two possible states: the image area is either black (i.e. printing) or white (i.e. non-printing). The continuous-tone signal produced by the scanner (between 0 and 255) must therefore be transformed into a binary value (1 or 0). The simplest possibility is to code all grey values above a certain threshold value with 1 and the remainder with 0. It is obvious that, in this way, a large part of the image information is lost. In order to obtain a better reproduction result, various dot frequency modulation screening techniques are suitable. They can be divided into the following subclasses: 1) Point to Point thresholding based techniques; 2) Error Diffusion along a line by line, column by column scan; 3) Error Propagation along a Hilbert scan; 4) Special techniques. These techniques have already been described extensively in an earlier patent application EP-A-94.203.587.4 (in the name of Agfa-Gevaert). Therefor, hereafter only a concise survey is given just to initiate some basic hinsight. The most representative technique of point to point thresholding is the halftoning based on the "Bayer" dither matrix, cfr. Bayer, B.E., "An optimum method for two level rendition of continuous-tone pictures", Proc. IEEE International Conference on Communications, Conference Record, pp. (26-11), (26-15), 1973. A number of variations on this technique are developed, as e.g. disclosed in US Serial No 08/ 106.333, in US Serial No 08/ 116.254 and in US 5,111,310. Another frequency modulation screening techniques suitable for use in connection with the present invention is the well known Error diffusion first described by Floyd and Steinberg "An adaptive algorithm for spatial grey scale" SID 75 Digest, Society for information display 1975, pp. 36-37. Details about the error diffusion screening method may be found e.g. in US-P-5,175,804. Another preferred variant of frequency modulation screening for use in connection with the present invention is a method similar to the error diffusion with the exception that the order in which the image rels are processed can be described by a space filling deterministic fractal curve or a randomized space filling curve. A suitable deterministic fractal curve is for example the socalled "Hilbert Curve" disclosed by Witten Ian H., and Radford M. Neal, "Using Peano Curves for Bilevel Display of Continuous-Tone Images", IEEE CG&A, May 1982, pp. 47-52. Another method comprises a recursively division of the image into smaller matrices untill the size of a matrix reaches an image rel. At every subdivision into smaller submatrices a random ordering of processing the matrices is assigned to every submatrix. More details on these screening techniques are disclosed in e.g. EP-A-571.010 and EP-A-93.201.114.1. A special technique is disclosed in DE 2,931,092, and in US 4,485,397. Now, attention is given to figure 5, which shows a circuit to perform a frequency modulation screening in combination with a binary recording device, e.g. an imagesetter. First different building blocks of this circuit are described, later on its operation will be explained. Block 15 (sometimes called "pixelmap") is a memory block containing the contone dot values of an image. Typically these are 8 bit values, organized as N lines with M columns. Block 20 (generally called "bitmap") is a memory block with the same lay-out as block 15, in which the the halftoned pixel values will be stored. In the case of a binary recording device, every halftoned pixel word has a length of 1 bit. Block 45 is a device capable of imagewise exposing a substrate e.g. a photographic film or a lithographic printing plate precursor using the information in block 20. Block 40 is an arithmetic unit capable of calculating the sum of the pixelvalue P(i,j) and the error E at the output of a delay register 35. The conversion of a contone pixel value into a halftoned pixel value takes place in block 25. This conversion may be based on a thresholding operation: if the contone value at point (i,j) is below the value of 128, a value "0" is stored in the halftone memory, otherwise a "1" is stored. Block 30 contains an arithmetic unit that is capable to calculate the error between the original contone value, and the halftoned pixel value, and to store it in the delay register 35. Block 8 is a counter that sequences the processing of the N*M pixels of the image. Block 10 is a LUT with N*M entries (one for every image pixel), and a unique combination of a row and column address that corresponds with one pixel position in the image. Block 5 is a clock. The operation of Fig. 5 is now explained. At every clock pulse, the counter 8 is incremented, and a new pair of coordinates i(n),j(n) is obtained from block 10. These coordinates are used as address values to the pixel memory 15, to obtain a contone pixel value P(i(n),j(n)). This pixel value is immediately added to the error E(i(n-1),j(n-1)), that was stored in register 35 after the previous halftone step, and the sum of both is compared to the threshold value 26 in block 25. The outcome of the thresholding operation determines the value H(i(n),j(n)) that will be written into the halftone pixel memory at position i(n),j(n). At the same time a new error E(i(n),j(n)) is calculated from the difference between P(i(n),j(n)) and H(i(n),j(n)), and stored in the delay register 35. The circuit is initialized by setting the counter 8 to "1", the error to e.g. "256", and the operation is terminated when the counter reaches the level N*M. After that, the halftone memory 20 is read out line by line, column by column, and its contents are recorded on a substrate by the recorder 45. Each contone or multiple tone image pixel is carrying information about an address and an image signal. Each image signal is transformed to a bitmap signal B, being represented by e.g. an 8 bit value, depending (amongst others) on the required quality, memory capacity and performance. The contone image signals have preferably values from 0 to 255 (or 1 to 256), whereas the halftone image signals preferably have lower values (generally only 2). But all other number of signal levels can be handled by the same method. The final density resolution or tone resolution will not however be better than the number of levels in the original (or input image). For example, the images that can be processed range from 1 bit pro pixel pure binary halftone images to 8 bit per pixel contone images or even 10 or 12 bits per pixel per color to represent high quality contone images. These images can be processed using the same concepts behind the current invention. In such an imagesetter a socalled "raster image processor" (RIP) builds a binary bitmap image in memory, representing the halftone values, before sending the information as individual scan lines to the exposure unit. The presence of small dot areas, negative or positive, in the bitmap image can easily be detected by a special algorithm to be included (by software or by hardware implementation) in the bitmap creation functions of the RIP. In the exposure unit the timing of the dot exposure can be controlled and automatically adjusted to enhance the rendition of small dot areas. So, in the raster image processor (RIP) the bitmap information is modified selectively to correct the rendition of small dots. Small halftone dots are obtained by exposing a small number of rels, e.g. a small dot of 2 by 2 rels is exposed by turning the laser exposure on for two rels in two consecutive scan lines. The exposure of a small number of rels can easily be detected by checking the timing of the exposure of consecutive rels in a scan line. When the exposure sequence stays below a predefined limit (e.g. two rels), the exposure time is made longer automatically by a complete dot time or a fraction of that time. The longer exposure times makes the small halftone dots larger without changing the rendition of the large halftone dot areas. The same method can be used to correct the inverse exposure timing, e.g. a small unexposed area surrounded by a large exposed area. A short sequence of unexposed rels in a scan line is made longer by delaying the start of the next exposure. This makes the unexposed area somewhat larger to make sure that small halftone dots in the shadows are rendered correctly. According to a preferred embodiment of the present invention, a method for generating a screened reproduction of a multiple tone image comprises the steps of frequency modulation screening said multiple tone image to obtain screened data representing tones of said multiple tone image in terms of halftone dots; reproducing said halftone dots on an imaging element by means of a scanwise exposure; characterised in that said scanwise exposure for rendering a halftone dot is time modulated. In reference to Fig. 8, which illustrates basic steps of this method, it may be indicated that said screened data represent (still uncompensated) halftone dots, that one or more light sources may be used, and that said modulating is carried out in accordance to a location of corresponding halftone dots on a tone scale of said imaging element. In another preferred embodiment of the present invention an exposure time for rendering halftone dots of low tones is lengthened relative to halftone dots of mid tones, whereas an exposure time for rendering halftone dots of high tones is shortened. In a further preferred embodiment of the present invention (see Fig. 9), said modulating comprises the steps of capturing time compensation data with respect to a tone value on a tone scale; and combining said screened data with said compensation data to obtain time modulated data representative of compensated halftone dots. Said time compensation data with respect to a tone value on a tone scale relate to a coverage; e.g. a coverage of 3% on a film. Because in one part of a tone scale of an imaging element or imaging system the number of halftone dots may be low and in another part of said tone scale the number of halftone dots may be high, different time compensation data may be applied, said time compensation data are indicated as being "depending on the the location" of the corresponding halftone dots on said tone scale of said imaging element. Thus in a preferred embodiment of the present invention, an exposure time for rendering halftone dots is varied in acccordance with a particular tone to be reproduced. It may be stated that in this embodiment, and also in the immediately following embodiment, a compensating manipulation is carried out on the screened data before using them in the exposing step. In other embodiments to be described later on, said compensating manipulation will be carried out in real time ("on the fly"). Actual imagesetters as Accuset, Selectset and Avantra (manufactured and sold by Agfa-Gevaert) have a setting to optimize the rel pulse width for matching horizontal, vertical and diagonal line widths. A preferred embodiment of the present invention provides multiple Pulse Width Modulation (abbreviated as PWM) circuits and dynamically selects between them using some fixed criteria based on neighbouring dot states. In one embodiment of the present invention, said selection criteria are based on an individual scan line and on evaluating the dot pattern surrounding each dot to be imaged (socalled left and right neighbours) to select an appropriate PWM circuit. In a further embodiment of the present invention, said selection criteria are based on a larger area, including adjacent dots in previous and succeeding scan lines (socalled previous and succeeding neighbours), to have a more comprehensive sample to select the PWM circuit for each halftone dot to be imaged. In a still further preferred embodiment of the present invention (see Fig. 10), said modulating comprises the steps of capturing screened data representative of a first halftone dot; detecting at least one other halftone dot within a neighbourhood of said first halftone dot; forming compensation data for said first halftone dot with respect to said detecting; combining said screened data with said compensation data to obtain time modulated data representative of compensated halftone dots; and repeating all steps for each halftone dot to be printed. In a foregoing embodiment, said compensating manipulation is carried out rather straight-on or "in feedforward" in that it relies solely on an aimed location on a tone scale; e.g. in order to make a good record of a halftone dot corresponding to 3% nominally, a time compensation is made corresponding to 3.5 %. In a following embodiment, said compensating manipulation is carried out with checking an actual presence or abscence of halftone dots within a neigbourhood; e.g. if adjacent to a (first) halftone dot corresponding to 3% nominally no other halftone is present, a time compensation is made corresponding to 3.5 % for said first halftone dot. Thus in a preferred embodiment of the present invention, an exposure time for rendering halftone dots is varied in accordance with the number of neighbouring halftone dots or their relative position or their configuration. In order to explain more deeply the time modulation of the scanwise exposure of an imaging element according to the present invention, attention is given now to the exposure system itself. As an example, but non-limitative as to the scope of protection, a method for preparing a printing plate is known in which the method comprises the steps of informationwise exposing an imaging element and thereafter processing the exposed imaging element. Such a method is known as a "computer to plate" method, and can be carried out with the aid of different types of scanning devices. Fig. 6 schematically shows a flat bed type scanning device for use in a method according to the present invention; wheras Fig. 7 schematically shows an internal drum type scanning device for use in a method according to the present invention. The exposing step comprises generating a modulated writing light beam 60 having a wavelength to which an imaging element 70 is sensitive and directing a writing light beam 60 to a moving mirror 80 to cause the writing light beam 60 to scan across the surface of the imaging element 70 in a first scanning direction X. A reference light beam 65 is generated and directed to the moving mirror 80 simultaneously with the writing light beam 60 to cause the reference light beam 65 to scan across the surface of a light detecting element 75 in the first scanning direction X. Since the writing light beam 60 and the reference beam 65 are both deflected by the same moving mirror 80, the reference beam 65 falling on the light detecting element 75 causes the generation of synchronising signals indicative of the position of the writing light beam 60 on the surface of the imaging element 70. The writing light beam 60 is modulated and the imaging element 70 is moved in a second scanning direction Y in response to the synchronising signals generated by the light detecting element 75. In this manner any imperfections in the surface of the moving mirror 80, or in its movement, are accounted for. The writing light beam 60 is modulated as the scanning process proceeds, in order to informationwise expose the imaging element 70. The reference light beam 65 is not modulated, but is of constant intensity. In a drum type scanning device (either internal drum, external or revolving drum) said reference beam is usually replaced by an encoding-and- decoding system. For sake of simplicity, in Fig. 7 no encoder has been illustrated. Imagewise exposure in accordance with the present invention may proceed by a scanwise exposure by means of e.g. Cathode Ray Tubes (CRT's), Light Emitting Diodes (LED-'s) or lasers directly on the printing plate precursor, or it may be performed by first exposing according to said screened data an intermediate photographic film of high contrast, generally a high contrast silver halide film, and then using the imaged photographic film as a mask for exposing a lithographic printing plate precursor to a conventional light source in a camera exposure or contact exposure. Examples of lasers that can be used in connection with the present invention are e.g. HeNe lasers, Argon ion lasers, semiconductor lasers, YAG lasers e.g. Nd-YAG lasers etc.. In a still further preferred embodiment of the present invention, said modulating is carried out by modulating an emission-time of said light source. This can be carried out by advancing or delaying a start and/or an end of emission of said light source, by changing a period (or duration) of emission, etc. In pratice, such can be done by influencing the light source itself, or by influencing any optical means (e.g. a shutter, a mirror, a deformable mirror device, a prisme, etc). This embodiment thus modifies the timing of individual dot start time and/or stop time to effect a change in the physical size of the halftone dot, mainly in one direction (e.g. X). This concept has been developed in order to minimize the loss of tonal range in the dark tints due to recorder dot gain and light tints due to dot loss. In a further preferred embodiment of the present invention, said scanwise exposure is carried out by an exposure unit continuously or discontinuously moving across the imaging element during said scanwise exposure (cfr. Figs. 6 & 7). In a further preferred embodiment of the present invention, said scanwise exposure comprises a single pulse exposure, the pulse-width thereof being modulated. Fig. 11 is a chart illustrating principally activating pulses in the case of pulse width activation. In a further preferred embodiment of the present invention, said scanwise exposure comprises a multiple pulse exposure, wherein said time modulation is carried out by varying a number of pulses. In a still further preferred embodiment of the present invention, said exposing said imaging element by a light source is executed duty cycled pulsewise, wherein each "on-time" is followed by an "off-time". See Fig. 12, which is a chart illustrating principally activation pulses in case of duty cycle activation. In a still further preferred embodiment of the present invention, the ratio of an on-pulse pulse width to the sum of said on-pulse width and the off-time following said on-pulse, also called "duty cycle δ", is changed during said scanwise exposing said imaging element. See Fig. 13, which is a chart illustrating principally activation pulses in case of duty cycle activation with a decreasing period and a constant duty cycle. In a still further preferred embodiment of the present invention, said time modulating is carried out with a variable "modulation-ratio". (which is defined as the ratio of a modulated value of a time or a duty cycle to an unmodulated value of said time or duty cycle). In a still further preferred embodiment of the present invention, said variable modulation-ratio is changed according to the location of the corresponding halftone dot on a tone scale of said imaging element. In a further embodiment of the present invention, a method for generating a screened reproduction of a multiple tone image comprises the steps of frequency modulation screening the multiple tone image to obtain screened data representing tones of said multiple tone image in terms of halftone dots; reproducing said halftone dots on an imaging element by means of a scanwise exposure; wherein said scanwise exposure is carried out in response to a bitmap comprising a resolution which is, at least in the mainscan direction X, higher than the resolution of the raster image processor RIP. Thereabove, the bitmap resolution is used in a tone selective way, formerly indicated as being "depending on the the location" of the corresponding halftone dots on the tone scale of said imaging element. For example, suppose that an imagesetter has a X-resolution of e.g. 2400 dpi (or "dots per inch") and a bitmap has a X-resolution of e.g. 3600 dpi. Then, it is possible to control the lowtones ( or highlights if said imagesetter relates to a socalled "positive" working system) and hightones (or shadows) at 3600 dpi and the midtones at 2400 dpi. Alternatively, each microdot can be differentiated into a number of "submicrodots" each with smaller dimensions and thus capable for an improved spatial resolution. In a further embodiment of the present invention, an extra bitmap is created which determines for each halftone dot, and in relation to the location or the coverage, which frequency or which resolution has to be applied. A remarkable advantage of this embodiment is situated in the fact that by applying this method even "usual" (i.e. non-ideal) imaging elements and "usual" imagesetters provide a visual perceptable quality on the final print which otherwise (thus without applying the present invention) could hardly and only be attained by using optimized imaging elements and optimized imagesetters. According to a further preferred embodiment of the present invention, a method for generating a screened reproduction of a contone image comprises the steps of frequency modulation screening said contone image to obtain a halftone image comprising halftone dots; compensating a size or a shape of said halftone dots; and scanwise exposing an imaging element by at least one light source according to said compensated halftone dots, wherein said compensating is carried out in accordance to an estimation of a local density in a neigbourhood of said halftone dots. In a still further preferred embodiment of the present invention, said estimation is based on a contone value of a pixel corresponding to said halftone dot or to a number of halftone dots within a halftone neigh-bourhood. It may be remarked that both foregoing embodiments can be applied in realtime (on the fly). Preferred embodiments for making a lithographic printing plate Imagewise exposure in accordance with the present invention may proceed by a scanwise exposure by means of e.g. a laser directly according to said screened data on the printing plate precursor (socalled computer to plate) or it may be performed by first exposing according to said screened data an intermediate photographic film of high contrast, generally a high contrast silver halide film, and then using the imaged photographic film as a mask for exposing a lithographic printing plate precursor to a conventional light source in a camera exposure or contact exposure. An example of such an intermediate photographic film (as imaging element) is marketed by Agfa-Gevaert NV under the name AGFASTAR. Examples of photosensitive lithographic imaging elements are, for example, the silver salt diffusion transfer (generally referred to as DTR) materials disclosed in EP-A-410500, EP-A-483415, EP-A-423399, imaging elements having a photosensitive layer containing diazonium salts or a diazo resin as described for example in EP-A-450199, and imaging elements having a photosensitive layer containing a photopolymerizable composition as described for example in EP-A-502562, EP-A-491457, EP-A-503602, EP-A-471483 and DE-A-4102173. A preferred method for making a lithographic printing plate from a contone original according to the present invention comprises an additional step of developing a thus obtained imagewise exposed imaging element. One process for obtaining a lithographic printing plate by means of a DTR process uses an imaging element comprising, in the order given, a support with a hydrophilic surface such as a grained and anodized aluminium foil, a layer of physical development nuclei and a silver halide emulsion layer. An example of such an imaging element is marketed by Agfa-Gevaert NV under the name LITHOSTAR. The imaging element of the present embodiment is imaged using a scanning exposure followed by a development step in the presence of development agent(s) and silver halide solvent(s) so that a silver image is formed in the physical development nuclei layer. Subsequently the silver halide emulsion layer and any other optional hydrophilic layers are removed by rinsing the imaged element with water so that the silver image is exposed. Finally the hydrophobic character of the silver image is preferably improved using a finishing liquid comprising hydrophobizing agents. More technical details about said development may be found in e.g. EP-A-93.201.305.5 and US-SN-08/303,670 (both in the name of Agfa-Gevaert). A second type of mono-sheet DTR material comprises on a support in the order given a silver halide emulsion layer and an image receiving layer containing physical development nuclei e.g. a heavy metal sulphide such as PdS. The image receiving layer is preferably free of binder or contains a hydrophilic binder in an amount of not more than 30% by weight. Subsequent to imagewise exposure, the mono-sheet DTR material is developed using an alkaline processing liquid in the presence of developing agents e.g. of the hydroquinone type and/or pyrazolidone type and a silver halide solvent such as e.g. a thiocyanate. Subsequently, the plate surface is neutralised with a neutralising liquid. Details about the constitution of this type of mono-sheet DTR material and suitable processing liquids can be found in e.g. EP-A-423399, US-P-4501811, and US-P-4784933. Lithographic printing plate precursors of this type are marketed by Agfa-Gevaert NV under the names SETPRINT and SUPERMASTER. It may also be clear that said scanwise exposing can be carried out by a light source with a visible spectrum or with an infrared spectrum or with an ultraviolet spectrum, depending on the actual imaging elements. Imaging elements suitable for DTR processing typically have a maximum sensitivity within the range of 400 to 800 nm, depending upon the nature of the imaging element. Thus, LITHOSTAR LAP-B has a maximum sensitivity at about 490 nm, LITHOSTAR LAP-O has a maximum sensitivity at about 550 nm, and SUPERMASTER imaging elements also have a maximum sensitivity at about 550 nm. Thus, the writing light beam 60 preferably has a wavelength within the range of 400 to 800 nm, such as from 450 to 600 nm. Sensitivity spectra for imaging elements which can be used in the method according to the present invention may be found in the already mentioned EP-A-94.203.326.7. Alternatively, a lithographic printing plate may be prepared from a "heat mode" recording material as a lithographic imaging element. Upon application of a heat pattern in accordance with image data and optional development the surface of such heat mode recording material may be differentiated in ink accepting and ink repellent areas. The heat pattern may be caused by a light source such as a laser. The heat mode recording material includes a substance capable of converting the light into heat. Heat mode recording materials that can be used for making a lithographic imaging elements are described for example in EP-A-92201633, DE-A-2512038, FR-A-1473751, Research Disclosure 19201 of April 1980 and Research Disclosure 33303 of January 1992. According to a further preferred embodiment of the present invention, a method for making a lithographic printing plate from a contone original comprises the steps of frequency modulation screening said original to obtain screened data, modulating said screened data to obtain time modulated data; scanwise exposing a printing plate precursor by a light source according to said time modulated data; and developing a thus obtained scanwise exposed lithographic printing plate precursor. In a still further preferred embodiment of the present invention, a frequency modulated halftone dot, or at least 75% of all frequency modulated halftone dot, with a greatest main dimension on said lithographic printing plate has a main dimension between 14 and 100 µm. In a still further preferred embodiment of the present invention, a frequency modulated halftone dot, or at least 75% of all frequency modulated halftone dots, has a variable main dimension between 14 and 100 µm. Fig. 15 gives a three-dimensional representation af a single halftone dot measured with a high precision microscopic densitometer. Said halftone dot was made on a transparent film according to a frequency modulation method as usable in the present invention. More specifically, the measurement was carried out with the aid of a Leitz Ergoplan microscope in an opto-electronic system with a very high spatial resolution and a severe calibration procedure. Both axis of the groundplane indicate respectively an X- and a Y-displacement, whereas the vertical axis indicates optical density values. Especially in FM screening, halftone dots are so small that they must be measured in microns ( or thousandths of a millimeter). From this Fig. 15 it also may be understood that in normal halftone screening often small dots will not be rendered optimally, especially with imaging elements which are sensitive to edge-sharpness defects. In order to clearly demonstrate some remarkable advantages of the present invention, now the results of some experiments will be discussed. Using an apparatus as shown in figure 6 or 7, a commercially available silver salt diffusion transfer lithographic printing plate LITHOSTAR LAP-B was informationwise exposed with screened cyan, magenta, yellow and black separations of a color image. The image was screened by frequency modulation using the method of the present invention and the imaging element was subsequently developed using the processing liquid G5000B and the finishing liquid G5300B, each available from Agfa-Gevaert NV. The soformed printing plates were used on a conventional printing machine Heidelberg GTO52 using Hartmann Irocart inks to print 50 copies on paper sheet. The dampening liquid used was an aqueous solution containing 100% Rotamatic (commercially available from Rotaprint). The printing paper used was Zanders Ikonorex. The printed copies were examined and assessed for quality, specifically for contrast, tone range and finest rendered microdot size. Fig. 16 illustrates the results of comparative experiments on different timings for same halftone dots. Herein, Fig. 16.1 was measured with a high precision microscopic densitometer on a screen of halftone dots generated with a low pulse width (cfr. timing); Fig. 16.2 was measured on a screen of halftone dots generated with a normal pulse width; whereas Fig. 16.3 was measured on a screen of halftone dots generated with a higher pulse width (cfr. longer timing). From these figures, it may be clearly seen the size of the halftone dots can be influenced remarkably by use of the present invention. Further applicability of the present invention As may be clear from the description given before, a selective distortion correction, which adjusts only the small dots and has no effect on the large dots, allows to improve the halftone rendition so that an optimum number of halftone levels can be obtained on film or plates. The present invention provides a method for generating a screened reproduction of a contone image with a minimized loss of tonal range in the light tints due to dot loss and in the dark tints due to dot gain. It also provides an improved halftone rendition so that an optimum number of halftone levels can be obtained on film or plates. The method described in this invention improves the faithful rendition of small dots by selective correction of small overexposure areas, thus eliminating the need for tightly controlled spot sizes or materials with high gradients. This implies that the correction method described in this invention can be applied to improve the output quality of less expensive laser recorders and direct to plate exposure systems, exposing plates which do not have the steep gradient and short toe characteristics of graphic arts films. It may be noticed that the problem of insufficient quality, particulary of tone range in the final print, arises especially with imaging elements which are sensitive to edge-sharpness defects. In case of a color image, the above described screening process is performed on each of the color separations of the image. Preferably the color image is separated in its Yellow, Magenta, Cyan and Black components. Each of these components may then be screened and used to imagewise expose four lithographic printing plate precursors according to the present invention. Four lithographic printing plates, one for each color separation, will thus be obtained. The color separations can then be printed over each other in register in a lithographic printing machine using the four plates. The present invention is mainly directed at FM screening, i.e. Crystal Raster (registered tradename of Agfa-Gevaert), but is also applicable to improve AM screening, i.e. Agfa Balanced screening (registered tradename of Agfa-Gevaert). In amplitude modulation screening small dots appear only in the shadows and highlights, midtones are exposed as a single large cluster of pixels. The impact of spotsize and film characteristics on image quality and halftone rendition will be most noticeable in the small dot areas (highlights and shadows). Theoretical bitmap representations of 3 % and 97 % halftones may be distorted by exposure on films or plates to respectively lower values, 0 % to 1 %, and higher values, 99 % to 100 %. By adjusting the exposure of only the small dots, without affecting the large dot areas (midtones), the full range of the halftone values can be rendered. According to a further preferred embodiment of the present invention, the modulating of said screened data is carried out by modulating an emission-time and simultaneously modulating an emission intensity of said light source.

Claims: A method for generating a screened reproduction of a multiple tone image comprising the steps of: frequency modulation screening said multiple tone image to obtain screened data representing tones of said multiple tone image in terms of halftone dots; reproducing said halftone dots on an imaging element by means of a scanwise exposure being time modulated, characterised in that said exposure time for rendering halftone dots of low tones is lengthened relative to halftone dots of mid tones, whereas an exposure time for rendering halftone dots of high tones is shortened. A method for generating a screened reproduction of a multiple tone image comprising the steps of: frequency modulation screening said multiple tone image to obtain screened data representing tones of said multiple tone image in terms of halftone dots; reproducing said halftone dots on an imaging element by means of a scanwise exposure being time modulated, characterised in that said exposure time for rendering halftone dots of low tones is shortened relative to halftone dots of mid tones, whereas an exposure time for rendering halftone dots of high tones is lengthened. A method according to claim 1 or 2, wherein an exposure time for rendering halftone dots is varied in accordance with a particular tone to be reproduced. A method according to claim 1 or 2, wherein an exposure time for rendering halftone dots is varied in accordance with the number of neighbouring halftone dots. A method according to claim 1 or 2, wherein said scanwise exposure comprises a single pulse exposure, the pulse-width thereof being time -modulated. A method according to claim 1 or 2, wherein said scanwise exposure comprises a multiple pulse exposure, said time modulation being carried out by varying a number of pulses. A method according to claim 1 or 2, wherein said scanwise exposure is carried out by an exposure unit moving across the imaging element during said scanwise exposure. A method according to claim 1 or 2, wherein said imaging element is a lithographic printing plate precursor having a surface capable of being differentiated in ink accepting and ink repellent areas upon scanwise exposure and an optional development step. A method according to claim 8 wherein said lithographic printing plate precursor contains a silver halide emulsion layer and an image receiving layer containing physical development nuclei and wherein subsequent to said scanwise exposure said lithographic printing plate is developed using an alkaline processing liquid in the presence of developing agent(s) and silver halide solvent(s).

Inventors: BOSSCHAERTS JACOBUS, DELABASTITA PAUL, GOVAERT RENE, BOSSCHAERTS, JACOBUS, DELABASTITA, PAUL, GOVAERT, RENE, Bosschaerts, Jacobus, c/o Agfa-Gevaert N.V.,, Delabastita, Paul, c/o Agfa-Gevaert N.V.,, Govaert, René, c/o Agfa-Gevaert N.V.,
Applicants: AGFA GEVAERT NV, AGFA-GEVAERT N.V.
Application year: 1995
Main classification: H04N1
IPC codes: G03F7, H04N1, G03F5, B41C1, B41N1