WO2001089196A2

WO2001089196A2 - Method for controlling calibration and image sharpening of printing plates having integral photomask layer

Info

Publication number: WO2001089196A2
Application number: PCT/US2001/015751
Authority: WO
Inventors: Mark R. Samworth
Original assignee: Artwork Systems, Inc.
Priority date: 2000-05-16
Filing date: 2001-05-16
Publication date: 2001-11-22
Also published as: WO2001089196A3

Abstract

A method for correcting image dimensional changes in flexographic printing plates is provided. The method includes obtaining and analyzing data for a particular plate pertaining to the changes in linear image size between digital images and the images reproduced in a photomask, and the changes between photomask images and plate images following exposure and processing of the plate. The method provides for compensating for linear dimensional changes, such that image modification due to various printing processes sum to zero. The invention also addresses changes that are relevant to halftone production, and provides a method, apparatus and computer program to correct for image sharpening of printing plates.

Description

TITLE

METHOD FOR CONTROLLING CALIBRATION AND IMAGE SHARPENING OF PRINTING PLATES HAVING INTEGRAL PHOTOMASK LAYER

CROSS REFERENCE TO RELATED APPLICATIONS.

This application claims priority based on United States provisional application serial number 60/204,635 filed May 16, 2000 the contents of which are expressly incorporated herein.

BACKGROUND OF THE INVENTION

The present invention is directed to the field of digital image processing to produce digital image date for driving an exposure machine such as an image setter to image a printing plate, preferably a flexographic printing plate, and more particularly to a method and related software product for exposing a photomask layer on a plate surface having the photomask layer as an integral part thereof.

The advent of small yet powerful computers with advanced graphic capabilities has introduced profound changes in the printing industry. Gradually, the old method of generating images photographically and assembling them into signatures representing printed pages by hand, has been replaced by what is commonly referred to as desk top processing, publishing or prepress.

In desk top processing, an image and associated text are stored as digital data in a computer memory. This data undergoes image processing which may involve a number of different operations. Such image processing, involves operations such as color correction, density changes, spot elimination and page assembly, whereby multiple images and associated text are combined to form a page or package as will appear when printed. All these operations are controlled by software and the results are images displayed on a display screen.

Once a desired image is achieved on screen, the digital data representing this image is saved and a printing plate is generated. The printing plate prints an image on a printing medium corresponding to the image as viewed by the operator on his display screen.

The process of creating a printing plate from the digital image data comprises three stages. First the digital image is processed to generate virtual color separations with the proper halftones and line art, as well as the proper corrections for trapping where necessary. This corrected digital data is then forwarded to a film exposure unit commonly known as an imagesetter. As used herein the term imagesetter is understood to include typesetters and platesetters as well as any other form of equipment designed to receive control signals and comprising an exposure source for use in exposing a photosensitive medium. The imagesetter receives the digital data, which represents pixel by pixel exposure information for exposure of a photographic film medium. The imagesetter uses scanning laser beam that is focussed to a small spot representing the ultimate resolution capabilities of the equipment, to generate corresponding halftones and line art on the film medium, depending on the digital image data received. Following exposure, the film medium is developed to produce a film color separation.

The third step involves the actual making of the printing plate. A blank printing plate is exposed in contact with the film color separation and developed. Following development the printing plate has an image on its surface comprising halftones and line art corresponding to the continuous tone and solids of the image displayed on the display screen. In the particular case of a flexographic printing plate, such halftones and line art are represented by raised portions of the plate that extend to a height above a base, or floor, of the printing plate. An image is produced by contacting the surface of the raised portions with ink and contacting the inked surface with a printing medium such as paper, cardboard, cellophane, polyethylene terephthalate sheet material etc. Ink transfers from the plate to the printing medium. The use of multiple color separation plates printed sequentially in superposition, each with a different color ink, permits the reproduction of multicolored images as is well known in the printing arts.

Recently, there have been introduced in the printing industry a new type of printing plate, one that includes an integral exposure photomask coated on a surface of the plate. Such plates obviate the need to use a film intermediate during the exposure of the plate to generate the image thereon. The photomask, which typically comprises an opaque layer of carbon black in some soluble binding matrix, replaces the old film intermediate. In use, the mask is exposed directly by the imagesetter laser beam. The beam intensity is adjusted to vaporize the photomask where exposed, creating an image consisting of masked and open areas on the plate surface. The plate is next exposed to radiation through the open areas. Following exposure, the plate is developed in a solvent that removes the photomask. Depending on whether the plate is a negative working or positive working plate, the unexposed printing layer or the exposed printing layer of the plate is also removed. United States patent number 5,262,275 issued November 16, 1993 to Fan and assigned to E.I. Du Pont de Nemours and Co. Inc. discloses and claims such a printing plate particularly suitable for use as a flexographic printing plate.

Plates containing an integral photomask are diserable because their use eliminates the film color separations. It is therefore desirable to use the imagesetter to directly expose the plates with the integral masks. However, while the process would appear straightforward and simple, there are a number of problems in going from the display image to the printed image in a way that preserves the accuracy of the reproduction. These problems, include the following.

The image data is stored in digital format. Solids such as line art are represented by a sequence of exposed pixels to a digital value of 255 (in an 8-bit system) while halftones are represented by digital values ranging from 0 to 255. The image displayed on the display screen is a continuous tone image produced by an RGB color system while the printed image is a halftone image in an MYCK color system. Assuming that the digital image values indeed represent an accurate transformation of the image between the two color systems, one is still faced with reproduction problems as one goes from the digital image to the printed image. A common type of problem is a change in the dimensions of an image as it is reproduced. Such problems are referred to as image growth herein, even though the actual problem may be one of image shrinkage rather than growth.

Image growth is a well known problem in the printing industry and numerous solutions have been proposed. Such solutions range from efforts to compensate for such growth by modifying the digital image prior to exposing the intermediate film medium in the imagesetter, to altering the intensity of the exposure source in the imagesetter. The exact manner in which such solutions are implemented varies depending on the particular printing process involved. Thus, in typical flexographic printing applications, where a high contrast intermediate film negative is used to expose the flexographic printing plate it is known to modify the laser intensity of the exposing beam of an imagesetter to compensate for the mechanical gain in image size resulting during the step of transferring the ink from the printing plate onto the receiving medium on the printing press. The process simply involves comparing a printed image with the digital image and adjusting the exposure of the negative film intermediate by an appropriate amount to compensate for the image growth in the press. The aforementioned correction process has been proven adequate in situations where the flexographic printing plate is imaged using a film intermediate. This approach, however, has been less successful when used with direct exposure flexographic plates, that is plates that incorporate an integral photomask that can be exposed in an imagesetter. When a photographic film is used, the size of an exposed spot on the film depends among other things on light diffusion around the spot edges so that changing the exposure intensity of the laser beam resulted in an enlargement of the spot size. In correcting for image gain, it has always been assumed that there is no substantial image change during the step of plate making during the exposure of the plate through the film. Such assumption was justified by direct observation and is reasonable because the exposure process is conducted in vacuum, that is in the absence of oxygen. Vacuum is used to assure good contact between the plate and the film. Thus a vacuum frame is used in all instances where the image is on a film that is placed on a printing plate during the exposure of the printing plate. As most flexographic printing plates photosensitive layers comprise a photopolymerizable layer, such plates exhibit a sensitivity to the presence of oxygen which acts as an inhibitor to the polymerization process. When an integral photomask is used, the plate exposure process is not conducted in a vacuum frame but in air. The presence of oxygen during exposure appears to effect the polymerization in the exposed areas and the plate exhibits an unanticipated shrinkage in these areas.

Such shrinkage now introduces new errors in transferring a digital image from computer to paper, which the prior methods that were based on the assumption that there is none, or, at most, an insignificant image growth during plate exposure, do not address. This error affects both line art work and halftones differently. There is, therefore still a need for a method to compensate for plate shrinkage in a flexographic printing plate of the type comprising an integral photomask so as to generate prints from a digital image in a consistent, predictable manner rather than a hit and miss approach. SUMMARY OF THE INVENTION

According to the present invention there is provided a method for correcting for image dimensional changes in a flexographic printing plate comprising an integral photomask. The method includes the following:

obtaining data for a particular plate including: (a) the change in linear image size value (DM) between a digital image (D) and the digital image reproduced in the photomask (M) of the plate using the digital image (D), as a function of exposure of the photomask; and (b) the actual plate shrinkage value MP representing the linear dimension change between the photomask image (M) and the image on the plate (P) following exposure and processing of the plate

identifying a press cutback value PS for a particular press (also known as a press compensation value) as the linear dimension change between the printed image and the image on the plate (P). In cases where the printed image is to be an exact reproduction of the digital image, then the press cutback PP will equal the linear dimension change between the digital image and the plate image; and

adjusting, preferably, the exposure of the photomask so that DM + PS + MP

O.

The above adjustment is adequate for line work as both the photomask and the plate dimensional changes are reasonably constant for different solid areas. However in cases where there is involved in addition to line work halftone work as well, the situation is more complex. When halftones are present, the same linear dimension change represents a different percentage change in the grey scale appearance of different % halftone dot areas, resulting in inaccurate halftone reproduction. The problem is further aggravated due to the different behavior of the photomask and plate combination when very small dots or holes are reproduced representing highlights and shadows in the printed image.

To compensate for such additional problems, there is further provided according to the present invention the additional steps including obtaining the following additional data:

a first transfer function representing actual % dots created on the photomask for a given exposure level of the photomask, as a function of requested digital image % dots;

a second transfer function representing actual % dots produced on the printing plate following exposure of the plate through the photomask and development of the plate, as a function of photomask % dots; and

a third transfer function representing a desired % dot press compensation curve.

These curves are then used to form a Look-Up-Table (LUT) representing digital % dots sent to the imagesetter as a function of desired printed % dots, and the LUT is used to calculate the digital % dots prior to transmitting the digital image information to the imagesetter.

There is also provided according to the present invention a computer program product including: a computer useable medium having computer readable program code means embodied therein for correcting for image dimensional change in a flexographic printing plate comprising an integral photomask. The computer readable program also includes computer readable code means for retrieving the data for the plate. The data includes: an actual image sharpening value MP as a first linear dimension change between a photomask image and a first actual image on said plate representing the photomask image following exposure and processing of the plate; and a change in linear image size value DM between a digital image and said digital image reproduced in said photomask of said plate, as a function of exposure of the photomask;

The computer program also includes a computer readable code means for retrieving a desired plate shrinkage value PS as a second linear dimension change between the printed image on a substrate and the image reproduced on the plate; and means for calculating an exposure level for the photomask calculated to compensate for the plate shrinkage, such that DM -I- PS + MP=0, and for providing an output indicative of the exposure level which can be used to obtain the desired DM value.

The program may further comprise a computer readable code means for retrieving digital image data from memory representing: a first transfer f nction representing actual % dots created on the photomask for a given exposure level of the photomask, as a function of requested digital image % dots; a second transfer function representing actual % dot produced on the printing plate following exposure of the plate through the photomask and development of the plate, as a function of photomask % dots; and a third transfer function representing a desired % press compensation curve.

The program may further comprise a computer readable code means for calculating an altered digital image data such that the sum of the photomask size change, plate size change and press gain for said printing press equals a desired value, preferably 0; and a computer readable code means for applying the altered digital data to an imagesetter to expose the photomask on the printing plate.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a top view of a printing plate exposed to a single line having a width. Figure 2 is a schematic representation of an elevation cross section of a printing plate having an integral mask exposed to a line having a width, taken along arrows 2-2 in figure 1

Figure 3 illustrates the plate of figure 1 following exposure and development.

Figure 4 is a schematic representation of an elevation cross section of the printing plate of figure 3 , taken along arrows 4-4 in figure 3.

Figure 5 is a diagram showing the use of three transfer functions in deriving a LUT according to this invention.

Figure 6 is a flow diagram of the steps performed to compensate for image shrinkage when both line art and halftones are present.

DETAILED DESCRIPTION OF THE INVENTION

The features, advantages and operation of the present invention will become readily apparent and further understood from a reading of the following detailed description with the accompanying drawings, in which like numerals refer to like elements.

The phenomenon addressed by the present invention is illustrated in FIGS 1, 2,3 and 4. Now referring to FIGS. 1 and 2, there is shown a schematic representation of a photopolymer printing plate 14 with a photomask 12 applied thereon and a digital image line 8. The digital image line 8 has a width 8' . Following exposure of the photomask layer 12, digital line 8 results in a mask line 16 of width 16'. Depending on the exposure, the width of the photomask line 16' may be different, usually greater, than the width of the digital image line 8'. Exposure as used in this description is the product of the radiation intensity times the time during which the radiation is incident onto the exposed area. The transfer of the digital image to the photomask may be achieved through a laser that serves to remove the mask layer in the desired area. The phenomenon of image size variation upon transfer from digital image to the photomask is hereinafter referred to in general as "broadening", although the variation is not necessarily line broadening. A characteristic value for the linear dimensional change from the transfer of a digital image (D) to the photomask image (M) is assigned the designation DM. This characteristic value DM can be expressed as a function of radiation exposure.

After radiation exposure, the plate is developed in an appropriate solvent, and the mask and undeveloped plate material are removed leaving behind raised portion 24 corresponding to the original line 8 as shown in figures 3 and 4. The plate development process causes a further change to the dimensions of the line 8. The plate image 24 that is formed as a result of the plate exposure and development process has smaller dimensions 24' than the image formed in the photomask. This is believed due to the radiation exposure efficiency and the chemical process of plate development causing a physical shrinking of the plate. A characteristic value for the dimensional change effect between the photomask image and the developed plate image is assigned the designation MP.

A characteristic value for the combined image variation from the digital image to the printing plate image is assigned the designation PS.

A third image dimensional change phenomenon occurs when the plate is used on a press to print an image. This change is the result of pressing the plate against the surface than is printed. Press compression results in image broadening, and is commonly referred to as press gain. This results in a broadening of the image on the plate transferred on the printed output medium. The printed output medium may be any type of printed material, including paper, coating materials, metal, etc.

To practice this invention there is needed data about the dimensional change MP between the photomask and the plate following exposure and development. The MP value is measured as a linear dimension change between a photomask image and an actual image formed on the plate from the photomask image following exposure and processing. The MP values will vary depending on several factors including but not limited to: image dimensions, plate materials, photomask materials, development process and materials, and whether the plate is a negative or positive working plate.

The method of optimizing radiation exposure of the present invention also includes obtaining data regarding the change in linear image size between a digital image and the digital image reproduced in the photomask of the plate. As discussed above this characteristic value is designated as the DM value and is a function of radiation exposure of the photomask. Empirical analysis and measurement, of the effect of altering the exposure of the photomask to the imagesetter source is preferably used. A series of exposures of a solid line having a specified imagesetter input width may be used to expose a particular photomask on a particular plate using different source intensity settings. The line width reproduced on the mask is next measured for each source setting and its difference from the input width recorded to provide the dimensional change as a function of exposure. The DM values are expressed as a function of exposure. The data representing DM = F (E) is stored for use in the method of the present invention, preferably in a memory in a computer.

The third factor needed to practice the present invention includes identifying a desired image sharpening value, PS, as a linear dimension change between the printed image and the image reproduced on the plate. Quite often the PS value will be dictated by the operator in the field based on his experience with particular clients, plates, inks, etc. Such operators have often developed their own "press cutting" or "press gain" data, which may be used to obtain a PP value for a particular application. Preferably the linear dimensions of the printed image are equal to the linear dimensions of the digital image. Once the appropriate characteristic values, DM, PS and MP are determined for the relevant printing application, the mask exposure can be adjusted by selecting an exposure from the relationship DM = F (E) such that DM + PS + MP = O most accurately produce the desired printed image.

The present invention provides for adjusting the exposure of the photomask so that DM + PS + MP = O. Since PS and MP are both linear values that are determined empirically, or through information provided by the manufacturer, the difference between the PS and MP values will be a specific number. DM, however is defined as a function of mask exposure (E), and thus the value of DM depends on mask exposure. This dependency on mask exposure allows the optimization of the printing process so that the variations in image size are corrected for in the final printed image.

Alternatively, the exposure of the plate through the mask may be used to provide a variable that can be used to satisfy the DM+PS+MP=0 relationship. In such case it will be necessary to obtain and store data showing MP=f(E). In the alternative the digital data may also be corrected to compensate for the image dimensional change by generating an digital input to the imagesetter such that the resulting DM will provide the needed value to balance the equation DM+PS+MP=0.

The method of this invention can be extended to the production of halftones in printing applications. In optimizing exposure for the production of halftones, additional steps are performed according to the present invention. However, because a linear change in the halftone environment effects different halftones differently, there is need to develop the dimensional change in each of the three steps, i.e. DM, MP & PS in the form of transfer functions giving the % dot change corresponding to the linear change. For example, a transfer function representing actual % dots created on the photomask for a given exposure level of the photomask is obtained as a function of the requested digital image % dots. A calibration transfer function curve is generated or obtained that allows one to calculate the % dots that are actually created on the photomask when the % dots of the digital image is provided. This function is analogous to the DM value discussed above in relation to solid lines.

In addition to the transfer function correlating % dots of the digital image to % dots of the photomask, a second transfer function is generated. A transfer function representing actual % dots produced on the printing plate resulting from exposure and development of the plate is generated as a function of photomask % dots. This transfer function, or calibration curve, is analogous to the MP value discussed above in regards to solid lines. Finally, another transfer function representing a desired press compensation curve may also be generated. This third transfer function correlates the increase in % dots that occurs between the printing plate and the printed output due to press gain.

Having determined the required exposure for the solids based on the relationship DM + MP + PS = O, there is formed a Look-Up-Table (LUT). Such a Look-Up- Table provides digital image % dots as a function of desired printed % dots. The LUT is used to provide the proper digital value output to an imagesetter for a given digital value requested such that the proper % dot will be reproduced in a system whose exposure has been adjusted to compensate for line work dimensional changes.

The compensation curves and transfer functions discussed above and their use according to this invention are illustrated in graphical form in FIG. 4. The four- quadrant diagram of FIG. 4 sets forth exemplary compensation curves for a mask O 01/89196

-14-

response curve (Quadrant I), a plate response curve (Quadrant II), a press compensation curve (Quadrant III), and LUT curve (Quadrant IV). The axis are % dots in digital image (A), % dots of photomask (B), % dots on formed on the plate (C), and % dots on the printed output (O). Each of the four axis, A, B, C and O have 0% dots at the center with each axis extending out to 100% dots in halftone production.

The mask response curve of Quadrant I represents the non-linear correlation of halftone reproduction during mask exposure. In the discussion relating to solid lines, the mask response was characterized as an image growth phenomenon. This image growth phenomenon causes a higher % dots measurement in halftone production. As shown in the exemplary mask response curve of Quadrant I in FIG. 4, the mask response changes non-linear ly with % dots of the digital image.

As discussed in relation to solid line production, the mask response is a function of mask exposure. Thus a number of curves are developed and stored for various exposures. This curve may be produced for a variety of mask materials and masking techniques. A mask response curve can be generated through empirical studies, projected calculations, or from available information, such as a manufacturer's product disclosure information. A response curve will correlate the % dots produced on a photomask by a digital image of a known % dots value for halftones from 0% to 100% dots. Once the response curve is generated, it may be categorized according to the appropriate parameters and stored for further reference. The particular curve to be used will be determined by the DM exposure setting selected for the compensation required for the line art.

The plate response and press compensation curves of Quadrants II and III may be generated in similar fashion as the mask response curve. Data collected either through empirical measurement or projected analysis of available data may be used to generate curves for various plates and presses and processes. These curves are also non-linear and correlate the % dots halftone change that occurs as a result of the corresponding process.

The general shape of the exemplary curves shown in FIG. 4 suggest that the plate response results in image shrinkage, and thus a lower % dots value on the plate compared with the mask % dots value, whereas the press compensation curve generates a greater % dots value on the output medium as compared to the plate % dots value. The shape of these exemplary curves also demonstrate the non-linearity of the response curves as a function of % dots value. However, the shapes of the curves are only used for illustration of this invention and are not limiting the invention to such curves only.

The combination of the four compensation curves provides for the optimization of halftone production in printed materials. For example, assume that a portion of an image requires a 40% dot size. The LUT is used to calculate the % dot size that will be requested from the imagesetter. The dotted line forming the smallest rectangle on the diagram intersects the O axis at 40% dots. By following the same rectangle to the LUT curve, it is seen that the % dots that should be used by the imagesetter exposing the photomask in order to achieve 40% dots in the printed output should be 20%. Continuing to follow the dotted line of the innermost rectangle, the rectangle intersects the mask compensation curve, which shows that such 20% digital dot will result to a 40% value on the B axis. Continue following the dotted line intersection. The plate compensation curve of Quadrant II shows that the 40% dots of the photomask change to 20% dots on the plate, as shown on axis C. Still following the dotted line in Quadrant III after accounting for the press gain, the 20% dots on the plate are reproduced as 40% dots halftone on the printed output.

Two other exemplary scenarios are represented by larger dotted line rectangles. The middle rectangle illustrates that in order to form 60% dots on the printed medium with these components, the digital image has 30% dots to form a mask with 52% dots, which correlates to a plate with 32% dots. In a similar manner the outer dotted rectangle shows the process for a requested 80% dot area.

Optimizing the exposure to appropriately compensate for image dimensional changes in flexographic printing can be facilitated with a computer program product that performs the analysis and determines the appropriate exposure. A computer program product of the present invention includes computer program code for correcting for image sharpening in a flexographic printing plate comprising an integral photomask by adjusting an exposure of the photomask, as described above. The computer program code may be stored in a computer accessible manner on a computer disk or on a sever site available for authorized retrieval, such as downloading from a secure website. The computer readable program of the present invention includes means for retrieving data relevant to the printing process. Appropriate data for the program includes the actual plate image dimensional change value as the linear dimension change between the photomask image and the image on the plate, referred to above as MP. The data include a DM value as a function of mask exposure.

The computer readable code also includes means for retrieving a desired image sharpening value, referred to above as PS, as well as program code for calculating the exposure for the photomask. The compensation calculated by the computer program is determined in part by satisfying the condition DM = PS - MP. The computer program provides an output indicative of the exposure required to satisfy this condition.

The computer program may additionally output machine-readable code, which can be received by an exposure device. This allows the computer program to directly adjust the mask exposure according to the compensation calculation.

The computer program of the present invention may also include code for calculating the altered digital image data such that the photomask size change, plate size change and press gain for the printing press sum to a desired value. The computer program also includes means for outputting the altered digital image data. The method of output may vary and includes a printout of critical information, or a screen display of the information. Preferably, the computer would output the calculated adjustments directly to the devices employed in the printing process. This would require additional communication capabilities and configuration of the program. These capabilities may allow the computer to apply the altered digital data to an imagesetter to expose the photomask on the printing plate.

Figure 5 shows a flow diagram of the process of the present invention as may be implemented in a computer implementation.

First the data for the dimensional change between the mask image and the plate image following exposure and development of one or a plurality of plate is obtained (52) and stored (54) as values MP. Similarly the data providing DM as a function of exposure is generated (56) and stored (58).

Before implementing the program, the operator inputs (60) the press gain PS value. The computer program next solves the relationship DM+PS+MP=0 (62) preferably by selecting the proper exposure that will give DM such as to satisfy the equation. (Depending on which parameters are amenable to controled variation, the equation may be solved for PS or MP as well). If the image is only line work the process ends here with either an output of the required exposure value or a direct control of the radiation source to produce the required DM value.

If the image includes halftones the program next uses the LUT (78) to provide digital data input correction to the imagesetter. The LUT has been derived from the transfer curves as described earlier in the specification (72, 74, and 76) and has been stored in a memory. The LUT correction factor is applied to the data (68) and the corrected data is supplied (70) to the exposure unit. In the alternative each of the transfer functions 72,74, and 76 may be stored and the LUT correction factor calculated for each digital value before this digital value is sent to the imagesetter.

Those having the benefit of the above description of my invention may perform different variations, such as controlling the three factors that must be summed to either 0 or a desired value in different ways. However all such changes that result in implementing the relationship DM+PS+MP=0 are considered to be within the scope of my invention in which I claim:

Claims

L A method for correcting for image dimensional change in a flexographic printing plate comprising an integral photomask, the method comprising:

I. obtaining the following data for said plate:

a. a linear dimension change in line art image size value MP equal to a linear dimension change between a photomask image and an actual image on said plate representing said photomask image following exposure and processing of said plate; and

b. a linear dimension change in line art image size value DM equal to a linear dimension change between a digital image and said digital image reproduced in said photomask of said plate, as a function of exposure of said photomask;

II. identifying a plate to press linear dimension change in line art image size value PS as a linear dimension change between a printed image and said actual image on said plate; and

III. adjusting DM, PS and MP so that PS+DM+MP=0.

2. The method according to claim 1 wherein PS is the same as a measured image growth between a linear dimension of said actual image on said plate and a digital image used for generating said actual image on said plate.

3. The method according to claim 1 wherein DM is derived as a function of exposure and DM is selected so that DM+PS+MP=0.

4. The method according to claim 1 wherein PS is an average value of a plurality of measured image growth values between a linear dimension of an image on a plurality of plates and the same image printed on a plurality of output mediums.

5. The method according to claim 1 wherein there is further provided the additional steps of:

IV) obtaining the following additional data:

(a) a first transfer function representing actual % dots created on the photomask for a given exposure level of the photomask, as a function of requested digital image % dots;

(b) a second transfer function representing actual % dot produced on the printing plate following exposure of the plate through said photomask and development of the plate, as a function of photomask % dots; and

(c) a third transfer function representing a % dot press compensation curve;

V) forming a Look-Up-Table (LUT) by combining said transfer functions to generate a correction factor for adjusting input digital % dots to compensate for said adjusted DM, PS and MP; and

VI) adjusting the digital % dots prior to transmitting said digital image information to the imagesetter using the LUT so that printed % dot values have desired printed % dot values.

6. The method according to claim 5 wherein said press compensation curve is a press growth compensation curve.

7. The method according to claim 5 wherein said press compensation curve represents a desired cut back curve.

8. A computer program product comprising:

a computer useable medium having computer readable program code means embodied therein for correcting for plate shrinkage in a flexographic printing plate comprising an integral photomask, the computer readable program comprising:

I. computer readable code means for retrieving the following data for said plate:

II. computer readable code means for identifying a plate to press linear dimension change in line art image size value PS as a linear dimension change between a printed image and said actual image on said plate; and

calculating program code means for calculating an adjustment for DM, PS and MP so that PS+DM+MP=0.

9. The computer program according to claim 8 further comprising machine readable output code means for adjusting an exposure of said flexographic printing plate.

10. The computer program according to claim 8 further comprising:

IV. computer readable code means for retrieving digital image data from memory representing :

(c) a third transfer function representing a % dots produced on the plate as a function of the digital image % dots value;

V. computer readable code means for calculating an altered digital image data such that the sum of the photomask size change, plate size change and press gain for said printing press equals a desired value; and

VI. computer readable code means for outputing said altered digital image data.

11. The computer program according to claim 10 wherein said readable code means for outputting said altered digital image data include machine readable code means for applying said altered digital data to an imagesetter to expose said photomask on said printing plate.