DE4303081A1 - Exposure calibration for colour recorder or printer - involves computation of darkening function for nominal grey values for determn. of corrective transfer function - Google Patents

Abstract

A front-end computer (10) comprising a scan generator (10') and calculation stage (10'') is used with a raster image processor (11), recorder (12), film processor (13) and measuring device (14). A transfer function is determined from pixel-by-pixel and line-by-line exposure of the recording material, using a linearly patterned scan and measuring the surface coverage of the developed pattern. Relative darkening increases are used to derive the effective increase of a scanning point with allowance for provided scan parameters. ADVANTAGE - Nearly automatic calibration can be performed with greater accuracy in shorter time, relieving operator of routine tasks.

Description

The invention relates to the field of electronic reproduction technology and relates to a method for exposure calibration of recording devices for pixel and line-wise exposure of screened areas such as Images or color separations on a recording material by at least an exposure beam generated in an exposure unit and an on direction to carry out the procedure. A recording device of this type is also called imagesetter, printer or recorder.

The recording device can, for example, be a color separation recorder Production of halftone color separations for multi-color printing, with the halftone color separations "yellow", "cyan", "magenta" and "black" and exposed line by line. When overprinting the color separations too A multi-color print can cause troublesome moiré phenomena. For The individual color separations are avoided with these moiré phenomena Grids of different screen angles and / or screen widths exposed, at for example the color separation "yellow" with a screen angle of 0 °, the color abstract "cyan" with a screen angle of + 15 °, the color separation "magenta" with a screen angle of -15 ° and the color separation "black" with a screen angle of 45 °.

A grid is made up of a large number of periodically repeating ones Grid mesh or grid cells together, depending on the Grayscale values to be reproduced (tonal values) differently sized screen dots (Grid spots) are generated. Each grid point within a grid mesh  is made up of pixels or surface elements that correspond to the exposure beam can be recorded. The respective ratio between the area the grid mesh and the area of that recorded in the grid mesh Grid point, called area coverage, determines the reproduced Grayscale.

In such a color separation recorder, the image signal values, which are the represent gray values to be recorded, fed to a raster generator, in which the image signal values after a raster function, also spot function called, converted into corresponding control signal values for the exposure unit be changed. During a point and line relative movement the tax switches between the exposure beam and the recording material signal values the exposure beam on and off and thus determine which pixels exposed as parts of the halftone dots on the recording material or Not. The raster function determines the size of the raster points of the gray values to be recorded as well as the halftone dot shape. The Rela tiv movement between exposure beam and recording material takes place in the in most cases through a continuous or gradual transport of the Recording material in a direction perpendicular to the line direction Feed direction. The exposure beam is, for example, a laser beam that by means of a modulator acted upon by the control signal values is turned off. The exposure unit can be designed so that it one exposure beam or multiple exposure side by side rays generated.

A raster process for recording color separations with any raster Screen angles and / or screen widths is, for example, from DE-A-28 27 596 known.

When recording raster images and color separations, they soften the real screen dot sizes actually generated in the recording material or gray values (actual gray values) of the desired, nominal gray values (Target gray values) since each pixel and therefore each raster point more or is recorded less enlarged. The reason for the enlargement of the pixels is a halo that forms around each pixel and the intensity of the exposure is dependent on radiation. The difference between the real gray values and  the nominal gray values in relation to the nominal gray values is as Blackening increase or also called gray value increase.

Because an increase in density leads to disturbing changes in the tone value of the reproduction leads to the increase in density in practice compensated for by calibrating in front of the actual exposure Phase a correction curve, also called transfer curve, through a Exposure calibration is determined and by the during the exposure Image signal values, which represent the nominal gray values, after the Transfer curve are corrected so that the real recorded gray values correspond to the nominal gray values.

With an exposure calibration, various parameters such as type of Recording device, resolution of the recording device, density setting of the recording material used in each case and the respectively used Screen parameters such as screen function, screen size (screen frequency) and screen angles are taken into account.

With conventional exposure calibration, only one has to be changed this parameter creates a new, current transfer curve over the entire Calibration process can be determined. This need turns out to be special disadvantageous with regard to the raster parameters, since these are changed frequently. In the Practice is therefore complex in manual density measurements and perform time-consuming calibration processes in which the recording device is not available for usable exposures.

The object of the present invention is therefore to provide a method for exposure Calibration of a recording device and a device for through to specify the procedure, with which an almost automatic sequence with greater calibration accuracy in less time and with less effort is feasible and with which the operator can perform routine work is burdened.

This object is achieved with respect to the method by the features of the claim 1 and solved with respect to the device by the features of claim 15.  

The main advantage of the method according to the invention is that the manual steps on density measurement for two Control grid and on the mandatory density setting of the recording reduce equipment. This allows the process to be largely automated sieren. Another major advantage is that the update the transfer curve over the entire calibration process when changing one of the Often changing grid parameters can be omitted, making the calibration accurate increased and the time required is significantly reduced.

Advantageous further developments are specified in the subclaims.

The invention is illustrated with reference to FIGS. 1 to 5.

Show it:

Fig. 1 shows an X-control grid and a Y-control grid in the form of line patterns,

FIG. 2a is a line element made of an X-control grid,

Fig. 2b is a line element consisting of a Y-control grid,

Fig. 3 is a graphical representation of the function Schwärzungszuwachs in dependence of the line width in the control grids,

FIG. 4a is a graphic representation for explaining the Schwärzungszuwachses on a real raster point,

FIG. 4b shows a further graph for explaining the Schwärzungszuwachses at an ideal grid point, and

Fig. 5 is a block diagram of a device for exposure calibration.

The procedure for exposure calibration of a recording device is explained in detail below with the aid of process steps [A] to [F].

In process step [A] with the recording device X control grid and a Y control grid in the form of orthogonal to each other oriented line patterns on the one intended for later useful exposure Recording material exposed.

Fig. 1 shows in a section an X-control grid (90 °), the lines (1) transversely, are oriented to the recording direction (2), preferably perpendicular direction to the recording (2) and a Y-control grid (0 °), whose Lines ( 1 ) run in the direction of the drawing ( 2 ). The grid areas ( 3 ) of the X control grid and the Y control grid each represent a nominal target gray value (G _xSoll ; G _ySoll ). The nominal target gray values (G _xSoll ; G _ySoll ) of the grid areas ( 3 ) are each determined by the ratio of the thickness (n _x ; n _y ) of a line ( 1 ) to the line interval (d _x ; d _y ) according to equations [1] certainly.

G _xset = n _x / d _x

G _yset = n _y / d _y [1]

The nominal gray values (G _xSoll ; G _ySoll ) of the grid areas ( 3 ) can have any value between 0% and 100% area coverage of the theoretically possible nominal gray value range and can be chosen to be the same or different for the grid areas ( 3 ) of the two control grids. For the evaluation of the control rasters, it has proven advantageous to expose the raster areas ( 3 ) in an area coverage area of 20% to 40%, preferably with an area coverage of 25%.

The lines ( 1 ) of the X control grid and the Y control grid are exposed pixel by pixel using the exposure unit of the recording device, each line ( 1 ) being formed from at least one row of pixels.

In practice it has been shown that a relevant, measurable blackening increase - due to the spot size of the exposure beam on the recording material, the set intensity of the exposure beam and the type of recording material (film; photo paper) - only with 2 or 3 side by side lying or overlapping pixels occurs. It has therefore proven to be expedient to form the lines ( 1 ) of the control grid from n = 2 to 3 pixel rows in each case in order to achieve a measurable increase in blackening in the control grid. It also proves expedient to only take pixel rows with <= n pixels into account when determining the density increases in raster points.

FIG. 2a shows a section of a line ( 1 ) from the X control grid and FIG. 2b shows a corresponding section of a line ( 1 ) from the Y control grid.

In the example shown, the lines ( 1 ) each consist of three overlapping rows of pixels ( 4 ). The thickness of a line ( 1 ) is three pixels ( 5 ) (line thickness n = 3). Each row of pixels ( 4 ) is recorded with a corresponding increase in density ( 6 ) due to the enlargement of the pixels ( 5 ). Of the increases in density of the individual pixel rows ( 4 ), only the increases in density of the edge rows contribute to the enlargement of the lines ( 1 ).

From the exposed and developed control grids, the following process steps [B] and [C] the blackening increases of the control Determine grid separately in X and Y direction.

From the X control grid, the measurement of the area coverage of the grid area ( 3 ) can be used to selectively determine the X density increase in the X direction perpendicular to the line direction, since the corresponding density increase in the line direction has no influence on the measurement of the area coverage of the grid area ( 3 ) of the X control grid.

Similarly, the Y control grid can be determined selectively by measuring the area coverage of the grid area ( 3 ), the Y density increase in the Y direction perpendicular to the line direction, since the corresponding density increase in the line direction has no influence on the measurement of the area coverage of the grid area ( 3 ) of the Y control grid.

In method step [B], the grid areas ( 3 ) of the exposed and developed X control grid and the Y control _{grid are determined in terms of density} to determine the actually exposed, real gray values (G _xIst ; G _yIst ) and from the nominal gray values (G _xSoll ; G _ySoll ) and the real gray values (G _xIst ; G _yIst ) the X-total _{blackening increase} (S _xControl ) and the Y-total _{blackening increase} (S _yControl ) of the grid areas ( 3 ) are calculated according to equations [2].

S _xControl = (G _xIst - G _xSoll ) / G _xSoll

S _yControl = (G _yIst - G _ySoll ) / G _ySoll [2]

In a process step [C], the relative X-density increase (S _xRel ) and the relative Y-density increase (S _xRel ) and the relative Y-density increase (S _yRel ) are determined from the total X- _{density increase} (S _xControl ) and the Y-total _{density increase} (S _yControl ) of the grid areas ( 3 ) of the control _grid ) for at least one pixel ( 5 ) of a pixel group, preferably for one pixel ( 5 ), calculated using a correction factor according to equations [3].

S _xRel = S _{xControl *} n _x

S _yRel = S _{yControl *} n _y [3]

The correction factor takes into account the line thickness n _x or n _y in the control rasters and allows the determined total blackening increases to be reduced to the relative blackening increases of a pixel. The correction factor is obtained from the function relative density increase = f (1 / n) with n = line thickness in pixels, which is shown in FIG. 3.

The determination of the relative density increase of a pixel from the Total darkening of the control grid is based on the following considerations underlying, the determination of the increase in density in a coordi naten direction (X direction; Y direction) is considered.

A row of pixels formed from n pixels (n = length of the row of pixels) considered. Each pixel in this row of pixels produces a pixel blackening and a relative increase in density in the direction of expansion of the row of pixels. The total blackening of the row of pixels then results from equation [4].

n _* pixel blackening + blackening increase of the series [4]

Taking into account the fact that only the first and last pixel on the The increase in density of the pixel series is involved, the increase in density the pixel row equals the relative density increase of a pixel, and the Total blackening of the row of pixels results from equation [5].

n _* (pixel blackening + relative blackening increase _pixel / n) [5]

It follows that the relative density increase of a row of pixels in Coordinate direction of the pixel row corresponds to the following function:

relative density increase _series = f (1 / n) [6]

The relative increase in density of a row of pixels is thus reversed proportional to the length of the row of pixels.

This functional relationship according to equation [6] can be explained in the Control grid and, as will be shown later, also on a grid point transfer.

A row of pixels appears in the lines ( 1 ) of the control grid. Fig. 2a and Fig. 2b respectively show such a pixel row (7), which consists of n = 3 pixels (5). The number n pixels of the row of pixels ( 7 ) corresponds to the thickness (n) of the lines ( 1 ). The relative increase in density of a pixel ( 5 ) in the control grid according to equation [7] is inversely proportional to the line thickness (n).

rel. Density increase + f (1 / n) [7]

FIG. 3 shows the relative density increase as a function of the line thickness in pixels for the control grid with a constant, nominal area coverage of 25%, which is achieved with a line thickness of 3 pixels and a line interval of 12 pixels. This function is calculated (ideal course) or determined experimentally by measurements (real course).

From the increase in density of a pixel determined in the control grid after process step [C], the corresponding can then according to the invention Blackening increases in the grid points of the grid to be recorded Process step [D] can be calculated. The following considerations.

Fig. 4a shows a real halftone dot (pixel group) from a number of overlapping pixels ( 5 ), each pixel ( 5 ) having an annular increase in density ( 6 ). The pixels ( 5 ) generated by the exposure beam have approximately the shape of ellipses. The deviation from the ideal circular shape causes different blackening increases in the X and Y directions, which can be determined differently, just as with the control grids in the X and Y directions.

For the calibration process, the real, elliptical pixel shape can be simplified with sufficient accuracy to an ideal rectangular shape, which is shown in Fig. 4b.

FIG. 4b shows an idealized raster point of a raster. The raster point represents no Schwärzungszuwachs (6) a certain nominal gray-scale value (G _Soll) and an enlarged real gray value (G _actual) with Schwärzungszuwachs (6).

The grid point is made up of rectangular pixels ( 5 ) oriented in the X and Y directions. Each pixel ( 5 ) can be addressed by coordinates (x; y) of an XY coordinate system ( 8 ). The pixels ( 5 ) are arranged in the X and Y direction oriented pixel rows ( 7 ) of different lengths.

The increases in density of the individual rows of pixels ( 7 ) can be recorded separately, only those on the increases in density ( 6 ) of the individual rows of pixels ( 7 ), as in the case of the lines ( 1 ) of the control grid, only those at the edge of the relevant pixels Row-lying pixels (boundary pixels) are involved and the different lengths of the pixel rows ( 7 ) are taken into account by the function specified in equation [7] and shown in FIG. 3.

From the blackening increases calculated for the individual pixel rows ( 7 ), the blackening increases for the screen dot are determined by averaging, whereby the screen parameters such as screen width, screen angle, screen function and positive / negative recording have to be taken into account.

The raster function is generally used to convert the gray values into below grids of different sizes can be understood. It is irrelevant the procedure for this implementation. For example, the Implementation with the help of a grid mountain (threshold mountains), from abge stored threshold value matrices or from those already saved at an angle Halftone dots are performed.

The entire process is described below in process step [D]. The relative blackening _gains (S _xRel ; S _yRel ) obtained in process step [C] are used as a basis for the effective blackening increases of the grid point to be determined after step [D], since the function blackening gain = f (1 / n) according to equation [7 ], as already explained, is valid both for the increase in density for the lines of the control raster and for the increase in density for the pixel rows within the raster points.

In process step [D], the effective density increases in X and Y direction and from this the effective total blackening increases for the grid points are determined, whereby only pixel groups or pixel rows with <= n pixels are taken into account.

The following applies to each row of pixels ( 7 ) in the X and Y directions of a raster point at a raster angle β = 0 °:

The increase in density (S _{x series} ; S _{y series} ) of a row of pixels ( 7 ) results from equations [8] from the relative increase in density (S _xRel ; S _yRel ) of a pixel ( 5 ) and from the number (n _x ; n _y ) Pixels ( 5 ) or from the length of this row of pixels ( 7 ). The lengths of the pixel rows ( 7 ) depend on the grid width (grid frequency) and the grid point sizes via the grid function of the grid used.

S _{x series} = S _{xRel *} 1 / n _x

S _{y series} = S _{yRel *} 1 / n _y [8]

A raster point is typically (see FIG. 4b) composed of several pixel rows ( 7 ) of different lengths in the X and Y directions. The increases in density in the X and Y directions for the raster point are calculated from the mean values of the increases in density (S _{x series} ; S _{y series} ) of the individual pixel rows ( 7 ). The averaging is carried out using form factors (r _x ; r _y ).

The increases in density (S _x ; S _y ) in the X and Y directions of a raster point therefore result from equations [9] from the relative increases in density (S _xRel ; S _yRel ) and the form factors (r _x ; r _y ) that depend on the contour of the grid point, ie on the grid function and possibly on the grid angle (β) of the rotated grid.

The form factors (r _x ; r _y ) are the ratio of the number of pixel rows ( 7 ) in the X and Y direction (X rows; Y rows) and the total number of pixels ( 5 ) in the raster point . Only pixel groups with <= n pixels are taken into account, ie for the number of pixels ( 5 ) only those pixels ( 5 ) in the pixel row ( 7 ) are evaluated which have a pixel sequence of <= n in the evaluated coordinate (x; y) of the XY coordinate system ( 8 ) lie ( Fig. 4b).

S _x = S _{xRel *} r _x

S _y = S _{yRel *} r _y [9]

with form factor r _x = number of X rows / number of pixels and
with form factor r _y = number of Y rows / number of pixels.

In the case of a grid rotated by a grid angle (β) with asymmetrical grid points (e.g. ellipses, lines, etc.), the number of X rows and of Y rows as well as the contour of the grid points and thus the form factors (r _x ; r _y ). The change in the halftone dot contour is so small, however, that it can be neglected when determining the density increases without serious loss of accuracy.

The changed form factors (r * _x ; r * _y ) taking into account the screen angle (β) of a rotated screen result from equations [10], whereby only the screen rotation in a quadrant is considered.

r * _x = (r _{y *} angle / 90 °) + r _{x *} (1 - angle / 90 °)

r * _y = (r _{x *} angle / 90 °) + r _{y *} (1 - angle / 90 °) [10]

with angle = screen angle β modulo 90 °.

The effective density increases (S _xEff ; S _yEff ) in the X and Y directions for a screen point, taking into account the screen angle (β), is according to equations [11]:

S * _xEff = S _{xRel *} r * _x

S * _yEff = S _{yRel *} r * _y [11]

and the effective total blackening increase (S * _Eff ) of the halftone dot according to equation [12]:

S * _Eff = S * _xEff + S * _yEff [12]

The density increase function is formed in method step [E].

For this purpose, the function S * _Eff = f (G _Soll ) is first calculated for nominal gray values of the entire gray value range between black and white, taking into account the current raster parameters (raster width, raster angle and raster function) for later useful exposure. Selected nominal gray values (G _target ) of the theoretically possible gray value range between 0 and 100%, for example gray values with increments of 5%, are called up. A corresponding fictitious model raster point ( FIG. 4b) is generated for each gray value called (G _Soll ), the shape and size of which is dependent on the respective nominal gray value via the raster function. It does not matter which grid method the nominal gray values are converted into grid points.

The effective overall blackening increase S * _Eff is then calculated for each generated model screen dot according to equations [8] to [12] given in method step [D], taking into account the screen parameters, the screen width, screen angle and screen Function influence the number of pixels in the pixel rows and the form factors.

A possible setting of the recorder to negative recording is taken into account by arithmetically inverting the respective black / white content of the pixels ( 5 ).

The determined relationships between effective total blackening increases S * _Eff and nominal gray values (G _target ) are recorded in a table.

The density increase function f _S then results from normalization

f _s = G _target * (1.0 + S * _Eff ) / 100.

In process step [F], the correction function (transfer function) f _{T is} formed from the blackening increase function f _S by conversion and stored for later exposure. During the exposure, the nominal gray values (G _target ) are then converted into corrected gray values (G _Kor ) after the transfer function f _{T in} such a way that the actually recorded gray values correspond to the nominal gray values.

Fig. 5 shows a basic embodiment of a device for carrying out the exposure calibration for a recorder or image setter.

The device essentially consists of a front-end computer ( 10 ), a raster image processor (RIP) ( 11 ), a recorder ( 12 ), a film processor ( 13 ) and a measuring device ( 14 ). The front-end computer ( 10 ) contains, among other things, a control grid generator ( 10 ') and a computing stage ( 10 ''). The raster image processor ( 11 ) has a calibration computer ( 15 ), a correction stage ( 16 ) and a raster generator ( 17 ).

The image signal values required for the exposure of the X control grid and the Y control grid after method step [A] are generated in the control grid generator ( 10 ′) and via a line ( 18 ) and via the correction stage ( 16 ), which are required for this is initially ineffective, fed to the raster generator ( 17 ). The grid generator ( 17 ) generates control signals for the exposure of the control grid, which are transmitted to the recorder ( 12 ) via a line ( 19 ). The recorder ( 12 ) carries out the pixel-by-line exposure of the control grid on the recording material, the control signals switching the exposure beam of the recorder ( 12 ) on and off. The exposed control grids are developed in the film processor ( 13 ). In the measuring device ( 14 ), the control grids ( 20 ) exposed to the recorder ( 12 ) and developed in the film processor ( 13 ) are measured in terms of density after method step [B]. The gray values measured as area coverage values (G _xIst ; G _yIst ) are transmitted to the front-end computer ( 10 ) via a line ( 21 ) or entered manually into the front-end computer ( 10 ).

In the computing stage ( 10 ′ ') of the front-end computer ( 10 ), the relative increases in density (S _xRel. ) Are determined from the measured area coverage values and the nominal gray values (G _xSoll ; G _ySoll ) according to process _steps [B] and [C] ; S _yRel ) calculated and transferred via a line ( 22 ) to the calibration computer ( 15 ) and stored there. The number n of pixels to be evaluated is also transferred to the calibration computer ( 15 ) via the line ( 22 ).

This transfer always takes place if one of the parameters such as type of Recorder, resolution of the recorder, density setting on the recorder changes.

Together with the set, the current raster parameters (raster width, raster angle, raster function and positive / negative recording) are transmitted from the front-end computer ( 10 ) via a line ( 23 ) to the calibration computer ( 15 ) and to the Raster generator ( 17 ) transmitted. The transfer function f _T is then calculated in the calibration computer ( 15 ) according to the method steps [D] to [F] and entered via a line ( 24 ) into the correction stage ( 16 ), which means that the exposure calibration for the current grid to be recorded is complete.

During the useful exposure, the image signal values required for the exposure are then processed in the front-end computer ( 10 ) and fed via the line ( 18 ) to the correction stage ( 16 ), in which they are corrected in accordance with the transfer function f _T.

Claims (15)

1. Method for exposure calibration of a recording device for the pixel and line-wise exposure of rastered areas on a recording material by means of an exposure unit acted upon by image signal values, in which a correction function is determined and which represents the nominal gray values (G _target ) to be recorded Image signal values are corrected according to the correction function in such a way that the real gray values (G _actual ) recorded on the recording material correspond to the nominal gray values (G _target ), characterized in that

• - before the actual exposure, at least one nominal gray value (G _xSoll ; G _ySoll ) representing control _grid (X control _grid ; Y control _grid ) is exposed pixel by line and line by line on the recording material intended for later recording,
• the real gray values (G _xIst ; G _yIst ) of the control _grids exposed on the recording material are measured as area coverage values,
• - from the real gray values (G _xIst ; G _yIst ) of the control _grid and the nominal gray values (G _xSoll ; G _ySoll ) the total _{blackening increases} (S _xControl ; S _yControl ) are determined,
• - The relative blackening increases (S _xRel ; S _yRel ) of at least one pixel ( 5 ) are obtained from the total blackening increases (S _xControl ; S _yControl ) of the control _grid ,
• - for nominal gray values (G _target ) of the entire gray value area between white and black, the effective total blackening increases (S * _Eff ) of the halftone dots assigned by the raster function from the relative blackening increases (S _xRel ; S _yRel ) of a pixel ( 5 ) Control grids are calculated taking into account the grid parameters (grid width, grid angle, grid function, positive / negative recording) intended for the later rasterized recording,
• - The density increase function S * _Eff = f _S (G _target ) is determined from the effective total blackening gains (S * _Eff ) and
• - The correction function (transfer function) G _Kor = f _T (G _target ) is obtained from the density increase function S * _Eff = f _S (G _target ) by conversion to compensate for the density increase during the later exposure of the screened areas .

2. The method according to claim 1, characterized in that the control grid exposed in the form of line patterns on the recording material. 3. The method according to claim 1 or 2, characterized in that

• - The lines ( 1 ) of a first control grid (X control grid) transverse to the line direction ( 2 ), preferably perpendicular to the line direction ( 2 ), and
• - The lines ( 1 ) of a second control grid (Y control grid) run in the line direction ( 2 ).

4. The method according to any one of claims 1 to 3, characterized in that the nominal gray value represented by a control _grid (G _xSoll ; G _ySoll ) each by the ratio of the thickness (n _x ; n _y ) of a line ( 1 ) of the control _grid a line interval (d _x ; d _y ) is determined. 5. The method according to any one of claims 1 to 4, characterized in that the nominal gray values represented by the control _grid (G _xSoll ; G _ySoll ) area coverage values in a range from 20% to 50% of the theoretically possible area coverage values from 0% (white) to Correspond to 100% (black). 6. The method according to claim 5, characterized in that the nominal gray values represented by the control _grid (G _xset ; G _yset ) correspond to an area coverage value of 25%. 7. The method according to any one of claims 1 to 6, characterized in that each line ( 1 ) of the control grid from at least one row of pixels ( 4 ) is constructed. 8. The method according to claim 7, characterized in that each line ( 1 ) of the control grid has a thickness (n) perpendicular to the line direction of two to three rows of pixels ( 4 ) or pixels ( 5 ). 9. The method according to any one of claims 1 to 8, characterized in that the total blackening increase in the exposed control grids (S _xControl ; S _yControl ) is determined separately in the _row direction ( 2 ) and perpendicular thereto by the total _{blackening increase} (S _xControl ) in Row direction ( 2 ) from the broadening of the lines (1) of the X control _grid and the total _{blackening increase} (S _yControl ) perpendicular to the _row direction ( 2 ) from the broadening of the lines ( 1 ) of the Y control grid is determined. 10. The method according to any one of claims 1 to 9, characterized in that the total _{blackening increases} (S _xControl ; S _yControl ) from the real gray values (G _xIst ; G _yIst ) and the nominal gray values (G _xSoll ; G _ySoll ) determined according to the following equations become: S _xControl = (G _xIst - G _xSoll ) / G _xSoll S _yControl = (G _yIst - G _ySoll ) / G _ySoll . 11. The method according to any one of claims 1 to 10, characterized in that the relative blackening increases (S _xRel ; S _yRel ) of a pixel ( 5 ) with the aid of a correction factor which determines the thickness (n _x ; n _y ) of the lines ( 1 ) the control grid is taken into account, the total _{blackening increases} (S _xControl ; S _yControl ) of the control _grid are determined according to the following equations: S _xRel = S _{xControl *} line _thickness n _x S _yRel = S _{yControl *} line _thickness n _y . 12. The method according to any one of claims 1 to 11, characterized in that the effective total blackening increases (S * _Eff ) of the halftone dots from the relative blackening increases (S _xRel ; S _yRel ) of a pixel ( 5 ) taking into account the intended for the later screened recording Raster parameters are calculated by each

• the relevant raster point is generated from pixels ( 5 ), the pixels ( 5 ) being arranged in rows ( 7 ) of pixels ( 7 ) oriented in the line direction ( 2 ) and perpendicular thereto,
• - The effective increases in density (S * _xEff ; S * _yEff ) of the raster point from the increases in density of the individual pixel rows ( 7 ) and from the relative increases in density (S _xRel ; S _yRel ) by averaging and, if necessary, taking into account the respective screen angle (β ) are determined and
• - The effective total blackening increase (S * _Eff ) of the halftone _{dot is} calculated by adding the effective blackening increases (S * _xEff ; S * _yEff ).

13. The method according to any one of claims 1 to 12, characterized in that the effective blackening increases (S _xEff ; S _yEff ) of the halftone dots each from the blackening increases of the individual rows of pixels ( 7 ) and from the relative blackening increases (S _xRel ; S _yRel ) with the help of form factors (r _x ; r _y or r * _x ; r * _y ), which take into account the averaging and, if applicable, the respective screen angle (β), are determined according to the following equations: S * _xEff = S _{xRel *} r * _x S * _yEff = S _{yRel *} r * _y where the form factors (r * _x ; r * _y ) take into account the respective screen angle (β) as follows: r * _x = (r _{y *} angle / 90 °) + r _{x *} (1 - angle / 90 °) r * _y = (r _{x *} angle / 90 °) + r _{y *} (1 - angle / 90 °) with angle = screen angle (β) modulo 90 °
and wherein the form factors (r _x ; r _y ), which take the averaging into account, each from the number of pixel rows ( 7 ) in the row direction ( 2 ) and perpendicularly thereto and from the number of pixels ( 5 ) which are allotted to the raster point can be determined as follows: r _x = number of X rows / number of pixels r _y = number of Y rows / number of pixels. 14. The method according to any one of claims 1 to 13, characterized in that the density increase function f _S by normalization results as follows: f _S = G _{target *} (1.0 + S * _Eff ) / 100. 15. Device for performing the method according to claim 1, consisting of a front-end computer ( 10 ) for providing the pixel and line-wise exposure of screened areas on image material required on a recording material, a raster generator ( 17 ) for converting the image signal values Control signals required for the rasterized recording for the exposure unit of a recorder ( 12 ) and from a correction stage ( 16 ) connected to the front end computer ( 10 ) and the raster generator ( 17 ) for storing a correction function of the image signal values representing the nominal gray values (G _target ) are corrected such that the real gray values (G _actual ) recorded on the recording material correspond to the nominal gray values (G _target ), characterized by

• - A control grid generator ( 10 ') in the front-end computer ( 10 ) for calling the pixel and line-wise exposure of control grids (X control grid; Y control grid) required image signal values on the recording material provided for later recording by means of the Recorders ( 12 ), the control _grids representing nominal gray values (G _xSoll ; G _ySoll ),
• a film processor ( 13 ) for developing the control grid exposed with the recorder ( 12 ),
• a measuring device ( 14 ) for measuring the real gray values (G _xIst ; G _yIst ) of the exposed and developed control _grids ( 20 ) as area coverage values,
• - An arithmetic stage ( 10 ′ ') in the front-end computer ( 10 ) for determining the total _{blackening increases} (S _xControl ; S _yControl ) from the real gray values (G _xIst ; G _yIst ) and the nominal gray values (G _Soll ; G _ySoll ) and for determining the relative blackening increases (S _xRel ; S _yRel ) of at least one pixel from the total blackening increases (S _xControl ; S _yControl ) and
• - One connected to the front-end computer ( 10 ) and the correction stage ( 16 ) calibration computer ( 15 ), in which
• - for nominal gray values (G _target ) of the entire gray value range between white and black, the effective total blackening increases (S * _Eff ) of the halftone dots assigned by the raster function from the relative blackening increases (S _xRel ; S _yRel ) of a pixel ( 5 ) the control grid is calculated taking into account the grid parameters (grid width, grid angle, grid function, positive / negative recording) intended for the later screened recording,
• - The density increase function S * _Eff = f _S (G _target ) is determined from the effective total blackening gains (S * _Eff ) and in which
• - from the density increase function S * _Eff = f _S (G _target ) by conversion, the correction function (transfer function) G _kor = f _T (G _target ) to compensate for the increase in density during the subsequent exposure of the screened areas for storage in the correction stage ( 16 ) is obtained.