DE19501727C2 - Method for calibrating the color components in a color printing output device - Google Patents

Description

The invention relates to a method for calibrating the color shares in a color printing output device, for example a color printer, a color copier, or the like, which a color correction table in a control unit for output of printing inks such as cyan, magenta, yellow and possibly Contains black, where a color test sheet on the color print output device is printed, the printed Image of the color test sheet using a supervisory densitome ters is measured in its colors and the pressure for each correction values derived from the measurement into the colors correction table can be entered.

Nowadays, more and more color printers, color copiers and similar devices for use, by means of which they are colored Templates to be reproduced or a hard copy should be created from screen images. In such Printers have to enter the red, blue and green signals into corresponding signals for the printing inks, namely cyan, Magenta and yellow, possibly also black, converted will. A great color fidelity, even for private purposes is not absolutely necessary, there is a need nis afterwards, the best and most accurate reproduction of the to achieve the original image. This is all the more the  Case if due to the printed copy created and in it contained color information conclusions or Assessments should be made.

This is of particular importance in medicine. In the Nuclear medicine, in computed tomography and in Magnetic resonance medicine is used with digital systems, which keeps the radiation exposure to the patient to a minimum to reduce. With the appropriate detectors System obtained a data field, which a computer Evaluation is fed and it to the treating doctor allowed to display images like the X-ray method consider, diagnose and make a diagnosis. To from to create a hard copy of this screen display means a picture that also goes to the patient record, for example can be placed according to the current state of the art the video signal, which is directly on the monitor, one so-called imager fed. This imager creates point for Dot a black and white copy of what is on the screen X-ray film. This procedure only allows a certain one Image size for which the imager was designed. Then the development of the X-ray film is in one Darkroom necessary, which is not only a cost Time, material and personnel are involved, but also Disposal problems cause what, for example, the Developer, fixer or the silver concerns. Colored Hard copies are possible, but either at high costs associated with each expression or not quality can deliver as they are necessary for a diagnosis. Color printing output devices are used that however, for every color gamut that the physician for his Diagnosis required, must be reset.  

Solutions are therefore sought, especially in the medical field, Grayscale and color levels can be printed on plain paper lichen, which have good color fidelity and sufficient luminosity Show strength and color linearity.

They are full color copiers as color printing output devices known, the inking is indicated by a raster image processor is controlled. Known raster image processors include in their control device a memory field in which a correction door table can be created. Through this correction table correction curves are defined for the color gradients, which should achieve the desired color linearity.

In a first step, a color test sheet from the Color printing output device printed on which the printing colors cyan, magenta, yellow and black arranged in columns are. With descending lines, these inks are in on increasing printing density arranged. By means of a supervisor The color components are measured on a sitometer. With the measurement The color density can then be set in 18 steps over the range of Zero can be determined up to a maximum value. These measurements are - generally in software - with a manufacturer compared specification and a correction curve will calculate determined risch. The corresponding values of the curve will be then stored in the correction table. By means of the present The correction values can then be the color and gray gradients can be set up linearly in itself, but it is for example a gray curve often ver over the entire area to red pushed. This is due to the fact that even with a gray the individual color dots run from all four printing inks be mixed together.  

With the components described so far, both the hardware as well as the software, and through the procedures carried out only a "basic state" of color printing Output device can be set. This includes the Color linearization, but does not necessarily create sufficient color-true representations.

FIG. 3 of EP-A 0 454 085 shows a table of how Shades or shades of gray to a desired usage pattern sters or the like are to be adjusted, that in column 7 approximately Interpolation method explained from line 27 onwards helps with a measured tint value compared to its two Neighbors a parameter value for the printing process (here: the Grid voltage).

EP-A 0 539 868 relates to the conversion of density values into other color systems.

US-A 5,115,229 describes a procedure in which a Gray balance is done by comparison on two screens.

It is the object of the invention to provide a method for Calibration of the color components in a color print output to provide direction with the next to the color line Aryanization also adjusted the gray balance can be.

This output is from a method according to claim 1 solved. Advantageous refinements are the subject of Subclaims.

According to the invention, it is provided that each value (ALTn) of the Correction table specified by direct indexing is, after reducing the available color space to color space actually desired with a largest, desired th density (MAX) of the paint application, by a value (NEW) is replaced by the one in the correction table Value through the equation

emerges, where m is a predetermined number greater than or equal to 1, X is the quotient of MAX and the one on the printed image of the color test sheet and Y = 1 - X. This calibration enables the correction to be reset by the initial correction curves of the "ground state" so distorted that an optimal Gray balance is achieved.

Alternatively, it can be provided that each value (ALTn) of the Correction table specified by direct indexing is reduced to the actually desired color space, by using an optionally created table for the Color values or print densities the first value is taken, the corresponds to the lowest desired pressure value, with the Entry of this value from the maximum print density value of the in subtracted values stored in the correction table and as corresponding value (NEW) at the position of the Maximum value minus current position is adopted.

According to the invention it is provided that each value of the correction table specified by direct indexing, after Reduction of the existing color space to the actual one desired color space is replaced by a value that consists of the value in the correction table by the equation

With the method according to the present invention can thus be achieved that a true hardcopy on Plain paper is created - and also foils for Lectures can be printed - that give the visual impression for example largely on the screen of a computer corresponds. Anyone can use it without changing the printing system Color tables are reproduced. Because the conventional X-ray film is detached, carries the solution according to the invention significantly to reduce costs in medical Documentation at. Another advantage is that this Method is "networkable", that is, it is from separate screen workstations from an identical color output to the central printer possible.

Because environmentally friendly print media are used (color foils and special paper are not necessary) and especially on Chemicals such as developer and fixer as well as on the The process can do without X-ray film an important contribution according to the present invention for environmental protection.

The invention is not limited to the field of medicine, but can be used wherever qualitative high quality color prints are required.

For most applications, it will be enough to work with a value m = 1, which is an extremely fast Calibration enabled.

The procedure is generally set up to: expires automatically. This offers a variety of options also changes in color image quality with changing surroundings to compensate for exercise conditions. So is an advantage  liable design of the method provided that dependent of temperature signals from the environment of color printing Output device the values NEWn are recalculated. It this recalculation can also prove useful depending on air humidity signals from the environment of the Color printing output device to perform.

In the following, the invention is intended to be based on examples are explained.

example 1

With a display of a maximum of 256 gray levels for the nuclear medical diagnostics a good differentiability for planar and tomographic slice scintigraphies reached. A linear gray value display with up to 256 Grayscale and a bandwidth of 0.1-2.0 UD is for that human eye ideal.

The basic machines are the color printing output device Devices AGFA XC305 and AGFA XC315 used. For control of the inking unit will be the Fiery 125i raster image processor EFI company used. The basic calibration continues to use the associated software from EFI.

With 256 gray levels (0 to 255), the correction tables for the colors cyan (c), magenta (m), yellow (y) and black (k) for example the following form:

These tables represent color values that range from 0 to 255 can be replaced by corrected values. Calculate this according to the formula given in claim 1, for example for m = 1. The values X and Y can be determined by densitometric Measure the gray balance fields determined in the color test sheet will.

For example, if the color axis of the calibrated printout shifted too far to magenta is on the color test sheet the maximum order in the magenta area determined and with a default value, for example that of the manufacturer, ver like. For example, be the manufacturer value 1.4 d, the If the measured value is 1.5 d, the factor X is calculated

X = 1.4 / 1.5 = 0.933.

This results in Y too

Y = 1 - 0.933 = 0.066.

If the manufacturer's specification is not sufficient for the exact determination is sufficient, another test sheet can be printed out in which the maximum possible densities in the gray wedge to pure white be generated. This also allows the maxima Determine the ink application values of the machine.  

Example 2

The following is another example of the calibration ver drive according to the invention specified:

In a first step, a table with the values ALTn created the entries according to the hexadecimal values from 0 to 255. This table is under / ctransfer. . . in the expression <. . . <specified. The printer unit now takes from the table for each color value to be printed the assigned value, which here is identical to the assigned value th index (= hexadecimal value in the table).

Now a test sheet is printed, for each ink Contains swatches that are side by side or one below the other increasing density from 0 to 100% (or already converted by values between 0 and 255 determined are). Such test sheets and their preparation are part of the State of the art. Depending on the desired application the expert becomes "density jumps" between the color fields choose suitable. Appropriate density jumps are 10%, in critical areas, finer divisions can be selected, for example of 2%. It is not necessary to exactly 256 Color swatches can provide a smaller number suffice.

Using a supervisory densitometer, these fields are now sequentially scanned; each box is in its colors measured by determining a printing density D. The Values of the print density D are over a hardware connection stored in the printer unit, for example in a Table, which is to be called MESSn here. Contains the table MEASNES fewer than 256 entries, the table supplemented by suitable interpolation with intermediate values in order to completely cover the range from 0 to 255.  

The completed table MESSn is now used to for each color value of a source image the print densities of the to determine the associated target image. Conversely, for everyone Print density pointed to the numerical value of the source image because the correspondence is unique.

Now the color balance and linearization are checked leads.

The color printing output device provides a specific one Color space available based on the actual color space is reduced. This is, for example, between 0 and 1.2 D.

Table 1

The color space actually required is now (to values NEW) expands so that percentages correspond to computer values according to the maximum print density required stand. This is done according to the formula already mentioned, if necessary with simplifications resulting from the quality requirements.  

Example 3

Should a mathematical method not suffice or pose the printing unit has sufficient microprocessor power Any value can be NEW by searching and Exchange can be determined.

To do this, proceed as follows:
The desired pressure value in D is determined for each value ALTn, as described in Example 2. The first value that corresponds to this is taken from a table MESSn that has also been created. The hexadecimal entry of this value is subtracted from 255 and entered as the corresponding value in the table NEW at position 255 minus the current position (index).

The reason for the given examples is the Inversion of the numerical values of the calibration in that the imaging systems used their numerical values after the Pass the RGB model. The table MEAS RGB space linearized to the CMY space and functional shown cropped.

Then the table ALTn is always replaced by NEWn.

This is the calibration of the color printing output device completed. Print control of color printing output direction uses the numerical value of a color as an index for the Table NEW. With the value stored there, the color pressure output device controlled; thus the get the desired print density without a connected imaging system must have knowledge of the printing characteristic.

Claims (5)

1. A method for calibrating the color components in a color printing output device, for example a color printer, a color copier or the like, which contains a color correction table in a control unit for outputting the printing colors, such as cyan, magenta, yellow and possibly black, in which a color -Test sheet is printed out via the color printing output device, the printed image of the color test sheet is measured in its colors using a supervisory densitometer and correction values derived from the measurement are entered in the color correction table for each of the printing inks, characterized in that each value ( ALTn) of the correction table, which is specified by direct indexing, after reduction of the available color space to the actually desired color space with a greatest, desired density (MAX) of the color application, is replaced by a value (NEUn), which is from that in the correction table present value by the G ease , where m is a predetermined number greater than or equal to one, X is the quotient of MAX and the value measured on the printed image of the color test sheet and Y = 1 - X. 2. The method according to claim 1, characterized in that m = 1.   3. Procedure for calibrating the color components in a Color printing output device, for example a color printer, a color copier or the like, which a Color correction table in a control unit for outputting the Printing inks such as cyan, magenta, yellow and optionally Black, which contains a color test sheet over the Color printing output device is printed out printed image of the color test sheet by means of a supervisor densitometers is measured in its colors and for each the correction values derived from the measurement in the color correction table can be entered, characterized in that each value (ALTn) of the correction table specified by direct indexing to the actually desired color space is reduced by off an optionally created table for the color values or Print densities the first value is taken, the lowest corresponds to the desired pressure value, the entry of this Value from the maximum print density value in the correction Subtracted values stored in the table and as corre sponding value (NEW) at the position of the maximum value tes minus current position is adopted. 4. The method according to claim 1 or 3, characterized in net that depending on temperature signals from the environment of the Color printing output device recalculates the values (NEUn) will. 5. The method according to any one of claims 1 to 4, characterized characterized that depending on air humidity signals the surroundings of the color printing output device the values (NEW) be recalculated.