Guideline for Prinect Axis Control for printing presses with Prinect
Contents
1. 37 38 Publishing Information Trademarks Printed in 07 07 Heidelberg the Heidelberg logotype Prinect the Prinect logotype Photographs Heidelberger Druckmaschinen AG Axis Control CP2000 Center and Speedmaster are registered Fonts HeidelbergGothicMl trademarks of Heidelberger Druckmaschinen AG in the U S Printed in Germany and other countries Mini Spot is a trademark of Heidelberger Druckmaschinen AG in Germany and other countries PANTONE is a registered trademark of Pantone Inc All other trademarks are property of their respective owners Subject to technical modifications and other changes Heidelberger Druckmaschinen AG Kurfuersten Anlage 52 60 69115 Heidelberg Germany Phone 49 6221 92 00 Fax 49 6221 92 6999 www heidelberg com 00 000 00002. Prinect Color and Quality H ID LB RG Guideline for Prinect Axis Control for printing presses with Prinect CP2000 Center Table of contents 1 Introduction 4 1 1 Definitions 4 1 2 Principles of the Heidelberg color control strips 6 1 3 Why is the color control strip or several measurement patches not detected 8 2 Functions and features of Prinect Axis Control 9 3 Basic rules to optimized printing 10 4 Basic settings of Prinect Axis Control 12 5 Typical make ready for a new job 14 5 1 Procedure 14 5 2 Changes during a job 16 6 Reference values 18 6 1 Color sets 18 6 2 Creating color sets 18 6 3 Ways of entering reference values 18 6 4 Reference values from a print series 19 6 5 Changing reference values 20 7 Control methods 22 8 Reporting and protocol functions 23 9 Quality the reason for color measurement and color control 24 9 1 The goals of color measurement and color control 24 9 2 Spectral measurement and colorimetry 24 10 Color theory 26 10 1 Human color perception 26 10 2 Spectral distribution of the visible light 27 10 3 Two control strategies density and Lab 28 10 4 What does AE mean 29 10 5 Lab values from actual printing inks 29 11 Types of measurement devices 30 11 1 Spectral color measurement 30 11 2 Density measurement 30 11 3 Comparability of results obtained withdifferent measurement systems 32 1 Introduction 1 1 Definitions 1The correct notation is L a b e Lab color space Lab values L
3. Paper class 4 ll a b E a b Black 16 0 0 31 1 il Cyan 54 36 49 58 25 43 Magenta 46 72 5 54 58 2 Yellow 88 6 90 86 4 75 Red M Y 47 66 50 52 55 30 Green C Y 49 66 33 52 46 16 Blue C M 20 25 48 36 12 32 29 11 Types of measurement devices Measurement parameters for the printing industry Measurement geometry 45 0 or 0 45 angle at which light strikes first the sample and then the measuring head e Light type Illuminant D50 modeled after natural daylight slightly reddish e Observer 2 standard observer 11 1 Spectral color measurement Spectral measurement is performed by bending the light through a lattice Prinect Axis Control captures light in the wavelength range from 380 to 730 nm with a resolution of 10 nm Procedure e Calibration to absolute white and adjustment according the standard paper white e Measurement of the sample e Calculation of the spectral reflectance values The values obtained are placed in relation to the corresponding values of the paper white for each wavelength range within the limits of the achievable resolution of 380nm 730nm This yields a spectral distribu tion that is independent of the actual light source used e Calculation of the Lab value Aseries of calculations is performed on the obtained spectrum to determine the Lab value e Calculation of the density Density filters stored in the device are applied to the spectrum to obtain the density values
4. 11 2 Density measurement Densitometry is color blind It uses various standardized filters to derive reflection values The density is calculated by applying the formula D log f degree of reflectance the ratio of reflected light to incident light Procedure e Calibration to paper white e Setting to polarized or unpolarized mode Polarized surface reflections are filtered out gt Same values are obtained for wet and dry sheets Unpolarized surface reflections are also measured gt Lower density value than in polarized mode gt Different measurement results for wet and dry sheets e Measurement of the sample 30 Density measurement of the process colors CMYK using appropriate standardized filters e There are standardized narrow band filters broad band filters etc Different filter types yield different results e The filters allow light to pass in the wave length range in which the color separation being measured remits very little light in other words in the absorption range It is here that the ink film thickness affects reflectance most strongly gt Cyan red filter gt Magenta green filter gt Yellow blue filter 1 0 0 8 g Density oo ee measurement range 50 El with red filter 04 0 2 400 500 600 700 Wavelength nm g Density S measurement range 3 with green filter x 400 500 600 700 Wavelength nm g Density c measurement range HA with blue filter S x
5. Online control of inking in individual printing units e Reliable control of special colors with high reflectance e Objective color measurement and control The limitations of Prinect Axis Control Prinect Axis Control cannot be reliably used to measure and control print results with e Metallic inks e Opaque inks e Transparent substrates plastic sheet e Metallized substrates e Color control strips with patches smaller than 5 x 6 mm a 3 10 3 Basic rules to optimized printing Use ofthe Prinect Axis Control color measurement system alone does not automatically guarantee that the results will meet expectations The reference values can only be approximated Color control works best with anintegrated workflow that begins in prepress and extends down through the production process to the press operator In the following the important aspects for getting the best possible results with Prinect Axis Control are described 1 Determine the dot gain in the production process Adjust platemaking to take account ofthe dot gain when printing 2 Getthe press ready e Put the press in a stable reproducible state e Determine optimum reference values color standards for standard process colors 3 Optimize pre inking and characteristic curves for presetting the inking e Pre inking get inking units in the right state for production e Presetting of inking determine how far the zones should be opened to achieve the desired co
6. measure and save first the paper whiteness value then the solid patches and if relevant the grey balance patches as well It is absolutely essential to measure the paper white first 6 3 Ways of entering reference values 1 Reference values from DIN ISO 12647 Lab reference values cannot be numerically keyed into Prinect Axis Control Reference values should be captured from a series of color repro ductions To comply with the stipulations of DIN ISO while printing the color closest to the value specified by the standard should be incorporated into a color set If necessary it can then be adjusted by an appropriate percentage Note the permissible tolerance of AE 5 is reduced by the color deviation AE between the set reference value and the values stipulated by DIN ISO 2 Color samples from color books Color sample collections are available for non process ink series It is possible to read in a sample and add the measured value to a color set Color samples can only be used ifthe substrate on which the book is printed are the paper used for the pressrun are ofthe same type glossy matt coated uncoated and ifthe book stock s whiteness is also measured before lt is advisable to print a specimen of a non process color visually assess it and then insert it into the color set This lets you take account of batch to batch fluctuations and printable ink film thicknesses 3 Measuring an OK sheet For recurring jobs it is poss
7. menu e Outlined bars for deviation within AE tolerances e Solid bars for deviation outside the tolerances The tolerance setting only affects how setting parameters are displayed on screen It does not influence the precision of any follow up recommendation 13 14 The green indicator and how to read it 5 Typical make ready for a new job 5 1 Procedure Make ready Set the stock format Assign colors to the printing units using the color symbols After about 130 sheets pull a sheet make sure that the green indicator is lit indicating that the inking units have stabilized see figure below Set the measurement conditions substrate class and tolerance narrow medium wide Measure substrate paper white Color assignment from a suitable color set see 6 2 Creating color sets in the color archive or from individual measurements using the function Sample Select a color control strip and position it When there is a connection to prepress via Prinect Prepress Interface and data is supplied for positioning the color control strip on the sheets the measuring head is automatically pre positioned Check whether the graphical and numerical values are plausible If not double check the settings Approve the follow up recommendation After two to three follow ups the reference colors are achieved Production Regular pull measure and follow up sheets Here too make sure that the green indicator i
8. sheets prematurely Make sure the green indicator on the Prinect CP2000 Center is Lit With optimized presettings the colors should be approx AE 5 compared to the reference levels It takes about three follow ups to achieve a deviation smaller than AE 2 which should then remain stable for individual zones over the print run as a whole It should be taken into account however that the printing conditions can change with longer runs Zones that are outside the printed image or contain a certain color only in the color control strip should be blocked as they are not relevant to the results of printing If the first inspection sheet pulled exhibits color deviations larger than AE 10 despite using data from Prinect Prepress Interface or Plate Image Reader and pre inking recheck the press settings If there is a clear tendency for over or under inking to occur the characteristic curve for presetting the inking and the pre inking settings should be adjusted Particularly over inking of the first sheet pulled is a critical sign It may be helpful to wash the press and restart the job with different presettings Be cause the inking units act as reservoir especially when running low coverage they can be slow to adjust and quite a few waste sheets may have to be printed before the lower inking level is reached If it is impossible to consistently reproduce the reference density this may be because the inking unit has not stabilized Possib
9. the Prinect 6GS are specifically designed for grey balance control and only have solid patches for CMYina small number of zones Advantages of grey balance control e Minimal color fluctuations and changes are registered during the production run The color balance is critical for the visual appearance of a printed image The grey balance reveals even the tiniest changes in printing conditions inking or register 8 Reporting and protocol functions Prinect Axis Control Reporting This software module permits detailed documentation of all follow ups and their specific data so they can be called upon anetworked PC The reporting module must be additionally licensed and enabled 4 CD 102 4 Prinect Press Reporting HEIDELBERG Reports Control history Every time you follow up the average values for density dot gain slurring doubling AE AE and AF are saved in the control history and are displayed there HEIDELBERG 23 24 9 Quality the reason for color measurement and color control 9 1 The goals of color measurement and color control gt gt gt To achieve specified reference values To faithfully reproduce originals To deliver demonstrably consistent print quality Colors are always controlled on the basis of changes in ink film thickness Two ways of accomplishing this are widely spread in the printing industry densitometry and colorimetry Densitometry is
10. 0 2 00 2 10 Solid density 19 20 Very important In color sets a distinction is made between the paper classes of glossy coated matt coated and uncoated The most important differences between these types have to do with surface gloss calculation of polarized and unpolarized density and ink absorption acceptance when overprinting different colors In order to properly use a color set these basic material properties must be identical in the source and the paper actually being used for the print run Each color set is therefore limited in its application to the same stock type for which it was defined The differences between glossy coated and matt coated are minimal however The value for the paper white must always be included in the color set Printing inks are translucent and the substrate consequently always affects the color Normally color sets are measured from the printed sheet and defined accordingly The paper white will be compensated by calculation so that the color set can also be used with other papers 6 5 Changing reference values Reference values contained in a color set can be changed if necessary There are various ways to do this 1 Changing reference values during job preparation Right when creating a job the reference density value can be manually changed to the desired value in steps of 0 01 The corresponding Lab values are automatically adjusted accordingly 2 From an OK shee
11. 400 500 600 700 Wavelength nm Density measurement of non process colors That filter is used that yields the highest density value This causes the following problems e Bright colors have high reflectance the density value obtained is therefore corresponding low and cannot be used very well for color control purposes because changes in the ink film thickness resultin only negligible changes in density e Two completely different colors can yield exactly the same density value despite being obviously different in appearance see figure below 31 32 Two completely different colors the deviation between them is AE 114 3 The same density value is obtained for both however D 1 60 L 60 2 a 56 8 b 62 9 D 1 60 L 59 4 a 56 7 b 49 3 Density measurement range with blue filter Reflectance Color sample HKS 8 400 500 600 700 Wavelength nm A y o a DO DO DO DO O O DE 0 8 IN see ae eee rrcaer 8 Density i 5 0 6 Measurement range rodeo El with blue filter y i 0 4 En EN nn nn X en ee 0 2 O Nee se Color sample HKS 65 400 500 600 700 Wavelength nm 11 3 Comparability of results obtained with different measurement systems Requirement identical measurement conditions Illuminant Standard observer Measurement geometry Calibration to paper white absolute white Same background base preferably black base behind sheet during measurement Same time elapsed between pu
12. ab will be used throughout this text Be To define a color precisely its location within the Lab color space is by way of simplification indicated by three coordinates L lightness or luminance 0 black 100 white a red green axis b yellow blue axis Neutral grey hues have a 0 and b 0 Left Lab color space as a 3 dimensional model Right a b color plane 270 20n screen the letter d is used instead of A e AF pronounced delta E describes the distance between two colors within the Lab color space Distances within the Lab color system correspond to visually perceived differences between colors In other words a given AE value will always be perceived the same regardless of the chrominance colors and lightness AE Perceived difference smaller than 0 2 not visible 0 2 1 0 very small 1 0 3 0 small 3 0 6 0 moderate larger than 6 0 large e AE Describes the distance between the defined reference value and the best value achievable best match with the combination of ink and paper white being used e Density D The density of a measurement patch is determined with the aid of an appropriate filter There are standardized filters for the process colors Black Cyan Magenta and Yellow The densities of special colors are determined using the filter that yields the largest values Z70 Indicates dot gain in the 70 tint patch of Prinect color control strips Tolerance values
13. are stored in a tolerance archive In order to obtain consistent results throughout a print run the dot gain must remain constant When making films and plates it is also necessary to take into consideration how the halftone dot areas will thicken when transferred to the substrate during printing Unless the plates are matched to the process it will be impossible to get the right colors S D Slur doubling values as percentages S D values are determined from the area coverage differences in the line patches also known as ladder targets Tolerance values are stored in a tolerance archive AF Control parameters as percentages AF values are determined by comparing the reference Lab values with the actually measured Lab values They describe how far the measured color deviates from the reference value expressed as a percentage Rules of thumb for the relationships among AE AF AD for the process colors 1 AE 3 5 AF 1 AE 3 100 AD 1 10 AD 12 AF These conversions are only intended to provide a general orientation These precise values will vary depending on the inks used the color separation and the ink film thickness HEIDELBERG This figure shows the screen Axis Control Measurements To the right of the screen you ll find numerical values for density slurring doubling Lab AE AE and AF in each case referring to the selected zones On the left there is a graphical representa tion ofthe AF values for the i
14. changes in the substrate being printed on by appropriately controlling to the AE value Spectral measurement is the only way to generate ICC profiles This enables prepress to make plates that are tailored to the inks and stock used A AE value can be intuitively understood as a measure of color deviation without possessing any specialist knowledge because it corresponds to normal human color perception Density variations can influence the appearance very differently depending on the color separation If you only had to deal with the production process itself it would definitely be possible to successfully apply different color control strategies at least for the process colors But when dealing with customers and struggling to ensure standardized printing conditions it is necessary to make objective statements about quality Lab color values and the deviation AE are the best way to make generally valid quality assessments Controlling the production run on the basis of colorimetric values makes it possible to directly implement and monitor the customer s quality stipulations right at the press Quality Customer 25 26 10 Color theory 10 1 Human color perception The human eye perceives colors with three different types of receptors so called cones each of which responds to light of different wavelength ranges When light enters the eye the corresponding cones are stimulated and relay nerve pulses to the brain Itis the
15. ed by CIE There are three such functions corresponding to the three types of color receptors in the human eye As can be seen in the figure the entire visible spectrum is captured E L 47 3 a 75 b 6 5 Wavelength nm Example reflectance curves of CMYK and paper whiteness Reflectance curve of a solid Magenta patch obtained with the corresponding standard narrowband filter Spectral pattern of a solid Magenta patch with standard spectral value functions 27 28 Change in reflectance as the ink film thick ness varies in the solid Magenta patch 10 3 Two control strategies Density and Lab Both D and Lab change depending on the ink film thickness Because the ink film thickness is the only controllable parameter in printing it is obvious that both values will always change at the same time For example when ink zones are opened further this increases the ink film thickness This affects the remitted spectrum and thus both the density and the Lab value 1 0 0 8 o 2 0 6 S o 2 Paper white e 0 4 Magenta at 0 2 different ink film thickness 380 430 480 530 580 630 680 730 Wavelength nm Density Within the range that is relevant for offset printing ink film thicknesses of around 1 um the density of the standard processes colors changes almost linearly in relation to the ink film thickness Changes in the film thickness can thus be directly expressed as diff
16. ence values should only be utilized when absolutely necessary however Instead of making manual changes it is better to define the color set so that the desired print results are achieved entirely by making measurements and to follow up the calculated recommendations It is also important to ensure a standardized printing process with consistent reference values This nearly eliminates the need for manual changes If Prinect Axis Control indicates substantial color deviations although a sheet looks fine this can have the following causes e Wrong color set selected so that the wrong reference values are being applied e Paper white not measured in an unprinted area of the sheet e Incorrect measurement conditions or the wrong paper class defined HEIDELBERG The view Measurement reference values with the buttons OK sheet zonal transfer and change by percentage 22 7 Control methods Prinect Axis Control offers a choice between solid control and grey balance control for the chromatic colors CMY Solid control Here control is based on the solid patches for each color separation Black and allnon process colors are always controlled using the solid patches Grey balance control The grey balance patches are used to control the balance among the process colors C M and Y These color separations are overprinted with area coverage values of C 70 M 60 and Y 60 Some color control strips including
17. erences in density For example increasing the ink film thickness by 10 normally also increases the density by approximately 10 However even if the reference density is precisely achieved this does not mean that the color will have the desired appearance This is only the case in absolutely stable standardized processes which hardly ever occur in actual practice Lab In the Lab color space there is also a virtually linear relationship between color coordinates and ink film thickness However the three dimensional nature of this color space makes it considerably more difficult to depict changes in ink film thickness The AE value is therefore used as an easily visualized way of representing changes in the color coordinates and thus also the ink film thickness The goal of colorimetric control is to approach the reference value as closely as possible The remaining color deviation which cannot be elimi nated with the printing process as it stands is indicated by AE 10 4 What does AE mean For every combination ofink and paper the curve of printable colors can be determined within the Lab color space by measuring prints with different ink film thicknesses and patches with different halftone values Colors that are not located on this curve cannot be achieved with that particular combination ofink and paper The distance from the best achievable point on the curve is defined as AE AE describes the discrepancy between
18. g setting and adjust it if appropriate Le i 1 Prepress y 8 2 Press y 3 Characteristic Curves 4 Presetting of ink zones O 5 Color control OO O 6 Operator 3All press settings should be performed again at least once a year For details on how to proceed see Prinect Color Solutions 11 None of the color control strips supplied with the equipment can be deleted only user defined bars can be erased See the User Manual for details on how to create your own color control strip 4 Basic settings of Prinect Axis Control Check calibration regularly recalibrate e Use the provided customized calibration card Heidelberg Service will perform the first calibration It should then be repeated every three months Define measurement conditions Job Axis Control New Options The defined measurement conditions are important for comparing results obtained using different devices Light e D50 daylight slightly reddish e D65 daylight slightly bluish e A standardized light bulb light reddish e C standardized daylight but without UV portions D50 is the standard applied in the European printing industry Observer Selectable between 2 and 10 standard observer The default setting for offset printing is the 2 standard observer Filter Selectable between polarized and unpolarized Polarization filters remove surface reflections from the ink As a result measurements of wet a
19. ible to scan in color patches from the OK sheet of the original job and store the results in a color set Please note that for process related reasons measurements are always made in unpolarized mode and are therefore different depending on whether the sheet is moist or dry Consequently controlling the colors of freshly printed sheets based on values read in from a dry OK sheet may yield results that deviate slightly from it 6 4 Reference values from a print series Reference values should always be determined from a series of prints The aim is to determine the value that yields the highest density in the solid patch without dot gain causing the 70 tint patch to fill in This permits high contrast printing Procedure e Print sheets with inking levels ranging from too low to too high e Apply the Murray Davies formula to calculate the maximum print contrast from 100 and 70 patches on different sheets Dy Dy _ I K x100 where K relative print contrast as Dy a percentage value Dy density of solid patch Dj density of 70 tint patch That color level is best at which the relative print contrast is the highest The reference value for the color separation can be taken from this sheet for the color set 60 50 40 30 20 Contrast 10 Idealized representation of a print series In this example the greatest print contrast is achieved at a color level of approx D 1 7 1 30 140 1 50 160 1 70 1 80 1 9
20. le causes e Zones have previously been manually adjusted e The rotational speed of the ink fountain roller or the use of oscillators has been changed The press speed has been changed but the characteristic curve for speed compensation is not appropriately defined e The feed rate or composition of the dampening solution has changed Premature pull see figure Sheets Inspection sheet pulled before the inking unit has stabilized The result inking is overcompensated 17 18 6 Reference values 6 1 Color sets The reference values for color control are stored in color sets Each color set should at least contain values for e The paper white e The process colors The grey balance patch It can also contain Lab values for non process colors Itis absolutely essential to measure the actual stock used for each job This value can then be used to adjust the stored color values Prinect Axis Control comes with standard color sets designated Heidelberg These only represent average values for various ink series It is therefore important for you to define your own color sets for the inks and paper classes used 6 2 Creating color sets e In Service Archive Color Archive you can copy an existing color set and assign a meaningful name e g name of the ink series used to the copy e Select the stock quality glossy matt coated uncoated e Use a sample preferably a freshly printed sheet with the reference colors to
21. lling and measuring a sheet especially when comparing Lab values since these cannot be measured with polarizing filters With densitometers setting to polarized unpolarized Measurement parameters for the printing industry Measurement geometry 45 0 or 0 45 angle of light striking measuring head e Light type D50 modeled after natural daylight Observer 2 standard observer Requirement correct functioning e Clean lenses e Regular calibration An appropriate calibration card is included e When calibrating to the absolute white regularly clean the tile used For density comparisons with Prinect Axis Control please note the polarized density values are calculated not measured Prinect Axis Control is not equipped with a physical polarization filter Because measure ments are always made without a polarization filter how wet or dry a sheet is invariably affects the results of measurement both when the system is set to unpolarized and when polarized values are calculated As a sheet dries the colorimetric values obtained can slightly change The amount of time that elapses between pulling and measuring a sheet it must therefore be kept consistent to ensure comparable results 33 34 35 36
22. lor standard 4 Use presetting data Preset the zones for the right area coverage values Sources for values Prinect Prepress Interface or Plate Image Reader 5 Color control only with Prinect Axis Control Do not change zones by hand this interferes with the control system and results in poorer quality 6 Responsibilities of the press operator e Make sure that the inking unit is in a stable state check for the green indicator on the Prinect CP2000 Center e Printing just short of where smearing begins minimize the dampening solution feed to obtain high contrast reproduction Under optimum conditions the desired colors of less than AE 5 compared to the reference values can be achieved after printing about 130 sheets From here on Prinect Axis Control can take over When press sheets are pulled for inspection the inking units must be in a stable state Sheets pulled too soon may exhibit excessive or insufficient inking because they are still increasing or decreasing That is why it is important to watch for the green indicator on the Prinect CP2000 Center display it s better to pull a bit later than too early If the level of less than AE 5 compared to the reference value is not attained in the first sheet pulled recheck the characteristic curve for presetting the inking If while printing a typical form with normal area coverage the inking units take considerably longer than 130 sheets to stabilize check the pre inkin
23. n values Prinect color control strips have a 70 tint patch but other values are also possible e Patches 9 10 lines at different angles to identify slurring and doubling defects In Heidelberg presses each ink zone is 32 5 mm wide so there are exactly 13 measurement patches within two zones When mounting the color control strip make sure that e Itis correctly aligned with the ink zones of the press and e There isa5 mm paper white margin on the left and right end of the strip Important for the Prinect 6GS and 8GS color control strips due to the large number of different patches and the fact that each patch is 5 mm wide it is not possible for every type of patch to be within each ink zone If a certain patch is not found in a zone its value is assumed to be the average of the corresponding values from the two neighboring zones No numerical values are displayed for deviations in these zones because inking is controlled based on the neighboring zones The graphical display in these zones shows the average of the neighboring zones The table below provides an overview of the different color control strips and their makeup Solid and greyscale patches Patch type Patch every zone every other zone 3 per control strip Prinect 4GS Solid patch K C M Y Greyscale patch C M Y Prinect 6GS Solid patch K XZ K M Y Greyscale patch C M Y Prinect6GS99 Solid patch X Z K C M Y Greyscale patch C M Y Prinec
24. nd dry sheets are nearly identical Polarization filters only affect the density values The filters are not applied for the Lab values Measurements with Prinect Axis Control though are always made in an unpolarized mode because Prinect Axis Control does not have a physical polarization filter The polarized density value is calculated from the unpolarized spectral measurement Density filter standards The following filters are defined for density measurement Selectable filter sets DIN 16536 NB DIN 16536 ANSI A ANSI T For Prinect Axis Control the recommendation is to use either DIN 16536 or AnsiT in North America Color control strips In Service Archive Color Control Bar Archive you can suppress unused color control strips The most frequently used bar can also be preselected For analog mounting use CPC color control strips e For digital mounting use Prinect or Prinect FOGRA color control strips Note unused color control strips should not be irretrievably deleted but only moved to the unused color control strips field They then disappear from the standard menu but if needed can be re enabled for use Defining a color set An appropriate color set must be defined for the ink series and stock quality used For information on the possibilities for determining reference values please read section 6 2 Creating color sets Tolerances Display of deviations in the measurement overview
25. ndividual zones and inking units The reference value is set as the zero line The bars indicate the magnitude ofthe deviations from the reference value In this example Cyan is under inked across the entire width Section of a 4GS color control strip with explanations 1 2 The principle of Heidelberg Color Control Strips In the following the color control strip Prinect 4GS and the functions of its individual patches are described in detail by way of example My CM CM 184 1C4 1M4 i ect 4GS Dipco 2 0 Format 52 2003 Heidell zn ia Center ofzone Zone number center of press ho Zone limits Color control strips consist of repeating sequences The color control strip Prinect 4GS for example consists of sequences with a total length of 104 patches corresponding to 520 mm or 16 zones The color control strip Prinect 4GS has various types of patches e Patches 1 4 solid patches i e patches printed 100 with one color in this example the process colors CMYK on the strip B stands for Black These patches are used to control solid colors e Patch 5 grey balance patch consisting of 70 Cyan 60 Magenta 60 Yellow Used for grey patch control of the chromatic colors CMY e Patches 6 7 solid overprint patches for determining ink acceptance when printing multiple colors on top of one another widely described as ink trapping e Patch 8 halftone tint patch here Magenta For determining dot gai
26. optimized for chromatic colors in offset printing Colorimetry by contrast can be universally used for all processes because it simulates the perceptions of the human eye see section 10 1 Human color perception 9 2 Spectral measurement and colorimetry Spectral measurement is the basis for calculating color measurement figures whether these are density or colorimetric values Prinect Axis Control also displays density values as well as parameters derived from them dot gain and slurring doubling Colorimetric values are used to control color by minimiz ing the colorimetric discrepancy AE between the actual print results and the reference value The goal is to achieve the best possible color match between both Colorimetric color control makes use ofa color model that combines the color appearance ofthe printed ink with its thickness on the sheet This process is computerized and automatic The press operator is thus free to concentrate completely on the results of control the recommendations made by Prinect Axis Control for adjusting the inking The advantages of this control strategy Only colorimetric control ensures compliance with DIN ISO standards The color balance can be controlled using the sensitive grey balance patches Control of special non process colors is significantly better than density based control Spectral measurement can compensate for ink soiling batch to batch fluctuations in the ink and
27. re that actual color perception results Spectral measurement devices simulate this process by spectrally meas uring the light striking them and using the results to calculate the standard color values XYZ These can in turn be used to directly calculate Lab color coordinates among others Light Human Measurement device Sample Eye Lens with li receptor Cones y Standard spectral function of Blue Green Red standard observer Excitation Standard color values XYZ Color perception Lab color coordinates 10 2 Spectral distribution of the visible light Prinect Axis Control measures the spectral distribution of the light with a resolution of 10 nm These values called reflectance values are the basis for allsubsequent density and Lab value calculations Measurement is performed by bending the light through a lattice Wave lengths between 380 and 730 nm are captured Black Cyan Magenta Reflectance Yellow Paper whiteness Wavelength nm From spectrum to density value Density values are defined by the reflectance of the light captured in the wave length range of the filter used The rest of the spectrum is ignored There are standard filters for the process colors Cyan Magenta Yellow and Black D 1 70 Reflectance 580 630 680 730 Wavelength nm From spectrum to Lab value The Lab values are calculated using the standard spectral value functions defin
28. s lit If necessary manually adjust reference values OK sheet zonal transfer change block zones see 6 3 Changing reference values ai Adjustment being initiated Adjustment being made Adjustment completed inking stable again HEIDELBERG This figure shows the screen Measure ment adjustment recommendation Shown here Dio from dF zonal profile and control algorithm In measurement dF i e colors relative to reference values Here the function Color Booster can be individual enabled and disabled for each inking unit machine settings adjusting pulling measuring Schematic of the cycle pull measure m follow up 15 16 Optimum make ready with presettings 5 2 Changes during a job If everything is done correctly as described in basic rules for optimized printing the colors will approach the reference values during printing roughly as shown here Coloring Pull measure and adjust with Prinect Axis Control 4 Makeread y _ Production Presetting lt Measurement and Control Sheets The first step toward reaching the reference values is made with pre inking the characteristic curve for presetting the inking and ink presetting data The inking units are filled with ink beforehand and allowed to stabilize for production IMPORTANT Don t pull
29. size Sheet and color control strip upside down or rotated Color control strip printed too close to edge of sheet There must be least 5 mm of blank white paper between the color control strip and the edge of the sheet on the left and right Measuring head incorrectly positioned Use the arrow keys on the console to precisely position the green mark in the middle of the patch Lamp defective Printed inks cannot be correctly measured e g metallic inks Actual measurement value is too far off the reference value gt 20 2 Functions and Features of Prinect Axis Control Prinect Axis Control is a color measuring system for all print shops wishing to efficiently monitor quality right at the control console of their Speedmaster Because it is fully integrated into the ergonomic design of the Prinect CP2000 Center it can be used on all modells Functionality and capabilities e Spectrophotometric color measurement in color control strips e Measurement of quality parameters colorimetric and density values for solid and grey balance patches slurring doubling dot gain trapping relative print contrast and metamerism e Storage of values for repeat jobs e Job reports for quality analysis and documentation with the add on tool Prinect Axis Control Reporting e Color database HKS Pantone e Digital measurement elements Dipco for prepress included e Integrated in Prinect CP2000 Center for direct process control e
30. t If a sheet has been defined as OK based entirely on a visual assessment which often happens when getting a customer s approval itcan be defined as the new standard After approval of an OK sheet inking is then no longer controlled based on the values stored in the color set butinstead using the OK sheet as the reference 3 Zonaltransfer Zones on the press sheet in which the colors have been defined asOK can be transferred to other zones Especially when printing multiple ups or image copies this function is very useful Please note however thatan average value derived from the selected source zones is transferred and not the actual zone profile When working with a perfector press in double sided mode you can use the function transfer to other side 4 Changing by a percentage value For measurement and color control the color level can be adjusted by a percentage As soon as you confirm the changes they are displayed as the new deviation to the reference Density and Lab values never change independently of one another Both are based on the spectral distribution of the light reflected by a patch and captured by the measuring head The Lab and density values are calculated from this spectrum If the thickness of the printed ink film changes its reflectance also changes The resulting different spectrum causes the Lab and density values calculated from it to change as well The possibilities for changing refer
31. t 6S Solid patch K C M Y X Z Greyscale patch Prinect 6S Solid patch WV S1 52 53 54 Greyscale patch Prinect 8GS Solid patch KCM XZ UV Greyscale patch C M Y Special patches Ink acceptance Dot gain patches Slurring doubling patches patches Prinect 4GS CM CY MY CMY K C M Y K C M Y Prinect 6GS CM CY MY CMY K C M Y X Z K C M Y X Z Prinect 6GS99 CM CY MY CMY KGIGAM EYE Xe Z K C M Y Prinect 6S K C M Y XZ Prinect 6S USI 2 535 54 Prinect 8GS CM CY MY CMY K C M Y X Z U V K C M Y Prinect Axis Control also lets you define your own customized color control strip that is structured differently from the Prinect color control strips How ever be aware that the individual patches may not be smaller than 5 x 6 mm For details on how to do this consult Creating and Editing Color Control Bars in the User Manual 1 3 Why is a color control strip or are several patches not detected Wrong reference value gt difference between reference and actual values too large Wrong color set loaded Soiled ink s Wrong measurement conditions set Swapped color separations Reference values are incorrectly assigned in Prinect Axis Control No calibration or incorrectly calibrated Paper or lenses soiled Wrong color control strip positioned If the color control strip is not in the archive it can be created and saved with the correct definition and
32. the measured color value and the achievable color value the best match e When the best possible result has been achieved with the ink used AE is equal to zero e When the achievable color falls short of the desired color the distance between them is indicated by the value AE This color deviation cannot be reduced by control The only way to reduce AE is to change the printing process for example by using a different ink e Consequently AE and AE are not interdependent values Actual O AE Target Color curve sy Reference Starting at the actual color the color curve passes through the colors that are achiev able by varying the ink film thickness Possible reasons for a large AE value e Soiled ink e Wrong reference value assigned e Incorrect settings for measurement conditions and paper class e Paper white incorrectly measured e Paper white has changed during the run e Different ink used e Ink batch is non standard 10 5 Lab values of actual printing inks The following values are provided only by way of example the Lab values will be different depending on the ink series the ink film thickness and the paper The process standard for offset printing according to DIN ISO 12647 can 6 Measurement as per DIN ISO 13655 light R ae type D50 2 observer geometry 0 45 or provide AN QNEntacon 45 0 measured on a black base Paper type Coated Uncoated Paper classes 1 and 2
Download Pdf Manuals
Related Search