Spectral densitometry

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The Spektraldensitometrie is an electro-optical measurement method which originally developed separately and used methods of color measurement and color density measurement combines. The measuring device class created for this, the spectral densitometer , combines the previous measuring device classes of spectrophotometers and color densitometers in terms of device and application technology .

Spectral densitometers are used in the printing industry, including packaging printing, as well as the paper and printing ink industry, in order to determine typical process parameters as part of quality assurance in the corresponding production processes.

Process description

Spectral measurement principle

The spectral densitometry uses the measuring principle of recording the spectral reflectance , from which all other colorimetric and densitometric quantities can be calculated. This means that the sensor must be sensitive to the entire visible range of the electromagnetic spectrum , i.e. all wavelengths of light. The sensitivity range starts at around 400 nm ( nanometers ) in violet and extends through the shades of blue-green, green and yellow to red at 700 nm. The sensitivity of the sensor does not have to be physically continuous, i.e. there can be small gaps between the bands occur between which measured values are interpolated .

The spectral reflectance values ​​are obtained for all wavelength segments (bands) simultaneously using a polychromator . Depending on the manufacturer, this device fans out the measured luminous flux into its spectral components, using a diffraction grating or many narrow-band filters, the signal strengths of which are measured and output as a spectral value curve as a whole.

Similarities with and differences to spectrophotometers and densitometers

What all three device classes have in common is the gloss-free measurement geometry of the light source (gas-filled lamp or LEDs ) via the probe to the sensor. It is 45 ° / 0 ° or 0 ° / 45 °. Only in the case of spectrophotometers with diffuse measurement geometry ( Ulbricht sphere ) for measurements on curved or highly reflective surfaces and with swiveling sample holder for the measurement of angle-dependent color changes (goniospectrophotometer) do the measurement geometries differ; however, these devices are not relevant for the usual applications in the printing industry and are not discussed further below.

In order to be able to record the spectral reflectance, a spectral densitometer must have a spectral measuring head, as is also used in spectrophotometers. This means that a spectral densitometer can in principle also be used as a color measuring device, provided that the manufacturer has enabled the function.

However, unlike spectrophotometers, spectral densitometers, just like classic densitometers, are also equipped with a polarization filter, which must be swiveled into the beam path in order to measure densitometric quantities in accordance with ISO . Only from the spectral measurement data obtained in this way can densitometric quantities be calculated and output. The basic spectral measurement leads to a further difference between the spectral densitometer and the classic densitometer. This difference consists in the exact measurability of special colors - i.e. printing inks that do not comply with the ISO scale 2846 -1: 2006 ( sheet-fed offset printing and heatset web offset printing ), 2: 2007 ( coldset newspaper offset printing ), 3: 2002 ( illustration gravure printing ), 4: 2000 ( screen printing ) or 5: 2005 ( flexographic printing ) and are therefore mostly printed as full tone colors and not as process colors . Previous densitometers could “estimate” the solid densities of spot colors rather than measure them.

application

Measurands

Spectral measurands

In the case of the printing and packaging industry, spectral measurement quantities refer to all quantities that - as described above - relate to the visible range of the electromagnetic spectrum (light). They are used to describe colorimetric phenomena, i.e. the mathematical description of color vision and color rendering in connection with multi-color printing on various substrates .

The basis of all these spectral measurements is the metrological recording of the spectral reflectance (see 1.1). In the corresponding "Colorimetry" measurement mode, a spectral densitometer behaves like the earlier spectrophotometers. This means that the polarization filter in the spectral densitometer is not swiveled into the beam path.

All other colorimetric variables can be calculated from the measured spectral reflectance, i.e. they are not recorded directly as measured variables, but are calculated indirectly from the spectral reflectance using known formulas and then output from the measuring device display or the associated software on a PC. The standardized framework conditions of the CIE are simulated in the measuring device during the measurement process as well as the subsequent calculations . These framework conditions are

The typical process colorimetric parameters of the industries mentioned are

Color measurements on print run copies are carried out on the full-tone fields of the printed control strip, on larger full-tone surfaces or at any meaningful point in the printed photos and graphics. Colorimetric measurements require calibration to absolute white before starting the measurement . The required white standard in the form of a ceramic tile with a known CIELAB color point is included in the scope of delivery of a spectral densitometer.

Color display photos from a spectral densitometer (Techkon SpectroDens): above, colorimetry mode (ISO 13655: 2009 “M0”, spectral reflectance), below, densitometry mode (ISO 13655: 2009 “M3” modified, solid spectral density of a special color).

Densitometric measurands

Densitometric measured variables, also known as density-based measured variables, are recorded exclusively in connection with printing processes. The color density measured in incident light is an indirect measure of the layer thickness of printing inks on the printing material. In the earlier color densitometer device class, the Luther condition was met exclusively by the selective sensitivities of the sensors in coordination with the filters: red, green and blue filters for the chromatic colors cyan, magenta and yellow as well as the "Vis" filter (characteristic corresponds to the brightness sensitivity curve ) for the achromatic drawing color Key (black).

Today, in the spectral densitometers, the same sensors that record the spectral reflectance during color measurement also scan the entire visible range of the electromagnetic spectrum when determining the densitometric values. Depending on the manufacturer, both physical filters can be swiveled into the beam path and virtual , i.e. mathematically simulated filters can be set. In addition, the standard-compliant density measurement requires the use of a polarization filter, which suppresses the differences in gloss between still wet and already dried ink layers. In this way, the measurements are made comparable regardless of the time proximity to the printing process.

In the "Densitometry" measurement mode, the spectral reflectance is recorded with the polarization filter swiveled in. As in the colorimetric measurement mode, the purely mathematical determination of the related parameters follows - here, however, the density-based one:

The measurability of the density spectrum is a unique feature of the spectral densitometer. Because densities are calculated by taking the logarithm of the ratio of reflected to radiated luminous flux , the spectral reflectance is subjected to this calculation in the spectral densitometer. Because the spectral reflectance is nothing other than the distribution of the luminous flux conditions over the wavelength, and the density spectrum appears as a mirrored curve of the spectral reflectance. In this way, a curve is calculated here which corresponds to the density value distribution in the visible spectrum. This spectral density curve is indispensable in practice, especially when measuring special colors. The curve maximum in the measured density spectrum corresponds to the actual solid tone density of the special color. The measuring device sets a virtual filter at this point on the curve, which is assigned to a specific wavelength.

This feature distinguishes the spectral densitometer from the earlier color densitometers. When measuring spot colors, these devices swiveled the color filter into the beam path that came closest to the complementary color of the spot color to be measured. Example: The red filter was used for the measurement on a green. Since the red filter is normally matched in its spectral characteristics to the measurement of the cyan (blue-green), the green density measured with it must logically be very error-prone. This shortcoming can now be overcome by using a spectral densitometer. Special colors are primarily printed in packaging printing, but also in the form of " corporate colors " on advertising materials.

Densitometric measurements on print run copies are carried out in the fields provided for this on the print control strip that is also printed as well as at significant points in the subject . In addition to full-tone fields, there are grid fields for quarter tones (for example 40%) and three-quarter tones (for example 70% or 80%), overprint full tone fields for the color acceptance of the colors printed one after the other, overprint grids for the gray balance and vertical and horizontal line fields for the detection of shifting and duplication phenomena. Densitometric measurements require calibration to the white of the substrate before starting the measurement.

Equipment solutions

Hand-held measuring devices for single measurements

The most common type of measuring device is the hand-held measuring device. It should be robust enough against external influences in the pressroom (paper dust, powder, falling). Before starting the measurement, the measurement task (i.e. the parameter to be output) must be selected using the menu. Depending on whether it is a colorimetric or densitometric variable, the swiveling in and out of the polarization filter must be taken into account. The devices of the different providers behave differently; For example, with the SpectroDens, this required procedure can be triggered automatically so that application errors in this regard are excluded. This is followed by calibration to the white standard or the printing material white, if necessary.

Spectral densitometer for the individual measurement of densitometric and colorimetric quantities. 1 - 0 ° / 45 ° measuring optics on red overprinted metal foil; 2 - measurement button; 3 - menu buttons; 4 - Color display, ready to show the measured color in the CIELAB color wheel.

The actual measurement - the measurement on the printing press as the most common practical case is explained here as an example - is carried out in the following steps:

  1. Pulling the sample (i.e. the printed copy) on the printing press;
  2. Depositing the sample on the viewing desk according to the alignment of the ink zones;
  3. Placing the spectral densitometer at the desired measuring point (high-quality devices are designed in such a way that this can be done quickly and precisely);
  4. Triggering the measurement process (either by means of a function key on the device or, if the device is connected to a PC, alternatively by clicking in the software dialog), whereby the measurement duration is in the range of one second;
  5. manual advancement of the measuring device to the next measuring point on the printed copy.

Because hand-held measuring devices carry out the measurement process on the stationary printed copy, a standard-compliant measurement document can be used depending on the requirements.

Depending on the manufacturer, function activation and optional PC software connection, handheld spectral densitometers have the following functions:

  • Measure CMYK densities (with polarization filter), special color densities (with polarization filter) or color dimensions (without polarization filter) one after the other and against reference values;
  • Generate, export and import color libraries ;
  • Carry out difference, statistical and tolerance evaluations ( pass / fail , mean ) based on spectral quantities ;
  • Create pressure curve ;
  • Measure the area coverage according to Yule-Nielsen using grid step wedges on the print sheet or on printing plates and calculate a characteristic curve from the measurement series.

Hand-held scanning devices

A hand-held scanning spectral densitometer rolls over a pressure control bar: 1 - 0 ° / 45 ° measuring head; 2 - button to trigger the measurement in front of the first measurement field; 3 - Print control bar.

Compared to the devices exclusively for single measurements, hand-held scanning spectral densitometers also have the ability to capture measuring fields that are expediently arranged in a line in one continuous movement. The devices automatically recognize the transition from one measuring field to the next and record the spectral reflectance of each field in fractions of a second - with the polarization filter swiveled out or in, depending on the measuring mode.

The Fogra print control strip DKL, for example, consists of typical measuring field arrangements, with which the colorimetric and / or densitometric values ​​for each color zone of the printing units of multicolor printing machines are clearly displayed during the set-up and production printing process. These measuring instruments are suitable but also for evaluating the Ugra / Fogra - Media Wedge or for measuring calibration and linearization -Streifengruppen ( "targets") for digital proofing systems .

In contrast to the spectral densitometers for single measurements, hand-held scanning spectral densitometers do not have a display because the amount of data recorded is too large (a print control bar can have well over 100 measuring fields). This is why these devices are only operated in conjunction with PC software. This software is often installed on the control room computer on printing machines. Within one to two seconds after the completion of a hand-held scan measurement, i.e. after the last measurement field has been recorded, all measurement fields are evaluated. The most important analysis options of complete print control bars are the representation of the solid tone densities or the density differences, also in relation to defined tolerance limit values, as well as their logging over the entire print run.

A measuring process - again explained using an example on the printing press - is carried out in the following steps:

  1. Pulling the sample on the printing machine;
  2. Depositing the sample on the viewing desk according to the alignment of the ink zones;
  3. Placing the spectral densitometer in front of the first measuring field of the print control bar;
  4. Start the measurement process using the function key on the device while simultaneously starting manual operation of the device;
  5. Manual guidance of the device from the first to the last measuring field, whereby the use of a metal rail, which is fixed over the printed sheet format, or the use of the sheet stop bar on the viewing desk is helpful for precise guidance.

Hand-held scanning spectral densitometers can be found in print shops, particularly in sheet-fed press format classes 00 to 1, and more rarely up to 3b. They are often sold by printing press manufacturers as optional OEM accessories .

Scan measuring devices driven by electric motors

A spectral densitometer driven by an electric motor moves over a pressure control bar. 1 - measuring head; 2 - dental splint; 3 - print control bar; 4 - white standard at the base station; 5 - Radio interface for transferring the measured values ​​to the PC with analysis software.
A hybrid system driven by an electric motor that has both a colorimetric spectral measuring head and a non-spectral densitometer head and therefore cannot precisely determine special color densities.

For medium and large-format sheet-fed offset printing machines, i.e. from format class 3b up to the currently largest (9), an electric motor drive for the scan spectral densitometer is recommended, because safe operation by hand simply reaches its ergonomic limits here. Such solutions are also available for smaller formats. Electrically driven scanning spectral densitometers are also available as an option from some printing machine manufacturers, including some OEM solutions even exclusively under the name of the printing machine manufacturer.

Depending on the measuring device manufacturer, different construction principles are offered, which can differ considerably in terms of technical complexity and thus price, although the measuring speed is almost the same for all solutions. The following solutions, starting with the least effort, are on the market:

  • The spectral densitometer is moved on a horizontal toothed or slide rail, i.e. arranged in the scanning direction (X-direction) and is driven by an electric motor either by means of a small gear in the measuring device or by means of a cable in the side rail holder. Depending on where the print control bar is located on the print sheet, the rail is fixed at the appropriate distance from the desk edge. This construction principle can be arranged directly on the color control desk of the printing press control station. If necessary, the measuring device can be removed from the device and used for individual measurements at any point on the printed copy.
  • The spectral densitometer is attached to a vertical rail device, i.e. at right angles to the scanning direction, and can be moved up or down (Y-direction) in this device depending on where the print control bar is on the print sheet. In the selected position, the measuring device together with the vertical rail is moved by an electric motor in a second, namely horizontal rail device in the scanning direction. This construction principle requires a separate desk as an installation area next to the printing press control station. Here, too, the measuring device can be removed from the device for individual measurements.
  • The second-mentioned construction principle can be expanded in that the spectral densitometer in the vertical rail device can also be moved up or down by an electric motor. With this electromotive XY control, in addition to scanning the print control bar, the sequential approach to any measurement points on the print copy can be programmed and then called up during the print run. Depending on the manufacturer, this construction principle requires a separate desk for setting up or can be installed on the color control desk. Only one manufacturer has designed the spectral densitometer not as a measuring head, but as a removable measuring device.
  • The most complex construction principle is embodied by independent measuring consoles that scan the entire area of ​​the printed sheet. Because of their high price, they are usually installed centrally in the pressroom and networked with the control room computer of several printing machines. Inside the electromotive scanning device, which moves over the printed sheet in the X-direction, there are several spectral measuring heads next to one another, which record printed copies up to format size 3b without gaps. No polarization filters can be swiveled into the measuring heads, so that the polarization filter effect either has to be simulated for the calculation of the color densities - a faulty process due to the different drying speed of printing ink series - or the incomparability between wet and dry prints is simply accepted.

Most of the German printing machine manufacturers also offer hybrid systems that integrate two measuring heads in one housing: One works as a pure spectrophotometer for determining the color values, the other as a classic filter densitometer. The supposed advantage lies in the saving of time, which consists in the fact that colorimetric and densitometric quantities can be measured at the same time. However, at the latest when special color densities have to be determined, this concept no longer works. The better compromise on this point is to be found with the spectral densitometer: Both measurement modes can be called up automatically one after the other: a spectral measurement without polarizing filter (colorimetric mode) when moving forward on the rail and a spectral measurement with polarizing filter (densitometric mode) when moving backward which also allows spot color densities to be correctly determined.

Inline measuring systems

Inline spectral densitometer for high-speed flexo and inkjet printing machines. 1 - measuring head; 2 - Print control bar on the edge of the paper web.
Inline color measurement camera with external referencing: 1 - Scan spectral densitometer (Techkon SpectroDrive) for ISO-compliant calibration of the system; 2 - color measurement camera (KBA QualiTronic); 3 - internally illuminated measuring slot above the clocked pressure control bar

While hand-held measuring devices for individual measurements, hand-held and electric motor-driven scanning measuring devices measure the stationary printed copy (static measurement), inline spectral densitometers record multiple measurement fields on the moving printed copy ( dynamic measurement ). This measuring concept was first implemented on web printing machines (offset, gravure, flexographic and digital printing ), where the measurement is carried out on the running printed paper web. Solutions for sheet-fed offset printing have also existed since around 2004.

Because the measuring heads are installed “inline”, i.e. in the printing press, more precisely immediately after the last printing unit, there is no time for removing and positioning the sample on a desk or for positioning the measuring device. This shortens the response time of the entire measuring system compared to static measurements.

The advantage in response time comes into play at least in the production printing process. In the process of setting up a print job, in particular on a sheet-fed offset printing machine, the advantage is often compensated for, because this measurement sequence leads to excessive printing speed before the actual start of production. This results in a lot of waste that is economically unacceptable . In this respect, inline spectral densitometers currently bring the desired profitability primarily on web printing machines.

The special technical challenges with inline spectral densitometers are that

  • the measurement time must be synchronized with the machine cycle and
  • The measuring time is extremely shortened due to the high speed of movement of the measuring object.

The clock synchronization is relatively easy to carry out in that the measuring fields are located at a predetermined position and the optics of the measuring head are focused on the measuring fields arranged close together. In order to achieve a short measuring time, a high-energy measuring light must be ensured - either in permanent irradiation or as a synchronized flash.

Inline spectral densitometers for web printing machines usually have immovable built-in polarization filters and can measure CMYK densities or special color densities. The polarization filter can only be swiveled out in a few makes so that colorimetric parameters can also be measured. In practice, measurements are usually only made at a selected point on the paper web by moving the measuring head to the desired position on a rail using an electric motor. Several measuring heads are seldom positioned offset on separate rails. Measuring heads that traverse the entire web width currently only use non-spectral measuring methods.

The inline measurement solutions currently available for sheet-fed offset printing are not spectral densitometers, but rather either several spectrophotometers arranged across the width of the format (in which the polarization filter effect is at best simulated and which rely on the use of a special print control bar) or red / green / blue LEDs -based densitometer heads with polarizing filter.

All inline solutions that can measure either non-spectrally or spectrally, but not completely ISO-compliant due to the geometry, share the need for referencing. This refers to the use of a handheld, handheld or motorized scanning measuring device with which the printer outside the printing press measures the required reference values ​​in accordance with ISO and transfers the results to the computer on which the inline system is installed (in usually the control level calculator). The setpoints for the later inline measurement are therefore specified by means of an external measurement method. This procedure is inevitable because comparison templates, such as color-accurate digital proofs, can only be measured outside the printing press and are not equipped with a print control bar anyway.

Implementation of industry standards

Printing industry including packaging printing on low-reflective surfaces

This industry information summarizes all printing processes whose process and color rendering standardization is anchored in ISO 12647 : sheet-fed offset printing, continuous, heatset and coldset web offset printing, illustration gravure, screen printing, flexographic printing and digital proof printing. Packaging printing means the printing of folding boxes and labels on slightly reflective material surfaces, i.e. on paper , cardboard and corrugated cardboard as well as on some plastic films on which the measuring geometry 45 ° / 0 ° still delivers reliable results.

Mandatory standard implementation

The following basic standards must be implemented in the device functions of the spectral densitometer in order to meet the minimum requirements in this industry:

  • ISO 5-3 "density measurement",
  • Simulation of ISO 5631-3 "Observation conditions D50 / 2 °",
  • ISO 11664-4 “CIELAB (1976) color space”;
  • ISO 13655 "spectral color measurement".

In the technical implementation of ISO 13655, gas-filled incandescent lamps with tungsten filaments were used before 2009 , the emission spectrum of which largely corresponds to standard illuminant A, i.e. only a low UV component, but a high infrared component. The standard illuminant D50, which is specified in the printing industry, has a UV component that was not present in the required level in the measuring devices. H. the lighting with standard light D50 was simulated purely by calculation: from the spectral radiation distribution of the incandescent lamp measuring light and the spectral sensitivity of the sensors.

Since optical brighteners are increasingly being used in the papers to be printed , which convert the UV component from the real standard light D50 into visible bluish light and thus raise the whiteness of the paper to over 100%, there is a growing need to have this visual effect as well to be represented physically and metrologically in the spectral densitometers. For this reason, among other things, ISO 13655 was revised in 2009. It redefines the four measurement mode settings in a spectral range from 340 nm to 780 nm:

  • M0, “A mode”: for conventional color measurement before 2009, i.e. H. spectral reflectance without polarizing filter under a gas-filled tungsten lamp with almost standard illuminant A , whereby the UV component can be mathematically supplemented if necessary;
  • M1, "D50 mode": for the physically correct color measurement from 2009, d. H. Spectral remission factor without polarizing filter under an LED light source, which physically simulates the standard light type D50 with the correct UV component as best as possible , whereby a UV source must be switchable;
  • M2, "UV-Cut-Modus": for the color measurement with fading out of the UV component in the measuring light, i. H. Spectral reflectance without polarizing filter under a light source that generates any type of standard light without a UV component or, if necessary, excludes the UV component with a UV blocking filter;
  • M3, "polarizing filter mode": for densitometry , i.e. H. the measurement of optical density (blackening), color density (process printing inks) and spectral density (special colors) using a selective filter set or a spectral measuring head.

Further details on the measurement modes:

The requirement in mode M1 for physically correct simulation of the standard illuminant D50 led to a surge in innovation in 2011 and 2012. Since a theoretically required F8 fluorescent lamp does not fit into a small measuring device, the D50 standard light type is practically simulated with several, altogether spectrally representative LEDs (including UV LEDs for the UV component); Any intensity deficits in the spectral radiation distribution function are mathematically compensated as before in interaction with the spectral sensitivity of the sensors ( Luther condition ).

The physical simulation of D50 in M1 by means of LED measuring light was first realized in 2011 in a handheld measuring device from Konica Minolta. The spectral densitometer was developed primarily for creating ICC profiles. Barbieri electronic followed the same purpose as the second manufacturer in large-format inkjet printing ( LFP ).

As a third manufacturer, Techkon implemented the M1 mode in 2012 with complete LED coverage of the visible spectrum plus UV component, whereby Techkon's spectral densitometers are primarily used on offset and flexographic printing machines, i.e. under robust production conditions. For the first time, Techkon enables an evaluation of optical brighteners in the circulation papers and a metrological comparison with digital proofs that contain optical brighteners or take into account in their ICC profiles. Techkon's pressroom spectral densitometers are the only devices on the market to date that automatically swivel the polarizing filters on (M3) or off (M0, M1, M2) when the mode is changed.

Optional standard support

The fixation of the spectral densitometer on the printing industry has induced the leading measuring device suppliers to support the standard-compliant quality assurance in the printing process as well as in the production of color proofs in the software, which can be purchased in connection with the hardware . This is generally done by applying the following standards:

  • the seven-part ISO 12647, in which the colorimetric data of the paper types and the process color scales and their handling are specified;
  • in the German-speaking and some European countries as a supplement to ISO12647-2 the process standard offset printing , in which u. a. the color sequence in the printing press, the selection of the printing inks, the tonal value control in the prepress stage, the use of the polarization filter during measurements and other processes are described as detailed work instructions;
  • the process printing ink scale ISO 2846 with its colorimetric target values;
  • for the European market the analysis of the Ugra / Fogra media wedge as a standardized control tool in the printing industry ( media standard printing );
  • for the American market the GRACoL standard with its G7 control means.

Related industries

Spectral densitometers - hand-held devices for individual measurements - are also sold in other industrial sectors in which colors have to be measured:

  • Production of printing, painting and artist's colors,
  • Manufacture of paper, cardboard and packaging materials,
  • Textile industry.

The colorimetric device functions are primarily in demand there. The following standards must therefore be implemented:

  • Color measurement according to DIN 5033 >
  • Simulation of ISO 11475 "Observation conditions D65 / 10 °".

Web links

Individual evidence

  1. ^ Fogra Forschungsgesellschaft Druck eV: FograCert test program, measurement documents , accessed on April 27, 2014
  2. ^ Forschungsgesellschaft Druck eV: Control means, digital print control strips , accessed on April 27, 2014
  3. ↑ QI Press Controls: Intelligent Density System IDS ( Memento of the original from April 27, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (CCD sensor-based inline density measurement system; PDF; 244 kB), accessed on April 27, 2014 @1@ 2Template: Webachiv / IABot / www.qipc.com
  4. ↑ ISO 5-3: 1995 Photography - Density measurements - Part 3: Spectral conditions (International Organization for Standardization), accessed April 25, 2014
  5. ↑ ISO 5631-3: 2008 Paper and board - Determination of color by diffuse reflectance - Part 3: Indoor illumination conditions (D50 / 2 degrees) (International Organization for Standardization), accessed on October 26, 2009
  6. ↑ ISO 11664-4: 2008 Colorimetry - Part 4: CIE 1976 L * a * b * Color space (International Organization for Standardization), accessed on April 25, 2014
  7. ↑ ISO 13655: 2009 (Revised 2013) Graphic technology - Spectral measurement and colorimetric computation for graphic arts images (International Organization for Standardization), accessed on January 18, 2016
  8. ↑ Fogra Forschungsgesellschaft Druck eV: Development of methods to overcome fluorescence-related deviations between proof and print runs. (AiF-No .: 16607 N), interim results, DfwG-Tagung Aachen 2012 , last accessed on January 18, 2016
  9. ↑ Techkon: SpectroDensitometer SpectroDens, Software SpectroConnect. Handbook 2015 ( Memento of the original from April 27, 2014 in the Internet Archive ) Info: The @1@ 2Template: Webachiv / IABot / www.techkon.com archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , last accessed January 18, 2016
  10. ↑ ISO 12647-2: 2013: Graphic technology - Process control for the production of half-tone color separations, proof and production prints - Part 2: Offset lithographic processes (International Organization for Standardization), accessed on January 18, 2016
  11. ↑ GRACoL 2013 Introduction (New Requirements and Applications for Commercial Offset Lithography) ( Memento of the original from January 24, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. IDEAlliance.org, accessed April 27, 2014 @1@ 2Template: Webachiv / IABot / www.idealliance.org
  12. ↑ ISO 11475: 2004 Paper and board - Determination of CIE whiteness, D65 / 10 degrees (outdoor daylight) (International Organization for Standardization), accessed April 27, 2014