Title: STUDY MATERIALS - Colour models in printing
Course: Pre-Press Procedure / e-design > Section: Part III – Colour models/colour management

After the online lectures, the study materials along with the map of new concepts will be uploaded to Moodle.

Colour models in printing

Colour models in printing

The most common colour models in printing are

        - RGB,
        - CMYK,
        - CIE XYZ,
        - CIE LAB,
        - HSL,
        - HSB.

There are two types of colour spaces. Device-dependent and device-independent colour spaces. A device-independent colour model is an abstract mathematical model that describes colours with numerical values.

The CIE XYZ mathematical colour model was developed in 1931 by CIE (International Commission on Illumination) after several experiments. This colour model was derived in 1920 from experiments by W. David Wright and John Guild, in which the RGB colour space was marked in the XYZ coordinate system. An absolute instrument, the CIE standard colorimetric observer, was defined. Given the theory of the three components of vision, the observer can find the correspondence to each monochromatic radiation as a mixture of three additive primary colours. Data for the definition of a standard observer were collected by colour matching experiments. For each colour, their XYZ coordinates were found during the experiments. All other colour models fit in the CIE XYZ model.

RGB colour model transmits all the colours around us.

Red, green and blue are the three colours that represent the whole range of colours around us. RGB is a colour model that combines red, green, and blue light. This is also called additive colour synthesis. The RGB colour space is mostly used by all light emitting devices. RGB is mainly used in electronics: televisions, monitors, photo and video equipment, scanners, etc. At the same time, the RGB model already had a solid theoretical basis before the digital era, which is based on human colour perception (see Colour synthesis). RGB is the basis for measuring colours in colorimetry. However, even the best colour rendering devices cannot deliver the colours described in the entire RGB colour model. Therefore, colour spaces based on an RGB device are described according to the colour rendering capability of the device. For example, monitors have their own colour space, digital devices have sRGB, and Adobe has RGB for colour conversion needed for printing.

The CMYK colour model represents colours that are transmitted from light-reflecting and light-absorbing objects such as paper, fabric, metal, plastic, and so on. These are colours that absorb a certain part of the colour spectrum and reflect the rest back, which creates a sense of colour. This is a subtractive colour synthesis (see Colour synthesis). CMYK, originally the CMY colour space, is derived from RGB contrasting colours. Cyan is obtained when Red is not present in the RGB space, Magenta is obtained when Green is absent, and Yellow when Blue is absent. Black, called Key, has been added to the colour space to allow the widest possible range of colours to be printed. The disadvantage of the CMYK colour space is the much smaller colour space compared to the RGB colour space. When converting colours from RGB colour space to CMYK, this process must be done with as little colour change as possible. An Adobe RGB colour space has been developed for this purpose, from which CMYK colours are obtained with the slightest colour change.

CIELAB (kasutusel ka CIE L*a*b* või Lab) 1976 aastal võttis CIE kasutusele Hunteri Lab koordinaadistiku. Täpsemalt – võeti kasutusel nn CIE LAB värvusmudel. See on inimese värvitajuga seotud sõltumatu värviruum. LAB-i kasutatakse niisuguse kolmemõõtmelise värviruumi tähistamiseks, milles saab kirjeldada praktiliselt kõiki inimese silma poolt eristatavaid värve. LAB-i värvusmudelis suudeti silma värvieraldusvõimet veelgi täpsemalt kirjeldada ning kasutusele võeti vastandvärvide teooria. Mudel koosneb kolmest kanalist, mille abil kirjeldatakse värvitooni. L* (Lightness) kirjeldab värvuse heledust (valge-musta telg), a* vastab punase-rohelise teljele ja b* kollase-sinise teljele. See mudel on laialdaselt kasutusel värvihalduses ja värvuste mõõtmisel.

HSL - Hue, Saturation, Luminance is a three-dimensional colour model based on the colour sphere. The sphere is divided into 360 sections, which show the colour tones along the sphere according to the CIE chromatic diagram, i.e. the size of the angle determines the colour shade from red to yellow, from yellow to green, from green to blue, from blue to purple and from purple to red again. The saturation value is related to the radius but is expressed as a percentage from 0 to 100, where 0 is the centre of the sphere. Now we add a third dimension that expresses brightness.

A colour model very similar to HSL is HSV – Hue, Saturation, Value, sometimes also HSB, where B is brightness. HSV and HSL are just different colour models that match the RGB colour space. These are device-dependent colour models.

Colour measurement

Measuring colours

The best instrument for assessing colours is the human eye. However, the visual perception of colours is a subjective process that depends on the difference in observation conditions as well as the characteristics of the observer's visual apparatus and is therefore not a suitable basis for industrial standardisation. Measurement is necessary to assess the accuracy of the processes. However, there are many areas where colour accuracy is very important and printing, photography and design are definitely among them. In order to decide whether a colour is correct or incorrect, it must be possible to measure it.

When measuring the printed colour, the colour of the paper must also be taken into account, as there is no ideal white paper.

Colourimetry is a science and technology that physically defines and describes human colour perception. It is based on the determination of the spectral composition of colour. In printing by colourimetry we mean the measurement and comparison of colours with a spectrophotometer.

Spectrophotometry examines the ability of substances and materials to affect electromagnetic radiation, which in the optical or visible part of the scale means colour. It is used to measure colour and describe differences. To measure colour, the CIELAB model is most often used.

Tolerance unit delta E

Colour differences are described in the CIELAB colour model with the colourimetric deviation unit delta E (ΔE). Delta is the fourth letter of the Greek alphabet and denotes a difference in mathematics; E comes from the German word Empfindung, which means perception. In a simplified way, the length of a space vector is measured in the case of comparable colour dots. ΔE expresses the mean difference of the LAB coordinates. The perception of difference is not the same throughout the colour space but depends on the colour tone, saturation and brightness because the colour perception of the eye is different in different parts of the spectrum. For example, for lighter tones, differences are much better perceived than for darker or saturated tones.

The perception of tonal differences is also related to the observation time. It takes the amateur 5 seconds to perceive ΔE15, 10 seconds to perceive ΔΔE10 and 15 seconds to perceive ΔE5. It is thought that ΔE2-5 is not realistically perceptible.

Measuring instruments

Measuring instruments

In order to offer clients the best printing results and ensure quality, the printing process is thoroughly standardised. In printing, various measuring instruments are used to measure colours and assess print quality.

Densitometer – densitometric measurement is the daily activity of a printing operator to achieve print quality. In printing, a densitometer is an instrument for measuring the intensity of colour tones on paper or for measuring the transparency of films.

There are two types of densitometers:

1. A reflection densitometer, which measures the reflection and absorption of light at a given arithmetic value.

2. A transmission densitometer for measuring light transmission and permeability. It is used to measure film and other transparent materials. Let's take a closer look at the reflection densitometer used in printing.

The densitometer measures the brightness of CMYK colour through various filters. The densitometer also has a lamp that lights up at the time of measurement and the reflected light is measured. Measurements with a densitometer also take into account the material on which the measurements are made. At the start of the measurement, the brightness of the material (paper white) is first measured, and when measuring colour, the densitometer subtracts the whiteness of the paper to measure only the optical density of the colour. A polarising filter is used to reduce deviations caused by light reflection. Reflections can be reduced with two perpendicular polarisation planes. Polarisation is maintained when light is reflected, but disappears when scattered.

In addition, there is a UV filter that takes into account the light in the room, i.e. does not allow UV rays to affect the light reflected back from the paper. A densitometer must be accompanied by a reference card for calibrating the densitometer.  The reference card is provided with printed and checked colour fields. The reference card must be updated every few years because the colours on the card age and oxidise during storage and when exposed to UV light and are not suitable for calibration.

A densitometer is used to measure the following:

1. Optical density of colours. The unit of measurement is D and the values are between 0 and 3. The densities used for offset printing are between 0.8 D and 2.3 D. Suitable and correct densities are found by measuring the main tones of the prints with a spectrometer and comparing the results with the colour values of the main tones in the icc profile for the respective paper and printing machine. Colour fields with ΔE of up to 2 compared to the profile are measured with a denistometer for optical density, which is a measured colour density.

2. Dot gain. The dot gain is the difference in the size of the raster dot on the plate compared to the one on the print. The dot on the print is and must always be bigger than on the plate. The increase in dot size must be within a certain range.

3.  Trapping (overprint of two colours) or stickiness. The correctness of the trap ensures the correctness of the halftones.

4.  Grey balance. A 50% black colour field and 50% CMY (C 50%, M 40% and Y 40%) are compared. If the difference is within the permissible range, then the whole print is balanced.

When measuring from paper, the background against which the measurements are made must be taken into account. The background is also taken into account in the profile descriptions, which specify the background that should be used for performing measurements. The problem is that the paper is somewhat transparent. When printing on a single-sided sheet-fed or web machine, the scales usually coincide on the sides of the paper. In order to exclude the effects of coinciding scales during measurement, a black background must be used for measurement. If there is no print on the other side of the paper behind the scale, it is measured on a white background. In the profile descriptions, the background colour is indicated by the English abbreviations WB (white backing) and BB (black backing). When comparing profiles, it is necessary to measure against the background specified in the profile descriptions whether or not there is a print behind the scale. The measuring boards of newer printing machines all usually have a black background.

A colourimeter – measures colourimetric values, i.e. numbers, which are based on the reactions of the components of the visual apparatus and based on which it can be assessed when metamerism occurs and when it does not.

Spectrophotometer – colours are distinguished in physics by the light wavelengths. A spectrophotometer is a tool that measures the light intensity depending on the wavelength. The spectrophotometer is provided with a lamp which lights up at the time of measurement.  The light is reflected back from the object to be measured, and the reflected light is converted through filters to colour in the RGB colour space. Next, the colour value is converted to the Lab colour space.

Each spectrophotometer is provided with a built-in calibration system. This is a white stone from which the reflected light is measured, and the results are used to generate an algorithm that starts calculating the correct colour values. Calibration is always performed when the spectrophotometer is started and when the measurement parameters are changed.  The calibration stone is associated with a specific measuring instrument, and an algorithm corresponding to this measuring instrument is created based on each stone. Calibration stones of similar instruments must not be interchanged. The measurement results may not be adequate in this case. If necessary, the spectrophotometers can only be calibrated in a specialist workshop. Each spectrophotometer is accompanied by a calibration certificate and instrument accuracy and the period of validity of the certificate. At the end of the certificate validity, the instrument must be recalibrated. Thereafter, a new certificate will be issued.

A spectrometer measures the colour values in the LAB and, if necessary, the difference ΔE between the two colours can also be seen on the instrument display. As the printing process is never absolutely accurate and there are constant colour fluctuations in the printing process due to various reasons, the standard also allows for tolerances. The permitted colour differences of digital proofs from the standard have also been determined. The human eye begins to distinguish colour differences when ΔE is greater than 2.5. Up to a value of ΔE 4, the colour differences are easily noticeable to the naked eye. Above ΔE 4, all colour differences are substantially noticeable.

Pursuant to standard 12647-2, a difference in basic tones (cyan, magenta, yellow and black) is allowed, compared to the standard ΔE 5, the average difference ΔE 4 on printed paper is up to ΔE 3, the difference between the lowest and the highest is ΔE 10.

Today, combined measuring instruments are used, where the function of the densitometer is also added to the spectrophotometer.  In many cases, the spectrophotometer is also built into the output devices. For example, new digital and photo printers have a built-in spectrophotometer that monitors and measures colour values during the process. Newer printing machines also have built-in spectrophotometers that allow measuring and correcting colour reproduction during the printing process. There are many different solutions for the use of spectrophotometers in printing machines, depending on the price and need.

One solution is a scanning spectrophotometer located on the table of the printing operator. There are two types: one that measures the digital print scale placed on the printing sheet and the other that allow the measurement of the entire printing sheet in Lab values. In addition, both web and sheet-fed printing machines are provided with built-in measuring instruments that measure and allow colour correction throughout printing. To use such scanning spectrophotometers, a so-called colour profile is generated in the RIP together with the plate files, in which the quantities of colour by colour areas of the printing machine are described. When a profile is sent to the computer controlling the spectrometer and the colour printer of the printing machine, the colour screws of the printing machine open according to the sent profile, and by exchanging information between the measurement results and the profile, colour transmission can be controlled automatically during the printing process.

Spectrodensitometers combine the characteristics of a photo spectrometer, colourimeter and densitometer and allow the control of colours, discolouration, paper quality, etc.

Printing halftones Bitmap (types of bitmap, bitmap density, bitmap angles, dot gain)

Printing halftones. Raster.

Since the invention of the art of printing, people have always wanted to print images in addition to text. Earlier illustrations were woodcut line images in which halftones could not be used and a maximum of two colour tones were used. The printing technology does not allow different brightness levels of the colour tone to be achieved by adjusting the thickness of the ink layer.

When printing CMY primary colours in full tones, we can only achieve 7 colour tones. These include the three primary tones of C, M, Y and the three secondary tones of C + M = bluish purple, C + Y = green, and M + Y = red. All three primary tones, when printed together, ideally result in black.

Under normal conditions, the primary colours are not perfectly pure, and in the case of registration, a brownish-grey colour is obtained. A neutral tone (K) is used for printing neutral black.

It is easy to imagine printing full-colour surfaces, but how are all the other colour shades in the prints achieved? Using four basic colours, it is possible to print much more than 7 tones.

We can print colour-rich photos and achieve tones that are completely absent from basic colours. Such a wide colour space is possible by the use of different brightness levels of the primary colours, which creates a number of colour shades or halftones. High requirements on image quality have become increasingly important in print and are one of the advantages of print media over digital.

Raster

In the 19th century, rasterisation technology was invented to transform the surface of an image into dots and colourless areas.   Adjusting the ratio of coloured to colourless surfaces allows printing halftones. The dots used to print halftones are called raster. Raster dots are small enough so that the human eye cannot distinguish them from the viewing distance and sees a uniform surface. It is therefore an intentional illusion. When we print magenta 20% as a raster, it looks like a pinkish tone to us.

Comparison of 100%, 50% and 20% magenta (with and without magnification)

When we design prints in a computer or process photos, a raster is not used to represent halftones at this stage. Halftones seen on a computer screen are the monitor's interpretation of the colour tone values of various vector objects or, in the case of pixel graphics, of the tone of each pixel.

A raster only comes into play when we start printing the design file. Different rasters are used by laser and inkjet printers as well as printers that are used to make image carriers. Rasterisation is performed in a special raster processor called an RIP (Raster Image Processor). In most cases, the image arrives in the processor in PostScript page description language, and the processor renders it as a raster image.

Thus, the RIP can also be called a translator, which makes the design file understandable to the printer. If the design file contains errors or techniques are used that are too complex for the RIP, rasterisation may fail or an incorrect raster image may be displayed. For this reason, PDF file standards set out requirements to ensure trouble-free rasterisation of an image.  A raster image is created in the form of high-resolution 1-bit graphics.

Raster types

Different brightness levels of raster surfaces are achieved by changing the size, shape or frequency of raster points. There are two main types of rasters, called AM (Amplitude Modulation) and FM (Frequency Modulation) rasters.

In the case of the AM raster (amplitude modulation raster), the distance between the raster dot lines is always the same, but the size of the dots changes. In the case of lighter colour tones, the raster dots are smaller and in areas without colour, the raster dots are bigger. In the case of darker colour tones, the raster dots are bigger and in areas without colour, the raster dots are smaller. The AM raster is also called a normal or conventional raster. The strengths of the AM raster are the well-controlled dot gain and smooth surfaces that can be printed with high quality, and the weakness is the loss of smaller details during rasterisation. In offset printing, an AM raster is usually used.

In the case of the FM raster (frequency modulation raster), all raster dots are of the same size but are located on the surface with different density. In light colour shades there are fewer raster dots and in darker shades there are more. This is also called a stochastic raster. The strength of the FM raster is the much better preservation of small details, and the weakness is the high dot gain, noise generation and, as a result, poorer results when printing even surfaces. FM rasters are often used in inkjet printers as well as in hi-fi offset printing when more than 4 colours are used and there are not enough moiré-free raster angles for the AM raster. For example, this is true in the case of 6-colour hexachrome printing, when orange and green are used in addition to traditional CMYK colours.

A hybrid raster is the newest type of a print raster and it seeks to combine the strengths of the AM and FM raster.

Raster density

The more raster points there are per unit area, the more natural and accurate the image will look. So one might think that the denser the raster, the better. Unfortunately, certain printing technologies and printed materials do not allow the use of very dense rasters. For example, a much denser raster can be used for offset printing than for screen printing.

In the case of offset printing, the size of the raster dots depends on the quality of the printing paper. The higher the quality and coverage of the paper, the smaller the dots that can be used. The measurement unit of a printing raster is l/cm (lines per cm), which describes how many raster lines are printed per 1 centimetre, or lpi (lines per inch), which describes how many raster lines are printed per 1 inch. In the case of lower quality newsprint, 85 lpi raster is used, in which case the eye can distinguish individual raster dots even without using a magnifying glass. For higher quality uncoated paper, raster 133 lpi is used, and for coated paper, 150 or 175 lpi. For particularly well-configured printing machines and high-quality paper, 200 lpi raster is sometimes used, which gives good results when printing photo books and reprographs of works of art, for example.

- 150-175 lpi - Coated paper
- 200 lpi - Glossy paper and extra high-quality print
- 128-133 lpi - Uncoated paper; Softer and textured paper
- 115 lpi - Lower quality newsprint
- 50 lpi - Screen printing
- 85 lpi - 600 dpi laser printer

Raster angles

Raster images of different colour tones cannot be printed exactly on top of each other with the AM raster, because the inaccuracy of the printing process would cause fluctuations of the colour tones. To prevent this, the rasters are printed at an angle to each other, which in turn can create an eye-catching pattern called the moiré effect.  A 45° angle between the rasters creates the least perceptible pattern for the eye. Typical raster angles of CMYK sub-colours for offset printing are C 15°, M 75°, Y 90°, and K 45°. In the case of precise registration, the raster dots form a rosette image.

Raster dot rosette and moiré  efekt

In the case of FM rasters, different raster angles are not used and thus there are no problems with them. In addition to AM and FM rasters, hybrid rasters are used that combine the best features of both raster types.

Dot gain

Dot gain is the increase in raster dot size during the printing process, which causes the image to become darker than required.

Dot gain depends the most on the printing paper – the lower the quality of the paper, the higher the dot gain. The dot gain of newsprint is much higher than that of glossy magazine paper.

Raster density also affects dot gain – the smaller the raster dot, the higher the dot gain.

The dot gain is therefore higher for the FM raster than for the AM raster. For the same raster and paper 300 lpi = 30%, 150 lpi = 15%, 75 lpi = 7.5%. The dot gain also depends on the shape of the raster. For example, the dot gain of elliptical raster dots in midtones is smaller than that of circular raster dots.

The dot gain is not similar for all halftones. The highest dot gain is in midtones (40-60%). Dot gain is measured with a densitometer. This is usually done by the 40% and 80% tone fields on the control strip on the edge of the printing sheet. The usual dot gain on a 50% raster surface for a 150 lpi raster is 15-18%. The dot gain cannot be avoided, but it can be compensated for. For example, ICC profiles set to specific paper types can change the tonality of an image so that we get a print with the correct brightness after the dot gain.

Colour separation (colour separation principles: UCR, GCR)

COLOUR RESOLUTION

In light-emitting devices, such as monitors and TVs, the RGB colour model must be used. RGB is also used in photo and video equipment and scanners.

A CMYK system is used in the printing industry and in colour printers.

CMYK and RGB colour spaces vary in scope. Compared to RGB, CMYK contains fewer red, green, and blue colours. However, CMYK has colours that are missing in RGB. Therefore, the colour spaces do not coincide and the colour conversion has to accept losses.

Colour resolution is the separation of the CMYK colour space into 4 sub-colours – cyan, magenta, yellow and black (K). Different algorithms can be used for colour conversion, and colour management is used to keep losses to a minimum.

The right moment for colour separation is after photo editing for several reasons:

- Photos are corrected in the RGB system because many Photoshop tools do not work in CMYK mode.
- From the same RGB source file, it is possible to easily convert different CMYK files, depending on where you want to use them.
- The CMYK colour profile used for colour separation determines the maximum colour coverage of the photo, i.e. the sum of colours, the black limit, and compensates for the darkening caused by the expected dot gain, so it is not correct to change the brightness or contrast of the photo after colour separation, as this may cause the colour coating to change again and cause printing problems. Using the correct separation profile guarantees the best CMYK colour matching of the image, taking into account both the properties of the paper (whiteness, coverage and capacity) as well as the printing machine and colour (dot gain, CMYK).

Before colour separation, it is recommended that you combine the layers of the photo to avoid any changes that may occur. Save a backup of the original file with layers. When you convert a file from one colour system to another, the colour space is changed, and some of the colour information is always lost and cannot be recovered by converting it back. Therefore, it is right to limit yourself to one conversion and to save the original file in RGB format before colour separation. For example, in RGB-CMYK-RGB conversion, the first and last file are almost never the same.

K or black

In CMYK, the fourth colour is marked by the letter K, which in one version means the colour model key “e” in English, and in the other version the abbreviation K is used because B could be confused with RGB blue.

Black is not very important in forming a colour image. The entire colour space is achieved with three primary colours. However, black is key in the sense that it gives the possibility to separate separations with different colours from the same source material, print high-quality black texts and add sharpness to details. Replacing coloured sub-colours with black for darker colours saves costs.

Principles of colour separation (UCR, GCR)

One of the differences between RGB and CMYK is that in RGB it is not possible to represent the same colour with different sub-colour ratios, but in CMYK it is possible to change the relationship between the black sub-colour and coloured sub-colours. Different colour separation algorithms determine how many of the coloured sub-colours we replace with black in darker areas as well as how many coloured sub-colours we keep in addition to black. Due to the different resolution techniques, very different CMYK files can be created from one RGB source file.

Colour separation algorithms are different because paper types with very different properties are used.  On coated paper many more colours can be printed than on uncoated paper. Different paper types also have different dot gain, which affects the printing result.

The black limit indicates the maximum amount of black.  Black plays an important role in detail and contrast, and therefore a limit higher than 95% is not normally allowed. If we allowed a maximum of 100% black, it would mean a loss of detail in the darkest areas.

The sum of colours is the allowable sum of the four sub-colours, which depends on the type of paper. 400% colour would be too much for any type of paper and cause smudging and drying problems, but only 235% is allowed for newsprint, for example. Therefore, for darker areas you must deal with much less colour quantity.

Skeleton black refers to a colour separation where black is used only in the darkest areas and coloured sub-colours are used to the maximum extent. In the analogue era, a method of separation was often used, the strength of which is that detail and sharpness are also present in coloured sub-colours, but the weakness is the high colour coverage, i.e. colour sum, which can cause drying problems and smudging.

UCR (under colour removal)

UCR (under colour removal) means that black is only used in areas that are very dark or in neutral shades and the coloured parts are not changed. As black is darker than colours, it can reduce the proportion of colours and achieve the same degree of darkness. The strength of this separation is the ability to adjust the tonality of the image during printing. The weaknesses are the easier tonal fluctuations that can occur during printing, and the accumulation of detail and sharpness only in the black sub-colour area, which requires skilled photo processing to achieve good printing results. UCR is suitable for printing on coated paper because it retains colour saturation even in dark areas.

GCR (grey component replacement)

GCR (grey component replacement) achromatic colour separation. Based on this separation principle, in addition to dark and neutral areas, black can also replace the part which is equal in all sub-colours. Of the three sub-colours, the one the quantity of which is the smallest makes the image darker and brings the colour tone closer to neutral grey.

With a light green colour, such as C40% M0% Y100%, there is nothing to do with black. However, if we take the darker green C90% M50% Y100%, then there is an equal share of C50% M50% Y50%. In real life, these are not perfectly pure colours, and when printing according to ISO 12647-2 densities, more cyan is required to get grey:  C62% M50% Y50%. This part is called the grey component and it can be replaced by black. In Heavy GCR separation, the grey component is largely replaced by black. In the case of medium GCR separation, more colours are retained.

For GCR separation, the black sub-colour is very strong and more like a black-and-white image. This allows the greatest savings when using coloured colours.  Images with neutral tonality are easier to print with GCR separation to avoid tonal fluctuations. However, images may lose much of their dark saturation with this separation.

UCR

Medium GCR

Heavy GCR

Maximum GCR

For smaller print volumes, GCR is not a recommended separation principle, as it requires very precise adjustment of the black colour during printing to achieve an equally high-quality result for both small text and images. The more adjustments needed, the more paper is needed to get the job done. The tonality of a print cannot usually be affected by this separation during printing.

Colour management

COLOUR MANAGEMENT

Purpose of colour management

Colour management is necessary to obtain similar or identical colour results as standard from different output devices. Each device operates in a specific colour space, which allows a specific colour gamut or range of colours. The aim is to achieve a situation where colour matching can be ensured throughout the production chain, the best contrast and exact colour matching of the transmitted colours, and the largest colour gamut allowed by the device.

When colour management is not used, colour transfer depends only on the output device. In order to use colour management, a mathematical model (standard) independent of devices is needed.

The mathematical CIE XYZ colour model was first used in colour management. However, with the development of technology, many colour rendering devices emerged, and such a model was not sufficient because it was a non-linear compressed colour model and did not allow all the colours to be transmitted to be adequately defined. Device- and material-based colour spaces were developed for different colour models such as RGB, HSL, HSB CMYK, etc. Such colour spaces are related and depend on different colour spaces or devices. Therefore, they are called device-dependent colour spaces.

Today, CIE LAB is an independent mathematical colour model in colour management. For colour management, colours are converted to the Lab system. To do this, it has to be described to the device how the colours are transmitted in that device. These descriptions are called colour profiles.

ICC profiles (profiles used for printing, profile creation)

ICC profiles

In 1993 the International Colour Consortium (ICC) was established by Adobe, Sun, Apple, Kodak, Microsoft and Silicon Graphics. This organisation began to develop different colour profiles for different output devices and papers.

These colour profiles are called icc profiles according to the extension.

These profiles specify the following:

- how colours are converted to a particular device or material,
- what the maximum amount of colour allowed is,
- how and how much black colour is generated.

In order to convert colours from one colour space to another according to ICC profiles, the PCS or Profile Connection Space is used. PCS – Profile Connection Space – a device-independent colour space that connects conversions.

PCS is defined as a part of the L*a*b* colour space. With the help of this, the colours are first converted to the Lab colour space and then to the new selected colour space. Using a printable Adobe RGB icc profile, the colours are converted to the Lab colour space using PCS, and from there, also with the help of PCS for example to Fogra39.icc profile using CMYK colour space.

Colour management and icc profiles are used for the following purposes:

1. In colour separation Icc profiles are used to convert colours from one colour space to another.

2. In colour correction of images If the image is in the required colour space, the image editor can use the print profile to see on the monitor screen what the image would look like when printed according to the profile. Of course, provided that the monitor is properly calibrated and profiled.

3. In screen comparison With the newer versions of Photoshop and Acrobat Pro, the print result can be mimicked on the screen particularly accurately, as it is possible to select the icc profile of the paper corresponding to the particular printing machine for such a simulation.

4.     For making digital proofs A digital proof printer can be set to mimic the colour rendering of a specific printing machine by calibrating the printer according to either the profile made for the printing machine or the ISO Fogra standard profiles.

The profiles are classified according to the ICC standard into 3 device profiles:

SCNR – input profiles (scanners, digital cameras) MNTR – monitor profiles (CRT. LCD and LED monitors) PRTR – output profiles (printers and printing machines)

The profiles of input device profiles are one-way, i.e. they contain information for conversion from the device to PCS. The profiles of output devices are two-way – for information conversion from the colour space of the devise to PCS space and vice versa.

ICC profile is present as a separate file or embedded in some image or graphic file format EPS, TIFF, JFIF, GIF ...

Creating ICC profiles

A special program and spectrophotometer are required to make Icc profiles. As software usually Profilemaker is used. The program comes with special scales and an interface for measuring with a spectrophotometer.

On the output device (printer, printing machine, scanner, monitor, etc.) a scale is displayed; for measuring a photometer is used and the measuring results are analysed with a profile program and compared with the Lab colour space. According to the results of the analysis, a profile is made which must ensure the correct colour rendering of the given device. After the profile is created, it will be taken into use. Using the profile, a new colour rendering with the appropriate scale is performed, and it is checked whether the profile corresponds to the selected colour space and whether it can accurately interpret and convert colours.

Icc profiles used for printing

In the case of printing machines, the profiles are made so that a scale used to make an icc profile is printed by using test sheets of different types of paper.  The sheets are measured with a spectrophotometer and analysed with a program designed for preparing or comparing the profile. According to the results, an icc profile is generated. For printing machines, icc profiles are always made for different types of paper because different types of paper can render the colour space differently. This method was largely used between 2004 and 2006.

From there on, the so-called standard colour profiles have been used. The problem is that the printing machines wear out during the work and the processes change, and unfortunately at some point it is no longer possible to print according to the developed profile. A new icc profile must be created after a certain period of time. However, as there are many printing houses, and publishers and advertisers use the services of several printing houses, such a large number of different profiles is confusing, especially if the icc profiles are changed.

In Europe, icc profiles have been developed for the average European printing machine and for different types of paper. It is easier to print and keep the printing machine in such a condition that it is possible to print according to a standard profile rather than to constantly make new ones. ECI – European Color Initiative is a company that has developed ISO standards for offset printing. ECI is a group of experts in device-independent colour management and digital publishing, set up in 1996 in Hamburg on the initiative of four German publishers: Bauer, Burda, Gruner + Jahr and Springer. To date, ISO-compliant icc profiles, scales and characteristics for measuring and comparing colours have been developed in collaboration with Fogra under the ECI name. Fogra is a German company that offers services for printing companies. Fogra is involved in standardisation, training, consulting, testing and inspection of prints. The Swiss Center of Competence for Printing and Media Technology (UGRA) is developing measuring scales and corresponding characteristics.  In 1992 a cooperation agreement was signed with Fogra for the marketing of control equipment and digital print scales under the Ugra/Fogra brand.

It is also easier for the client to use the corresponding standard profiles than to use the personal profiles of many different printing houses.

The following ISO standards are available for printing

- ISO 12647-2 – general offset printing standard
- ISO 12647-3 – newspaper offset printing standard
- ISO 12647-4 – gravure standard
- ISO 12647-5 – screen printing standard

ISO standards for printing

ISO 12647-2 standard

Here icc profiles for various types of paper have been developed. In addition to the profiles, there are corresponding Fogra characteristics for each profile to measure digital proofs or prints – Media Wedge control files. They describe the absolute values of the colour fields on the scales in Lab values to check and compare profiles.

The scales are Ugra Fogra-Media Wedge.

The scales are made differently according to the measuring instruments. An appropriate scale must be selected for each instrument. The number of colour fields and colour values are the same for all. Until 2008, there were two versions of the scales with 46 colour fields and  23 colour fields in two lines. The first line contains the main tones with halftones and the second line contains trapping, i.e. overprints of colours with the corresponding halftones, four colours of grey, black and paper colour tone. In 2008, the third line was added and 24 colour fields were placed in each line - a total of 72 colour fields. The third version includes all 46 colour fields present in the second version. In addition, colour fields have been added to better measure colour intensity and grey balance. These scales are only used to calibrate and check digital proofs and to check colours on test prints. These scales are not used for daily printing. Until 2012, the icc profiles developed pursuant to standard 12647-2 for 5 paper types were valid:

Coated glossy and matt offset printing papers corresponding to paper types 1 and 2. Until 2006, profile Fogra27.icc and ISO coated.icc per ECI-ISO marking was used for this type of paper. In 2006, the profiles Fogra39.icc and ISOcoated_v2_eci.icc were replaced accordingly. Magenta and greyscale conversions were improved in the profile.

With new profiles it is easier to get a print meeting the standard. One more profile ISOcoa-ted_v2_300_eci.icc has been created based on Fogra 39. It differs in the maximum colour amount allowed. If ISOcoated_v2_eci.icc allows a maximum colour amount of 330%, then in ISOcoated_v2_300_eci.icc the maximum allowed colour amount is 300%. The latter profile is used for coated roll papers in heatset printing.

Paper type 3 includes coated LWC with a weight of 54 g/m2 and above roll paper. For this paper PSO_LWC_Standard_eci.icc profile and the corresponding Fogra 46L MediaWedge control file are used. (PSO - Process Standard Offset printing) In the case of lightly coated improved LWC paper, PSO_LWC_Improved_ eci.icc profile and the Fogra45L control file are used accordingly.

Uncoated white offset printing paper corresponds to paper type 4. PSO_Uncoated_ISO12647_eci.icc is used here and the corresponding control file is Fogra47L.

Paper type 5 is a slightly yellow uncoated offset printing paper. The profile is ISOuncoatedyellowish.icc and the control file is Fogra30L.

Paper types are marked by combinations of 2-4 letters.

The most commonly used markings are:

WFC – Woodfree Coated

MWC – Medium Weight Coated, LWC Improved – Low Weight Coated Improved - low weight, coated, improved

LWC Standard – Low Weight Coated Standard - low weight, coated, plain

MFC – Machine Finished Coated – machine finished papers WFU – (Woodfree Uncoated) – wood free, uncoated

SC – SuperCalandered – supercalendered, uncoated

INP – Improved NewsPrint – improved newsprint, uncoated SNP – Standard NewsPrint – plain newsprint, uncoated

A new ISO/DIS 12647-2:2012 paper classification standard was developed.  The standard gives a classification of white printing papers for sheet-fed and heatset printing. In total there are eight paper grades - four coated and four uncoated paper grades.

The standard specifies for each paper grade:

- printing conditions,
- L*a*b* values for full colours,
- dot gain sizes and
- other parameters necessary to standardise printing.

There is a standard colour profile (ICC profile) for each paper grade).

Paper grades are marked PS1 to PS8.

PS print substrate description.

PS1 – PS4 is coated papers,

PS5 – PS8 is uncoated papers.

PS1-WFC, improved MWC

High quality, double or triple coated, glossy or silk, sheet-fed paper with gram weight 80-250 (g/m2), colour profile - ISOcoated_v2_eci.icc

PS2 – improved LWC and MWC

lightly coated heatset printing paper, with gram weight up to 50-80 (g/m2), colour profile PSO_LWC_Improved_eci.icc

PS3 – plain LWC, glossy

Plain LWC, glossy or semi-gloss heatset printing paper with gram weight 48-70 (g/m2), colour profile - PSO_LWC_Standard_eci.icc

PS4 – MFC, plain LWC, matt

Plain, coated matt, heatset printing paper with gram weight 51-65 (g/m2), colour profile - PSO_MFC_Paper_eci.icc

PS5 – WFU

Uncoated white offsetpaper, sheet-fed and heatset printing paper with gram weight 70-250 (g/m2), colour profile - PSO_Uncoated_ISO12647_eci.icc

PS6 – SC (supercalendered)

Uncoated, supercalendered heatset printing paper with gram weight 38-60 (g/m2), colour profile - SC_paper_eci.icc

PS7 – INP (improved newspaper)

Uncoated, INP – improved newsprint for heatset printing with gram weight 40-56 (g/m2), colour profile - PSO_INP_Paper_eci.icc

PS8 - SNP (regular newspaper)

Uncoated. SNP – plain newsprint for heatset printing with gram weight 40-52 (g/m2), colour profile - PSO_SNP_Paper_eci.icc