Color management

With color management (English colo ( u) r management) is to be achieved by creating a template that has been recorded with any input device, is reproduced as similar as possible on any output device.

Basics

A high similarity of colors between the input and output of an image is called color fidelity. To achieve this accuracy, color management systems ( CMS german, color management system) are used, which, however, can never provide a 100 % match.

Use color management systems

• Device-dependent color descriptions ( device profile )
• Device-independent interchange color spaces ( eng. Profile Connection Space, PCS).

The purpose of a color management system is the device-dependent color descriptions ( the input and output devices ) using the device-independent interchange color space to convert into each other. This ensures that each device is approximately equal to the colors in a color management system.

A simple example is the expression of colored documents that look nearly identical to a color management system on the screen and on the printout:

• As a device profile usually come ICC profiles for use.
• The color models involved are often the RGB color model ( for digital cameras and monitors), and the CMYK color model (for printer ).
• The color spaces involved (RGB and CMYK) are components of the mentioned color models.
• The device-independent CIE Lab color space is used in this case as a link between the other color spaces.

In addition to the L * a * b * color space on which to base the common CM- systems, there are other media-independent color spaces such as L * u * v *, which is in contrast to the L * a * b rather used for the measurement of light colors will. Also XYZ and xyY are those physical spaces, which have in common, all perceptible by the human eye colors, so the visible light, to be able to represent.

Color Management is for example often used in the printing, photographic and advertising industry. The demand for color management solutions is steadily increasing not only the professionals but also for amateur photographers and serious amateurs.

Color profiles

As each person perceives colors individually, also devices, at least in classes of devices, different color spaces in which they register or colors represent. Such individuality is due to differences in construction and production fluctuations. Color profiles can reflect the color data of a device class or the individuality of a special device. The standard format for color profiles developed by the ICC ( International Color Consortium engl. ) and internationally standardized in the ISO standard 15076. Every one involved in the conversion device (monitor, digital camera, scanner, etc. ) needs its own profile. It contains translation tables or calculation parameters by which performs the conversion of color data from or into the PCS ( profile connection space, connection color space). As a PCS XYZ and LAB mainly be used.

With regard to their application to different input profiles (RGB → PCS), output profiles (PCS → RGB or CMYK) and device link profiles that allow a direct gamut mapping without going through a PCS between two CMYK color spaces.

In terms of its internal structure, a distinction matrix - LUT profiles and profiles ( look up table ). Matrix 's profile is preferably used for devices whose color behavior is dependent on relatively few influences and can therefore be described sufficiently well, for example in the form of a 3x3 Umrechungsmatrix. The file size of matrix profiles is relatively small ( a few kilobytes ). Find LUT profiles for device application, the color behavior depends on many factors and is too complex that it could be described by a simple matrix transformation sufficiently accurate. LUT profiles can be up to several megabytes. It should be borne in mind that a profile is only valid for a particular state of that device. Thus, if for example, the paper changed from a white to a yellowish paper, the same CMYK values ​​lead to different colors. The same applies to monitors when rotated, for example, the brightness control.

Profiling

The profiling is based on a color measurement. Here, colors, the exact color values ​​are known, by the device shown (monitor, printer) or measured ( scanner), and then compared with the known values. This results in, inter alia, the gamut, which describes the ability of the color reproduction of a device. Depending on the device type is done the creation of profiles in different ways. Profiles must be regularly regenerated, because change, especially monitors over time. Company profiles are about suitable only for the series, but not for the specific device.

Scanner

To create a scanner profile you use a template with lots of little different colored boxes ( IT8 target ), which was measured by the manufacturer using a spectrophotometer. The scanner reads this template, and compares the read color values ​​with the reference values ​​of the target. From the differences of these values, then the ICC profile (L * a * b * color space ) is calculated and thus is now the device-specific color space ( RGB color space ) to a device independent color space linked such that the color values ​​read by the scanner, can be accurately reproduced.

Monitors

To monitor profiling come a colorimeter and a matching software used. The color measuring device is connected to the measuring computer and software, and is normally positioned in the center of the monitor. After starting the test run, the software is on the screen one after the other colors, the exact RGB value of the software is known. The color measuring device returns the value of the CIELab color actually visible to the software. After this procedure has been run for all RGB values ​​, each possible RGB color a CIELab value can be assigned. For example, the software provides a perfect red = RGB ( 255,0,0 ) Represents the meter returns that the monitor displays the value CIELab (0.73, 0.26 ). Thus, each value can be translated from RGB to CIELab.

• RGB ( 255,0,0 ) = CIELab (0.73, 0:26 )
• RGB ( 254,0,0 ) = CIELab (0.72, 0.25)
• RGB ( 253,0,0 ) = CIELab (0.71, 0:24 )
• RGB ( 252,0,0 ) = CIELab (0.71, 0:23 )
• ...

Not all monitors are able to fully reproduce the RGB values. The result is that in these different monitors RGB values ​​equal CIELab values ​​are assigned.

• RGB ( 255,0,0 ) = CIELab (0.70, 0:20 )
• RGB ( 254,0,0 ) = CIELab (0.70, 0:20 )
• RGB ( 253,0,0 ) = CIELab (0.70, 0:20 )
• RGB ( 252,0,0 ) = CIELab (0.70, 0:20 )
• ...

This characteristic, device-specific peculiarities of the color reproduction are why color management is used at all.

Printer

Even if all printers possibly also other colors work ultimately based on the principle of subtractive color mixing with the inks CMYK and so presents itself to the vast majority of the budget for the home and office environment devices as an RGB device to the operating system. The required color separation ( v. conversion RGB to CMYK ) is performed by the driver or the printer hardware, without the user having it influence. Only printer for professional purposes, such as proof systems or large format printer (usually via PostScript driven ) appear to the system as a true CMYK devices. The generation of ICC profiles for printers is done by printing out a test chart with many color fields whose color values ​​are known. Subsequently, the L * a * b * values ​​of the color patches are measured with a spectrophotometer. Thus, a relationship between the printed RGB or CMYK data and the visible CIE L * a * b * color values ​​is made. It is thus known that color impression (L * a * b * value) is formed when a certain ink or toner combination is output to the printer. In a profiling program, the measured data will be put into a form that corresponds to the specification of the ICC (International Color Consortium). It created standardized tables that allow a conversion from RGB or CMYK in the PCS (CIELAB or XYZ) and vice versa. Note that you have to create a separate profile for each Tinten/Toner- and paper combination to obtain predictable and accurate printing results.

A second possibility for printer profiling software offers higher quality scanners from different manufacturers using a standard profiling. Thus, an existing scanner assuming no additional hardware required to make an ICC profile of the printer.

Implementation

If the profiles for input and output device before, so can be done with the help of the Color Management Module (CMM ) is a conversion of the color descriptions. The color management module is the color computer, which reads the values ​​from the tables ( color profile ) and if necessary, makes adjustments. In this way, pixel by pixel conversion from RGB to CIELab color values ​​and ultimately into CMYK values ​​are achieved for the relevant input and output device.

With the help of color management, it is possible to combine any devices to each other and still get the best possible result ( depending on the capabilities of the output device ). The procedure described here is today in the printing practice standard.