See color management.
The color depth indicates how many different color levels are available for each individual pixel of a graphic. Since the “fineness” of the gradations depends on how much memory is used per pixel, the color depth is specified in bits.
With 8 bits, for example, 256 color shades can be distinguished for one color channel. A color is created by mixing several color channels of a color space. For computer graphics, the RGB color space is usually used, in which colors are composed by additive mixing of the three primary colors red, green and blue. Even most common computer monitors can only distinguish 8 bits per channel.
One speaks of a true-color representation if the current color depth has at least 24 bits, i.e. 8 bits per color channel (red, green, blue). A color depth of 24 bits corresponds to approx. 16 million colors; this means that practically every conceivable color can be reproduced true to life.
In the field of color management, the terms calibration and profiling are often used interchangeably, although they are actually two different processes.
In digital imaging, all input devices (e.g. digital camera, scanner) as well as output devices (e.g. monitor, printer) should be calibrated and profiled to avoid color errors due to faulty devices.
During calibration, devices are trimmed to technical boundary conditions (on the screen, for example, compliance with a specific color temperature); during profiling, on the other hand, the devices are measured and an associated ICC profile is created. Strictly speaking, calibration takes place before profiling, but is often performed in a single pass using special tools.
The difference between calibration and profiling can be clearly understood using the example of a digital bathroom scale: When the scale is turned on, it is usually calibrated automatically so that when it is completely unloaded, the display shows 0 kg. Profiling, on the other hand, would mean that reference measurements are created for a certain number of reference weights (10 kg, 20 kg, 30 kg, etc.) and that a correction value is in turn stored in a profile for each reference weight (e.g. that the scale displays 5% too much when loaded with a weight of 10 kg, only 3% too much when loaded with 50 kg, and only 1% too much when loaded with 100 kg).
A professionally performed color calibration serves to optimize and measurably compare the image quality of a scanner.
The perception of colors by the human eye is extremely subjective and also dependent on the surrounding lighting conditions. In addition, every imaging device, whether digital camera, scanner, monitor or color printer, has device-specific color interpretations that are reflected in different color models, color spaces or color tones.
Therefore, a professional color calibration, individually adapted to the respective camera sensor, when installing a scanner guarantees accurate colors right from the start.
Color calibration is usually performed using a color target that contains standardized color patches. This is scanned and then the calibration software compares the colors determined by the scanner with the target colors (the actual colors of the individual color patches on the calibration slide are standardized). From the comparison of the actual colors with the target colors, a device-specific color profile (ICC profile) is created, which is now used for each subsequent scan. In this way, individual color errors of a scanner can be corrected. Such an ICC profile is always created for a specific scanner only and cannot be transferred to another device of the same construction.
The color calibration should always be done using professional color targets with color reference standards that correspond to the common digitizing standards.
In the professional digitizing sector, the creation of a color profile is part of the standard scope of delivery.
Unlike the human eye, which can perceive almost any number of different colors, each image-processing input and output device has its own finite color space, called a device-specific color space. For example, a normal RGB screen represents colors from a combination of 256 red, green and blue tones each; this corresponds to a maximum number of 16,777,216 displayable color tones. But even with this enormous number, not every color that the human eye perceives can be represented. Moreover, even devices that work in the same color space reproduce colors differently.
Therefore, in order to ensure uniform color reproduction across different devices – for example within the process chain of scanner, image-processing PC, desktop publishing viewer and printer – the image data must be digitally matched or offset against each other. This is done by special color management modules. They create color profiles for the respective devices, which describe the colors in relation to a reference color space. The conversion is done on the basis of ICC profiles with so-called rendering intents.
Every imaging device, whether a digital camera, scanner, monitor or color printer, has device-specific color interpretations that translate into different color models, color spaces or hues. Input devices such as digital cameras and scanners as well as many output devices such as monitors and displays use the RGB color model, while printers use the CMY or CMYK color model.
A color profile is used to translate colors from one color space to another without distorting the color fidelity of the original. A color profile stores how a particular device represents colors against a device-independent color space. The color profiles are created device-specifically by color management systems, and the conversion is usually based on ICC profiles with so-called rendering intents.
The aim of a color profile is thus to achieve unchanged color reproduction on all devices connected in an imaging process chain (e.g. scanner, image-processing PC, desktop publishing viewer and printer).
In the professional digitizing sector, such color profiles are part of the standard scope of delivery.
The Universal Test Target (UTT) is a single test chart developed by the Dutch National Library (KB) in collaboration with Image Engineering Dietmar Wueller (IE) and the Fachverband für Multimediale Informationsverarbeitung e. V. (FMI) as part of the Metamorfoze initiative.
The aim was to design a new, uniform test chart that would incorporate the five standard test charts that had been in use until then and thus simplify handling. This was to provide an insight into the overall image quality of the scan results of all types of high-end cameras and scanners, based on current ISO standards. These are captured and analyzed with special software to provide information on technical aspects such as OECF, MTF, noise and color accuracy.
UTT is available in two versions: measured and unmeasured. The “measured” version comes with individually measured reference data for the particular chart purchased. The “unmeasured” version is produced in respect of the Metamorfoze standards and tolerances.
The UTT is available with a variety of options in sizes from DIN A3 to A0. Because the UTT is applicable to all types of digitization projects and preservation, it is particularly important for libraries, archives and museums.
The aim of the developers of the UTT was to save time and improve quality by using the unified target during the digitization process. However, it should be noted that the UTT is extremely error-prone in practice due to its simple design with individually affixed test charts and must therefore be handled with extreme care.
Test charts are needed to create device characterization for a camera, scanner or printer when creating a color management system. They are used to objectively measure the accuracy or characteristics of an image processing system to ensure its effective operation and provide long-term assurance.
Test charts can consist of physical templates or be integrated into the image processing system as windows.
In the digitization sector, physical test charts are usually used for quality measurement. It should be noted, however, that these are subject to aging processes and can fade. They should therefore be stored carefully and protected from light.
Test charts contain line, dot or other patterns as well as a defined number of color patches for checking colors, the composition of which is known from the process colors cyan, magenta, yellow and black. A distinction must also be made between ordered (visual) and unordered (random) test charts in which the color patches are arranged randomly.
Depending on the structure and complexity of the test charts, various criteria of the camera or lens used can be tested: among others, sharpness in critical areas such as image center and image edge, chromatic aberration, distortion, vignetting, resolution, color reproduction, dynamic range, white balance, autofocus problems and image noise at different ISO settings.
Various commercially available standardized test charts are mostly used to match the specific requirements. As a universal standard, the Universal Test Target (UTT) has also been developed on the basis of current ISO standards to provide an insight into the overall image quality of the scan results of all types of high-end cameras and scanners. However, it should be noted that the UTT is extremely error-prone in practice due to its simple design with individually affixed test charts and thus has to be handled with extreme care.
Another combination of different targets is also offered by the ISA Golden Thread Device Level Target.
At book2net we prefer to work from a combination of the following test charts:
- Applied Image Inc® ISO Resolution Chart (T-10) (determination of resolution)
- ImatestTM Scanner SFR & OECF QA-62 (edge sharpness and MTF analysis)
- ColorChecker® Digital SG (color management)
A gray chart is used to measure the dynamic range or tonal value differentiation of a digital camera, scanner or monitor. It is a test chart in which there is a smooth or multi-graded (gray scale chart) transition between dark black and bright white.
In photography, the dynamic range describes the difference between the lightest and darkest point within an image. It is given as the ratio of the darkest to the lightest point.
In an original (slide, negative, photo), the brightest point has a so-called minimum density and the darkest point has a maximum density. The difference between the maximum density and the minimum density is then the so-called density range, or dynamic range.
The dynamic range of the subject or the original is crucial for determining the correct exposure: Only if the exposure range of the sensor or film is greater than or equal to the dynamic range of the subject can all the details of the subject be captured. Otherwise, parts of the subject will be imaged in black and/or white.