Category Archives: C

Color depth

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.

Calibration

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).

Color calibration

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.

Color management

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.

Color profile

Jedes bildgebende Gerät, ob Digitalkamera, Scanner, Monitor oder Farbdrucker hat gerätespezifische Farbinterpretationen, die sich in unterschiedlichen Farbmodellen, Farbräumen oder Farbtönen bemerkbar machen. Eingabegeräte wie Digitalkameras und Scanner sowie Ausgabegeräte wie viele Monitore und Displays nutzen das RGB-Farbmodell, währen Drucker dagegen das CMY- bzw. CMYK-Farbmodell verwenden.

Ein Farbprofil dient nun dazu, Farben von einem Farbraum in einen anderen Farbraum zu übersetzen, ohne dass die Farbechtheit der Ausgangsvorlage dadurch verfälscht wird. In einem Farbprofil ist gespeichert, wie ein bestimmtes Gerät Farben gegenüber einem geräteunabhängigen Farbraum darstellt. Die Erstellung der Farbprofile erfolgt gerätespezifisch durch Farbmanagementsysteme, die Umrechnung in der Regel auf Basis von ICC-Profilen mit sogenannten Rendering Intents.

Ziel eines Farbprofils ist somit, eine unveränderte Farbwiedergabe auf allen in einer Imaging-Prozess-Kette (z.B. Scanner, bildbearbeitendem PC, Desktop-Publishing-Viewer und Drucker) miteinander verbundenen Geräten zu erreichen.

Im professionellen Digitalisierungsbereich gehören solche Farbprofile zum Standard-Lieferumfang.

Color temperature

The color temperature is a measure for the quantitative description of the color impression of light sources; the unit of measurement of the color temperature is the temperature unit kelvin (K).

The spectrum of an ideal thermal radiator (“black body”, “black body radiator” or “Planckian radiator”) serves as the reference model for determining the color temperature. This emits electromagnetic radiation in the visible and invisible range, whose wavelength distribution is determined solely by the temperature. For real thermal light sources (flame, light bulb, sun) this is approximately true.

When a black body is slowly heated, it passes through a color scale from dark red, red, orange, yellow, white to light blue. The temperature of the black body at which there is the best possible color match with the light source to be determined is the color temperature of the illuminant. Each natural or artificial light situation can thus be assigned approximately a temperature, which can then be used to describe a light situation mathematically.

Since reddish colors are perceived as “warm” and bluish colors as “cool,” a higher color temperature corresponds to a “cooler” color. Common light sources have color temperatures in the order of magnitude of less than 3,300 K (warm white), 3,300 to 5,300 K (neutral white) to more than 5,300 K (daylight white).

For the practice of photography and digitizing, this means that depending on the existing lighting conditions of the location, a certain color temperature must be set in order to achieve a correct reproduction of colors. In digital photography, this process is called white balance.

Chromatic abberation

In optics, chromatatic aberration (also abbreviated to CA) is a failure of a lens that occurs when light of different wavelengths or colors is refracted to different extents. This can lead to lateral chromatic aberration, which manifests itself primarily at image edges in green and red or blue and yellow color fringes at light-dark transitions. Longitudinal color errors can also occur in the form of different discolorations in front of and behind the focal plane.

apochromatic image of a building

Comparison of an image without and with chromatic aberration, in this case a lateral chromatic aberration.

Source: Wikimedia Commons (unchanged)
Copyright: Creative Commons Attribution-Share Alike 3.0 Unported.

 

Since such color errors should be avoided when digitizing, our systems work with a chromatic-corrected lens that compensates for this phenomenon. Otherwise, colored fragments would show around the edges of black letters, for example, and the scan would not be identical with the original.

RGB color space

What is a color space?

Most likely you are reading this article on the screen of your computer, laptop or your smartphone. Do you see the colors in the illustrations? These colors are defined on your screen by the use of a color space. A color space is a defined range of colors. Color space means the use of a specific color model. A color model is a method of generating many colors from a defined group of primary colors. Each color model has a range of colors that it can generate. This area is the color space. The most common systems are RGB and CMYK.

When choosing which color space to use, the basic question is: Are you working in digital or print format? Digital devices such as cameras and monitors use a color space called RGB.

The RGB color space is composed of three basic colors to which the light-sensitive cones in the human eye react most sensitively: red, green and blue. Theoretically it is possible to decompose every visible color into combinations of these three “primary colors.” Color monitors, for instance, can display millions of colors simply by mixing different intensities of red, green and blue. It is most common to place the range of intensity for each color on a scale from 0 to 255 (one byte). The range of intensity is also known as the “color depth”. Multiplying all available color gradations per channel results in 2563 or 16,777,216 color combinations. One often finds the statement: 16.7 million colors.

The possibilities for mixing the three primary colors together can be represented as a three-dimensional coordinate plane with the values for R (red), G (green) and B (blue) on each axis. This coordinate plane results a cube called the RGB color space.

Source: Wikimedia Commmons Copyright: GNU-Lizenz für freie Dokumentation. 

The RGB color space is based on colored light. The three colors of light combine in different ways to create color. It is an additive process, a look at the pictures shows why:

 

If all three color channels are set to their maximum values (255 at a one byte color depth), the resulting color is white.

If all three color channels have a value of zero, it means that no light is emitted and the resulting color is black (on a monitor, for example, it cannot be blacker than the surface of the monitor producing 0 light).

This type of color mixing is also called “additive color mixing”.

 

What types of RGB color spaces are existing?

Source: Wikimedia Commons (unbearbeitet)
Copyright: Creative Commons Attribution-Share Alike 3.0 Unported 

Different color spaces allow for you to use a broader or narrower range of those 16.7 million colors used in an image. If you think about it, there is a nearly infinite number of ways you can mix different colors together. If you add just a little more green here or there, you have got a new color. Take away a bit more red, and you have just created yet another color. What most people do not know is that they can choose the level of color detail their camera records. A bigger color space captures more colors than a smaller one.

Color spaces differ in the number of colors that can be visualized within a color space. When it comes to working with digital devices, sRGB, AdobeRGB and ECI-RGB are among the most important and well-known color spaces:

The smallest of these color spaces, is sRGB. The sRGB color space was originally developed as a color space for CRT monitors in order to display images created in sRGB as similarly as possible on all monitors.

AdobeRGB is able to represent about 35% more color ranges than sRGB is able to. The color gamut was primarily improved in the green tones, including the blue-green area, i.e. the so-called cyan tones.

The ECI-RGB V2 color space is one of the standardised RGB colour spaces. It is the recommended color space in the Metamorfoze Preservation Imaging Guidelines and the only one allowed at the highest level of these imaging standards. As a working colour space for professional image processing ECI-RGB V2 covers practically all printing processes as well as all widespread display technologies. ECI-RGB thus particularly fulfils the requirements for true colour reproduction. A corresponding ICC profile for integration in image processing programs can be downloaded free of charge from the ECI website and allows constant colour reproduction on all output devices.

Camera slider

The majority of our scanners are designed with a fixed geometry. This minimizes mechanical movements and allows the systems to work without wear for several years. One disadvantage, however, is that formats and resolutions are fixed. If, for example, you scan an A4 sheet with an A1+ scanner, you “give away” a large part of the recording area with the background, which is detected and removed by the software when automatic scanning is activated.

For institutions with different originals and especially changing format sizes in their collections, which always want to get the best possible resolution, we therefore offer flexible reprographic systems. These are equipped with adjustable camera sliders to which our capturing unit is attached. This allows the user to easily adjust the height of the camera, either manually or motorized, depending on the model. For large originals, the camera unit is moved upwards, i.e. away from the original; for small photos or slides, downwards, thus increasing the resolution of the scans.

Our software allows defining individual camera positions and calibration settings, so that the user can conveniently vary between the different options with just one mouse click.

The perfect solution for archives and institutions with changing and heterogeneous holdings.