Category Archives: C

Color rendering index (CRI)

Color rendering is a characteristic used to define the quality of artificial light compared to natural light. The color rendering index indicates how faithfully the colors are reproduced by a light source in its own environment and how fully the light spectrum is displayed with all its color components. Compared to a natural light source, such as the sun, artificial light sources often have the problem that the color rendering deviates far from the natural light that is used as a reference.

The quality of color rendering of light sources of equivalent color temperature is described by the color rendering index (CRI).

An artificial light source would have optimal color rendering if its light contained all spectral colors as in sunlight and the colors of the illuminated objects looked correspondingly natural. In nature, perfect white is achieved when the sun falls perpendicularly on the earth at noon, i.e. no wave ranges are refracted away into the atmosphere; as a reference, this corresponds to value of CRI 100. This means that the higher the Ra value of an artificial light source, the more natural the colors appear and the higher the quality of color rendering.

With book scanners, developers try to get as close as possible to the value 100 for the light source, usually a CRI of 80-95 is achieved. However, this value is not solely responsible for the quality of the light; valid statements can only be made in connection with the color temperature. The values can also fluctuate depending on the current operating temperature of the light source.


Candela (cd) is an internationally standardized physical unit of measurement for the luminous intensity of a light source in a certain direction.

Candela is used to describe more precisely the luminous flux emitted by a light source in a certain direction. Candela is the Latin word for candle; 1 candela therefore corresponds approximately to the amount of light emitted by a candle. Since a light source does not emit its light uniformly in all directions, another unit of measurement to lumens is necessary. While the unit of measurement lumen generally indicates the luminous flux of a light source, this value still says nothing about how focused or wide the emitted light beam is.

The candela value, which measures the intensity of light at a specific beam or solid angle, helps in this determination. An analysis of the light intensity in relation to all directions can be modeled into a so-called luminous intensity distribution curve. Depending on the design of a light source, the luminous intensity is distributed uniformly or irregularly over a given solid angle.

The higher the candela number given, the more focused the light beam. 1 candela of a light source is when its illuminance with a distance of one meter is 1 lux; i.e. lux and candela are interdependent in consequence.

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.


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

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.

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.