Compression of video signals is used in order either to reduce the bandwidth of the transmission channel (e.g. satellite TV or network) or to reduce the amount of data to be saved (e.g. DVD or Blu-ray). There are two different kinds of compression – lossless and lossy. Lossless compression allows the encoded data to be decoded without any differences to the original. Lossy compression implies that not all the data from the original has to be present in the decoded video in order to achieve good perceptual quality.

Nearly all digital cameras use a compression algorithm that is know as “JPEG-Compression”. How does this work?

"What is the maximum theoretical resolution I am able to measure with the Siemens star chart?" is a commonly asked question. The small table below gives the answer for some typical situations.

The term "light" describes the part of the electromagnetic spectrum visible to the human eye. This region ranges between 380nm (violet-blue light) and 780nm (red light). Similar to the science field of radiometry, photometry studies the measurement of the electromagnetic radiation. However, in addition to the fact that the light is only a small part of the electromagnetic spectrum, the human eye is not equally sensitive to this radiation. Its sensitivity depends on the wavelength of the incident light.

The Dynamic Range is the maximum contrast a camera is able to reproduce from the point the camera reaches it’s maximum output level (saturation) down to the point where the signal is just as high as the noise level so that noise and signal cannot be distinguished anymore D = S(max)/σ(dark). It is measured using a gray scale target with a contrast greater than the expected Dynamic Range of the camera under test.

Modern digital cameras have very powerful noise reduction algorithms implemented in their signal processors. All of them have one thing in common: The algorithm needs to decide “What is noise and what is image content”. Have a look at the concept.

As the refractive index of a transmittive medium is dependent on the wavelenght of light, dispersion occurs, when white light transmits such a medium. Refraction is stronger for light of short wavelengths, for example blue, and less intensive for light of long wavelngths, for example red.

Different kinds of glasses cause refraction or dispersion of various intensities. In photographical lenses the the same effect occurs. So it is neccessary to correct these abberations, otherwise you get to see these effects in the images.

In ISO 12233 [1], as slanted edge is used for measuring the Spatial Frequency Response (SFR). When this standard has been developed, this method was know from scanner performance measurement and was adopted for camera measurement. The defined test target has a lot of different structures that can be used for visual inspection and also has slanted edges in horizontal and vertical orientation.

Generally, the function of rotationally symmetric optical systems is to form an image that is geometrically similar to the object, except for some particular systems, such as fish-eye lenses and eyepieces, where this condition is deliberately not maintained. Ideally, this function is accomplished according to the geometry of perspective projection. Departures from the ideal image geometry are called distortion.

In order to describe a color of a not self-luminous source it is important to have detailed knowledge of the illuminant used. The International Commission on Illumination CIE (Commission Internationale de l'Éclairage) have defined a number of spectral power distributions, referred to as CIE standard illuminants, to provide reference spectra for colorimetric issues. The illuminants are denoted by a letter or a letter-number combination. Their spectral power distributions (SPD) are normalized to a value of 100 at a wavelength of 560 nm in following figures.

A convenient solution that provides the standard types of illumination and that is large enough to illuminate a whole table top scene is offered by the lightSTUDIO. 

As we all know cameras and scanners see colors different than the human eye. Therefore colors are corrected in the camera or scanner software using a 3x3 matrix and sometimes an ICC color profile.

From time to time we do get the question how to clean a test chart.