Optical filter

Optical filters select the incident radiation according to certain criteria, such as on the wavelength, the polarization state or (more often as a side effect ) the direction of incidence. The better-known optical filter ( color filters, UV filters ) are presented in the article Filter ( Photography ).

Graduated filters

Graduated filter having a filter surface of the continuously variable filter effect. They are used to achieve infinitely variable attenuations, for example, as a displaceable wedge along a narrow beam path - where the gradient of the filter is negligible, or the beam profile is not affected (for example, a laser beam if its intensity distribution of the Gaussian bell-shaped curve follows only laterally offset ).

The graduated filters also include the usually neutral gray center filter used in photography, in particular at extreme wide-angle lenses to compensate for the light falloff occurring there due to the Cos4 law.

Edge filter

An edge filter has two more or less sharply separated from each other spectral regions in which transmitted the filter ( permeable ) or absorbed ( impermeable ). It is usually designed as a high-pass, low-pass edge filter can only be realized with poor Offband suppression. Since in the optical system of the wavelength of the frequency is more of the speech is called instead of high-pass or low-pass more of long-pass (cut-off filter), and the short -pass filter ( cut- in filter), depending on whether the long-wavelength or the short-wavelength spectral components are transmitted. An edge filter specific wavelength is referred to, which separates the absorption and transmission range of each other.

Polarization filter

The polarization filter is made of anisotropic films, or from operating in reflection dielectric surfaces. It is used in photography to affect reflections.

Interference filter

The interference filter is a filter that is based on the optical interference effects in thin films. Besides edge filters, polarizing filters and graduated filters interference filters can be produced with a very narrow passband and a high degree of suppression of Offband area. The passband (or used area ) is one wavelength range ( or spectrum ) in which the filter has a high transmittance (transmission (Physics) ) has. The spectral range outside of this permeability range is called offband. This shows the quality of a filter in lower transmission values ​​: in the case of interference filters this radiation is reflected, absorbed in "normal" color filters. Interference filters are used for example for the suppression of infrared radiation in image sensors in digital cameras and projection systems. For the specification of optical interference filters, there is the International Standard ISO 9211 ( Optics and photonics - Optical coatings ). This consists of the parts

• Part 1: Definitions
• Part 2: Optical properties
• Part 3: Environmental durability
• Part 4: Specific test methods.

Bayer filter

This is a structured color filter vorschaltet each pixel of a CCD or CMOS sensor its own color separation filter that only one per wavelength range through (red, green or blue). Each four photosensitive pixels are combined to form a color pixel, see Bayer sensor.

Complementary color filter

This filter is also structured as described above Bayer filter but works instead with the primary colors of additive color mixture (red, green, blue) with their ( additive ) complementary colors (magenta, cyan, yellow). Thus, the light sensitivity is doubled, because a primary color filter blocks 2/3 of the spectral range ( a green filter, for example, locks red and blue), while a complementary color filter only 1/3 blocks (eg, blocks a yellow filter only blue). This advantage comes with a more complex electronic color decoding.

Complementary color filters are, inter alia, built into camcorders so still large zoom factors ( 25-fold and more) to realize compact dimensions and correspondingly very small sensor surfaces without letting increase the electrical signal amplification in the readout circuit and thus the signal noise excessively. In current digital cameras, however, complementary colors filters are hardly to be found.