Apodization

From Wikipedia, the free encyclopedia
Jump to: navigation, search

Apodization literally means "removing the foot". It is the technical term for changing the shape of a mathematical function, an electrical signal, an optical transmission or a mechanical structure.

Apodization in electronics[edit]

Apodization in signal processing[edit]

The term apodization is used frequently in publications on FTIR (Fourier Transform InfraRed) signal processing. An example of apodization is the use of the Hann window in the Fast Fourier transform analyzer to smooth the discontinuities at the beginning and end of the sampled time record.

Apodization in digital audio[edit]

An apodizing filter can be used in digital audio processing instead of the more common brickwall filters, in order to remove the pre-ringing that the latter introduces.

Apodization in optics[edit]

In optical design jargon, an apodization function is used to purposely change the input intensity profile of an optical system, and may be a complicated function to tailor the system to certain properties. Usually it refers to a non-uniform illumination or transmission profile that approaches zero at the edges.

Apodization in photography[edit]

The diaphragm of a photo camera is not strictly an example of apodization, since the stop doesn't produce a smooth transition to zero intensity, nor does it provide shaping of the intensity profile (beyond the obvious all-or-nothing, "top hat" transmission of its aperture).

The Minolta/Sony Smooth Trans Focus 135mm f/2.8 [T4.5] lens, however, is a special lens design, which accomplishes this by utilizing a concave neutral-gray tinted lens element as apodization filter, thereby producing a pleasant bokeh. The same optical effect can be achieved combining depth-of-field bracketing with multi exposure, as implemented in the Minolta Maxxum 7's STF function.

Simulation of a Gaussian laser beam input profile is also an example of apodization.

Photon sieves provide a relatively easy way to achieve tailored optical apodization.[1]

Apodization in astronomy[edit]

Apodization is used in telescope optics in order to improve the dynamic range of the image. For example, stars with low intensity in the close vicinity of very bright stars can be made visible using this technique, and even images of planets can be obtained when otherwise obscured by the bright atmosphere of the star they orbit.[2][3] Generally, apodization reduces the resolution of an optical image; however, because it reduces diffraction edge effects, it can actually enhance certain small details. In fact the notion of resolution, as it is commonly defined with the Rayleigh criterion, is in this case partially irrelevant. One has to understand that the image formed in the focal plane of a lens (or a mirror) is modelled through the Fresnel diffraction formalism. The classical diffraction pattern, the Airy disk, is connected to a circular pupil, without any obstruction and with a uniform transmission. Any change in the shape of the pupil (for example a square instead of a circle), or in its transmission, results in an alteration in the associated diffraction pattern.

See also[edit]

References[edit]

Apodization in Optics

  1. ^ Hewett, Jacqueline (2007-06-01). "Photon sieves benefit space telescopes". Optics.org. Retrieved 2007-06-05. 
  2. ^ FIRST RESULTS FROM VERY LARGE TELESCOPE NACO APODIZING PHASE PLATE: 4 μm IMAGES OF THE EXOPLANET β PICTORIS b* The Astrophysical Journal (Letter)
  3. ^ Planet hunters no longer blinded by the light. spacefellowship.com Note: this article includes several images of such a phase plate