Siemens Star
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A Siemens star, or spoke target, is a device used to test the resolution of
optical instrument An optical instrument is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common examples include periscopes, microscopes, telescopes, and camera ...
s,
printers Printer may refer to: Technology * Printer (publishing), a person * Printer (computing), a hardware device * Optical printer for motion picture films People * Nariman Printer (fl. c. 1940), Indian journalist and activist * James Printer (1 ...
, and
displays A display device is an output device for presentation of information in visual or tactile form (the latter used for example in tactile electronic displays for blind people). When the input information that is supplied has an electrical signal ...
. It consists of a pattern of bright "spokes" on a dark background that radiate from a common center and become wider as they get further from it. In concept, the spokes only meet at the exact center of the star – the spokes, and the gaps between them, become narrower the closer to the center one looks, but they never touch except at the center. When printed or displayed on a device with limited resolution, however, the spokes touch at some distance from the center. The smallest gap visible is limited by the smallest dot of ink the printer can produce, making the Siemens star a useful tool for comparing two printers' resolutions (
DPI DPI may refer to: Organizations * Department of Public Information, related to U.N. *Daffodil Polytechnic Institute, an educational institution of Bangladesh * Dhaka Polytechnic Institute, an educational institution of Bangladesh * Disabled Peop ...
). Similarly, it can be applied to a
camera A camera is an instrument used to capture and store images and videos, either digitally via an electronic image sensor, or chemically via a light-sensitive material such as photographic film. As a pivotal technology in the fields of photograp ...
's optical resolution by taking photographs of a Siemens star printed at high resolution and comparing photographs from different cameras, to see which retained the center detail the closest. In the field of
video production Video production is the process of producing video content. It is the equivalent of filmmaking, but with video recorded either as analog signals on videotape, digitally in video tape or as computer files stored on optical discs, hard drives, SSDs, ...
, where it is often called a back focus chart, the Siemens star is widely used to adjust the back focus of removable lenses. It is also used during film or video shoots to help setting the
focus Focus (: foci or focuses) may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in East Australia Film *Focus (2001 film), ''Focus'' (2001 film), a 2001 film based on the Arthur Miller novel *Focus (2015 ...
in special situations. Siemens stars are similar to the sunburst pattern used as a background in graphic design, as in the Japanese Naval Ensign,
Russian Air Force The Russian Air Force () is a branch of the Russian Aerospace Forces, the latter being formed on 1 August 2015 with the merging of the Russian Air Force and the Russian Aerospace Defence Forces. After the dissolution of the Soviet Union, the reb ...
flag and Jordanian Royal Standard. They are useful in drawing the eye to a point on the page. Under optical blur from
defocus In optics, defocus is the aberration in which an image is simply out of focus. This aberration is familiar to anyone who has used a camera, videocamera, microscope, telescope, or binoculars. Optically, defocus refers to a translation of the f ...
, a Siemens star (like any periodic pattern) gives rise to the phenomenon of ''spurious resolution'' above the resolution limit, i.e. toward the center of the Siemens star. (Spurious resolution appears similar to
aliasing In signal processing and related disciplines, aliasing is a phenomenon that a reconstructed signal from samples of the original signal contains low frequency components that are not present in the original one. This is caused when, in the ori ...
, but it is a purely optical phenomenon, so it occurs without need of pixels.) This results in inverted polarity of the stripe pattern: black stripes appear in the place of white stripes and vice versa (and further polarity inversions occur further inward). (The illustration under
Optical transfer function The optical transfer function (OTF) of an optical system such as a camera, microscope, human eye, or image projector, projector is a scale-dependent description of their imaging contrast. Its magnitude is the image contrast of the Sine and cosine ...
shows spurious resolution caused by blurring.) When looking at the Siemens star with slightly blurred vision, e.g., without spectacles or with defocus from staring, this is seen as a shimmering ring around the Siemens star's center that changes size with viewing distance. The star was developed by Siemens & Halske AG (today
Siemens Siemens AG ( ) is a German multinational technology conglomerate. It is focused on industrial automation, building automation, rail transport and health technology. Siemens is the largest engineering company in Europe, and holds the positi ...
) in the 1930s to test the lenses of Siemens narrow-film cameras.Alexandra Kinter, Siemens AG, Siemens Archives in Munich, Germany.


See also

*
Secchi disk The Secchi disk (or Secchi disc), as created in 1865 by Angelo Secchi, is a plain white, circular disk in diameter used to measure water transparency or turbidity in bodies of water. The disc is mounted on a pole or line and lowered slowly dow ...


References

{{Reflist, 30em


External links


ISO 15775 chart (pdf)
resolution test chart featuring a
vector Vector most often refers to: * Euclidean vector, a quantity with a magnitude and a direction * Disease vector, an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematics a ...
Siemens star
Siemens star with n=314 sectors for high resolution and easy calculation
(vector graphic in PDF) Visual perception Optical illusions