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((More citations needed |date=July 2019))
[[File:siemens_star.svg|thumb|A Siemens star]][[File:Spurious resolution.gif|thumb|alt=A Siemens star with gradually increasing defocus error shows increasing spurious resolution.|A Siemens star with defocus error shows spurious resolution, in the form of alternating bands.]]
A '''Siemens star''', or '''spoke target''', is a device used to test the [[optical resolution|resolution]] of [[optical instrument]]s, [[computer printer|printers]], and [[display device|displays]]. 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 –&nbsp;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 ([[dots per inch|DPI]]). Similarly, it can be applied to a [[camera]]'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.

[[File:Collimator-Siemens Chart.JPG|thumb|left|200px|A Siemens chart filter in a [[collimator]], used for auto-focus calibration of [[digital camera|digital still cameras]]]]
In the field of [[video production]], 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 (optics)|focus]] in special situations.

Siemens stars are similar to the [[sunburst|sunburst pattern]] used as a background in graphic design, as in the [[Flag of Japan#Military flags|Japanese Naval Ensign]], [[Russian Air Force]] flag and [[Flag of Jordan|Jordanian Royal Standard]]. They are useful in drawing the eye to a point on the page.

[[File:Siemens star (128 spokes) & Matlab code.svg|thumb|A Siemens star with 128 spokes. It shows [[aliasing]] in the center, where the spokes are smaller than the pixel pitch. (It is not blurred or aberrated, so it does not show spurious resolution.)]]
Under [[optics|optical blur]] from [[defocus]], a Siemens star (like any periodic pattern) gives rise to the phenomenon of ''spurious resolution''<ref name="blur_unblurred">((cite journal |last1= Strasburger |first1= Hans |last2= Bach |first2= Michael |last3= Heinrich |first3= Sven P. |title= Blur Unblurred – A Mini Tutorial |journal= i-Perception |volume= 9 |issue= 2 |pages= 204166951876585 |year= 2018 |doi= 10.1177/2041669518765850 |pmid= 29770182 |pmc= 5946648))</ref><ref>((cite journal |last1= Smith |first1= George |title= Ocular defocus, spurious resolution and contrast reversal |journal= Ophthalmic and Physiological Optics |volume= 2 |pages= 5–23 |year= 1982|issue= 1 |pmid= 7088556 ))</ref> above the resolution limit, i.e. toward the center of the Siemens star. (Spurious resolution appears similar to [[aliasing]], but it is a purely optical phenomenon, so it occurs without need of pixels.)<!-- Bach, Waltenspiel, Schildwaechter, 1989, p. 562, write, "Spurious resolution (Westheimer, 1964; Goodman, 1968) is a purely optical phenomenon (not to be confused with aliasing) that occurs with any periodic pattern."--> 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]] 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]]) in the 1930s to test the lenses of Siemens narrow-film cameras.<ref>Alexandra Kinter, Siemens AG, Siemens Archives in Munich, Germany.</ref>

==See also==
*[[Secchi disk]]

==References==
((Reflist|30em))

==External links==
*[https://web.archive.org/web/20051218202316/http://www.ps.bam.de/IE13/E1-1CD22.PDF ISO 15775 chart (pdf)] resolution test chart featuring a [[vector graphics|vector]] Siemens star
*[https://claclaclack.files.wordpress.com/2015/05/siemensstern_n314_cl_v3.pdf Siemens star with n=314 sectors for high resolution and easy calculation] (vector graphic in PDF)

[[Category:Visual perception]]
[[Category:Optical illusions]]

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