The Siemens star is a radial pattern used to test the resolution of optical instruments. Its luminance profile is rectangular and under optical blur (by, e.g., bad accommodation or uncorrected short-sightedness) gives rise to spurious resolution. The latter lets one see stripes towards the center of the Siemens star that are not physically present and have the opposite polarity relative to the actual stripes. That is, they are white where the actual stripe is black and vice versa. See our paper Blur Unblurred (Strasburger, Heinrich, Bach; i-Perception 2018; open access) for explanations. To work properly, a Siemens star should best be given as vector graphics (svg, pdf, eps, emf, ai, etc.), so that only the resolution of the displaying device comes into play. Beware that many vector formats nowadays support (or are rendered as) raster graphics; you will recognize pure vector format by its small file size, or that they can be enlarged indefinitely without jagged contours.
For playing around, here are:
The svg version with its Matlab code were also uploaded to Wikipedia (English and German).
Hans Strasburger, Sep. 2019