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The irradiation ( lat. , Radiation ') is an optical illusion which results in bright objects on a dark background and larger dark objects on a bright background appear smaller than they actually are.

The name of this illusion of the visual sense comes from Hermann von Helmholtz , who traced the phenomenon back to the scattering of light in the eye, whereby a bright area of ​​the visual field can irritate a larger area on the retina than an otherwise identical but darker area.


The effect of irradiation shows itself at every distance of the perceived object and the apparent enlargement is more noticeable the brighter it appears in the field of vision; however, it no longer increases significantly above an illumination density that corresponds approximately to daylight. According to Helmholtz's explanations in 1867, the circles of confusion caused by the dioptic apparatus of the eye, which still occur even with complete accommodation due to the spherical and chromatic aberration of the eye, are sufficient as an explanation . Joseph AF Plateau assumed for the irradiation in 1839 a spread of the light impression on the retina of our eye and ascribed it as a visual illusion to the receptive processing of the retinal image. The phenomenon itself was known earlier as an illusion; so mentioned it by Galileo Galilei in 1632 in his dialogue concerning the two world systems .


One observes the irradiation particularly conspicuously in the dark of night in the light shape of the crescent moon , which seems to belong to a disk of larger radius than the rest of the moon ; therefore, the tips of a narrow sickle occasionally appear as if they were reaching over halfway - an aspect known as luna cornuta , the croissant-shaped moon, or an effect of irradiation that misinterprets the geometry of the day-night boundary (Terminator) of the moon can lead.


  • Welcker : About irradiation. Giessen 1852.

Web links

Commons : Optical illusion  - album with pictures, videos and audio files

supporting documents

  1. ^ Franz Bruno Hofmann: The doctrine of spatial sense. J. Springer, Berlin 1925, p. 125.
  2. see Helmholtz: Handbuch der Physiologische Optik. L. Voss, Leipzig 1867, p. 98 ff.
  3. see Joseph Antoine Ferdinand Plateau: Mémoire sur l'irradiation , Brussels 1839.
  4. see Galileo: Dialogo sopra i due massimi sistemi del mondo , Florence 1632.