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The irradiation illusion is an illusion of visual perception in which a light area of the visual field looks larger than an otherwise identical dark area. It was named by Hermann von Helmholtz around 1867; but the illusion was familiar to scientists long before then; Galileo mentions it in his 1632 book Dialogue Concerning the Two Chief World Systems. It arises partly from scattering of light inside the eye. This has the effect of enlarging the image of a light area on the retina.

References

1. Hermann von Helmholtz, Handbook of Physiological Optics (J. P. C. Southall, Trans. 3rd ed.), New York (1910, 1962), Dover
2. Galileo Galilei, Dialogue Concerning the Two Chief World Systems (1632), Day 1.

v t e Optical illusions (list)
Illusions	Afterimage Ambigram Ambiguous image Ames room Autostereogram Barberpole Bezold Café wall Checker shadow Chubb Cornsweet Delboeuf Ebbinghaus Ehrenstein Flash lag Fraser spiral Gravity hill Grid Hering Impossible trident Irradiation Jastrow Lilac chaser Mach bands McCollough Müller-Lyer Necker cube Oppel-Kundt Orbison Penrose stairs Penrose triangle Peripheral drift Poggendorff Ponzo Rubin vase Sander Schroeder stairs Shepard tables Spinning dancer Ternus Vertical–horizontal White's Wundt Zöllner
Popular culture	Op art Trompe-l'œil Spectropia (1864 book) Ascending and Descending (1960 drawing) Waterfall (1961 drawing) The dress (2015 photograph)
Related	Accidental viewpoint Auditory illusions Illusions Tactile illusions Temporal illusion

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