The Delboeuf illusion is an optical illusion of relative size perception. In the best-known version of the illusion, two circles of identical size have been placed near to each other and one is surrounded by an annulus; the surrounded circle then appears larger than the non-surrounded circle if the annulus is close, while appearing smaller than the non-surrounded circle if the annulus is distant. A 2005 study suggests it is caused by the same visual processes that cause the Ebbinghaus illusion.
It was named for the Belgian philosopher, mathematician, experimental psychologist, hypnotist and psychophysicist Joseph Remi Leopold Delboeuf (1831–1896), who created it in 1865.
A debunked research study related to the Delboeuf illusion claimed that eating off small plates tricks diners into eating less. In June 2018, researchers at Israel's Ben-Gurion University of the Negev published a paper calling the claim into question.
- Roberts B, Harris MG, Yates TA (2005). "The roles of inducer size and distance in the Ebbinghaus illusion (Titchener circles)". Perception. 34 (7): 847–56. doi:10.1068/p5273. PMID 16124270.
- Delboeuf, Franz Joseph (1865). "Note sur certaines illusions d'optique: Essai d'une théorie psychophysique de la maniere dont l'oeil apprécie les distances et les angles". Bulletins de l'Académie Royale des Sciences, Lettres et Beaux-arts de Belgique. 19: 195–216.
- "The Delboeuf Illusion: Common Weight Loss Trick Debunked in Study". newsweek.com. 2018-07-17. Retrieved 2018-07-18.