Entoptic phenomena (from Ancient Greek  ἐντός (entós) within, and  ὀπτικός (optikós) visual) are visual effects whose source is within the human eye itself. (Occasionally, these are called entopic phenomena, which is probably a typographical mistake.) In Helmholtzs words: "Under suitable conditions light falling on the eye may render visible certain objects within the eye itself. These perceptions are called entoptical." Overview
Entoptic images have a physical basis in the image cast upon the retina. Hence, they are different from optical illusions, which are caused by the visual system and characterized by a visual percept that (loosely said) appears to differ from reality. Because entoptic images are caused by phenomena within the observers own eye, they share one feature with optical illusions and hallucinations: the observer cannot share a direct and specific view of the phenomenon with others. Helmholtz[1] commented on entoptic phenomena which could be seen easily by some observers, but could not be seen at all by others. This variance is not surprising because the specific aspects of the eye that produce these images are unique to each individual. Because of the variation between individuals, and the inability for two observers to share a nearly identical stimulus, these phenomena are unlike most visual sensations. They are also unlike most optical illusions which are produced by viewing a common stimulus. Yet, there is enough commonality between the main entoptic phenomena that their physical origin is now well understood.
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A phosphene is the perception of light without light actually entering the eye, for instance caused by pressure applied to the closed eyes.A phenomenon that could be entoptical if the eyelashes are considered to be part of the eye is seeing light diffracted through the eyelashes. The phenomenon appears as one or more light disks crossed by dark blurry lines (the shadows of the lashes), each having fringes of spectral colour. The disk shape is given by the circular aperture of the pupil. See also
References
Sources
Jan E. Purkyně, 1823: Beiträge zur Kenntniss des Sehens in subjectiver Hinsicht in Beobachtungen und Versuche zur Physiologie der Sinne, In Commission der J.G. Calveschen Buchhandlung, Prag. H. von Helmholtz, Handbuch der Physiologischen Optik, published as "Helmholtzs Treatise on Physiological Optics, Translated from the Third German Edition," ed. James P. C. Southall; 1925; The Optical Society of America. Leonard Zusne, 1990: Anomalistic Psychology: A Study of Magical Thinking; Lea; ISBN 0-8058-0508-7 [12]
Becker, O., 1860, “Über Wahrnehmung eines Reflexbildes im eigenen Auge [About perception of a reflected image in your own eye],” Wiener Medizinische Wochenschrift, pp. 670 672 & 684 688. M. Tscherning, 1920, Physiologic Optics; Third Edition, (English translation by C. Weiland). Philadelphia: Keystone Publishing Co. pp. 55–56. White, Harvey E., and Levatin, Paul, 1962, "Floaters in the eye," Scientific American, Vol. 206, No. 6, June, 1962, pp. 199 127. Duke Elder, W. S. (ed. ), 1962, System of Ophthalmology, Volume 7, The Foundations of Ophthalmology: heredity pathology diagnosis and therapeutics, St. Louis, The C.V. Mosby Company. p450.