Entoptic phenomena are visual effects whose source is within the eye itself. In Helmholtz's words:
"Under suitable conditions light falling on the eye may render visible certain objects within the eye itself. These perceptions are called entoptical."
(Occasionally, these are called entopic phenomena, which is probably a typographical mistake; see entopic.)
Entoptic images have a physical basis in the image cast upon the retina. Hence, they are different from optical illusions, which are perceptual effects that arise from interpretations of the image by the brain. Because entoptic images are caused by phenomena within the each person's 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. from Wikipedia.
William Hundley.