phi

fii Φ, the 21st letter of the Euclidean alphabet|Euclidean and Greek alphabet|modern Greek alphabet, usually romanized as "ph".
puhekieltä The golden ratio.
A visual illusion whereby a sequential pattern of lights produces a false sense of motion.
1963, Psychology Through Experiment (George Humphrey, ‎Jaroslav Antonio Deutsch), page 90:

(..) tolerance (Frenkel-Brunswik 1949) for it showed that those persons with the strongest defences against internal conflict, i.e. obsessionals and conversion hysterics, were those least able to perceive the reality-conflicting phenomenon of phi.

The illusion of movement in a highway construction sign is an instance of the phi phenomenon, which is also at work in motion pictures and television.

Is it the phi phenomenon or the persistence of vision? Originally, the illusion of motion that movies excel at was thought to be the effect of the persistence of vision.

(ux)