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How it works.
Color afterimages are one of the most studied phenomena in visual neuroscience. When you stare at a highly saturated color for an extended period (20-60 seconds), the photoreceptor cells responsible for detecting that color — the cone cells tuned to that wavelength — become fatigued and temporarily reduce their sensitivity.
When you then look at a neutral grey or white surface, the fatigued cones fire at a reduced rate compared to their neighbors. Your visual system, which encodes color as the ratio of activity across cone types (the opponent-process model: red-green and blue-yellow axes), interprets this imbalance as the complementary color.
Red stare → cyan afterimage. Green stare → magenta afterimage. Yellow stare → blue afterimage. This is the opponent-process theory of color vision, first proposed by Ewald Hering in 1878 and later confirmed by measuring retinal ganglion cell responses.
The duration and vividness of the afterimage scale with both the saturation of the original stimulus and the exposure duration — which is why the generator lets you control both. A 100% saturated image held for 60 seconds produces an afterimage that can last 10-20 seconds on a blank white surface.
Science fact The three types of cone cells (S, M, L) have peak sensitivities at roughly 420nm, 530nm, and 560nm. The afterimage is generated by differential fatigue across these three cell populations.