In
general, one's
retinas don't stop responding to a
stimulus (i.e.
light) as soon as the stimulus has gone. Instead, it
decays so that the brain can fill in the time in between. This is why projected displays look like a single
image and why
animation and
movies and the like appear to be a
moving picture rather than a series of disconnected
frames. It is not why after a bright
flash you still see purplish-greenish
spots in your field of view where the flash was; that is where it's taking longer for your
retina to readjust to the lack of
stimulus due to retinal fatigue. On the topic of fatigue, the inventor of the
zoetrope was actually mostly-
blind in one eye when he invented it because he was doing lots of self-
research on persistence of vision, seeing how long it would take for the
spot in his eye to disappear after staring at the
sun for different amounts of time. His last data point was for an exposure of
half an
hour, and the spot never went away - he had actually
burned the retinal
cells.
There is also a free(beer)/opensource (but not free(speech)) raytracer called Persistence of Vision, or POVray, which really has very little to do with the concept of persistence of vision, but the name sounds cool.