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.