Flicker fusion threshold
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The flicker fusion threshold (or flicker fusion rate) is a concept in the psychophysics of vision. It is defined as the frequency at which all flicker of an intermittent light stimulus disappears.
Like all psychophysical thresholds, the flicker fusion threshold is a statistical rather than an absolute quantity. There is a range of frequencies within which flicker sometimes will be seen and sometimes will not be seen, and the threshold is the frequency at which flicker is detected on 50% of trials.
The flicker fusion threshold varies with brightness (it is higher for brighter lights) and with location on the retina where the light falls: the rod cells have a faster response than the cone cells, so flicker can be seen in peripheral vision at higher frequencies than in foveal vision. The flicker fusion threshold is also higher for a fatigued observer.
The flicker fusion threshold also varies between species. Pigeons have been shown to have higher threshold than humans, and the same is probably true of all birds. Many mammals have a higher proportion of rods in their retinae than humans do, and it is likely that they would also have higher flicker fusion thresholds.
Flicker fusion is important in all technologies for presenting moving images, nearly all of which depend on presenting a rapid succession of static images (e.g. the frames in a cine film or a digital video file). If the frame rate falls below the flicker fusion threshold for the given viewing conditions, flicker will be apparent to the observer, and movements of objects on the film will appear jerky. For the purposes of presenting moving images, the human flicker fusion threshold is usually taken as 16 Hertz. The frame rate used in cine projection (48 Hz, each frame shown twice), television (50 or 60 Hertz interlaced), and computer displays (60 to 110 Hertz) therefore allows a reasonable margin for error or unusual viewing conditions.