Flicker fusion threshold

The flicker fusion threshold (or flicker fusion rate) is a concept in the psychophysics of vision. It is defined as the frequency at which an intermittent light stimulus appears to be completely steady to the observer (this article centers around human observers). Flicker fusion threshold is related to Persistence of vision.

Explanation
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 is proportional to the amount of modulation; if brightness is constant, a brief flicker will manifest a much lower threshold frequency than a long flicker. The threshold also varies with brightness (it is higher for a brighter light source) and with location on the retina where the perceived image falls: the rod cells of the human eye have a faster response time than the cone cells, so flicker can be sensed in peripheral vision at higher frequencies than in foveal vision. The flicker fusion threshold also is 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.

Importance
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 cinema film, TV show, 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 (Hz). In actual practice, movies are recorded at 24 frames per second, and TV cameras operate at 25 or 30 frames per second, depending on the TV system used. Even though motion may seem to be continuous at 25 or 30 fps, the brightness may still seem to flicker objectionably. By showing each frame twice in cinema projection (48 Hz), and using interlace in television (50 or 60 Hz), a reasonable margin for error or unusual viewing conditions is achieved in minimising subjective flicker effects.

Flicker is also important in the field of domestic (alternating current) lighting, where noticeable flicker can be caused by varying electrical loads, and hence can be very disturbing to electric utility customers. Most electricity providers have maximum flicker limits that they try to meet for domestic customers.

Subjectivity of flicker
Modern computer CRT displays usually operate at a vertical scan rate well over 60 Hz (modern ones are around 100Hz), and can thus be considered flicker-free. Other display technologies do not flicker noticeably so the frame rate is less important. LCD flat panels do not seem to flicker at all as the backlight of the screen operates at a very high frequency of nearly 200 Hz, and each pixel is changed on a scan rather than briefly turning on and then off as in CRT displays.

Some individuals, particularly autistics, can still notice a flicker in CRTs or even in fluorescent lights; a few find fluorescent lights generally uncomfortable for this reason.

In some cases, it is possible to indirectly detect flicker at rates well beyond 60 Hz in the case of high-speed motion, via the stroboscopic effect. Human factors experts refer to this effect as a Phantom Array. Fast-moving flickering objects zooming across view (either by object motion, or by eye motion such as rolling eyes), can cause a dotted or multicolored blur instead of a continuous blur. A common example of this phenomenon is the DLP Rainbow Effect. Some special effects, such as certain kinds of electronic glowsticks commonly seen at outdoor events, have the appearance of a solid color when motionless but produce a multicolored or dotted blur when waved about in motion. Some particularly sensitive people claim to be able to see the flicker of office fluorescent lighting or street lighting, which occur at 100 or 120 Hz. Other times, people can indirectly detect the presence of a high-speed flicker via the mere existence of a wagon-wheel effect.