The
flicker fusion threshold (or
flicker fusion rate) is a concept in the
psychophysicsPsychophysics is a discipline within psychology that investigates the relationship between physical stimuli and their subjective correlates, or percepts...
of
visionVisual perception is the ability to interpret information and surroundings from visible light reaching the eye. The resulting perception is also known as eyesight, sight or vision...
. It is defined as the frequency at which an intermittent light stimulus appears to be completely steady to the
observerObservation is either an activity of a living being , consisting of receiving knowledge of the outside world through the senses, or the recording of data using scientific instruments. The term may also refer to any datum collected during this activity.-Observation in science:A scientific method...
(this article centers around human observers). Flicker fusion threshold is related to
persistence of visionPersistence of vision is the phenomenon of the eye by which an afterimage is thought to persist for approximately one twenty-fifth of a second on the retina....
.
Explanation
When fusion is attained, all that is needed to change the intensity is change the relative periods of light and darkness. One can prolong the dark periods and thus darken, therefore the effective and average brightness are equal; this is the Talbot-Plateau law. Like all
psychophysical thresholdPsychophysics is a discipline within psychology that investigates the relationship between physical stimuli and their subjective correlates, or percepts...
s, 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.
Different points in the visual system have very different critical flicker fusion rate (CFF) sensitivities. Each cell type integrates signals differently. For example, photoreceptors are very slow and sluggish whereas some retinal ganglion cells can maintain firing rates up to 250 Hz.
http://www.bio.net/bionet/mm/neur-sci/2007-July/061925.html
The flicker fusion threshold is proportional to the amount of
modulationAmplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
; 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
retinaThe vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
where the perceived image falls: the
rod cellRod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. Because they are more light sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated...
s of the human eye have a faster response time than the
cone cellCone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser towards the periphery of the retina....
s, so flicker can be sensed in
peripheral vision Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision...
at higher frequencies than in
foveaThe term fovea comes from the Latin, meaning pit or pitfall. As an anatomical term, there are several foveae around the body, including in the head of the femur.Fovea of the eye=...
l vision. This is essentially the concept known as the Ferry-Porter law, where it may take some increase in brightness, logarithmically, to require as many as 60 flashes to achieve fusion, while for rods, it may take as little as four flashes, since in the former case each flash is easily cut off, and in the latter it last long enough, even after 1/4 second, to merely prolong it and not intensify it.
The flicker fusion threshold also is lower for a fatigued observer. Decrease in the critical fusion frequency has often been used as an index of central fatigue.
Display frame rate
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 videoDigital video is a type of video recording system that works by using a digital rather than an analog video signal.The terms camera, video camera, and camcorder are used interchangeably in this article.- History :...
file). If the
frame rateFrame rate, or frame frequency, is the measurement of the frequency at which an imaging device produces unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems...
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
hertzThe hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
(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 frame/s, the brightness may still seem to flicker objectionably. By showing each frame twice in cinema projection (48 Hz), and using
interlaceIn the domain of mechanical television, the concept of interlacing was demonstrated by Léon Theremin. He had been developing a mirror drum-based television, starting with 16 lines resolution in 1925, then 32 lines and eventually 64 using interlacing in 1926, and as part of his thesis on May 7, 1926...
in television (50 or 60 Hz), a reasonable margin of error for unusual viewing conditions is achieved in minimising subjective flicker effects.
Display refresh rate
Computer
CRTThe cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen...
displays usually operate at a vertical scan rate well over 60 Hz (modern ones are around 100Hz), and can thus be considered flicker-free. Most people do not detect flicker above 75Hz.
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.
Lighting
Flicker is also important in the field of domestic (
alternating currentIn alternating current the movement of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again...
) 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.
Fluorescent lampA fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light....
s using conventional magnetic ballasts flicker at twice the supply frequency. Electronic ballasts do not produce light flicker since the phosphor persistence is longer than a half cycle of the higher operation frequency of 20kHz. The 100–120 Hz flicker produced by magnetic ballasts is associated with headaches and eyestrain.
Individuals with high critical flicker fusion threshold are particularly affected by magnetic ballasts: their EEG alpha waves are markedly attenuated and they perform office tasks with greater speed and decreased accuracy. The problems are not observed with electronic ballasts. Ordinary people have better reading performance using high-frequency (20–60 kHz) electronic ballasts than magnetic ballasts.
The flicker of fluorescent lamps, even with magnetic ballasts, is so rapid that it is unlikely to present a hazard to individuals with
epilepsyEpilepsy is a common chronic neurological disorder characterized by recurrent unprovoked seizures...
. Early studies suspected a relationship between the flickering of fluorescent lamps with magnetic ballasts and repetitive movement in autistic children. However, these studies had interpretive problems and have not been replicated.
Visual phenomena
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 "phantom array" effect. 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, as if they were multiple objects.
StroboscopeA stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. The principle is used for the study of rotating, reciprocating, oscillating or vibrating objects...
s are sometimes used to induce this effect intentionally.
Some special effects, such as certain kinds of
electronic glowsticksGlowsticking is a form of dancing with glowsticks or other glowstick-like instruments that share the same qualities: durability, consistency in light, safety to toss around, and the material of which they are made, often a soft and pliant plastic....
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. These are typically LED based glow sticks. For a single color, flashing an LED, rather than a constant on state uses less power for the same perceived brightness. The multicolored effect is where a combination of different color LEDs are used. A combination of red, green and blue LEDs allow almost any color to be produced. Yellow for example is a combination of red and green. When moving the glow stick, timing differences between the on/off state of the different LEDs becomes evident, and the colors are separated into the separate components.
A related phenomenon is the DLP Rainbow Effect, where different colors are displayed in different places on the screen for the same object due to fast motion.
The stroboscopic effect is sometimes used to "stop motion" or to study small differences in repetitive motions.
Non-human species
The flicker fusion threshold also varies between
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
. Pigeons have been shown to have higher threshold than humans, and the same is probably true of all
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
s. Many
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s 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. This has been confirmed in dogs.
See also
- Persistence of vision
Persistence of vision is the phenomenon of the eye by which an afterimage is thought to persist for approximately one twenty-fifth of a second on the retina....
- Tweening
Inbetweening or tweening is the process of generating intermediate frames between two images to give the appearance that the first image evolves smoothly into the second image. Inbetweens are the drawings between the keyframes which help to create the illusion of motion...
- CDR Computerized Assessment System
The CDR Computerized Assessment System is a computerized battery of cognitive tests designed in the late 1970s by Professor Keith Wesnes at the University of Reading in Berkshire, England, for repeated testing in clinical trails...
- Wagon-wheel effect
The wagon-wheel effect is an optical illusion in which a spoked wheel appears to rotate differently from its true rotation. The wheel can appear to rotate more slowly than the true rotation, it can appear stationary, or it can appear to rotate in the opposite direction from the true rotation...
External links