Flicker fusion threshold
Encyclopedia
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 on human observers). Flicker fusion threshold is related to persistence of vision
.
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 sluggish, whereas some retinal ganglion cells can maintain firing of 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 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 cell
s of the human eye have a faster response time than the cone cell
s, so flicker can be sensed in peripheral vision
at higher frequencies than in fovea
l vision. This is essentially the concept known as the Ferry-Porter law, where it may take some increase in brightness, by powers of ten, 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 lasts long enough, even after 1/4 second, to merely prolong it and not intensify it. From a practical point of view, if a stimulus is flickering, such as computer monitor, decreasing the intensity level will eliminate the flicker.
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.
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 frame/s, 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 of error for unusual viewing conditions is achieved in minimising subjective flicker effects.
displays usually operate at a vertical scan rate well over 60 Hz (modern ones are around 100 Hz), and can thus be considered flicker-free. Most people do not detect flicker above 75 Hz.
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, 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 lamp
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 light from fluorescent fixtures that have 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 epilepsy
. 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.
s are sometimes used to induce this effect intentionally.
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. 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 their 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.
. 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. This has been confirmed in dogs.
Psychophysics
Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they effect. Psychophysics has been described as "the scientific study of the relation between stimulus and sensation" or, more completely, as "the analysis of perceptual...
of vision
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of 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 observer
Observation
Observation is either an activity of a living being, such as a human, 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 data collected during this activity...
(this article centers on human observers). Flicker fusion threshold is related to persistence of vision
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....
.
Explanation
When fusion is attained, all that is needed to change the intensity is to change the relative periods of light and darkness. One can prolong the dark periods and thus darken the image; therefore the effective and average brightness are equal. This is known as the Talbot-Plateau law. Like all psychophysical thresholdPsychophysics
Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they effect. Psychophysics has been described as "the scientific study of the relation between stimulus and sensation" or, more completely, as "the analysis of perceptual...
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 sluggish, whereas some retinal ganglion cells can maintain firing of 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 modulation
Amplitude modulation
Amplitude 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 retina
Retina
The 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 cell
Rod cell
Rod 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 visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
s of the human eye have a faster response time than the cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s, so flicker can be sensed in peripheral vision
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 fovea
Fovea
The fovea centralis, also generally known as the fovea , is a part of the eye, located in the center of the macula region of the retina....
l vision. This is essentially the concept known as the Ferry-Porter law, where it may take some increase in brightness, by powers of ten, 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 lasts long enough, even after 1/4 second, to merely prolong it and not intensify it. From a practical point of view, if a stimulus is flickering, such as computer monitor, decreasing the intensity level will eliminate the flicker.
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
Digital video is a type of digital 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 rate
Frame rate
Frame rate is 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 hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
(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 interlace 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 CRTCathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
displays usually operate at a vertical scan rate well over 60 Hz (modern ones are around 100 Hz), and can thus be considered flicker-free. Most people do not detect flicker above 75 Hz.
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 currentAlternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
) 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 lamp
Fluorescent lamp
A 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. A fluorescent lamp converts electrical power into useful...
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 light from fluorescent fixtures that have 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 epilepsy
Epilepsy
Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...
. 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. StroboscopeStroboscope
A 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 glowsticks
Glowsticking
Glowsticking 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 their 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 speciesSpecies
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. 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. This has been confirmed in dogs.
See also
- Persistence of visionPersistence 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....
- TweeningTweeningInbetweening 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 key frames which help to create the illusion of motion...
- CDR Computerized Assessment SystemCDR Computerized Assessment SystemThe 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 effectWagon-wheel effectThe 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
- IEC Flicker Meter
- The Flicker Fusion Factor Why we can't drive safely at high speed
- Webvision's section concerning the psychophysics of time in vision