Flicker fusion threshold: Difference between revisions
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== Visual phenomena == |
== Visual phenomena == |
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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 |
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. [[Stroboscope]]s are sometimes used to induce this effect intentionally. |
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Some special effects, such as certain kinds of [[glowsticking|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.{{Fact|date=April 2009}} The multicolored effect is where a combination of different color LEDs are used. A combination of red, green and blue LEDs allow |
Some special effects, such as certain kinds of [[glowsticking|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.{{Fact|date=April 2009}} 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. |
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A related phenomenon is the [[Digital_Light_Processing#The_color_wheel_.22rainbow_effect.22|DLP Rainbow Effect]], where different colors are displayed in different places on the screen for the same object |
A related phenomenon is the [[Digital_Light_Processing#The_color_wheel_.22rainbow_effect.22|DLP Rainbow Effect]], where different colors are displayed in different places on the screen for the same object due to fast motion. |
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The [[stroboscopic effect]] is sometimes used to "stop motion" or to study small differences in repetitive motions. |
The [[stroboscopic effect]] is sometimes used to "stop motion" or to study small differences in repetitive motions. |
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Revision as of 22:14, 15 October 2009
 | This article may need to be rewritten to comply with Wikipedia's quality standards, as -- Incoherent, somewhat contradicting explanations. See talk page. (May 2009) |
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
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.[1] 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.
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. [1]
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. 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.[2]
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.[3]
Technological considerations
 | This article needs attention from an expert in Technology. Please add a reason or a talk parameter to this template to explain the issue with the article. (November 2008) |
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 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 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 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. 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 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.
Fluorescent lamps 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.[4]
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.[5] Ordinary people have better reading performance using high-frequency (20–60 kHz) electronic ballasts than magnetic ballasts.[6]
The flicker of fluorescent lamps, even with magnetic ballasts, is so rapid that it is unlikely to present a hazard to individuals with epilepsy.[7] Early studies suspected a relationship between the flickering of fluorescent lamps with magnetic ballasts and repetitive movement in autistic children.[8] However, these studies had interpretive problems[9] 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. Stroboscopes 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.[citation needed] 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 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. This has been confirmed in dogs.[10]
See also
References
- ^ "eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
- ^ "eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
- ^ Ernst Simonson and Norbert Enzer, Measurement of fusion frequency of flicker as a test for fatigue of the central nervous system, J. Indus. Hyg. Tox., 23, 1941, 83-89.
- ^ "Full-spectrum Fluorescent lighting : A review of its effects on physiology and health". Retrieved 2008-04-23.
- ^ Küller R, Laike T (1998). "The impact of flicker from fluorescent lighting on well-being, performance and physiological arousal". Ergonomics. 41 (4): 433–47. doi:10.1080/001401398186928.
- ^ Veitch JA, McColl SL (1995). "Modulation of fluorescent light: flicker rate and light source effects on visual performance and visual comfort". Light Res Tech. 27 (4): 243–256. doi:10.1177/14771535950270040301. Retrieved 2007-08-12.
- ^ Binnie CD, de Korte RA, Wisman T (1979). "Fluorescent lighting and epilepsy". Epilepsia. 20 (6): 725–7. doi:10.1111/j.1528-1157.1979.tb04856.x. PMID 499117.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Colman RS, Frankel F, Ritvo E, Freeman BJ (1976). "The effects of fluorescent and incandescent illumination upon repetitive behaviors in autistic children". J Autism Child Schizophr. 6 (2): 157–62. doi:10.1007/BF01538059. PMID 989489.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Turner M (1999). "Annotation: Repetitive behaviour in autism: a review of psychological research". J Child Psychol Psychiatry. 40 (6): 839–49. doi:10.1017/S0021963099004278. PMID 10509879.
- ^ http://www.onpointradio.org/2009/09/a-dogs-eye-view,
External links
- IEC Flicker Meter
- The Flicker Fusion Factor Why we can't drive safely at high speed