- Flicker fusion threshold
The flicker fusion threshold (or flicker fusion rate) is a concept in the
psychophysicsof 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.
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. [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
retinawhere 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 visionat higher frequencies than in foveal vision.
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. [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.]
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 cinema film, TV show, or a
digital videofile). If the frame ratefalls 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 interlacein television (50 or 60 Hz), a reasonable margin of error for 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
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.
LCDflat 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.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 stroboscopiceffect. 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. 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. The 100–120 Hz flicker produced by magnetic ballasts is associated with headaches and eyestrain. [ cite web
title = Full-spectrum Fluorescent lighting : A review of its effects on physiology and health
url = http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=81689
accessdate = 2008-04-23 ] 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. [cite journal |journal= Ergonomics |volume=41 |issue=4 |pages=433–47 |year=1998 |title= The impact of flicker from fluorescent lighting on well-being, performance and physiological arousal |author= Küller R, Laike T |doi= 10.1080/001401398186928] Ordinary people have better reading performance using high-frequency (20–60 kHz) electronic ballasts than magnetic ballasts. [cite journal |journal= Light Res Tech |volume=27 |issue=4 |year=1995 |pages=243–256 |url=http://irc.nrc-cnrc.gc.ca/pubs/fulltext/nrcc38944/ |accessdate=2007-08-12 |author= Veitch JA, McColl SL |title= Modulation of fluorescent light: flicker rate and light source effects on visual performance and visual comfort]
The flicker of fluorescent lamps, even with magnetic ballasts, is so rapid that it is unlikely to present a hazard to individuals with
epilepsy. [cite journal |journal=Epilepsia |year=1979 |volume=20 |issue=6 |pages=725–7 |title= Fluorescent lighting and epilepsy |author= Binnie CD, de Korte RA, Wisman T |pmid=499117 |doi=10.1111/j.1528-1157.1979.tb04856.x] Early studies suspected a relationship between the flickering of fluorescent lamps with magnetic ballasts and repetitive movement in autisticchildren. [cite journal |journal= J Autism Child Schizophr |year=1976 |volume=6 |issue=2 |pages=157–62 |title= The effects of fluorescent and incandescent illumination upon repetitive behaviors in autistic children |author= Colman RS, Frankel F, Ritvo E, Freeman BJ |doi=10.1007/BF01538059 |pmid=989489] However, these studies had interpretive problems [cite journal |journal= J Child Psychol Psychiatry |year=1999 |volume=40 |issue=6 |pages=839–49 |title= Annotation: Repetitive behaviour in autism: a review of psychological research |author= Turner M |pmid=10509879 |doi= 10.1017/S0021963099004278] and have not been replicated.
Persistence of vision
* [http://www.enernex.com/staff/docs/FlickerPQA2004.pdf IEC Flicker Meter]
* [http://pages.cthome.net/rwinkler/fff.htm The Flicker Fusion Factor] Why we can't drive safely at high speed
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