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 average human observer. Flicker fusion threshold is related to persistence of vision. Although flicker can be detected for many waveforms representing time-variant fluctuations of intensity, it is conventionally, and most easily, studied in terms of sinusoidal modulation of intensity. There are then 6 parameters that determine the ability to detect the flicker:
- the frequency of the modulation;
- the amplitude or depth of the modulation (i.e., what is the maximum percent decrease in the illumination intensity from its peak value);
- the average (or maximum-these can be inter-converted if modulation depth is known) illumination intensity;
- the wavelength (or wavelength range) of the illumination (this parameter and the illumination intensity can be combined into a single parameter for humans or other animals for which the sensitivities of rods and cones are known as a function of wavelength using the luminous flux function);
- the position on the retina at which the stimulation occurs (due to the different distribution of photoreceptor types at different positions);
- the degree of light or dark adaptation, i.e., the duration and intensity of previous exposure to background light, which affects both the intensity sensitivity and the time resolution of vision.
As long as the modulation frequency is kept above the fusion threshold, the perceived intensity can be changed by changing 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 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; the overall threshold frequency for perception cannot exceed the slowest of these for a given modulation amplitude. Each cell type integrates signals differently. For example, rod photoreceptor cells, which are exquisitely sensitive and capable of single photon detection, are very sluggish, with time constants in mammals of about 200 ms. Cones, in contrast, while having much lower intensity sensitivity have much better time resolution than rods do. For both rod- and cone-mediated vision, the fusion frequency increases as a function of illumination intensity, until it reaches a plateau corresponding to the maximum time resolution for each type of vision. The maximum fusion frequency for rod-mediated vision reaches a plateau at about 15 Hz, whereas cones reach a plateau, observable only at very high illumination intensities, of about 60 Hz
In addition to increasing with average illumination intensity, the fusion frequency also increases with the extent of modulation (the maximum relative decrease in light intensity presented); for each frequency and average illumination, there is a characteristic modulation threshold, below which the flicker cannot be detected, and for each modulation depth and average illumination, there is a characteristic frequency threshold. It should be noted that these values vary with the wavelength of illumination, because of the wavelength dependence of photoreceptor sensitivity, and they vary with the position of the illumination within the retina, because of the concentration of cones in central regions including the fovea and the macula, and the dominance of rods in the peripheral regions of the retina.
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, 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 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
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. However, the nature of the back-lighting used can induce flicker - LEDs cannot be easily dimmed, and therefore use pulse-width modulation to create the illusion of dimming, and the frequency used can be perceived as flicker by sensitive users.
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 20 kHz. 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, although the effect was small except at high contrast ratio.
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.
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. 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.
The flicker fusion threshold also varies between species. Pigeons have been shown to have higher threshold than humans (100 Hz vs. 60 Hz), and the same is probably true of all birds, particularly birds of prey. 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. Research also shows that size and metabolic rate are two factors that come into play.
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