Flicker fusion threshold

The flicker fusion threshold, also known as critical flicker frequency, flicker modulation threshold, or flicker fusion rate, is the frequency at which a flickering light appears steady to the average human observer. It is a concept studied in vision science, more specifically in the psychophysics of visual perception. The effect was traditionally called "persistence of vision", but that term has also been used to describe positive afterimages and motion blur. 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 seven 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 on the retina);

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;

physiological factors such as age, sex, and fatigue.

Explanation

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.