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The flicker fusion threshold, also known as critical flicker frequency 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 .
In visual perception, flicker is a human-visible change in luminance of an illuminated surface or light source which can be due to fluctuations of the light source itself, or due to external causes such as due to rapid fluctuations in the voltage of the power supply (power-line flicker) or incompatibility with an external dimmer.
So long as the flicker rate is greater than the human flicker fusion threshold, and the LED is stationary relative to the eye, the LED will appear to be continuously lit. Varying the on/off ratio of the pulses is known as pulse-width modulation (PWM).
However, this instantaneous variation in the transmitted color is not to be humanly perceptible, because of the limited temporal sensitivity in the human vision — the "critical flicker fusion threshold" (CFF) and the "critical color fusion threshold" (CCF), both of which cannot resolve temporal changes shorter than 0.01 second. The LEDs ...
8: hecht s, verrijp cd. intermittent stimulation by light : iii. the relation between intensity and critical fusion frequency for different retinal locations. j gen physiol. 1933 nov 20;17(2):251-68. 9: hecht s, shlaer s, verrijp cd. intermittent stimulation by light : ii. the measurement of critical fusion frequency for the human eye.
The flicker of a CRT monitor can cause various symptoms in those sensitive to it such as eye strain, headaches [9] in migraine sufferers, and seizures in epileptics. [10]As the flicker is most clearly seen at the edge of our vision there is no obvious risk in using a CRT, but prolonged use can cause a sort of retinal shock where the flickering is seen even when looking away from the monitor.
This perception of modulated light as steady is known as the flicker fusion threshold. However, when the modulated light is non-uniform and contains an image, the flicker fusion threshold can be much higher, in the hundreds of hertz. [3]
In order for a display to not have any noticeable flicker, the refresh rate for each LED must be greater than the Flicker fusion threshold; 50 Hz is often used as an approximation. As an example, 8 tri-state pins are used to control 56 LEDs through Charlieplexing, which is enough for 8 7-segment displays (without decimal points). Typically, 7 ...