Flicker-free

Flicker-free is a term given to video displays, primarily cathode ray tubes, operating at a high refresh rate to reduce or eliminate the perception of screen flicker. For televisions, this involves operating at a 100 Hz or 120 Hz hertz field rate to eliminate flicker, compared to standard televisions that operate at 50 Hz (PAL, SÉCAM systems) or 60 Hz (NTSC), most simply done by displaying each field twice, rather than once. For computer displays, this is usually a refresh rate of 70–90 Hz, sometimes 100 Hz or higher. This should not be confused with motion interpolation, though they may be combined – see implementation, below.

Televisions operating at these frequencies are often labelled as being 100 or 120 Hz without using the words flicker-free in the description.

The term is primarily used for CRTs, especially televisions in 50 Hz countries (PAL or SECAM) and computer monitors from the 1990s and early 2000s – the 50 Hz rate of PAL/SECAM video (compared with 60 Hz NTSC video) and the relatively large computer monitors and close viewing distances of 1990s/2000s computer monitors (hence making more of the screen in the viewer's peripheral vision) make flicker most noticeable on these devices.

Contrary to popular belief, modern LCD monitors are not flicker free, since most of them use pulse-width modulation (PWM) for brightness control. As the brightness setting is lowered, the flicker becomes more noticeable, since the period when the backlight is active in each PWM duty cycle shortens. The problem is much more pronounced on modern LED backlit monitors, because LED backlight reacts faster to changes in current.

The goal is to display images sufficiently frequently to exceed the human flicker fusion threshold, and hence create the impression of a constant (non-flickering) source.

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