Earlier LCDs used fluorescent lamps which flickered at 100–120 Hz newer fluorescently backlit LCDs use an electronic ballast that flickers at 25–60 kHz which is far outside the human perceptible range, and LED backlights have no inherent need to flicker at all.
Screens that use opposite polarity per-line or per-pixel can reduce this effect compared to when the entire screen is at the same polarity, sometimes the type of screen is detectable by using patterns designed to maximize the effect. In practice, there is a small difference, which means that every pixel flickers at about 30 Hz. Ideally, this wouldn't be noticeable because every pixel has the same brightness whether a positive or a negative voltage is applied. However, in order to prevent accumulated damage LCDs quickly alternate the voltage between positive and negative for each pixel, which is called 'polarity inversion'. In LCD screens, the LCD itself does not flicker, it preserves its opacity unchanged until updated for the next frame. The plasma pixels fade in brightness between refreshes. Use of refresh rates above 120 Hz is uncommon, as they provide little noticeable flicker reduction and limit available resolution.įlatscreen plasma displays have a similar effect. Most people find that refresh rates of 70–90 Hz and above enable flicker-free viewing on CRTs. A refresh rate of 60 Hz on most screens will produce a visible "flickering" effect. For most devices, the screen's phosphors quickly lose their excitation between sweeps of the electron gun, and the afterglow is unable to fill such gaps – see phosphor persistence. Flicker occurs on CRTs when they are driven at a low refresh rate, allowing the brightness to drop for time intervals sufficiently long to be noticed by a human eye – see persistence of vision and flicker fusion threshold.
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