However, in order to prevent accumulated damage LCDs quickly alternate the voltage between positive and negative for each pixel, which is called 'polarity inversion'.
The standard framerate of 24 fps produces very obvious flicker, so even very early movie projectors[example needed] added additional vanes to the rotating shutter to block light even when the film was not moving.
Video projectors typically use either LCDs which operate similarly to their desktop counterparts, or DLP mirrors which flicker at 2.5–32 kHz,[5] though "single-chip" color projectors switch between displaying a frame's red, green, & blue channels at as little as 180 Hz using a color wheel or RGB lightsource.
For stereoscopic 3D, a single-image system can only display the left-eye or right-eye image at once, switching between them at 90–144 Hz, though this does have the advantage of reduced crosstalk versus two-image 3D projection.
The exact refresh rate necessary to prevent the perception of flicker varies greatly based on the viewing environment.
In a completely dark room, a sufficiently dim display can run as low as 30 Hz without visible flicker.
[citation needed] At normal room and TV brightness this same display rate would produce flicker so severe as to be unwatchable.
Video hardware outside the monitor can also cause flicker through many different timing and resolution-related artifacts such as screen tearing, z-fighting and aliasing.
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.