Editing Flicker (screen) (section)

==Occurrence==
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&nbsp;– see [[persistence of vision]] and [[flicker fusion threshold]]. For most devices, the screen's [[phosphor]]s quickly lose their excitation between sweeps of the [[electron gun]], and the afterglow is unable to fill such gaps&nbsp;– see [[phosphor persistence]]. A refresh rate of 60&nbsp;[[Hertz|Hz]] on most screens will produce a visible "flickering" effect. Most people find that refresh rates of 70–90&nbsp;Hz and above enable [[flicker-free]] viewing on CRTs. Use of refresh rates above 120&nbsp;Hz is uncommon, as they provide little noticeable flicker reduction and limit available resolution.

Flatscreen [[plasma display]]s have a similar effect. The plasma pixels fade in brightness between refreshes.

In [[liquid crystal display|LCD]] screens, the LCD itself does not flicker, it preserves its opacity unchanged until updated for the next frame. However, in order to prevent accumulated damage LCDs quickly alternate the voltage between positive and negative for each pixel, which is called 'polarity inversion'. Ideally, this wouldn't be noticeable because every pixel has the same brightness whether a positive or a negative voltage is applied. In practice, there is a small difference, which means that every pixel flickers at about 30&nbsp;Hz.<ref>{{Cite web |url=http://machineryequipmentonline.com/video-equipment/liquid-crystal-display-lcdpolarity-inversion/ |title=Liquid crystal display (LCD):Polarity inversion &#124; video equipment |access-date=2018-03-05 |archive-date=2018-03-06 |archive-url=https://web.archive.org/web/20180306142315/http://machineryequipmentonline.com/video-equipment/liquid-crystal-display-lcdpolarity-inversion/ |url-status=dead }}</ref> 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.<ref>{{Cite web|url=http://www.lagom.nl/lcd-test/inversion.php|title=Inversion (pixel-walk) - Lagom LCD test|website=www.lagom.nl}}</ref>

More of a concern is the [[LCD backlight]]. Earlier LCDs used [[fluorescent lamp]]s which flickered at 100–120&nbsp;Hz; newer fluorescently backlit LCDs use an electronic ballast that flickers at 25–60&nbsp;kHz which is far outside the human perceptible range, and [[LED-backlit LCD|LED backlights]] have no inherent need to flicker at all. On top of any inherent backlight flicker, most fluorescent and LED backlight designs use digital [[Pulse-width modulation|PWM]] for some or all of their dimming range by switching on and off at rates from several kHz to as little as 180&nbsp;Hz,<ref>{{Cite web|url=https://www.tftcentral.co.uk/articles/pulse_width_modulation.htm|title = Pulse Width Modulation (PWM)|date = 17 March 2015}}</ref> though some flicker-free designs using true analog DC dimming exist.<ref>{{Cite web|url=https://iristech.co/flicker-free-monitors/|title=Flicker free monitors &#124; IrisTech|date=2 April 2016}}</ref>

Flicker is necessary for a film-based [[movie projector]] to block the light as the film is moved from one frame to the next. The standard framerate of 24&nbsp;fps produces very obvious flicker, so even very early movie projectors{{Such as?|date=November 2022}} added additional vanes to the rotating shutter to block light even when the film was not moving. Most common is 3 vanes raising the rate to 72&nbsp;Hz. Home film movie projectors (and early theater projectors) often have four vanes, to raise the 18&nbsp;fps used by silent film to 72&nbsp;Hz. Video projectors typically use either LCDs which operate similarly to their desktop counterparts, or [[Digital Light Processing|DLP]] mirrors which flicker at 2.5–32&nbsp;kHz,<ref>{{Cite web |date=November 1, 2022 |title=3D printing & direct imaging products |url=https://www.ti.com/dlp-chip/3d-printing-direct-imaging/overview.html |access-date=November 1, 2022 |website=Texas Instruments}}</ref> though "single-chip" color projectors switch between displaying a frame's red, green, & blue channels at as little as 180&nbsp;Hz using a color wheel or RGB lightsource. For [[Stereoscopy|stereoscopic]] 3D, a single-image system can only display the left-eye or right-eye image at once, switching between them at 90–144&nbsp;Hz, though this does have the advantage of reduced crosstalk versus two-image 3D projection. Movie projectors typically use an [[incandescent lamp]] or [[arc lamp]] which does not itself flicker noticeably.

Older televisions used [[interlaced video]], so among other artifacts, the image jumped one line at half the rate (25 or 30&nbsp;Hz) that the image changes (50 or 60&nbsp;Hz).

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&nbsp;Hz without visible flicker.{{cn|date=April 2024}} At normal room and TV brightness this same display rate would produce flicker so severe as to be unwatchable.

The human eye is most sensitive to flicker at the edges of the human field of view (peripheral vision) and least sensitive at the center of gaze (the area being focused on). As a result, the greater the portion of our field of view that is occupied by a display, the greater is the need for high refresh rates. This is why computer monitor CRTs usually run at 70 to 90&nbsp;Hz, while CRT TVs, which are viewed from further away, are seen as acceptable at 60 or 50&nbsp;Hz (see [[Analog television#Standards|analog television standards]]).<ref>{{Cite web |title=Why Is the US Standard 60 Hz? - News |url=https://www.allaboutcircuits.com/news/why-is-the-us-standard-60-hz/ |access-date=2022-11-03 |website=www.allaboutcircuits.com |language=en}}</ref>

Chewing something [[Crunchiness|crunchy]] such as [[tortilla chip]]s or [[granola]] can induce flicker perception due to the vibrations from chewing synchronizing with the flicker rate of the display.<ref>{{Cite web |last=Campbell |first=Todd |date=November 3, 2022 |title=Answer Geek:How Chewing Affects TV Flicker |url=https://abcnews.go.com/Technology/story?id=119579&page=1 |access-date=2022-11-03 |website=ABC News |page=1 |language=en |type=Website |publication-place=United States of America |edition=1st}}</ref>