Flicker (screen)

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Flicker is a visible change in brightness between cycles displayed on video displays. It applies to the refresh interval on cathode ray tube (CRT) televisions and computer monitors, as well as plasma computer displays and televisions.

Contents

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 – see persistence of vision and flicker fusion threshold. 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. A refresh rate of 60  Hz on most screens will produce a visible "flickering" effect. Most people find that refresh rates of 70–90 Hz and above enable flicker-free viewing on CRTs. Use of refresh rates above 120 Hz is uncommon, as they provide little noticeable flicker reduction and limit available resolution.

Flatscreen plasma displays have a similar effect. The plasma pixels fade in brightness between refreshes.

In 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 Hz. [1] 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. [2]

More of a concern is the LCD backlight. 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. On top of any inherent backlight flicker, most fluorescent and LED backlight designs use digital PWM for some or all of their dimming range by switching on and off at rates from several kHz to as little as 180 Hz, [3] though some flicker-free designs using true analog DC dimming exist. [4]

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 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. Most common is 3 vanes raising the rate to 72 Hz. Home film movie projectors (and early theater projectors) often have four vanes, to raise the 18 fps used by silent film to 72 Hz. 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. 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 Hz) that the image changes (50 or 60 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 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.

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 Hz, while CRT TVs, which are viewed from further away, are seen as acceptable at 60 or 50 Hz (see analog television standards). [6]

Chewing something crunchy such as tortilla chips or granola can induce flicker perception due to the vibrations from chewing synchronizing with the flicker rate of the display. [7]

Software artifacts

Software can cause flicker effects by directly displaying an unintended intermediate image for a short time. For example, drawing a page of text by blanking the area to white first in the frame buffer, then drawing 'on top' of it, makes it possible for the blank region to appear momentarily onscreen. Usually this is much faster and easier to program than to directly set each pixel to its final value.

When it is not feasible to set each pixel only once, double buffering can be used. This creates an off-screen drawing surface, drawing to it (with as much flicker as you want), and then copying it all at once to the screen. The result is the visible pixels only change once. While this technique cuts down on software flicker, it can also be very inefficient. [8]

Flicker is used intentionally by developers on low-end systems to create the illusion of more objects or colors/shades than are actually possible on the system, or as a speedy way of simulating transparency. While typically thought of as a mark of older systems like 16-bit game consoles, such flicker techniques continue to be used on new systems, as in the temporal dithering used to fake true color on most LCD monitors.

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.

Health effects

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 can create a sort of motion sickness, a discrepancy between the movement detected by the fluid in the inner ear and the motion we can see. Symptoms include dizziness, fatigue, headaches and (sometimes extreme) nausea. [10] The symptoms usually disappear in less than a week without CRT use, and usually only last a few hours unless the exposure has been over a long period.

Related Research Articles

<span class="mw-page-title-main">Computer monitor</span> Computer output device

A computer monitor is an output device that displays information in pictorial or textual form. A discrete monitor comprises a visual display, support electronics, power supply, housing, electrical connectors, and external user controls.

<span class="mw-page-title-main">Liquid-crystal display</span> Display that uses the light-modulating properties of liquid crystals

A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly but instead use a backlight or reflector to produce images in color or monochrome.

<span class="mw-page-title-main">Interlaced video</span> Technique for doubling the perceived frame rate of a video display

Interlaced video is a technique for doubling the perceived frame rate of a video display without consuming extra bandwidth. The interlaced signal contains two fields of a video frame captured consecutively. This enhances motion perception to the viewer, and reduces flicker by taking advantage of the phi phenomenon.

<span class="mw-page-title-main">Plasma display</span> Type of flat panel display

A plasma display panel (PDP) is a type of flat panel display that uses small cells containing plasma: ionized gas that responds to electric fields. Plasma televisions were the first large flat panel displays to be released to the public.

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 concept studied in vision science, more specifically in the psychophysics of visual perception. A traditional term for "flicker fusion" is "persistence of vision", but this has also been used to describe positive afterimages or motion blur. Although flicker can be detected for many waveforms representing time-variant fluctuations of intensity, it is conventionally, and most easily, studied in terms of sinusoidal modulation of intensity.

<span class="mw-page-title-main">Video projector</span> Device that projects video onto a surface

A video projector is an image projector that receives a video signal and projects the corresponding image onto a projection screen using a lens system. Video projectors use a very bright ultra-high-performance lamp, Xenon arc lamp, metal halide lamp, LED or solid state blue, RB, RGB or remote fiber-optic RGB lasers to provide the illumination required to project the image. Most modern projectors can correct any curves, blurriness and other inconsistencies through manual settings.

The refresh rate, also known as vertical refresh rate or vertical scan rate in reference to terminology originating with the cathode-ray tubes (CRTs), is the number of times per second that a raster-based display device displays a new image. This is independent from frame rate, which describes how many images are stored or generated every second by the device driving the display. On CRT displays, higher refresh rates produce less flickering, thereby reducing eye strain. In other technologies such as liquid-crystal displays, the refresh rate affects only how often the image can potentially be updated.

<span class="mw-page-title-main">Backlight</span> Form of illumination used in liquid crystal displays

A backlight is a form of illumination used in liquid-crystal displays (LCDs) that provides illumination from the back or side of a display panel. LCDs do not produce light by themselves, so they need illumination to produce a visible image. Backlights are often used in smartphones, computer monitors, and LCD televisions. They are used in small displays to increase readability in low light conditions such as in wristwatches. Typical sources of light for backlights include light-emitting diodes (LEDs) and cold cathode fluorescent lamps (CCFLs).

An output device is any piece of computer hardware that converts information or data into a human-perceptible form or, historically, into a physical machine-readable form for use with other non-computerized equipment. It can be text, graphics, tactile, audio, or video. Examples include monitors, printers, speakers, headphones, projectors, GPS devices, optical mark readers, and braille readers.

<span class="mw-page-title-main">Active shutter 3D system</span> Method of displaying stereoscopic 3D images

An active shutter 3D system is a technique of displaying stereoscopic 3D images. It works by only presenting the image intended for the left eye while blocking the right eye's view, then presenting the right-eye image while blocking the left eye, and repeating this so rapidly that the interruptions do not interfere with the perceived fusion of the two images into a single 3D image.

<span class="mw-page-title-main">Screen burn-in</span> Disfigurement of an electronic display

Screen burn-in, image burn-in, ghost image, or shadow image, is a permanent discoloration of areas on an electronic display such as a cathode ray tube (CRT) in an old computer monitor or television set. It is caused by cumulative non-uniform use of the screen.

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 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.

This article discusses moving image capture, transmission and presentation from today's technical and creative points of view; concentrating on aspects of frame rates.

<span class="mw-page-title-main">Raster scan</span> Rectangular pattern of image capture and reconstruction

A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems. The word raster comes from the Latin word rastrum, which is derived from radere ; see also rastrum, an instrument for drawing musical staff lines. The pattern left by the lines of a rake, when drawn straight, resembles the parallel lines of a raster: this line-by-line scanning is what creates a raster. It is a systematic process of covering the area progressively, one line at a time. Although often a great deal faster, it is similar in the most general sense to how one's gaze travels when one reads lines of text.

<span class="mw-page-title-main">Large-screen television technology</span> Technology rapidly developed in the late 1990s and 2000s

Large-screen television technology developed rapidly in the late 1990s and 2000s. Prior to the development of thin-screen technologies, rear-projection television was standard for larger displays, and jumbotron, a non-projection video display technology, was used at stadiums and concerts. Various thin-screen technologies are being developed, but only liquid crystal display (LCD), plasma display (PDP) and Digital Light Processing (DLP) have been publicly released. Recent technologies like organic light-emitting diode (OLED) as well as not-yet-released technologies like surface-conduction electron-emitter display (SED) or field emission display (FED) are in development to supercede earlier flat-screen technologies in picture quality.

Display motion blur, also called HDTV blur and LCD motion blur, refers to several visual artifacts that are frequently found on modern consumer high-definition television sets and flat panel displays for computers.

A multiple-sync (multisync) monitor, also known as a multiscan or multimode monitor, is a raster-scan analog video monitor that can properly synchronise with multiple horizontal and vertical scan rates. In contrast, fixed frequency monitors can only synchronise with a specific set of scan rates. They are generally used for computer displays, but sometimes for television, and the terminology is mostly applied to CRT displays although the concept applies to other technologies.

<span class="mw-page-title-main">Monochrome monitor</span> Type of CRT computer monitor

A monochrome monitor is a type of computer monitor in which computer text and images are displayed in varying tones of only one color, as opposed to a color monitor that can display text and images in multiple colors. They were very common in the early days of computing, from the 1960s through the 1980s, before color monitors became widely commercially available. They are still widely used in applications such as computerized cash register systems, owing to the age of many registers. Green screen was the common name for a monochrome monitor using a green "P1" phosphor screen; the term is often misused to refer to any block mode display terminal, regardless of color, e.g., IBM 3279, 3290.

<span class="mw-page-title-main">LED-backlit LCD</span> Display technology implementation

An LED-backlit LCD is a liquid-crystal display that uses LEDs for backlighting instead of traditional cold cathode fluorescent (CCFL) backlighting. LED-backlit displays use the same TFT LCD technologies as CCFL-backlit LCDs, but offer a variety of advantages over them.

References

  1. ↑ "Liquid crystal display (LCD):Polarity inversion | video equipment". Archived from the original on 2018-03-06. Retrieved 2018-03-05.
  2. ↑ "Inversion (pixel-walk) - Lagom LCD test". www.lagom.nl.
  3. ↑ "Pulse Width Modulation (PWM)". 17 March 2015.
  4. ↑ "Flicker free monitors | IrisTech". 2 April 2016.
  5. ↑ "3D printing & direct imaging products". Texas Instruments. November 1, 2022. Retrieved November 1, 2022.
  6. ↑ "Why Is the US Standard 60 Hz? - News". www.allaboutcircuits.com. Retrieved 2022-11-03.
  7. ↑ Campbell, Todd (November 3, 2022). "Answer Geek:How Chewing Affects TV Flicker". ABC News (Website) (1st ed.). United States of America. p. 1. Retrieved 2022-11-03.
  8. ↑ "Tutorials | Catch22". Archived from the original on 2014-07-14. Retrieved 2007-01-25.
  9. ↑ Leosoft Ltd. "Eye Saver". www.eye-saver.net.
  10. 1 2 "Monitor Flicker". Image Science. Retrieved 2022-11-03.
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