Flicker (screen)

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Flicker is the change seen in the brightness between the cycles shown on the video display. This is especially true for refresh intervals on cathode ray tube television (CRT) and computer monitors, as well as computer screens and Plasma televisions.

Video Flicker (screen)

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Flicker occurs in CRTs when they are driven at low refresh rates, allowing the brightness to drop for long enough intervals to be noticed by the human eye - see the persistence of flicker vision and flicker thresholds. For most devices, screen phosphorus quickly loses excitation between electron gun sweeps, and incandescent light can not fill such a gap - see the persistence of phosphorus. The 60 Hz refresh rate on most screens will produce a visible "flicker" effect. Most people find that refresh rates of 70-90 Hz and higher allow a flicker-free display on CRTs. The use of refresh rates above 120 Hz is rare, as they provide little flicker reduction and limit available resolution.

The Flatcreen Plasma screen has the same effect. Plasma pixels fade in brightness between refreshments.

On the LCD screen, the LCD itself does not blink, it keeps the opacity unchanged until it is updated for the next frame. However, to prevent the accumulation of LCD screen malfunction quickly replace the voltage between positive and negative for each pixel, called 'polarity reversal'. Ideally, this will not be noticeable because each pixel has the same brightness whether a positive or negative voltage is applied. In practice, there is a small difference, which means that each pixel blinks about 30 Hz. Screens using per-line or per-pixel polarity can reduce this effect compared to when the entire screen is on the same polarity, sometimes the type of screen can be detected by using a pattern designed to maximize the effect.

More worrying is the LCD backlight. The previous screen uses fluorescent lights that blink at 100 or 120 Hz; most modern screens use electronic ballasts that blink at 25-60 kHz which is far beyond human reach. The LED backlight should not blink at all, although some designs may "dampen" themselves by turning on and off quickly and feels.

Flicker is required for the film projector to block the light when the film is moved from one frame to the next. The standard 24 fps framerate produces a very clear flicker, so even very early film projectors add an extra propeller to the rotating shutter to block the light even when the film does not move. The most common is the 3 propellers raising the level to 72 Hz. Home movie projectors (and early theater projectors) often have four propellers, to raise the 18 fps used by silent films to 72 Hz. Digital projectors usually use DLP mirrors that do not have flicker. Some 3D projections quickly alternate between images for each eye (alternating polarization at the same rate) at 144 Hz. Film projectors usually use incandescent or arc lights that do not blink themselves.

The television uses interlaced video so the screen flashes twice as fast (50 or 60 Hz) so that the image changes (25 or 30 Hz). This is considered necessary even on the first television using very slow phosphorus.

Appropriate levels of refreshment are required to prevent flicker perceptions vary greatly based on the viewing environment. In a really dark room, a dimly lit screen can run as low as 30 Hz without a flicker look. In a normal room and TV brightness, this same display level will produce a flicker that is so severe that it can not be penetrated.

The human eye is most sensitive to the flicker on the edge of the human field of view (peripheral vision) and least sensitive at the center of the view (the area that is focused on). As a result, the larger the portion of our field of view occupied by the screen, the greater the need for high refresh rates. This is why CRT computer monitors typically run from 70 to 90 Hz, while TVs, viewed from a distance, look acceptable at 60 or 50 Hz (see Analog Television Standards).

Chewing something crisp like tortilla or granola chips can cause flicker perception due to the vibration of chewing chewing with the flicker level on the screen.

Maps Flicker (screen)

Software artifact

The software can cause flicker effects by instantly displaying unwanted intermediate images for a short time. For example drawing a text page by clearing the first white area in the frame buffer, then drawing 'above', allowing blank areas to appear for a moment on the screen. Usually this is much faster and easier to program than to directly set each pixel to its final value.

When it is not possible to set each pixel only once, double buffering can be used. It creates a surface of the image on the outside of the screen, draws it there (with as many flickers as you want), and then copies it all at once onto the screen. The result is a pixel that looks only changed once. While this technique reduces flicker software, it can also be very inefficient.

Flicker is used intentionally by developers on low-end systems to create the illusion of more objects or colors/shades than is actually possible on the system, or as a quick way to simulate transparency. Although usually considered a sign of older systems such as a 16-bit game console, flicker techniques continue to be used on new systems, such as temporary dithering that is used to fabricate original colors on most new LCD monitors.

Video hardware outside the monitor can also cause flicker through various times and artifacts related to resolutions such as tear screens, z-fighting and aliasing.

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Health effects

Flicker from a CRT monitor may cause symptoms in those who are sensitive to it such as headaches in migraine sufferers and seizures in patients with epilepsy.

Since flicker is most clearly visible on the edge of our sight, there is no clear risk in using CRTs, but long-term use can cause a kind of retinal shock where flickering is visible even when looking away from the monitor. It can create a kind of motion sickness, the difference between the movement detected by the fluid in the inner ear and the movements we can see. Symptoms include dizziness, fatigue, headache and (sometimes extreme) nausea. Symptoms usually disappear in less than a week without the use of CRT, and usually only last a few hours unless exposure has been long.

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References

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External links

• Predict the threshold for the video display terminal

Source of the article : Wikipedia