Flicker (screen)
Encyclopedia
Flicker is a visible
fading between cycles displayed on video displays, especially the refresh interval on cathode ray tube
(CRT) based computer screens
. 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. 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. A similar effect occurs in PDPs
during their refresh cycles.
For example, if a Cathode Ray Tube computer monitor's vertical refresh rate is set to 60 Hz
, most monitors will produce a visible "flickering" effect, unless they use phosphor with long afterglow. 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.
Since the shutters used in liquid crystal display
s for each pixel stay at a steady opacity, they do not flicker, even when the image is refreshed. The backlights of such displays typically operate in the range of 150–250 Hz.
The lighting used in film projectors
is typically an incandescent lamp or arc lamp
, which does not flicker, but some degree of flicker is desirable to help decrease the flicker fusion threshold
comfortably below film's typical framerate of 24 fps. This is usually accomplished with a shutter which causes the lamplight to apparently strobe on and off at a multiple of the framerate, most often 48–96 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. At normal room and TV brightness this same display rate would produce flicker so severe as to be unwatchable.
Another factor in detecting flicker is peripheral vision. The human eye is most sensitive to flicker at the edges of our field of view, 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 TVs, which are viewed from further away, are seen as acceptable at 60 or 50 Hz (see PAL
and NTSC
.)
's failure to consistently maintain its graphical
state. For example, the practice of blanking an area directly in the frame buffer, then drawing 'on top' of it, makes it possible for the blank region to appear momentarily onscreen.
When it is not feasible to set each pixel only once, double buffering
can be used. The method involves creating an off-screen drawing surface, drawing to it, and then copying it all at once to the screen. While this technique cuts down on software flicker, it can also be very inefficient.http://www.catch22.net/tuts/flicker.asp
Flicker is also 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, the flicker technique continues to be used on new systems, such as the temporal dithering used to fake true color on most new 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
.
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
. 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.
Health effects specific to FLKR FRAME VIDEO include some reports or seizures, but due to the small pixel sized video adjustment, no accurate example of this actually happening has been found. Reports of this come from anecdotal stories from email and the internet.
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
fading between cycles displayed on video displays, especially the refresh interval on cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
(CRT) based computer screens
Computer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...
. Flicker occurs on CRTs when they are driven at a low refresh rate
Refresh rate
The refresh rate is the number of times in a second that a display hardware draws the data...
, allowing the brightness
Brightness
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target...
to drop for time intervals sufficiently long to be noticed by a human eye. For most devices, the screen's phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
s quickly lose their excitation between sweeps of the electron gun
Electron gun
An electron gun is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in television sets and computer displays which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle...
, and the afterglow is unable to fill such gaps. A similar effect occurs in PDPs
Plasma display
A plasma display panel is a type of flat panel display common to large TV displays or larger. They are called "plasma" displays because the technology utilizes small cells containing electrically charged ionized gases, or what are in essence chambers more commonly known as fluorescent...
during their refresh cycles.
For example, if a Cathode Ray Tube computer monitor's vertical refresh rate is set to 60 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, most monitors will produce a visible "flickering" effect, unless they use phosphor with long afterglow. 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.
Since the shutters used in liquid crystal display
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
s for each pixel stay at a steady opacity, they do not flicker, even when the image is refreshed. The backlights of such displays typically operate in the range of 150–250 Hz.
The lighting used in film projectors
Movie projector
A movie projector is an opto-mechanical device for displaying moving pictures by projecting them on a projection screen. Most of the optical and mechanical elements, except for the illumination and sound devices, are present in movie cameras.-Physiology:...
is typically an incandescent lamp or arc lamp
Arc lamp
"Arc lamp" or "arc light" is the general term for a class of lamps that produce light by an electric arc . The lamp consists of two electrodes, first made from carbon but typically made today of tungsten, which are separated by a gas...
, which does not flicker, but some degree of flicker is desirable to help decrease the flicker fusion threshold
Flicker fusion threshold
The flicker fusion threshold is a concept in the psychophysics of vision. It is defined as the frequency at which an intermittent light stimulus appears to be completely steady to the observer...
comfortably below film's typical framerate of 24 fps. This is usually accomplished with a shutter which causes the lamplight to apparently strobe on and off at a multiple of the framerate, most often 48–96 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. At normal room and TV brightness this same display rate would produce flicker so severe as to be unwatchable.
Another factor in detecting flicker is peripheral vision. The human eye is most sensitive to flicker at the edges of our field of view, 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 TVs, which are viewed from further away, are seen as acceptable at 60 or 50 Hz (see PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
and NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
.)
Software artifacts
Flicker, a flashing effect displeasing to the eye, often occurs through flaws in software, with no hardware faults involved. Flicker in software is caused by a computer programComputer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
's failure to consistently maintain its graphical
Computer graphics
Computer graphics are graphics created using computers and, more generally, the representation and manipulation of image data by a computer with help from specialized software and hardware....
state. For example, the practice of blanking an area directly in the frame buffer, then drawing 'on top' of it, makes it possible for the blank region to appear momentarily onscreen.
When it is not feasible to set each pixel only once, double buffering
Double buffering
In computer science, multiple buffering is the use of more than one buffer to hold a block of data, so that a "reader" will see a complete version of the data, rather than a partially-updated version of the data being created by a "writer"...
can be used. The method involves creating an off-screen drawing surface, drawing to it, and then copying it all at once to the screen. While this technique cuts down on software flicker, it can also be very inefficient.http://www.catch22.net/tuts/flicker.asp
Flicker is also 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, the flicker technique continues to be used on new systems, such as the temporal dithering used to fake true color on most new 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
Z-fighting
Z-fighting is a phenomenon in 3D rendering that occurs when two or more primitives have similar values in the z-buffer. It is particularly prevalent with coplanar polygons, where two faces occupy essentially the same space, with neither in front...
and aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
.
Flicker Frame Video (FLKR FRAME)
Flicker Frame Video, often called FLKR Frame, is a method of modifying poor-quality video files into files that trick the human eye into seeing a clearer and better image in a video. This is done by doubling the total frames in a video file, and reducing the time of each frame by half. Every other frame is then off-set from the frame before it by 1 or more pixels. When the file is played back, this off-set makes the human eye see clearer edges and a more improved overall image in the film. Contrast, saturation, color and brightness can also be adjusted in opposite frames to increase picture quality in the final playback video file.Health effects
The constant refreshing of a CRT monitor can cause various symptoms in those sensitive to it such as headaches in migraine sufferers and seizures in epileptics, if they are photosensitive. Screen filters are available to reduce these effects. A high refresh rate (above 75 Hz) also helps to negate these particular effects. Frequent users of TFT monitors usually are more sensitive to the low refresh rate of CRT monitors as the light behind a TFT monitor (backlight) usually is driven at 40–50 kHz, which is unnoticeable to the human eye.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
Motion sickness
Motion sickness or kinetosis, also known as travel sickness, is a condition in which a disagreement exists between visually perceived movement and the vestibular system's sense of movement...
, a discrepancy between the movement detected by the fluid in the inner ear and the motion we can see. Symptoms include dizziness
Dizziness
Dizziness refers to an impairment in spatial perception and stability. The term is somewhat imprecise. It can be used to mean vertigo, presyncope, disequilibrium, or a non-specific feeling such as giddiness or foolishness....
, fatigue, headaches and (sometimes extreme) nausea
Nausea
Nausea , is a sensation of unease and discomfort in the upper stomach with an involuntary urge to vomit. It often, but not always, precedes vomiting...
. 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.
Health effects specific to FLKR FRAME VIDEO include some reports or seizures, but due to the small pixel sized video adjustment, no accurate example of this actually happening has been found. Reports of this come from anecdotal stories from email and the internet.