Flicker is a visible fading between cycles displayed on video displays,
especially the refresh interval on cathode ray tube 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 – 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 similar effect occurs in PDPs during their
refresh cycles. For example, if a cathode ray tube's
vertical refresh rate is set to 60 Hz, most screens 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 displays 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. But,
to save the crystals from deterioration caused by constant current, voltage is
constantly reversed, which may cause flicker. "In a pixel on an LCD monitor,
the amount of light that is transmitted from the backlight depends on the
voltage applied to the pixel. For the amount of light, it doesn't matter
whether that voltage is negative or positive. However, applying the same
voltage for a long period would damage the pixel. For example, electricity
decomposes water into oxygen and hydrogen gas. A comparable similar
effect could happen inside the liquid crystals that are in the pixels. In
order to prevent damage, LCD displays quickly alternate the voltage between
positive and negative for each pixel, which is called 'polarity inversion'.
Ideally, the rapid polarity inversion wouldn't be noticeable because every
pixel has the same brightness whether a positive or a negative voltage is
applied. However, in practice, there is a small difference, which means that
every pixel flickers at about 30 hertz." 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. Chewing something crunchy such as
Doritos or granola can induce flicker perception due to the vibrations from
chewing synchronizing with the flicker rate of the display.
Another factor in detecting flicker is peripheral vision. The human eye is most
sensitive to flicker at the edges of human field of view, and least sensitive
at the center of gaze. 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.
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 program'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.
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. 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 also helps to negate these particular
effects. Flat-screen monitors do not have this problem as the pixels change
directly from one color to the next without going black, and the light
behind a TFT monitor 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, 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 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. References
External links Predicting flicker thresholds for video
display terminals