Flicker (screen): Difference between revisions

Flicker is visible fading between cycles displayed on video displays, especially the refresh interval oncathode ray tube (CRT) based computer screens. Flicker occurs on CRTs when the they are driven at a low refresh rate, allowing the screen's phosphors to lose their excitation (afterglow) between sweeps of the electron gun. A similar effect occurs in PDPs during their refresh cycles.

For example, if a CRT 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 displays for each pixel stay at a steady opacity, they do not flicker, even when the image is refreshed. The backlights of such displays do flicker, but typically operate in the range of 150-250 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)

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 data in the video hardware, resulting from program drawing, may momentarily include a white rectangle before the text that belongs there is 'drawn.' Part or all of a flickering region is frequently and completely blank or in some other intermediate state.

The CS_VREDRAW and CS_HREDRAW window class styles direct Windows to cause an entire program window to be redrawn when the window is resized. Disabling these styles causes only the areas uncovered by resizing to be invalidated for painting. However, flicker may still occur at the edges or when data is updated for other reasons.

The practise of blanking an area, 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 offscreen drawing surface (in GDI, a bitmap and device context), drawing to it, and then blitting it all at once to the screen. While this technique cuts down on software flicker, it can also be very inefficient.[1]

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.

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 150-250 Hz, which is virtually 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 (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.

• Predicting flicker thresholds for video display terminals