Flicker arises because the video display images are not generated continuously, but are rather redrawn at a rate that typically varies between 50 to 85 times per second (Hz), in a process called screen refresh. The screen brightness decays between the refresh cycles. The resulting flutter in brightness, called flicker, gives rise to a well documented cause of visual fatigue. The threshold for detecting flicker varies from person to person, with the persistence of the screen phosphor, and with the observing conditions. For most people, flicker is not apparent beyond 70 Hz, however, under some circumstances it can be visible upwards of 100 Hz. In addition, subliminal flicker, which is just below the threshold of conscious detection, may also produce visual fatigue.

Sensitivity     Interlace     Settings     Image Lag

Flicker Sensitivity

The sensitivity to flicker varies from person to person, but it is generally higher for younger observers. For a given individual, the sensation of flicker generally increases with any of the following:

• as the screen intensity is increased
• as the background room lighting is decreased
• with the use of fluorescent room lighting
• as the video screen size increases
• as you move closer to the screen
• as you use your peripheral vision by looking at the screen off-axis.

The final step is to reverse the above process, which enhanced the flicker, in order to reduce it to a point that is well below your detectable threshold. You may need to change the room lighting and the screen brightness. Finally, you may need to increase the refresh rate by using the video board manufacturer's install or setup utility. Never set the refresh rate higher than the minimum required. If you increase the refresh rate too high, image sharpness will be reduced. Also, make sure that you don't exceed the capabilities of your monitor. For most people a good starting value for the refresh rate is 75 Hz.

Interlace

A cheap trick to reduce flicker that works well for television images but not for computer displays is called interlaced scanning. Instead of drawing each raster line in turn top to bottom, the even lines are drawn the first time around, and then the odd lines are drawn in the second pass. The even field and the odd field comprise one entire image, which is called a frame. When the displayed images vary smoothly, such as in a television picture, the eye perceives the flicker frequency to be at the field rate, which is twice the frame rate. The entire image is still transmitted at the frame rate, so the flicker frequency has been doubled without doubling the data rate or increasing the cost of the system. Unfortunately, computer images are highly structured, so the information on the even lines is often different from the adjoining odd lines. As a result, interlace smoothing doesn't work well, and the perceived flicker rate falls back down to the frame rate.

Although interlaced images generally have detectable flicker, which is annoying, it also allows that monitor to operate at a higher pixel resolution that it could in a non-interlaced mode. So interlacing is a compromise with a well defined penalty. None of the standard video systems, CGA through VGA, utilize interlace for modes up through 640 x 480. At resolutions of 1024 x 768 and higher some displays do utilize interlacing with frame rates of only 43.5 Hz, and 30 Hz in some cases. Before upgrading to such a video system, you may wish to consider whether interlacing and flicker will be a problem for you.

Hardware Settings

If you buy a video board or a computer system without a monitor or display, the video refresh rate is most likely set by the factory to 60 Hz, the lowest common denominator for monitors. It is also possible that some high resolution modes may be disabled so that you don't inadvertently damage a monitor that can't handle the higher scan rates. Under these circumstances you'll need to set the refresh rate yourself using the video board manufacturer's install or setup utility. A good starting value for the refresh rate is 75 Hz, provided your monitor and video board can handle that rate. Computer systems sold as complete integrated packages that include a monitor generally have the refresh rates appropriately set for the supplied monitor, but check to make sure.

The amount of flicker produced by a video display varies with the type of display and the refresh rate. For CRTs it depends strongly on the persistence of the screen phosphors. Too long a persistence will produce another problem called image lag, which is discussed below. Active Matrix LCD displays often have some flicker because their refresh rates are reduced for performance reasons. Flicker on certain AC gas plasma displays is significantly less than other types of displays because they can make use of a memory effect in the plasma. Dual Scan LCD displays also generally have reduced flicker, simply because they are so slow that the image cannot change rapidly enough to cause this problem. However, both gas plasma and LCD displays can produce noticeable flicker when pulse width modulation techniques are used to produce intensity variations in the display.

Image Lag

Image lag arises when the screen light output responds slowly to a changing image. It is a problem for CRTs that are designed for low refresh rates and have long persistence phosphors. It is also a problem for LCD displays, except those with a TFT active matrix. Phosphors with varying degrees of persistence are used to minimize the appearance of flicker in the image. When the refresh rate is under 60 cycles per second, the persistence may be unusually long, giving rise to afterimages and image trails that can last a few seconds in dim lighting. The afterimages can make the screen unreadable if the screen content changes rapidly, such as during scrolling, video clips or animation. In some cases the decaying color of a phosphor will be different from its normal emitting color; this can be particularly annoying with some white phosphors that have yellow or green light tails. DisplayMate has a persistence test for evaluating image lag.

For LCDs the problem is worse, because it takes a noticeable amount of time for the image to reach full contrast after it is drawn, besides taking a noticeable amount of time to disappear after the image is removed. If the image is changing rapidly as during scrolling, panning, fast screen updates, video clips, or animation, then the image may appear very dim or possibly may even be invisible. A moving or blinking Mouse cursor may also be difficult to see. Afterimages like those of CRTs with long persistence phosphors may also be present. DisplayMate has scrolling, panning, and persistence tests that will allow you to evaluate the severity of these effects.