Information about Flicker (screen)

Flicker is visible fading between image frames displayed on cathode ray tube (CRT) based monitor. Flicker occurs when the monitor's CRT is driven at a low refresh rate, allowing the screen's phosphors to lose their excitation (afterglow) between sweeps of the electron gun.

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 70-80 Hz and above enable flicker-free viewing on CRTs. Refresh rates above 120 Hz are uncommon, as they provide no noticeable flicker reduction.

Since flat panel displays use Active-matrix liquid crystal displays which use a transistor for each pixel make the pixel keep its state, they do not flicker, at least not based in the screen update rate. They may however show flicker from the backlight, but it should normally be refreshed at rather high frequencies.

The exact refresh rate necessary to prevent the perception of flicker varies greatly based on the viewing environment because phosphor. 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 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 80 Hz, while TVs, which are viewed from further away, are seen as acceptable at 60 or even 50 Hz. (see PAL and NTSC TV)

Refresh rate versus Frame rate

In evaluating flicker, it is important to distinguish between refresh rate - how often a screen is illuminated, and frame rate - how often the image being displayed can change. Motion picture film has a standardized frame rate of 24 Hz, meaning that there are 24 photographs projected in each second. However, to reduce the flicker that such a low refresh rate would cause, each image is illuminated twice before the film advances to the next frame. The result of this is that the viewer does not notice the 48 black periods per second anymore.

Analog television sets never vary their refresh rate - NTSC always uses 59.94 Hz, so source material which is slower, such as film, needs to be repeated as needed to produce the NTSC refresh rate. Thus, low frame rate cannot cause flicker on TV, but can cause jerky motion, as is the case with old silent film that runs at 12-20 Hz.

Hardware artifacts

Flicker can also refer to the phenomenon which occurs on computers and gaming consoles when many objects must be displayed and hardware limitations prevent all of them from being rendered simultaneously. In these cases, one screen refresh may display the first set of objects while the next screen refresh will display the remaining ones. When observed in real time, the viewer will see all of the objects, though they appear and disappear rapidly.

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 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]

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 become more sensitive to the flickering than those used to CRTs as the light behind their TFT usually refreshes at ~200 Hz.

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.

Slang

Flicks is an old English expression referring to the Cinema or building where films are shown. Example usage, "Are you going to the flicks tonight?". Less often used today. More recently replaced with the words cinema or "The Pictures" (which is in turn derived from "the picture house").

External links

• Predicting flicker thresholds for video display terminals