Expressed in Hertz (Hz), in interlaced mode this is the number of fields written to the screen every second. In non-interlaced mode it is the number of frames (complete pictures) written to the screen every second. Higher frequencies reduce flicker, because they light the pixels more frequently,reducing the dimming that causes flicker. Also called vertical frequency.

Same as vertical frequency, is measured in Hertz (Hz) and represents the number of frames displayed on the screen per second. Too few, and the eye will notice the intervals in between and perceive a flickering display. The world-wide accepted refresh rate for a flicker-free display is 70Hz and above, although standards bodies such as VESA are pushing for higher rates of 75Hz or 80Hz.

A CRT has three electron guns whose streams must converge faultlessly in order to create a sharp image. There are no convergence problems with an LCD panel, because each cell is switched on and off individually. This is one reason why text looks so crisp on an LCD monitor. There's no need to worry about refresh rates and flicker with an LCD panel - the LCD cells are either on or off, so an image displayed at a refresh rate as low as between 40-60Hz should not produce any more flicker than one at a 75Hz refresh rate.

Liquid crystal displays

Liquid crystals were first discovered in the late 19th century by the Austrian botanist, Friedrich Reinitzer, and the term 'liquid crystal' itself was coined shortly afterwards by German physicist, Otto Lehmann.

Liquid Crystal Display: a display technology that relies on polarising filters and liquid crystal cells rather than phosphors illuminated by electron beams to produce an on-screen image.

Liquid crystals are almost transparent substances, exhibiting the properties of both solid and liquid matter. Light passing through liquid crystals follows the alignment of the molecules that make them up - a property of solid matter. In the 1960s it was discovered that charging liquid crystals with electricity changed their molecular alignment, and consequently the way light passed through them; a property of liquids.

An LCD consists of two polarising filters with their lines arranged perpendicular (at 90 degrees) to each other, which, as described above, would block all light trying to pass through. But in-between these polarisers are the twisted liquid crystals. Therefore light is polarised by the first filter, twisted through 90 degrees by the liquid crystals, finally allowing it to completely pass through the second polarising filter. However, when an electrical voltage is applied across the liquid crystal, the molecules realign vertically, allowing the light to pass through untwisted but to be blocked by the second polariser. Consequently, no voltage equals light passing through, while applied voltage equals no light emerging at the other end.

The crystals in an LCD could be alternatively arranged so that light passed when there was a voltage, and not passed when there was no voltage. However, since computer screens with graphical interfaces are almost always lit up, power is saved by arranging the crystals in the no-voltage-equals-light-passing configuration.

An LCD screen that has its own light source from the back of the screen, making the background brighter and characters appear sharper. This backlight is added, typically in the form of cold-cathode fluorescent tubes mounted along the top and bottom edges of the panel, the light from these being distributed across the panel using a plastic light guide or prism. The image which appears on the screen is created by this light as it passes through the layers of the panel.

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