Refresh rate

Refresh rate is a measure of the number of times a display's image is 'repainted' per second. The refresh rate is measured in hertz. A display's refresh rate depends on the video card used (although displays do have a maximum refresh rate).

The original standard for refresh rates, developed by the Video Electronics Standards Association (VESA), was 60 Hz. At 60 Hz the display's image flickers noticeably which can cause eye fatigue and headaches. A new standard set the refresh rate to 75 Hz. IIRC, under European Law it is illegal for an employer to force an employee to use a display with a refresh rate of less than 70 Hz.

DRAM modules also have a refresh rate. A DRAM module is made up of electrical cells which must be refreshed thousands of times per second to prevent them losing data. Some DRAM modules are able to refresh themselves independently of the processor. These types of DRAM are commonly used in laptops due to their lower power consumption.

On video monitors, there are actually two refresh rates, but you normally only hear about one. The horizontal refresh rate is the inverse of the amount of time it takes to draw a single line on the screen:

A typical monitor has a 31.5 KHz maximum horizintal 
frequency, as determined by a crystal installed on the 
motherboard.

  31.5KHz = 31500Hz

        1
     -------  =  0.000031746 seconds
      31500

So, in 0.000031746 seconds, it draws a single line across the screen. Most people have heard about the vertical refresh rate, because these numbers are touted by the sales advertising.

To determine the vertical refresh rate:


    Horizontal Frequency (In Hertz)
  -----------------------------------  = Vertical Refresh
      The number of rows (lines)


Since a typical SVGA monitor can display 640 x 480:

               31500
              -------   =   72Hz
                480

Continuing using similar numbers:

 A monitor running 800 x 600  =  72Hz (rounded)
 A monitor with 1024 x 768    =  70Hz

Normally, a higher refresh rate gives you better picture quality and less flicker. You do not want to have a 60Hz vertical refresh rate or less, especially since flourescent lights also operate at 60Hz, which gives pronounced flicker and an almost stop-motion scan line that progresses down your screen. In countries that have a 50Hz electrical system, you don't have to worry as much, since I personally don't know of any sub-60Hz monitor refresh rates.