The primary output device in a graphics system is a video monitor. The operation of most video monitors is based on the standard cathode ray tube (CRT) design.
The Cathode Ray Tube
The electron gun emits a beam of electrons which are focused to a point on the screen phosphor. The beam is positioned on the screen by a deflection system which operates in the horizontal and vertical directions. The intensity of the beam is controlled by the intensity signal on the control grid. When the phosphor is hit by the electron beam it absorbs energy and jumps to a higher quantum-energy level. As it returns to its normal level it emits visible light i.e. it phosphoresces. In the phosphors used in graphics devices the persistence of the phosphorescence is typically 10-60 microseconds.
Before the human visual system can see a transient image it must be continually redrawn (refreshed) at a rate higher than the critical fusion frequency of the human visual system. To allow the human visual system to see a continuously refreshed image without flicker the refresh rate has to be at least 60 c/s.
To allow continuous refreshing of an image there must be some stored representation of the image from which the refresh system can obtain the graphical information required to re-draw the image. This representation nowadays is invariably a set of values of Intensity / colour at each of a discrete set of points laid out in a rectangular array covering the screen.
While it may seem a disadvantage to continually refresh the image there are some very important advantages of such refresh type systems. For example it is possible to edit an image by changing the stored representation between refresh cycles for what appears to be instantaneous updating of the image. Compare this with some earlier systems in which the only way to carry out an edit was to clear the whole screen and then redraw the whole image. Also by changing the stored representation between refresh cycles animation is possible
