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In a computer display, to refresh is to redraw the image information from memory. Computer or television displays have to be refreshed because they don't have the capacity to hold a stable image. Electron guns in the cathode ray tube (CRT) constantly sweep across the screen, redrawing the display. The RAMDAC (random access memory digital-to-analog converter) in the graphics card determines a refresh rate: how many times per second the information will be drawn and the image repainted. At adequate refresh rate levels, a display appears stable, but if a refresh rate is too low a display will flicker and can cause eye strain and headaches.