Time it takes for the electron beam in a cathode ray tube device to move from the end of one horizontal line down and to the left of the screen to begin drawing the next line. During this time, the electron beam is shut off so that no other lines are accidentally created as the beam scans down and left. cathode ray tube paints lines one at a time from left to right and top to bottom. When it completes one horizontal line, it must begin again at the next lower line and at the left of the screen. The beam must be refocused and redirected to this new point so that it can paint the next new line. The time it takes for this to occur is known as the horizontal-blanking interval. hen the last horizontal line is drawn at the bottom of a screen, the beam moves to the top left of the screen for the next field to be generated. The time it takes to do this is called the vertical blanking interval.