Tektronix had originally developed their storage tubes in the late 1950s as a way to store images on oscilloscope displays for study, although the same system had already been used in radar displays. The basic concept uses a conventional CRT layout but with two sets of electron guns. One, the "flood gun", provides a constant flow of low-energy electrons across the entire screen, causing it to glow slightly. The second, the "write gun", is the normal gun of a black and white TV, and its beam was moved about on the display surface in the conventional fashion using electromagnets. However, this gun is set to a higher than normal energy. When its beam strikes the screen, it causes an effect known as photoemission that drives electrons out of the phosphorus towards the front of the display where they are picked up by a thin transparent electrode. This area now contains fewer electrons than normal, giving it a positive charge relative to its surroundings. This causes some of the electrons from the flood gun to be strongly attracted to that spot, which keeps it lit up. The display using this technique is instantly recognizable by the bright flash of the write gun beam as it moves about the display.