It is known that various techniques have been proposed in the field of flat video screens. The ideal system should be capable of generating screens of both small and large dimensions, of being compatible both for black and white as well as for color, of having small electric consumption and of being simple of manufacture.
The conventional television tube with electron beam scanning cannot be reduced in thickness for physical reasons, i.e. image distortion if the beam falls at a low angle onto the screen and lack of precision for reaching the mask on a screen in case of color. Furthermore, the screen dimensions cannot be significantly increased due to vacuum requirements, in other words relating to strength of the materials under pressure.
Therefore, the tendency is to form images not with a scanned beam but by a line and column addressed matrix of points(spots).
In this field of art, liquid crystals are attractive since they require very little electric consumption of power, but on the other hand, to be visible they require an external light source. Moreover, it is very difficult to obtain gradations in grey levels and to produce color images.
Other methods have been proposed for the realization of flat screens. One of them uses a plasma micro-discharge in a gas as an electron source, such electrons being thereafter attracted toward a fluorescent screen. Line and column addressing permits illumination of the desired spot on the screen. Unfortunately, utilization of a plasma as the electron source is delicate, since the plasma either occurs or does not occur, i.e. the spot is either illuminated or extinguished. The result is that grey levels cannot be obtained.