In a known type of cathode ray tube, the electron stream from the gun is divided by an electrostatic screen into two distinct beams, which are then independently controlled by separate deflecting plates. It is found, however, that each beam is liable to be affected, to some extent, by the deflecting voltages applied to the other beam.
The inventors state that this undesirable inter-modulation effect, which is particularly in evidence when the deflecting plate circuits are of high impedance, is due to the collection by each of the plates of secondary electrons emitted by the, electrostatic screen. The platter is inevitably subjected to a certain amount of bombardment by the original electron stream, especially at those parts nearest to the gun.
The source of the trouble is removed (a) by carbonising the surface of the dividing screen, so as to minimise secondary emission; and (b) bv extending the upper end of the screen into close proximity with the anode of the tube, thereby reducing the penetration of each deflecting field into the region swept by the other beam.
A C Cossor, Ltd., E E Sheltogn and H Moss. Application date July 231/d, 1940. No. 553131.