Modular display apparatus with means for preventing brightness variations

Claim 1

A display apparatus for successively producing a series of image frames, said apparatus comprising a flat panel cathode-ray display tube including a luminescent screen, a plurality of electron-emitting means and an apertured control plate disposed between the electron emitting means and the screen, said display tube comprising a plurality of modules, each including at least one of the electron-emitting means and a portion of the control plate associated with a respective area of the screen, for producing from distributions of emitted electrons respective areas of the image frames, the improvement comprising: (a) a plurality of parallel switching electrodes in each module associated with respective rows of apertures in the control plate for controlling the passage of current through said apertures, said switching electrodes being disposed on a side of the control plate facing the electron-emitting means; (b) at least one brightness control electrode in each module associated with a respective column of apertures in the control plate for controlling the magnitudes of any currents passing through said apertures, said at least one brightness control electrode being disposed on a side of the control plate facing the screen and being oriented transversely to the switching electrodes in the respective module; (c) means for sequentially measuring the magnitudes of the currents passing through selected apertures in said rows as the switching electrodes are sequentially activated, between successive image frames; (d) comparing means for determining the differences between the magnitudes of the measured currents and a reference magnitude; and (e) biasing means for applying bias voltages representing the differences to the respective brightness control electrodes associated with the apertures for which the differences were determined, in synchronism with sequential activation of the switching electrodes associated with said apertures, during production of each image frame, to prevent variations in image brightness corresponding to non-uniform electron distributions in the modules.