GB687940A - Improvements in or relating to television pick-up tubes - Google Patents

Abstract

687,940. Cathode-ray tubes. TOWNSEND, F. H., and CATHODEON, Ltd. Nov. 9, 1950 [Nov. 18, 1949], No. 29530/49. Class 39(i) In a television pick-up tube of the high velocity scanned type having a charge storage surface capable of storing energy in the form of a charge pattern corresponding to the light distribution in an optical image, a thermionic source of electrons for emitting electrons towards the storage surface, and means for accelerating the emitted electrons towards the storage surface so as to charge the storage surface negatively, the thermionic source comprises a plurality of thermionic cathodes arranged around and in front of the storage surface, means being provided for individually adjusting the electron emission from the respective cathode, whereby the concentration of electrons directed upon different parts of the storage surface may be adjusted to obtain a predetermined selective negative charging of different parts of the storage surface. As shown the pick-up tube comprises an evacuated envelope containing a semitransparent photo-cathode 2 and a storage target spaced therefrom consisting of a mica sheet 3, which may be coated with a material to make it highly secondary emissive, backed with a conductive layer 4 forming the signal plate. Photoelectrons released from the photo-cathode 2 by an optical image projected thereon are accelerated towards the storage surface 3 by the wall coating 7 which is at a positive potential and are focused upon the storage surface by the focusing coil 8. The storage surface is scanned by a high velocity beam from the electron gun 6, and a collector electrode 9; which may be a wall coating, collects secondary electrons released from the storage surface 3. Four thermionic cathodes 10 are disposed around the area defined by the storage surface 3, being spaced in front thereof and so positioned that they do not interfere with the formation of the electron image on the surface 3, nor with the scanning by the cathode-ray beam. The collector electrode 9 is maintained positive with respect to the cathodes 10 and directs and accelerates the electrons released therefrom towards the storage surface with such velocities that the surface is negatively charged, a grid electrode 12, maintained at a positive potential also controlling the acceleration. Alternatively the electrode 9 may be omitted and its function performed by the electrode 12. The emission from the--cathode 10 is individually adjusted to obtain the desired negative charging on the storage surface. In the case of an iconoscope the electrons would irradiate the photo-sensitive mosaic target instead of the secondary emitting target of the image iconoscope.