Cathode ray tube burn compensation

#This invention relates to cathode ray tubes (CRTS) and in particular to compensation for burning of CRT phosphor by the CRT electron beam. The invention is particularly applicable to cathode ray tubes used in flying spot scanning equipment.

#When a flying spot CRT is operated for prolonged periods (several hours) the efficiency of the phosphor generally falls over those parts of the tube which have been scanned.

#It is known to correct for the variations in brightness so produced in a film scanner by means of a photosensor directed at the face of the CRT, providing a correction signal, which is then divided into the video signal from the main photosensors (which receive light through the film being scanned). It is also known to avoid the horizontal bands which would otherwise result when the telecine is operating with the film moving (the scan height is reduced to compensate for the film movement), by slowly moving the scan up and down the CRT face and compensating for this motion by modulating the movement of the film. The scan may be moved by up to one half of the film frame height.

#An embodiment of the invention will now be described with reference to the accompanying drawings, in which:

#FIG. 1 is a simplified block diagram of parts of a flying spot telecine relevant to the present invention; and

#FIG. 2 is a flow chart explaining operation of the embodiment of the invention.

#The telecine illustrated in part in FIG. 1 is a flying spot telecine sold under the trade mark URSA by Rank Cintel Limited of Ware, Hertfordshire, England. Aspects of the scanning circuitry and other features of the URSA telecine are described in International Patent Application W090/07842, the content of which is incorporated herein by reference.

#The telecine comprises a flying spot scanner including a cathode ray tube 10 the beam of which is driven by scanning amplifiers 12 under the control of a scan control processor 14 and an operator controlled control system 16. The telecine operates by scanning the CRT to produce an unmodulated raster, which is then imaged through lens 18 onto the cine film 20. The light passing through the film is collected and converted into video signals by three photomultipler tubes 22 R, G, B. The film is thus scanned in line sequential fashion one film frame at a time, the conventional line interlaced television signal being produced by manipulation of the signal in a frame store 24. When the film is stationary the telecine by convention repeatedly scans one film frame. In this embodiment of the invention the scan is alternated between its normal position and a position where the lines are traced midway between the lines of the normal scan. In other words, the scan is displaced by half a line.

#The shifting of the scan to produce an interline scan is performed by the scan control processor 14 which adds a 1/2 line offset to the scan signal. The URSA scanning system as described in W090/07842 uses a digital scanning map and the offset is achieved by incrementing the vertical scan address by an amount equivalent to one half line spacing.