Apparatus for use in determining a characteristic of a cathode ray tube

Claim 1

Apparatus for use in ascertaining the characteristic of a cathode ray tube relating luminous spot position to deflection current, said apparatus comprising: (A) a locating means for a cathode ray tube; (B) a first photosensor; (C) optical means defining a first optical path between the screen of a cathode ray tube located by said locating means and the first photosensor; (D) a graticule disposed in said first optical path, (i) said graticule containing a predetermined spatial distribution of light transmissive and non-transmissive areas, (ii) the first optical path including imaging means operative to form a real image of a luminous spot on said cathode ray tube screen at a corresponding position on said graticule so that the output of said first photosensor has a value dependent on the position of said spot on the cathode ray tube screen; (E) a second photosensor; (F) further optical means which defines a second optical path between the screen of the cathode ray tube and the second photosensor, (i) said second optical path having a predetermind constant light transmissivity so that the output of said second photosensor has a constant value independent of the position of said spot on said cathode ray tube screen; (G) a first signal comparator responsive to the outputs from said first and second photosensors; and (H) means responsive to the output of said first comparator for producing a signal for application to a beam deflection means of the cathode ray tube to position said spot and hence said image of the spot on the graticule such that the outputs produced by said first and second photosensors are in a predetermined relationship.