Multiplier phototube with calibrating electron beam

Claim 1

IN A MULTIPLIER PHOTOTUBE HAVING PHOTOCATHODE ADAPTED TO RECEIVE RADIATION FROM A SOURCE OF UNKNOWN INTENSITY AND TO CONVERT THE SAME TO A FIRST ELECTRON BEAM, ELECTRON MULTIPLYING MEANS INCLUDING A PLURALITY OF DYNODE ELEMENTS, VOLTAGE SUPPLY MEANS FOR APPLYING PROGRESSIVELY HIGHER VOLTAGES TO SAID DYNODE ELEMENTS THEREBY TO MULTIPLY THE ELECTRONS IN SAID BEAM, A TARGET ELECTRODE, AND OUTPUT CIRCUIT MEANS COUPLED TO SAID TARGET ELECTRODE FOR PROVIDING AN OUTPUT SIGNAL RESPONSIVE TO THE MULTIPLIED BEAM: MEANS FOR CALIBRATING SAID PHOTOTUBE COMPRISING MEANS FOR CONTINUOUSLY PROVIDING A SECOND ELECTRON BEAM IN SAID TUBE SIMULTANEOUSLY WITH SAID FIRST BEAM AND HAVING A PREDETERMINED BEAM CURRENT, SAID SECOND BEAM BEING NORMALLY DIRECTED AWAY ROM SAID MULTIPLIER MEANS, AND MEANS SELECTIVELY DEFLECTING BOTH SAID FIRST BEAM AWAY FROM SAID MULTIPLYING MEANS AND SAID SECOND BEAM ONTO THE SAME SO THAT ONLY SAID SECOND BEAM IS MULTIPLIED.