Optical path length difference compensated flying-spot scanner

Claim 1

A COMPENSATED FLYING-SPOT SCANNER COMPRISING: (A) CATHODE RAY TUBE MEANS, HAVING A SHORT PERSISTENCE SCREEN AND INTENSIFYING MEANS, FOR SWEEPING A VARIABLE INTENSITY ELECTRON BEAM TO ILLUMINATE SPOTS ON THE SCREEN, (B) DIFFERENTIAL COMPARATOR MEANS, HAVING TWO INPUTS, AND AN OUTPUT CONNECTED TO THE TUBE INTENSIFYING MEANS FOR VARYING THE INTENSITY OF THE BEAM, (C) A REFERENCE SIGNAL CORRESPONDING TO A DESIRED BEAM INTENSITY CONNECTED TO ONE COMPARATOR MEANS INPUT, (D) PHOTOSENSITIVE MEANS CONNECTED TO THE OTHER COMPARATOR MEANS INPUT AND DISPOSED FOR RECEIVING SPOT ILLUMINATION AND IN RESPONSE THERETO PRODUCING A PROPORTIONAL SIGNAL WHICH DRIVES SAID COMPARATOR MEANS TO DECREASE THE BEAM INTENSITY AND SPOT ILLUMINATION WHEN THE PHOTOSENSITIVE MEANS SIGNAL IS GREATER THAN THE REFERENCE SIGNAL AND TO INCREASE THE BEAM INTENSITY AND SPOT ILLUMINATION WHEN THE PHOTOSENSITIVE MEANS SIGNAL IS LESS THAN THE REFERENCE SIGNAL, AND (E) FILTER MEANS DISPOSED IN THE PATH BETWEEN THE SCREEN SPOTS AND THE PHOTOSENSITIVE MEANS FOR ATTENUATING THE SPOT ILLUMINATION BEFORE IT IS RECEIVED BY SAID PHOTOSENSITIVE MEANS IN AN AMOUNT THAT IS INVERSELY PORPORTIONAL TO A FUNCTION OF THE DISTANCE BETWEEN THE SPOT AND SAID PHOTOSENSITIVE MEANS.