Automatic electron beam focusing system

Claim 1

IN AN ADJUSTABLE FOCUS IMAGING DEVICE WHEREIN AN INTERROGATING BEAM IS SEQUENTIALLY SCANNED OVER AN IMAGE IN ORDER TO DERIVE ELECTRICAL SIGNALS REPRESENTATIVE OF SAID IMAGE, APPARATUS FOR AUTOMATICALLY CONTROLLING THE FOCUSING OF SAID INTERROGATING BEAM COMPRISING: FIRST MEANS FOR PROVIDING AS PART OF SAID IMAGE A REPRESENTATION OF A REFERENCE OBJECT; SECOND MEANS FOR DEVELOPING A PLURALITY OF ELECTRICAL SIGNALS INDIVIDUALLY REPRESENTATIVE OF THE SCANNING BY SAID INTERROGATING BEAM OF THE PART OF SAID IMAGE REPRESENTATIVE OF THE REFERENCE OBJECT AT DIFFERENT FOCUS ADJUSTMENTS OF SAID INTERROGATING BEAM; COMPARISON MEANS FOR COMPARING AT LEAST TWO OF SAID PULARITY OF ELECTRICAL SIGNALS DEVELOPED AT DIFFERENT FOCUS ADJUSTMENTS OF SAID INTERROGATING BEAM ACCORDING TO A COMMON CHARACTERISTIC OF SAID SIGNALS; AND MEANS RESPONSIVE TO SAID COMPARISON MEANS FOR DETERMINING WHICH OF SAID COMPARED SIGNALS CORRESPOND TO THE MORE DESIRED FOCUS SETTING AND FOR DEVELOPING A SIGNAL TO ADJUST SAID FOCUS IN THE DIRECTION OF THE MORE DESIRED SETTING; THE SYSTEM BEING SO CONSTRUCTED AND ARRANGED SO AS TO CONTINUOUSLY COMPARE ELECTRICAL SIGNALS INDICATIVE OF THE FOCUS SETTINGS AND TO READJUST THE FOCUS ACCORDING TO SAID COMPARISON UNTIL THE DESIRED FOCUS IS OBTAINED.