In {HEAD ~ an alternative embodiment} of {TAIL ~ the invention}, this difference voltage is continuously sensed and compared with a reference voltage for producing a signal for operating a servo motor that is operative to lock in the focal spot size during an exposure 
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In { HEAD ~ an alternative embodiment } of { TAIL ~ the invention } , this difference voltage is continuously sensed and compared with a reference voltage for producing a signal for operating a servo motor that is operative to lock in the focal spot size during an exposure
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