Apparatus for automatically controlling exposure in photographic printing machines

Claim 1

APPARATUS FOR CONTROLLING THE EXPOSURE IN A PHOTOGRAPHIC PRINTING MACHINE OF THE KIND IN WHICH THE DURATION OF EXPOSURE IS CONTROLLABLE BY MEASUREMENT OF THE LIGHT PASSED FROM THE ORIGINAL TO THE COPY, COMPRISING A FIRST LIGHT SOURCE FOR ILLUMINATING SAID ORIGINAL, AN ADDITIONAL LIGHT SOURCE ADAPTED TO PROVIDE AN INTENSITY REFERENCE, A PHOTOSENSITIVE ELEMENT INTENDED TO RECEIVE LIGHT PASSED FROM SAID ORIGINAL AND LIGHT FROM SAID ADDITIONAL LIGHT SOURCE, MEANS FOR DIRECTING AT LEAST PART OF THE PRINTING LIGHT AND THE REFERENCE LIGHT ALTERNATELY AND FOR BRIEF PERIODS ONTO SAID PHOTOSENSITIVE ELEMENT, SAID PHOTOSENSITIVE ELEMENT BEING ARRANGED TO TRANSFORM THE LIGHT PULSES INTO ELECTRICAL SIGNALS, A THYRATRON-GATE ADAPTED TO BE FED WITH SAID ELECTRICAL SIGNALS AND ARRANGED TO SPLIT THEM INTO A PICTURE PULSE TRAIN AND A REFERENCE PULSE TRAIN, A COMPARATOR, A FIRST CHANNEL FOR SAID PICTURE PULSE TRAIN, A SECOND CHANNEL FOR SAID REFERENCE PULSE TRAIN, EACH SUCH CHANNEL LEADING FROM SAID THYRATRON-GATE TO THE FIRST AND SECOND INPUT OF SAID COMPARATOR RESPECTIVELY, SAID FIRST CHANNEL INCLUDING AMPLIFIER MEANS IN SERIES, ADAPTED TO MULTIPLICATIVELY INFLUENCE THE AMPLITUDES OF SAID PICTURE PULSE TRAIN IN SUCH CHANNEL, MEANS FOR ADJUSTING THE GAIN FACTOR OF SAID AMPLIFIER MEANS TO REQUIRED CORRECTION VALUES, AN INTEGRATOR IN SAID FIRST CHANNEL DISPOSED ADJACENT TO THE FIRST INPUT OF SAID COMPARATOR, AND A SWITCHING-OFF DEVICE OPERATIVELY CONNECTED TO THE OUTPUT OF SAID COMPARATOR FOR SWITCHING OFF SAID FIRST LIGHT SOURCE WHEN THE SIGNAL AMPLITUDE OF THE FIRST INPUT OF SAID COMPARATOR REACHES THE SIGNAL AMPLITUDE OF THE SECOND INPUT OF SAID COMPARATOR.