Photoelectric inspection and sorting machines

Claim 1

IN APPARATUS FOR DETECTING DEFECTS IN MOVING SHEET MATERIALS OF SUBSTANTIALLY UNIFORM REFLECTION CHARACTERISTICS WHICH SENSES BY PHOTOELECTRIC MEANS VARIATIONS IN THE INTENSITY OF LIGHT REFLECTED FROM THE SURFACE OF SUCH A MATERIAL WHEN PASSING OVER AN INSPECTION SURFACE, AN INSPECTION HEAD HAVING A LOWER SURFACE PARALLEL, AND CLOSELY ADJACENT, TO THE SURFACE OF SAID MATERIAL WHEN PASSING OVER SAID INSPECTION SURFACE, A PLURALITY OF PHOTOELECTRIC CELLS POSITIONED ABOVE SAID LOWER SURFACE AND ADJACENT THERETO, EACH SUCH CELL VIEWING AN ASSOCIATED SECTION OF SAID MATERIAL SURFACE THROUGH A SLIT-LIKE APERTURE FORMED IN SAID LOWER SURFACE, SAID APERTURE EXTENDING AT RIGHT ANGLES TO THE DIRECTION OF MOTION OF SAID SHEET MATERIAL AND HAVING A DIMENSION IN THE DIRECTION OF MOTION OF SAID SHEET MATERIAL OF THE SAME ORDER OF MAGNITUDE AS THE DIAMETER OF THE SMALLEST FULLY BLACK DEFECT TO BE DETECTED BY SAID APPARATUS, LAMPS ILLUMINATING EACH SAID ASSOCIATED SECTION, SHIELDING MEANS SHIELDING EACH SAID PHOTOELECTRIC CELL FROM SUBSTANTIALLY ALL EXCEPT LIGHT FROM SAID LAMPS REFLECTED FROM ITS ASSOCIATED SECTION THROUGH SAID APERTURE, AT LEAST ONE BALANCED PUSH-PULL TYPE AMPLIFIER HAVING TWO BALANCEABLE INPUT ELECTRODES EACH CONNECTED TO AN EQUAL NUMBER OF PHOTOELECTRIC CELLS, AND TWO OUTPUT ELECTRODES, AND AN UTILIZATION CIRCUIT CONNECTED ACROSS SAID OUTPUT ELECTRODES, WHEREBY IN PHASE AND EQUAL AMPLITUDE SIGNALS APPLIED SIMULTANEOUSLY TO EACH INPUT ELECTRODE PRODUCE NO OUTPUT SIGNAL ACROSS SAID UTILIZATION CIRCUIT.