Focal plane shutters

Claim 1

IN AN AERIAL CAMERA FOR AUTOMATICALLY TAKING A SERIES OF OVERLAPPING PICTURES FROM A FLYING AIRPLANE IN RAPID SEQUENCE ON A DRIVEN ROLL FILM, IN COMBINATION, AN OBJECTIVE, MEANS DEFINING AN EXPOSURE GATE IN THE FOCAL PLANE OF SAID OBJECTIVE, A FILM DRIVE MECHANISM TO CONTINUOUSLY ADVANCE THE FILM IN SAID FOCAL PLANE, IN OPERATION, IN THE DIRECTION THE IMAGE TRAVELS AND ACROSS SAID EXPOSURE GATE AT A CONSTANT RATE DURING AND BETWEEN SUCCESSIVE EXPOSURES; MEANS FOR ADJUSTING THE SPEED OF MOVEMENT OF THE ROLL FILM ACROSS SAID EXPOSURE GATE SO THAT IT EXCEEDS THE SPEED OF MOVEMENT OF THE IMAGE DUE TO THE RELATIVE SPEED OF THE FLYING AIRPLANE AND THE TERRAIN BEING PHOTOGRAPHED BY A PERCENTAGE AT LEAST EQUAL TO THE PERCENTAGE OF OVERLAP OF SAID PHOTOGRAPHS THEREBY CONTINUOUSLY OVERCOMPENSATING FOR IMAGE MOTION, AND A FOCAL PLANE SHUTTER FOR PERIODICALLY EXPOSING THE FILM IN SAID FOCAL PLANE AT INTERVALS AT LEAST EQUAL TO THE TIME REQUIRED FOR THE FILM TO TRAVERSE SAID EXPOSURE GATE AT SAID CONSTANT SPEED, SAID FOCAL PLANE SHUTTER COMPRISING A PAIR OF COAXIAL, AXIALLY SPACED, ADJACENT, LIGHT-IMPERVIOUS, CIRCULAR DISCS ROTATABLY DRIVEN IN PARALLEL PLANES ABOUT A COMMON AXIS EXTENDING PERPENDICULAR TO SAID FOCAL PLANE, SAID DISCS BEING DISPOSED A SHORT DISTANCE IN FRONT OF SAID FOCAL PLANE AND EACH HAVING A PERIPHERAL APERTURE EXTENDING RADIALLY INWARDLY TO EFFECTIVELY EXPOSE THE FULL HEIGHT OF SAID EXPOSURE GATE WHEN DRIVEN PAST SAID GATE, THE APERTURE IN ONE OF SAID DISCS DIVERGING LINEARLY TOWARD THE PERIPHERY OF SAID ONE DISC AND BEING SUBSTANTIALLY NARROWER THAN THE APERTURE IN THE OTHER DISC, MEANS FOR ROTATABLY DRIVING THE TWO DISCS IN A COMMON DIRECTION AT DIFFERENT SPEEDS HAVING A FIXED RATIO TO EACH OTHER TO PERIODICALLY MOVE SAID APERTURES CONCURRENTLY PAST SAID EXPOSURE GATE TO CAUSE PERIODIC EXPOSURES OF SAID FILM SEPARATED BY TIME INTERVALS DURING WHICH EACH OF THE DISCS MAKES MORE THAN ONE REVOLUTION AND AN EFFECTIVE EXPOSURE TIME HAVING A DURATION FOR EACH POINT IN SAID EXPOSURE GATE EFFECTIVE TO LIMIT THE UNSHARPNESS CAUSED BY THE OVERCOMPENSATION OF SAID IMAGE MOTION.