Photoelectric exposure meters with light ray limiters

Claim 1

IN A PHOTOELECTRIC EXPOSURE METER, A PHOTOCELL, A LIGHT RAY LIMITER, AND MEANS INTERCONNECTING SAID PHOTOCELL AND LIGHT RAY LIMITER SO THAT LIGHT RAYS PASSING THROUGH SAID LIGHT RAY LIMITER IMPINGE UPON SAID PHOTOCELL, SAID LIGHT RAY LIMITER COMPRISING A TRANSPARENT PLATE HAVING A SERIES OF HORIZONTALLY EXTENDING PARALLEL PRISM SURFACES ON ONE SIDE THEREOF FORMING A PRISMATIC SCREEN FACING THE PHOTOCELL WITH THE STEPS OF SAID HORIZONTALLY EXTENDING PARALLEL PRISM SURFACES BEING INCLINED TO THE NORMAL OF SAID PHOTOCELL AND IN A DIRECTION AWAY FROM THE DIRECTION OF THE REQUIRED PENETRABILITY CHARACTERISTIC WHEREBY SAID LIGHT RAY LIMITER WILL RESTRICT A LIGHT RAY BEAM WHICH HAS AN INCIDENT ANGLE THE UPPER BOUNDARY OF WHICH IS DISPOSED WITHIN THE UPPER PORTION OF THE SURROUNDING FIELD OF AN OBJECT SECTION TO BE PHOTOGRAPHED, AND A TELECENTRICAL OPTICAL SYSTEM LOCATED IN FRONT OF SAID PHOTOCELL, A POSITIVE MEMBER IN SAID OPTICAL SYSTEM, AND A NEGATIVE MEMBER IN SAID OPTICAL SYSTEM, SAID NEGATIVE MEMBER HAVING A FIRST SURFACE WHICH IS CONCAVE AND NONSPHERICAL AND HAVING A SECOND SURFACE ON WHICH ARE FORMED SAID PARALLEL PRISMATIC SURFACES OF SAID PRISMATIC SCREEN.