Optical cryptographic devices

Claim 1

A OPTICAL CRYPTOGRAPHIC DEVICE COMPRISING MEANS DEFINING A LIMITED FIELD IN A SELECTED OBJECT PLANE, AND A SCREEN COMPRISING RELATIVELY SMALL OPTICAL IMAGE FORMING ELEMENTS WHICH ARE SMALL RELATIVE TO SAID FIELD AND SPACED FROM SAID FIELD AT AN OBJECT DISTANCE, ELEMENTS OF SAID SCREEN FORMING AERIAL IMAGES OF RELATIVELY SMALL PORTIONS OF SAID FIELD AT AN IMAGE DISTANCE WHICH IS CONJUGATE TO SAID OBJECT DISTANCE, THE REFRACTIVE SURFACES OF SAID ELEMENTS DEFINING DIFFERENT RESPECTIVE POWERS AND THE OPTICAL AXES OF SAID ELEMEMTS INTERSECTING THE LOCUS OF THE AERIAL IMAGE AT A PLURALITY OF LATERAL LOCATIONS, DIFFERENT ONES OF SAID ELEMENTS DEFINING DIFFERENT RESPECTIVE PERIPHERAL SHAPED, AND SAID SCREEN BEING SO CONSTRUCTED AND ARRANGED THAT THE AERIAL IMAGES FORMED BY SAID SCREEN ARE DIFFERENTLY ORIENTED RELATIVE TO EACH OTHER FROM THE RELATIVE ORIENTATIONS OF THE CORRESPONDING RESPECTIVE PORTIONS OF SAID FIELD.