Albada type viewfinder having undulating reflecting mask frame

Claim 1

IN AN ALBADA TYPE VIEWFINDER INCLUDING A FRONT TRANSPARENT OPTICAL ELEMENT HAVING AN EYE FACING CONCAVE SURFACE CARRYING A PARTIALLY-LIGHT-PERMEABLE MIRROR, AND A REAR TRANSPARENT FRAME CARRIER OPTICAL ELEMENT SPACED AXIALLY FROM THE FRONT ELEMENT AND HAVING AN OBJECT FACING SURFACE CARRYING AN IMAGE LIMITING FRAME COMPRISING A BORDER STRIP BOUNDING AN OPEN IMAGE VIEWING AREA, THE FRAME BEING DISPOSED IN SUBSTANTIALLY THE FOCAL PLANE OF THE MIRROR FOR IMAGING THEREBY AT SUBSTANTIALLY INFINITY: THE IMPROVEMENT COMPRISING, THOSE PORTIONS OF THE FRAME CARRIER SURFACE OF SAID REAR OPTICAL ELEMENT UNDERLINING THE FRAME BEING UNDULATED AND SPECULAR, AND THE FRAME HAVING AN UNDULATING AND REFLECTING SURFACE CONFORMING TO THE FRAME CARRIER SURFACE; THE VIEWING AREA BOUNDED BY THE FRAME BEING CLEAR AND TRANSPARENT WHEREBY DIFFUSION OF LIGHT COMING FROM THE FIELD OF VIEW OF THE VIEWFINDER IS AVOIDED, AND THAT PORTION OF SUCH LIGHT IMPINGING UPON THE FRAME IS REFLECTED BY A PLURALITY OF UNDULATED SURFACES AS A CONCENTRATED REFLECTION SO THAT THE MAJOR PART OF THE LIGHT IMPINGING UPON THE FRAME IS REFLECTED TO SAID MIRROR FOR RE-REFLECTION TO THE EYE OF THE OBSERVER; THE LENGTH OF THE WAVES OF THE UNDULATED FRAME CARRIER SURFACE BEING FROM 0.10 TO 0.50 THE WIDTH OF THE FRAME BORDER STRIP AND WITHIN THE RANGE OF FROM 5 TO 50 TIMES THE WAVE LENGTH OF LIGHT; THE HEIGHT OF THE WAVES OF THE UNDULATED FRAME CARRIER SURFACE BEING FROM 0.10 TO 0.25 THE LENGTH OF THE WAVES OF THE UNDULATED FRAME CARRIER SURFACE.