CPC B42D 25/324 (2014.10) [B42D 25/21 (2014.10); B42D 25/29 (2014.10); G07D 7/12 (2013.01); G07D 2207/00 (2013.01)] | 9 Claims |
1. An optical security element made of a refractive transparent or partially transparent optical material and comprising an optical assembly of a caustic layer having a light-redirecting surface with a relief pattern of given depth and a focal length fc and an adjacent lens element of focal length fL configured to redirect incident light received from a point-like light source through it and form a projected image containing a caustic pattern directly on a retina of an observer looking at the point-like source through the optical security element; wherein
the optical security element comprises one of the following:
a) the caustic layer has a positive focal length (fC>0) and the lens element has a negative focal length (fL<0), or
b) the caustic layer has a negative focal length (fC<0) and the lens element has a positive focal length (fL>0); and
wherein a relationship between the focal length of the lens element and the focal length of the caustic layer satisfies following equation:
![]() where:
R is distance between the caustic layer and an eye of the observer;
ds is a distance between the point-like light source and the optical security element; and
dR is a comfortable reading distance from the eye, which is at least 25 cm.
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