A METHOD OF DESIGNING A LIGHT-REDIRECTING SURFACE OF A CAUSTIC LAYER, AN OPTICAL SECURITY ELEMENT COMPRISING THE DESIGNED LIGHT-REDIRECTING SURFACE OF THE CAUSTIC LAYER, A MARKED OBJECT, USE AND METHOD OF AUTHENTICATING THE OBJECT

The invention relates to a method of designing a refractive transparent or partially transparent light-redirecting surface, or a reflective light-redirecting surface, of a caustic layer comprising providing a discrete representation of an input target image, calculating a generalized power diagram for a set of image pixels pi of the target image and computing a piecewise light-redirecting surface of the caustic layer based on a calculated optimal set of weights minimizing a cost function associated with the set of image pixels pi. The invention also relates to an optical security element, a marked object, a method of visually authenticating an object and use of the optical security element for authenticating or securing against counterfeiting.