Authentication of security documents by means of photochromic dyes

The value document substrate has a photochromic security feature and two foil layers, particularly cover layers (3,4), where one of the cover layers is transparent or translucent. The photochromic security feature is enclosed between the both cover layers. The both cover layers are provided to cover two surfaces of a core layer (5) respectively. A photochromic dye is provided as a retinal-based dye, particularly a rhodopsin. The rhodopsin has bacteriorhodopsin or proteorhodopsin. An independent claim is also included for a value document, particularly banknote, which is selected from a group comprises hybrid banknote, a polymer-banknote, a foil patch, a security thread, a label and a transfer element.