Security document with positive and negative authentication tilt images
Disclosed is a security document including an image layer with in a first image area an identification image. The document has in a second image area an authentication image that is composed of a first authentication image that is a positive version of the first image and a second authentication image that is a negative version of the first image. The authentication images can be observed at different angles and form a tilt image. Mutual comparison of the positive and negative tilt images and comparison with the identification image provides improved detection of unauthorized alterations.
1. A method of providing a security document ( 1 ), comprising steps of:
providing an image layer ( 2 ) with an array of cylindrical lenses ( 10 );
providing an image processing unit ( 18 ) with an identification image having image elements with predetermined brightness values (Bp) on a positive brightness scale;
providing the image processing unit ( 18 ) with a positive authentication image ( 7 ) corresponding to the identification image ( 4 ) with picture elements having brightness values that correspond to brightness values of the identification image ( 4 ), and a negative authentication image ( 8 ) corresponding to the identification image and with picture elements having brightness values (Bn) that correspond to a reversed brightness scale in which brightest one of the picture elements in the positive brightness scale appear darkest and darkest one of the picture elements appear brightest; and
laser engraving the positive authentication image ( 7 ) and the negative authentication image ( 8 ) through the cylindrical lenses ( 10 ) in the image layer ( 2 ) at first and second angles (α 1 , α 2 ), respectively, such that both the identification image ( 4 ) and the first authentication image ( 7 ) are observable at a perpendicular viewing direction, and the second authentication image ( 8 ) is observable at a non-perpendicular viewing direction.
2. The method according to claim 1 , wherein in the image processor unit ( 18 ), the first and second authentication images ( 7 , 8 ) are transformed such that they are viewable in similar positions by a user observing the security document at different angles.
3. The method according to claim 1 , wherein the first and second authentication images ( 7 , 8 ) are laser engraved in the image layer ( 2 ) from mutually different angles (α 1 , α 2 ) relative to a perpendicular viewing direction ( 17 ).
4. The method according to claim 3 , wherein in the processing unit ( 18 ), a transformation is carried out on one of the first and second authentication images ( 7 , 8 ) to correct a width of the one of the first and second authentication images so that respective widths of the first and second authentication images ( 7 , 8 ) are equal.
5. The method according to claim 2 , wherein the first and second authentication images ( 7 , 8 ) are laser engraved in the image layer ( 2 ) from mutually different angles (α 1 , α 2 ) relative to a perpendicular viewing direction ( 17 ).