The invention relates to the marking of a material (18) with identifier
marks (16), the material thus marked, and verification of the genuineness
of a product on the basis of the marks. In accordance with the invention,
an optical brightener is incorporated in the material (18), and marking
is performed by reducing the brightness of the material at a selected
location by directing local heating to this location, the mark (16) thus
produced appearing with a darker shade than its environment in
ultraviolet light (20). The marking is based on partial or complete
destruction of the brightening effect of the optical brightener under
heating. The invention is suitable for providing e.g. coated paper and
board containing an optical brightener with identifier marks for
preventing falsifications.