Sensor for verification of genuineness of security paper

A method for approving a document, such as a banknote (1) with a watermark (2a, 2b), the pattern of said watermark consisting of two characteristically shaped neighbouring areas (2a, 2b) with a local area density (mass per unit area) which is markedly both higher and lower than the principal average area density of said banknote (1) in the watermark region, whereby said watermark or at least a characteristic section thereof is brought to a position corresponding with a two-part capacitive sensor device (4, 6, 7), which sensor device consists of a common, flat metal plate (7) as one capacitor side and the other capacitor side is divided into two metal plates (4, 6) situated both in a common plane and being electrically separated, with insignificant separation distance (5) compared to the other areawise dimensions of said two plates (4, 6), and the change in capacitance caused by the watermark is observed and compared with a change caused by a correct watermark, characterized in that the watermark or said characteristic section thereof is brought in position with a doubly active capacitive sensor device (4, 6, 7) in which the two plates (4, 6) are situated in a common fixed plane and are adapted in shape to each one of said two characteristically shaped neighbouring areas (2a, 2b) or said characteristic sections thereof, that a preset symmetry property of the double output signal from said sensor device is disturbed in a predetermined manner when a correct watermark coincides with the two sensor plates (4, 6), and that the symmetry property is continuously monitored by signal processing equipment connected to said sensor device.