An apparatus and method are provided for checking the authenticity of the security element of a value document. For checking the authenticity of the security element, reversed bright/dark modulation is examined. For this purpose, for determining a combination course, the remission course and the transmission course along the longitudinal direction of the security element are set off against each other pixel by pixel and the combination course obtained by setting off pixel by pixel is checked with regard to an intensity modulation along the longitudinal direction of the security element.