A document (e.g. banknote) authenticating apparatus comprises a transport system (32–34, 50, 51) for transporting banknotes. An inspection device (35) is provided past which banknotes are transported by the transport system, the inspection device including a UV source (1) such as a LED and a UV detector (4, 5) arranged to irradiate a banknote and to detect reflected UV respectively. A processor (11) is responsible to the reflected UV to determine the authenticity of the banknote. The inspection device (35) includes a reference surface (3) over which the banknotes are transported in use, the reference surface being exposed to UV radiation from the source (1) in the absence of a banknote so as to generate a reference level signal, and being oriented such that the banknotes are delivered at an acute angle to the surface whereby passage of a banknote across the surface effects a cleaning action on the surface.