Abstract: A method and apparatus for monitoring the diffuse reflectivity of a portion of a surface. The method is particularly applicable to soil detection of banknotes. The method comprises exposing the surface portion (3) to a beam of radiation from a source (2). At least one part of the diffuse radiation reflected by the surface portion (3) is sensed by a sensor (6) located closely behind an aperture (7) such that the aperture (7) defines, in effect, a sensing position situated such that the intensity of the diffusely reflected radiation sensed is substantially invariant within a working range of distances (D) between the aperture (7) and the surface portion (3). A value representative of the diffuse reflectivity of the surface portion (3) is then determined from the sensed intensities.