An apparatus and a method for optically scanning a surface of an object under an adverse external condition to which optical components are subjected, such as cameras and lighting sources sensitive to soiling, make use of a first imaging sensor unit having its optical components subjected to the adverse external condition, and being configured for generating reflection intensity image data associated with a target surface. A second imaging sensor unit having its optical components substantially not subjected to the adverse external condition, is configured for generating reflection intensity image data associated with a reference surface. Correction data are calculated from a comparison between the reflected intensity image data associated with the target and reference surfaces, which correction data are then applied to the reflected intensity image data associated with the target surface to generate corrected intensity image data compensating for the adverse external condition.