An apparatus for optically characterizing a surface structure of a board or sheet, includes a light source arranged at a first side of a said board and adapted to illuminate a first area of a surface of said board or sheet by emitting towards said surface a collimated light beam at a first, oblique angle of incidence relative to said surface of said board or sheet. A light receiver arranged at a second side of a said board or sheet, said second side being substantially opposite to said first side in respect of said board or sheet, is adapted to receive at least a part of said collimated light beam being reflected off said first area of said surface of said board or sheet, and includes a spatial detector adapted to receive by different parts of said detector light of said collimated light beam being reflected off respective, different parts of said first area.