8 9 impinge at a slant on surface 2 that is at an angle of incidence u03b1 1 at measuring point 3 and at an angle of incidence a 2 at measuring point 4 angle u03b1 is measured on the surface relative to a vertical plane axis of incidents l in this case l 1 and l 2 light rays 8 9 at two measuring points 3 4 are reflected in this area on surface 2 and the reflected light rays 10 11 emerge again in a so called angle of reflection u03b2 1 and u03b2 2 respectively individual angle of reflection u03b2 1 or u03b2 2 respectively is also determined relative to vertical l 1 or l 2 respectively on surface 2 incident light rays 8 or 9 respectively verticals l 1 l 2 as well as emergent or reflected rays 10 11 are located in one plane for measuring point 3 they are in plane e 3 and for measuring point 4 they are in plane e 4 depending upon the opacity of surface 2 light rays 8 9 can either be reflected completely or a part is transmitted and absorbed reflective light rays 10 11 are captured by detector device 12 which is allocated to both measuring points 3 and 4 detector device 12 may be in various embodiments it may be an image capturing device 13 image capturing device 13 may for example be in the embodiment of a camera with which reflected light rays 10 or 11 respectively are captured simultaneously for individual measuring points 3 and 4 whereby the relationship between image detection 13 and measuring points 3 and 4 is provided by an.