apart the focusing apparatus of which produces a bell shaped intensity distribution along a line parallel to the line connecting the two sources their intensities add to give a constant value for the same distance from this line the plane 17 defined by each source axis and their line of connection is a plane of maximum illumination intensity the two sources shown in fig4 can be separately adjusted to adjust the optimal illumination in fig4 plane 17 of maximum illumination intensity intersects the conveyor belt along a line 22 which viewed in the transport direction of the belt lies behind the intersection 21 of image plane 10 with conveyor belt 4 the exact and desirable arrangement of these planes 10 17 has already been mentioned and will now be explained with reference to fig5 and 6 fig5 again shows the illumination and imaging situation illuminating means 12 can for example consist of a series of point sources whose light is focused and directed downwardly onto conveyor belt 4 where the plane 17 defined above cuts the surface 9 of an article the illumination intensity is strongest on this surface along a line parallel to the series of point sources the illumination intensity is constant even.