Note that the spread of the reflected beam in the direction perpendicular to the plane made by the incident beam and beam that a flat surface would reflect is a few orders of magnitude smaller than the spread in this plane, along the exit angle direction. As a result, the complex 2D description can be safely reduced to 1D, and only one direction of the 2D detector is considered: that which the specular reflection would give from a flat surface. The general case of a 3D curved surface can thus be reduced to that of a cylinder with an incident beam perpendicular to its axis.