It is also closer in principle to the more sophisticated approaches used when dealing with real bulk crystalline solids, where the first step is either to integrate contributions to the ECD over constant energy surfaces in a wave-vector space (k -space), or to integrate contributions over the relevant surface Brillouin zone. The Forbes approach is equivalent either to integrating over a spherical surface in k -space, using the variable K p to define a ring-like integration element that has cylindrical symmetry about an axis in a direction normal to the emitting surface, or to integrating over an (extended) surface Brillouin zone using circular-ring elements.