Figure 4.4 — Angular spectrum representation Fig. 4.4 — Angular spectrum representation. Optical fields are evaluated in planes perpendicular to an arbitrarily chosen axis, in this case the z-axis. The optical fields in the plane z = 0 (top left) result from the superposition of various plane waves (top right). The optical fields in another plane with z constant (bottom left) can be evaluated by summing up the propagated plane waves (bottom right).