![Fig. 4.4 — Angular spectrum representation](https://i0.wp.com/opticaltweezers.org/wp-content/uploads/2015/11/Fig4_4.png?fit=660%2C441)
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).
Fig. 4.4 — Angular spectrum representation