Thicknesses

Thicknesses are the relative distance to the next surface vertex in lens units. Thicknesses are not cumulative, each one is only the offset from the previous vertex along the local z axis. The orientation of the local z axis can change using coordinate breaks (see Coordinate Break) or surface tilts and decenters (see the Tilt/Decenter section of the Surface Properties).

Thicknesses corresponding to real propagation (see "Real propagation") always change sign after a mirror. After an even number of mirrors (including zero mirrors), thickness are positive for real propagations and negative for virtual propagations (see "Virtual propagation"). After an odd number of mirrors, thicknesses are negative for real propagations and positive for virtual propagations. This sign convention is independent of the number of mirrors, or the presence of coordinate breaks. This fundamental convention cannot be circumvented through the use of coordinate rotations of 180 degrees.