This keyword is used when the ionisation source is from a plane and not from a point source. real1 must contain the value of the incident ionizing flux above υ = real2 [Ryd] (constant at each point on the plane) in units of [phot/s/cm^{2}]. If real2 = 0. then the real1 is assumed to be the bolometric flux.

When this keyword is specified the density distribution must be defined via the densityFile option. The ionizing plane is the x-z plane. the energy packets are reflected when they hit the y-z and the y-x planes and can escape from the x-z planes. (This can be easily changed to suit. please contact me if your work requires it to be different)