It is assumed that absorbed 'high energy photons' are converted to 'low energy photons' without any displacement in space, i.e. a possible re-emission takes place at the same position as the absorption. Furthermore, a complete isotropic emission with random polarization is assumed.
The amount of created low energy photons is determined by the so-called 'internal efficiency' which gives the probability for emission at a certain wavelength. Internal efficiencies are defined by the superposition of oscillator terms, user-defined formulas or imported data sets.
Here is an example. In the range 500 to 1000 nm (with 101 spectral points, i.e. 5 nm resolution) the formula EXP(-SQR((X-800)/100)) defines the following broad peak (note that x refers to the wavelength, in this case):
This means that most of the emitted photons have a wavelength around 800 nm. Almost no radiation at 500 nm is produced internally.