A statistical equilibrium radiative transfer code, RADEX, is made available for public use as part of the Leiden Atomic and Molecular Database (LAMDA). RADEX is a one-dimensional non-LTE radiative transfer code, that uses the escape probability formulation assuming an isothermal and homogeneous medium without large-scale velocity fields. RADEX is comparable to the LVG method and provides a useful tool in rapidly analyzing a large set of observational data providing constraints on physical conditions, such as density and kinetic temperature.

RADEX provides an alternative to the widely used rotation diagram method which relies upon the availability of many optically thin emission lines and is useful only in roughly constraining the excitation temperature in addition to the column density. RADEX makes no extrapolation of collisional rate coefficients outside the temperature range specified in the molecular data file used as input. Instead they are kept constant.

RADEX is offered both as a on-line version for quick estimations of physical conditions, and as source code for extensive grids of calculations.

The formalism adopted in RADEX is summarized in Van der Tak, F.F.S., Black, J.H., Schöier, F.L., Jansen, D.J., van Dishoeck, E.F. 2007, A&A 468, 627 (arXiv:0704.0155v1).