Test of externally heated models of different optical depth, as indicated by the A_V values on the x axis. The noise of the temperature estimates is shown for the centre (red symbols, values scaled by 0.1), half radius (black symbols), and outer surface (blue symbols) of the model clouds. For each A_V, results are shown (each group of connected symbols, from left to right) for the default method, the use of fixed k_A = 2 and density-dependent k_A(ρ) parameters, and for the FFI method.