Evolution of the IR flare T_b spectrum for the model with δ = 5 and E_c = 30 keV. For λ> 50 μm, the brightness temperature tends to reflect the actual electron temperature in the optically-thick upper chromosphere. The inset planel shows the flare excess emission, after subtracting the pre-flare spectrum. This highlights the positive slope of the short-wavelength branch, T_b(ν) ∝ ν², that can be identified with the Rayleigh-Jeans law expected from an optically-thin slab.