Relative response functions of model SU0. Optical depth, τ, scales at the peak wavelength of the Mg I 12.32 μm line intensity, and τ₅₀₀ is the continuum optical depth at λ = 500 nm. Note that the scale of τ₅₀₀ is uneven. Panel (a): Relative response function of normalized intensity to temperature, ${\stackrel{\sim }{R}}_{I_{\mathrm{norm}},T}$. Panel (b): Relative response function of intensity to temperature, ${\stackrel{\sim }{R}}_{I,T}$ (10⁻¹¹ W m⁻² Hz⁻¹ sr⁻¹). Panel (c): Relative response function of normalized intensity to density, ${\stackrel{\sim }{R}}_{I_{\mathrm{norm}},\rho }$. Panel (d): Relative response function of intensity to density, ${\stackrel{\sim }{R}}_{I,\rho }$ (10⁻¹¹ W m⁻² Hz⁻¹ sr⁻¹). The relevant definitions are given in Eq. (12).