Modelled effect of scattering on the MIR optical-depth estimates towards filament centre. Frame a shows the model optical depth, which corresponds to a peak column density of N(H₂) = 10²⁴ cm⁻² with the CMM dust model. The other frames show the relative error in τ(8μm) estimates. Frame b includes only an isotropic radiation field with χ = 10. Frames c–e show the errors, when a point source, with luminosity 50 times the default value, is included at Δy = 2.09 pc and 0.93 pc behind the filament (frame c), in front of the filament (frame d), or to one side.