Spatial distribution of the spectral slope of the polarisation spectrum in OMC-1. The reddish region shows a rising polarisation spectrum, whereas the blueish region shows a spectrum with a falling slope. This map is overlayed with dust temperature (solid black lines with T_d ≥ 60 K with a step of 5 K) and gas density (dashed black line with N(H₂) = 1, 2, 3, 6, 10, 20 × 10²² cm⁻² or A_V ≥ 12.4 magnitude). The blue dots indicate the 12 positions where the spectra are used to compare with numerical models.