Colour excess ratios in H, I₁, and I₂ for the sub-regions of Orion A as defined in Fig. 8. From left to right, overall the galactic longitude of the regions decreases. The dashed horizontal lines represent the averages across the entire cloud. The slopes for L1641/ L1647 are systematically larger than the average in H (smaller for I₁ and I₂), while for regions near the ONC, we observe the opposite behaviour.