Our sub-region layout to study variations in the colour excess ratio. The top panel shows a dust optical depth map, the bottom panel the dust temperature derived by fitting a modified black-body to Herschel and Planck far-infrared data (Lombardi et al. 2014a,b). The analysis for the sub-regions is split into two parts: First, we study variations of the colour excess ratio by splitting the cloud in east-west direction at l = 210° (dashed line). Subsequently, the entire cloud is split into multiple sub-regions, outlined here by the boxes drawn with solid lines. Regions are labelled only in the top panel and the Spitzer survey contours for I₁ and I₂ are also drawn. toggle labels.