H − K colours of the background stars as a function τ_250  μm from Herschel assuming a dust emissivity index of β = 1.8. A linear fit to the data points is shown with a green line. The red curve shows the relationship implied by the dependence of on N_H found in Orion A by Roy et al. (2013). The mean error of the data points is indicated with a cross in the bottom right.