Projected surface density of bona-fide background stars (left panel) and of bona-fide cluster members (right panel), as selected in Sect. 3. The contour labels give the density of stars in arcmin^-2. In the direction of the Trapezium cluster (the star symbol indicates the position of Θ¹Ori-C, assumed to be the cluster center) we find the minimum density of background stars, while it increases towards the outskirts of the cloud. This is indicative of the decreasing optical thickness of the OMC-1 with increasing projected distance from the cluster center. Conversely, the surface density of cluster members steeply increases with decreasing distance from Θ¹Ori-C.