We introduce photon measurement density functions (PMDF) as a generalization of photon sampling volumes in near-IR transillumination of scattering tissue. For a given source-detector pair, the PMDF identifies the regions within the tissue contributing to the measurement signal. The knowledge of these regions of sensitivity is important for both spectroscopic applications where the penetration depth of the probing light must be known, and for imaging applications where the PMDF can be used to select the measurement types best suited for the reconstruction of a given tissue parameter. We have developed analytical models which allows to calculate PMDFs for certain geometries and a variety of measurement types in 3D, and a finite element model (FEM) which can be applied to arbitrarily complex and inhomogeneous 2D cases. As an example, we present calculations performed on a FEM mesh generated from an MRI image of a human head.