Direct transmission and backscatter differ in how the radiation interacts with the material and where the detector sits, so the readings reflect different parts of the sample and require separate calibration. In direct transmission, radiation travels through the entire thickness of the material and is detected on the far side. The measured intensity depends on the material’s overall attenuation, which relates to its density and moisture along the transmission path. Because the signal samples the full thickness, the calibration links transmitted strength to density and moisture that affect that path length. In backscatter, the detector is near the source and most of the signal comes from radiation that is scattered back toward the detector. This makes the reading mainly sensitive to the near-surface region and to surface moisture and density, with the scattering geometry and interaction probabilities giving a different relationship between signal and the material properties than in transmission. Because the contributing physics and the sampled volume are different, backscatter requires its own calibration. Since they use different geometries and interaction mechanisms, they are not identical, and the appropriate calibration must be chosen for each mode.