The way in which 3-D information is produced in a line-scan based stereoscopic X-ray imaging system is examined. The concept of a depth plane is derived from the volume element (voxel) structure created in the image production process. Thus a stereoscopic X-ray image can be considered to be comprised of a series of parallel depth planes. The results presented here quantify the incremental distribution in range between these planes together with the implied lateral resolution present in each plane. This research is part of an ongoing programme [1] to quantify 3-D data in security X-ray screening applications.