Abstract: This paper presents a memory-efficient auxiliary data structure for ray tracing called a lightweight bounding volume hierarchy, or LBVH. The new data structure reduces memory requirements in three ways: using implicit indexing, limited precision numbers, and a high branching factor. We show that LBVHs can be nearly as effective as standard bounding volumes in terms of speed while using significantly less memory. C++ source code for a ray tracer that implements LBVHs is provided online.