Lumen uses Software Ray Tracing against Signed Distance Fields by default. This tracing representation is supported on any hardware supporting Shader Model 5 (SM5), and only requires Generate Mesh Distance Fields to be enabled in the project settings.
The renderer merges Mesh Distance Fields into a Global Distance Field to accelerate tracing. By default, Lumen traces against each mesh's Distance Fields for the first two meters for accuracy, and the merged Global Distance Field for the rest of each ray. Projects with extreme overlapping instances can control the method Lumen uses with Software Ray Tracing Mode in the project settings.