A detector device includes a detector module and a lens module. The detector module includes an optical emitter and a photon detector spaced apart from the optical emitter. The lens body includes a first light-transmissive surface, a second light-transmissive surface, and a third light-transmissive surface. The optical emitter is operable to emit a light beam passing through the first light-transmissive surface and the second light-transmissive surface toward a surface so as to define a field of emission on the surface. The photon detector is aligned with the first and third light-transmissive surfaces so as to define a field of view on the surface. An extent of overlap between the field of view and the field of emission varies in accordance with a distance between the optical emitter and the surface.