1. A system for servicing photon map queries, comprising: a memory storing an acceleration structure including nodes that respectively define surfaces that each spatially bound a respective selection of a plurality of photons spatially located in a 3-D scene, the selections of varying relative granularity, and the nodes arranged in a graph with edges connecting pairs of nodes; a processor configured for executing an interface for accepting photon queries from one or more code modules or shaders, each of the photon queries defining a respective locus, a spatially located volume in the 3-D scene that includes the respective locus, and a maximum number (k) of photons to be returned responsive to the photon query, wherein k>=1; and an acceleration structure traversal resource configured for processing the accepted photon queries by traversing the acceleration structure with collections of photon queries from the accepted photon queries, wherein each collection is associated with one or more nodes of the acceleration structure, the traversal performed, collection-by-collection, for the photon queries by testing each of a plurality of child nodes of the node(s) associated with a selected collection of photon queries for overlap with each of the spatially located volumes of the photon queries of that collection, and updating a status of collections maintained in a memory by referencing each photon query in a respective collection associated with each child node found to overlap with the spatially located volume of that photon query, wherein the acceleration structure traversal resource is operable to identify, as an output of traversal, for each of the photon queries, identifiers for up to k photons that are closest to the locus in the 3-D scene defined by that photon query and within the spatially located volume defined by that photon query.