[[Photon]] mapping was created as a two-pass global illumination algorithm that is more efficient than raytracingray tracing.<ref name=":19">Wann Jensen, Henrik (1996). "[http://graphics.ucsd.edu/~henrik/papers/photon_map/global_illumination_using_photon_maps_egwr96.pdf Global Illumination using Photon Maps] {{Webarchive|url=https://web.archive.org/web/20080808140048/http://graphics.ucsd.edu/~henrik/papers/photon_map/global_illumination_using_photon_maps_egwr96.pdf |date=2008-08-08 }}" (PDF). ''Rendering Techniques ’96'': 21–30.</ref> It is the basic principle of tracking photons released from a light source through a series of stages.<ref name=":19" /> The first pass includes the photons being released from a light source and bouncing off their first object; this map of where are the photons are located is then recorded.<ref name=":18" /> The photon map contains both the position and direction of each photon which either bounce or are absorbed.<ref name=":19" /> The second pass happens with [[Rendering (computer graphics)|rendering]] where the reflections are calculated for different surfaces.<ref name=":20">{{Cite web|url=https://web.cs.wpi.edu/~emmanuel/courses/cs563/write_ups/zackw/photon_mapping/PhotonMapping.html|title=Photon Mapping - Zack Waters|website=web.cs.wpi.edu|access-date=2019-11-08}}</ref> In this process, the photon map is decoupled from the geometry of the scene, meaning rendering can be calculated separately.<ref name=":18" /> It is a useful technique because it can simulate caustics, and pre-processing steps do not need to be repeated if the view or objects change.<ref name=":20" />
