We report on a systematic study of upconverting fluorescence signal generation within turbid
phantoms and real tissues. An accurate three-point Green's function, describing the forward model
of photon propagation, is established and experimentally validated. We further demonstrate, for the
first time to our knowledge, autofluorescence-free transillumination imaging of mice that have
received biocompatible upconverting nanoparticles. The method holds great promise for artifact-free
whole-body visualization of optical molecular probes.