  We present a comprehensive study of the temperature and magnetic-field
dependent photoluminescence (PL) of individual NV centers in diamond, spanning
the temperature-range from cryogenic to ambient conditions. We directly observe
the emergence of the NV's room-temperature effective excited state structure
and provide a clear explanation for a previously poorly understood broad
quenching of NV PL at intermediate temperatures around 50 K. We develop a model
that quantitatively explains all of our findings, including the strong impact
that strain has on the temperaturedependence of the NV's PL. These results
complete our understanding of orbital averaging in the NV excited state and
have significant implications for the fundamental understanding of the NV
center and its applications in quantum sensing.
