  Nitrogen-vacancy centers in diamond allow for coherent spin state
manipulation at room temperature, which could bring dramatic advances to
nanoscale sensing and quantum information technology. We introduce a novel
method for the optical measurement of the spin contrast in dense
nitrogen-vacancy (NV) ensembles. This method brings a new insight into the
interplay between the spin contrast and fluorescence lifetime. We show that for
improving the spin readout sensitivity in NV ensembles, one should aim at
modifying the far field radiation pattern rather than enhancing the emission
rate.
