12 ns [14]. It is known, however, that the spontaneous emission rate is slowed down, and thus τ becomes larger in subwavelength dielectric media [15]. Furthermore, as the size of the dielectric medium is decreased, the dependence of the spontaneous emission rate on the shape of the medium as well as the emitter orientation and position becomes less significant [16]. Figure 3a displays a typical example of measurements on a single DNC. This measurement is fitted using a background and a double exponential with 1 /e times of 4 . 2 ± 0 . 3 ns and 22 . 8 ± 0 . 3 ns, respectively. We attribute the deviation from a simple exponential decay to the inhomogeneity of emitting transitions in a single DNC. In particular, the shorter lifetime component is most likely due to quenching effects for some NV centers. A plausible cause of this might be the presence of a thin graphite layer on the DNCs [11]. Another source of inhomogeneity can stem from the coexistence of a transition at 575 nm, which is attributed to the neutral charge state of NV centers, and the usual transition of the negatively-charged center at 637 nm [17]. Indeed, we confirmed the presence of both lines by measuring the emission spectra of individual DNCs. To our knowledge, no data are available on the fluorescence lifetime of the neutral color centers. A third phenomenon that can cause different fluorescence lifetimes in a DNC is the orientation of the transition dipole moments of the individual NV centers with respect to the glass substrate [18]. To examine this effect, we have performed rigorous finite-difference time-domain (FDTD) calculations to compute 4 FIG. 2: Fluorescence (a,c) and iSCAT (b,d) images of the same diamond nanoparticles on a glass cover slide in air (a,b) and covered with scattering particles (c,d). a c b d ScatteringGLYPH<.notdef>(arb.GLYPH<.notdef>units) FluorescenceGLYPH<.notdef>(counts/s)