  We present a technique to nanomechanically vary the distance between a
fluorescent source and a mirror, thereby varying the local density of optical
states at the source position. Our method can therefore serve to measure the
quantum efficiency of fluorophores. Application of our technique to NV defects
in diamond nanocrystals shows that their quantum yield can significantly differ
from unity. Relying on a lateral scanning mechanism with shear-force
probe-sample distance control our technique is straightforwardly implemented in
most state-of-the-art near-field microscopes.
