  We demonstrate that a simple silver coated ball lens can be used to
accurately measure the entire distribution of radiative transition rates of
quantum dot nanocrystals. This simple and cost-effective implementation of
Drexhage's method that uses nanometer-controlled optical mode density
variations near a mirror, not only allows to extract calibrated
ensemble-averaged rates, but for the first time also to quantify the full
inhomogeneous dispersion of radiative and non radiative decay rates across
thousands of nanocrystals. We apply the technique to novel ultra-stable
CdSe/CdS dot-in-rod emitters. The emitters are of large current interest due to
their improved stability and reduced blinking. We retrieve a room-temperature
ensemble average quantum efficiency of 0.87+-0.08 at a mean lifetime around 20
ns. We confirm a log-normal distribution of decay rates as often assumed in
literature and we show that the rate distribution-width, that amounts to about
30% of the mean decay rate, is strongly dependent on the local density of
optical states.
