  This paper presents a method for determination of the size distribution for
diamond nanocrystals containing luminescent nitrogen-vacancy (NV) centers using
the luminescence intensity only. We also revise the basic photo physical
properties of NV centers and conclude that the luminescence quantum yield of
such centers is significantly smaller than the frequently stated 100\%. The
yield can be as low as 5\% for centers embedded in nanocrystals and depends on
their shape and the refractive index of the surrounding medium. The paper also
addresses the value of the absorption cross-section of NV centers.
