  Optical fluorescence imaging is capable of measuring both the spatial and
rotational dynamics of single molecules. However, unavoidable measurement noise
will result in inaccurate estimates of rotational dynamics, causing a molecule
to appear to be more rotationally constrained than it actually is. We report a
mathematical framework to compute the fundamental limit of accuracy in
measuring the rotational mobility of dipole-like emitters. By applying our
framework to both in-plane and three-dimensional methods, we provide a means to
choose the optimal orientation-measurement technique based on experimental
conditions.
