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\end{equation}  where $\eta$ is the solvent viscosity, $k$ is the Boltzmann constant and $T$ is the absolute temperature.  Although time-resolved anisotropy measurements are a well established tool in molecular biology, only a few studies report applications in ophthalmology.\cite{McLaughlin2013,Danysh2010} In this study the effective hydrodynamic radii of three important posterior segment drugs -- ranibizumab, aflibercept and bevacizumab -- were measured by $\mu$s time-resolved fluorescence anisotropy and compared to radii calculated from the MW. Due to the large molecular weight of these drugs, short-lived nanosecond lifetime fluorescent dyes are not suitable for this study, as they fluorescence will have decayed by the time the drug has moved noticeably. Therefore, we used long-lifetime phosphorescence from a ruthenium based dye \cite{Baggaley2014} which allows us to measure the rotational diffusion of molecules with large molecular weight.\cite{Hoepker2014, Terpetschnig1995} weight.\cite{Hoepker2014,Terpetschnig1995}  A well-characterised protein bovine serum albumin (BSA) of comparable size (66.5~kDa) with known radius \cite{Axelsson1978} was also measured to validate the results.