  An absolute measurement of the components of the shear rate tensor
$\mathcal{S}$ in a fluid can be found by measuring the photon correlation
function of light scattered from particles in the fluid. Previous methods of
measuring $\mathcal{S}$ involve reading the velocity at various points and
extrapolating the shear, which can be time consuming and is limited in its
ability to examine small spatial scale or short time events. Previous work in
Photon Correlation Spectroscopy has involved only approximate solutions,
requiring free parameters to be scaled by a known case, or different cases,
such as 2-D flows, but here we present a treatment that provides quantitative
results directly and without calibration for full 3-D flow. We demonstrate this
treatment experimentally with a cone and plate rheometer.
