We report dynamic light scattering experiments on turbid colloidal sus
pension under stationary and laminar flow, as well as in the regime of
flow instabilities. It is shown that the time autocorrelation functio
n C1(t) = [E(0)E(t)]/[\ E(0)\ 2] of the scattered light field E(t) is
not sensitive to the mean velocity flow but rather to the root mean s
quare of velocity gradient. C1(t) is characterised on the level of eac
h scattering event by the correlation time required by a pair of scatt
erers initially separated by a transport mean free path to move a rela
tive distance of optical wavelength due to the velocity gradient. We v
erified this theoretical analysis using planar Couette flow as an exam
ple for homogeneous velocity gradients, and planar Poiseuille flow for
inhomogeneous velocity gradients. Agreement between experiment and th
eory is excellent. Finally, this technique is applied to spatially var
ying velocity gradient fields for measuring the threshold and wave num
ber of the Taylor-Couette instability. This illustrates the possibilit
y of studying hydrodynamic instabilities and quasi-local velocity grad
ients even under conditions of strong multiple scattering.