  Backscatter limits many interferometric measurements, including gravitational
wave detectors, by creating spurious interference. We describe an experimental
method to directly and quantitatively measure the backscatter interference. We
derive and verify experimentally a relation between backscatter interference,
beam radius and the scattering sample bidirectional reflectance distribution
function. We also demonstrate that our method is able to measure backscatter
from high quality optics for angles as low as 500 urad with a 160 urad angular
resolution.
