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 500μ r a d with an angular resolution of 160μ r a d.