On a complex optical system such as an interferometer, coherent Stray Light is a pitfall in many ways because it can affect the accuracy and resolution of the measurements, or their dynamics. The purpose of this work is to develop the demonstrator of an instrumentation able to measure the stray light optical amplitude, whatever its origin, in the photoreceivers of the interferometric system of the LISA space mission, the MOSA (Moving Optical SubAssembly). We use a single-mode laser source which optical frequency is swept over time, and inject this beam in the optical system under test. During the optical frequency scan, we record all the outgoing signals (both optical and electrical). If one or more of these signals show a fringe-like behaviour, this modulation is interpreted as due to the presence of a stray light amplitude, interfering with the nominal light amplitude. The nominal/SL optical path difference (OPD) is retrieved from the frequency of these interference fringes, as in coherent reflectometry. The list [SL amplitude vs. OPD] of the different SL contributions is retrieved by spectral analysis of the recorded photo-signals. The present set up fulfills most of the requirements for MOSA stray light tests by reaching a measurement floor in fractional amplitude SL/nom under 10−5 (10−6 above 10 Hz) on the measurement range [0–200 Hz] and by achieving a resolution in OPD SL/nom under 1 mm.