Light path optical length measurement for aircraft aerodynamics study

The length measurement method involves using the interference of measurement beams. An output beam is produced with two collinear components of independent polarisation and different frequencies. As each successive interfering beam is formed from the previous one the polarisations with different frequencies are interchanged, the interfered light converted into an electrical signal, and the phase of the signal measured. The method uses a laser interferometer comprising a laser (4), reflecting elements (1,2) either side of a doubly refracting plate (3) in the direction of the beam, a beam separation polarisation element (11) and a photoelectric transducer (10) and phase measurement (12) arrangement. The method is applicable to field density investigations in gas for aerodynamic measurements in flow channels or of flow around aircraft. Precise automatic contactless optical path length measurement is achieved in the presence of foreign material, acoustic, thermal and other effects.