Patent ID: 7400418

Claim:
A method of determining the clearance between a tip of a moving turbine blade and a turbine casing comprising the steps of (a) emitting a low-coherence light from a light source, (b) directing the low-coherence light through a collimator lens to the blade tip wherein the back surface of the collimator lens is placed at the level of the surface of the turbine casing, (c) directing light reflected from the collimator lens and from the blade tip to a beam splitter and directing a first resulting beam to a reference beam and a second resulting beam to a delay arm, (d) shifting the frequency of the reference beam by means of an acousto-optical modulator to a predetermined frequency, (e) scanning the time delay of the delay beam by means of a variable delay line, (f) combining the reference beam and the delay beam by means of a beam combiner, (g) detecting the power spectrum due to the interference of the light reflected from the collimator lens and from the moving blade tip at the frequency determined by the acousto-optical modulator, (h) calculating from the obtained power spectrum the distance between the turbine casing and the moving turbine blade tip.