An indirect interferential method for measuring the deformations of structures in temperature

The dimensional stability of a base of ceramic glass, designed to constitute a stable reference frame, has been determined with interferometric measurements in flowing air with variable temperature. An interferometric arrangement composed by one beam splitter and two mirrors was assembled directly on the base. The irradiance I(t) as a function of temperature at constant pressure was collected in the highest order of the interference pattern, I(t) shows a cyclic behavior, due to the superposition of the air refractivity variations and of the mechanical stretching of the frame. The component of the signal due to the air refractivity alone can be regarded as the reference signal, with a valuable periodicity in the inverse-temperature space, i.e. in the expansion coefficients space. The remaining component corresponding to the deformations of the supporting structure can be evaluated as a phase modulation of the reference signal. Preliminary results of measurements show that an equivalent thermal expansion coefficient of the order of magnitude of ±2·10–8 °C–1 can be probed. The theoretical and actual resolving power of the method, the unavoidable effects due to the presence of air, and some algorithms useful for noise reduction are discussed.