AN INDIRECT INTERFERENTIAL METHOD FOR MEASURING THE DEFORMATIONS OF STRUCTURES IN TEMPERATURE

The dimensional stability of a base of ceramic glass, designed to cons titute a stable reference frame, has been determined with interferomet ric measurements in flowing air with variable temperature. An interfer ometric arrangement composed by one beam splitter and two mirrors was assembled directly on the base. The irradiance I(t) as a function of t emperature at constant pressure was collected in the highest order of the interference pattern. I(t) shows a cyclic behavior, due to the sup erposition of the air refractivity variations and of the mechanical st retching of the frame. The component of the signal due to the air refr activity alone can be regarded as the reference signal, with a valuabl e periodicity in the inverse-temperature space, i.e. in the expansion coefficients space. The remaining component corresponding to the defor mations of the supporting structure can be evaluated as a phase modula tion of the reference signal. Preliminary results of measurements show that an equivalent thermal expansion coefficient of the order of magn itude of +/-2.10(-8)degrees C-1 can be probed. The theoretical and act ual resolving power of the method, the unavoidable effects due to the presence of air, and some algorithms useful for noise reduction are di scussed.