Patent ID: 6417927
Filing Date: 2002-07-09
Classification: G01B,G03F

Abstract:
An interferometric apparatus comprising:a source for generating at least two beams having different wavelengths; dispersion interferometer means having a measurement path whose length may be varied, a first mode of operation in which it provides information to enable the calculation of initial and updated values of inverse dispersive power, &Ggr; , and its equivalent, and a second mode of operation in which it provides information about short term index of refraction variations of air in said measurement path at both of said wavelengths and to measure the length of said measurement path using the current value for &Ggr; as determined in said first mode of operation; refractometer means proximate said measurement path for directly measuring the long-term variations in refractive index of air outside of said measurement path for at least said first wavelength while said dispersion interferometer means makes measurements in said measurement path; and means for: (1) placing said dispersion interferometer in said first mode of operation in which said measurement path is varied between at least two positions, receiving measurements from said refractometer means about long-term variations in the refractive index of air near said measurement path and averaging said long-term index variations over a predetermined time period while said dispersion interferometer is in its first mode of operation, using said average value of said long-term index of refraction to calculate an initial value of &Ggr; on the assumption that said average value of said long-term index of refraction is substantially the same as the index of refraction in said measurement path when said refractometer means is in said second mode of operation, and storing said initial value of &Ggr; for subsequent computational use, and (2) placing said dispersion interferometer in said second mode of operation in which said dispersion interferometer means generates measurements about short-term variations in the index of refraction in said measurements path, averaging said short term measurements and determining the difference between the instantaneous value of said short-term index value and its average value; continuously comparing said difference with the average value of the long-term index of refraction and determining if a comparison limit is exceeded and, if so, updating the current value of &Ggr; by re-entering said first mode of said dispersion interferometer means and, if not, remaining in said second mode of operation of said dispersion interferometer means and calculating the physical length of said measurement path using the current value for &Ggr;.