Measurement of the changes in air refractive index and distance by means of a two-color interferometer.

<p>We discuss the measurements of the changes in air refractive indices and geometrical distances by means of a two-color phase-modulated fringe-counting interferometer. We analytically classified the optical path lengths for IR and visible light into the changes in the air refractive index and the distance by using a dispersion formula for air. The interferometer was tested over a 235-m path in the testing tunnel of the National Research Laboratory of Metrology, which has a sensor system for measuring air conditions.</p> <p>The refractive-index change by the two-color interferometer agreed with the value calculated by the air conditions within an accuracy of 0.15 ppm.</p>