9.6 - A dual wavelength optical distance measuring instrument which corrects for air density

An instrument has been constructed which simultaneously measures optical path length at two wavelengths and so permits determination of average refractive index over the common path and hence distance to
1 \times 10^{-6}
or better without extensive meteorological observations. Using a He-Ne laser (6328 Å) and a filtered high-pressure mercury arc lamp (3681 Å), the instrument is expected to measure distances to 15 kilometers with accuracies as high as a few parts in ten million. Preliminary measurements over a 5.3-km path give a standard deviation of 0.15 cm, or better than
3 \times 10^{-7}
.