Optical measurement technique is widely used in length measurement, such as laser interferometers and electronic distance meters. The correction of air refractive index is necessary because the optical path length should be converted to the geometrical length in most vases. In order to obtain an average refractive index through the optical path, Edlen's formulae have been widely used with environmental measurements of air temperature, pressure, humidity and carbon dioxide concentration. In recent days, more precise formulae for calculating a refractive index of air have been desired in IR region. However, the measurement accuracy for near IR region is not sufficient.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.