The results of a comparison between calculated and measured values of the refractive index of air

When optical techniques are applied to high-accuracy length measurement in the free atmosphere it is essential to apply a correction for the refractive index of the air. This can be obtained either by calculation using Edlen's equation or by direct measurement using a gas refractometer. An investigation of the accuracy achievable using Edlen's equation has been made using a high-precision interference gas refractometer developed at the NPL. The results have shown that a correction to the water vapour term in Edlen's equation is required to achieve the highest accuracy.