VALD3 stores vacuum wavelengths of all the transitions. This allows uniform handling of extraction across the whole spectral range. On the other hand the selection tools in VALD3 include options for returning the air wavelengths. Furthermore, some of the original lines list include wavelengths measured in the air. This calls for conversion tools. Such tools must be uniformly accurate and reversible across the whole spectral range.

For the vacuum to air conversion we used the formula from Donald Morton (2000, ApJ. Suppl., 130, 403) for the refraction index which is also the IAU standard:

n = 1 + 0.0000834254 + 0.02406147 / (130 - s²) + 0.00015998 / (38.9 - s²), where s = 10⁴ / lambda_vac and lambda_vac is in Angströms

This formula comes from Birch and Downs (1994, Metrologia, 31, 315) and applies to dry air at 1 atm pressure and 15C with 0.045% CO₂ by volume. The corrections to Edlén (1953, J. Opt. Soc. Am., 43, 339) are less than 0.0001 AŽ at 2000 AŽ and less than 0.001 AŽ at 30000 AŽ.