The difference in height between the axis of the observed space (optical axis) and the burner tip
). The optical axis of the instrument should be the same as the optical axis of the observation space. Other definitions which express the observation height as a fraction of the whole flame height are not recommended because the height to the tip of the flame is not well defined. The observation height should be stated in the method.
PAC, 1986, 58, 1737
(Quantities and units in clinical chemistry: Nebulizer and flame properties in flame emission and absorption spectrometry (Recommendations 1986))
on page 1741
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook