The volume of that part of the flame that is observed through the optical device (in µl).
See also: observation space (in flame emission and absorption spectrometry)
Source:
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 (https://doi.org/10.1351/pac198658121737)