observation volume, \(V_{\text{obs}}\)

in flame emission and absorption spectrometry
https://doi.org/10.1351/goldbook.O04273
The volume of that part of the flame that is observed through the optical device (in \(\unicode[Times]{x3BC}\text{l}\)).
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 [Terms] [Paper]