The intersection of the optical beam and that part of the flame where the net signal
is at least half of the maximum net signal. The characteristics of the observation
space depend on the temperature of the flame, the
stoichiometry of the gases and the properties of the processed fluid (presence of
proteins for instance). The observation space in atomic absorption is analogous to the observation
space of a cuvette in molecular 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