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Laser-induced fluorescence (LIF) is one of the most popular techniques for flames diagnostics. Temperature measurements are in general obtained by scanning the laser wavelength through a molecular absorption band in order to probe the ground-state rotational population. OH is frequently used as thermometric species because of its favorable spectral characteristics and of its recurrence in combustion processes. Minor species concentration measurements are also performed by LIF, owing to its great sensitivity. While excitation spectra for temperature measurements must be carried out with lowintensity probing beams (linear regime), absolute concentration measurements require high-power excitation to overcome quenching problems (saturation regime). Moreover this latter regime allows maximum sensitivity and minimum dependence from laser intensity fluctuations. However, the quantitative interpretation of saturated LIF signals poses some problems not yet completely resolved (see Ref. 5 and references therein). The most typical is the evaluation of the effective probe volume due to nonuniformities in the intensity distribution of focused laser beams.

In recent years the two-line atomic fluorescence method (TLAF) has received increasing attention for performing local temperature measurements in flames with a good spatial resolution even with low power sources.