<p>Quantitative measurements of minor species are essential for understanding flame propagation and emission formation, and for validation of chemical kinetic models. Laser-induced fluorescence-based methods are widely employed due to their ability to selectively excite specific species and achieve high signal-to-noise ratios. However, these techniques are inherently susceptible to collisional quenching, which complicates signal quantification. Bi-directional laser-induced fluorescence (BD-LIF) was proposed decades ago as a promising approach to obtain absolute species concentrations while preserving spatial resolution. Despite its potential, initial measurements showed deviations of 50 – 60 % compared to equilibrium calculations and 1D simulations. We present a generalized quantification strategy for BD-LIF based on the general form of Beer’s law that explicitly accounts for wavenumber-dependent absorption and the resulting spatial evolution of the overlap between laser and absorption line, due to the stronger absorption near the line center. The method is demonstrated by measuring hydroxyl (OH) radicals following excitation in the A–X(1,0) system in the post-flame region of laminar CH₄-air flames. The results show very good agreement with simulated OH concentrations, underscoring the robustness of the generalized approach and its potential for broader application in combustion diagnostics.</p>