Thermal exposure from hydrogen flames is determined by the spectral properties of the combustion products, the optical thickness of the flame, and the attenuation of the emitted radiation during propagation through the surrounding atmosphere. This licentiate thesis investigates the effect of flame thickness on emitted spectral radiance, the fractional radiative contribution of species present in hydrogen flames, and the propagation of the emitted radiation through humid air, using line-by-line spectral calculations based on the HITRAN/HITEMP spectroscopic databases and Voigt line profiles.

The effect of flame thickness on the emitted spectral radiance is analysed by evaluating the spectral optical thickness, , and determining the fractions of the spectrally integrated radiance associated with optically thin, intermediate, and optically thick regimes. For flame thicknesses 𝐿 ≤ 0.1 m, more than 90% of the emitted radiation satisfies the optically thin criterion ≤ 0.1. As flame thickness increases, most of the  radiation that no longer satisfies the optically thin criterion enters the intermediate regime, whereas only a very small fraction reaches the optically thick limit.

The fractional radiative contributions of the species present in hydrogen flames are determined. The emitted radiation is dominated by water vapour, which contributes more than 96.6% of the total, followed by CO₂ at approximately 3.0% and OH at approximately 0.4%. Each of the remaining species contributes less than 0.01%.

The propagation of the emitted radiation through humid air is analysed. Since H₂O is the dominant radiating species in the flame and an absorbing species in humid air, the attenuation is determined by the spectral mismatch between the hydrogen flame emission spectrum and the absorption spectrum of ambient water vapour. Depending on relative humidity (RH), humid air absorbs approximately 10–20% of the emitted radiation within the first metre and approximately 20–40% over a 10 m propagation path.