A technique for the absolute calibration of a time-resolved spectrographic system has been developed at OinetiQ, specifically designed to be relevant to spectral acquisition from within the interior of translucent gun-propellant samples. The technique has shown itself to be particularly useful in the realm of propellant ignition as it allows for the precise determination of the moment that propellant combustion processes begin, as well as measuring the incident radiative flux leading up to ignition. Scope exists to extend its use for high-pressure measurements of the incident radiative flux during both conventional propellant burn and high-powered electrothermal-chemical (ETC) discharges. This paper sets out to describe both the technique and some of the pitfalls encountered during the development of the technique. The use of this technique in some of the experimental work performed at OinetiQ, including the results of measurements that compare the incident radiative flux with propellant ignition during both ETC discharges and conventional gunpowder burn, have been published separately; references for this experimental work are given in this paper.