A new method has been described elsewhere for measuring transient temperature changes using the absorption of the 254‐nm resonance radiation from an optically thick Hg lamp by traces of Hg vapor. The method is based upon the fact that the Hg resonance lines broaden in a known way with increasing temperature and pressure. The sensitivity and dynamic range of the temperature measurement depends upon the extent of line reversal which varies with the optical depth of the Hg lamp. It is shown here that this can be controlled by means of a proper lamp design. The mounting assembly for the lamp and the circuitry required for this purpose are described. It was demonstrated that the output of such a lamp is stable and reproducible with line profiles (optical densities) that could be varied over about a 20‐fold range. Within such a range of operating parameters, the measurement of transient temperature to 1500 K in gases at pressures below 100 Torr could be made.