A design is proposed for a stable infrared source suited to absorbance measurements. The source is a parallelepipedic cavity of tantalum, heated to 2600 K, with an apparent emissivity in excess of 0.7. It is observed to be three times brighter at 3000 cm−1than the 1440‐K SiC bar. A model is constructed in order to estimate the temperature distribution. For this kind of source, the temperature and emissivity are found to be quite uniform. A comparison between estimated and experimental values of temperature and emissivity is discussed.