Thermal detectors of infrared (IR) radiation are generally covered with a highly absorbing “black” coating in order to assure that the incident radiation is absorbed.1–3Essentially, the black material is the prime radiation sensor which becomes warmed by the photothermal effect and in turn warms a secondary sensor or “detector.” There have consequently been ongoing efforts to devise inproved, suitable “blacks”, e.g.4–5.The observations accompanying such work have involved measurements of the absorptive properties of the blacks, which are made independently of measurements of the electrical or other responses of the black/detector assemblies.5–7The purpose of the present letter is to demonstrate the possibility of measuring the absorptive properties of the black in situ on the detector.