In a photoacoustic1measurement, radiation is normally passed onto an absorbing sample, and one observes changes in the sample's properties or the effects they induce elsewhere. The sample itself is the prime sensor, and basically what is measured is the net energy transfer between sample and source. In practice, the source is made “hot,” i. e., a much stronger emitter than the sample. In principle, the source and detector are interchangeable, in terms of emittance or location, and also sometimes in practice. For example, infrared emission spectra of minerals were measured with hot (40°C) samples and a cold (30°C) detector2and also with cold (20°C) samples and a hot (24°C) detector3.