A description is given of the properties of a linear thermal detector irradiated by a modulated radiation field and operated simultaneously with a periodic bias current. The method is a generalization of the more common operation of passive thermal detectors with a constant dc bias and may be applied for barretters with a sufficient small time constant. A barretter has been developed with a time constant of approximately 14 &mgr;sec in air and 3 &mgr;sec in a hydrogen atmosphere such that modulation frequencies up to 10 kc, and for hydrogen filling up to 50 kc, are still feasible.