Another widely used thermal detector is the bolometer. The bolometer is a resistive element constructed from a material with a very small thermal capacity and large temperature coefcient so that the absorbed radiation produces a large change in resistance. In contrast to the thermocouple, the device is operated by passing an accurately controlled bias current through the detector and monitoring the output voltage. The change in resistance is like to the photoconductor, however, the basic detection mechanisms are different. In the case of a bolometer, radiant power produces heat within the material, which in turn produces the resistance change. There is no direct photonelectron interaction.