A novel concept of a fiber optic distributed sensor is presented. In particular, we describe a distributed thermal sensor formed by inducing a deliberated absorption in an infrared optical fiber. The increased absorption coefficient αab of this modified fiber determines the linear sensing length of the distributed thermal sensor. When part of the modified fiber is heated, infrared radiation is generated in the fiber and transmitted through it to its distal face. Measuring this radiation power with a radiometer, we are able to determine the average spatial temperature distribution along the fiber sensing length.