This paper formulates a general radiometric theory of multimode step index fibers, covering in particular the region of mid- and far IR fibers. The optical fiber is treated both as a passive waveguide, guiding the external radiation injected into it, as well as an active waveguide, generating internal thermal radiation which is guided to both fiber endfaces. Several fiber absorption profiles are considered. In other words, the thermal radiation sources coupling radiation into the guided modes of the fiber are in one case considered to be distributed in the core, and in another, to be distributed in the cladding. The model is based on 3-D optical geometry of bounded and tunneling skew rays and yields an analytical expression for the angular power distribution along the length of the fiber. The radiation emissivity of multimode fibers is formulated. Based on the model, the theory of a new fiber optic distributed thermal sensor is presented. This sensor needs no external source of radiation for its operation and is based on the self-generation of thermal radiation in a modified IR fiber. Such a sensor can be produced by deliberately inducing surface or bulk absorption in the fiber core or coating (cladding) a bare fiber core with an IR absorbing material.