Abstract: An arrangement for sensing temperature changes at an object, specifically within a transformer or the like, is disclosed herein. In one embodiment, this arrangement utilizes as one of its main components a sensor including a solid glass medium which changes in optical density with changes in temperature, a single optical fiber for directing a beam of light into the glass media, a single optical fiber for collecting the light transmitted through the medium, and an optical means for imaging the transmitted light to the collecting fiber. In addition, in this embodiment, the light is caused to pass through the glass medium such that the amount of light reaching the collecting fiber is linearly dependent upon the temperature at the medium with respect to temperatures within a wide range.