A fiber optic dosimeter in which an electron trapping material is held in a probe on the tip of an optical fiber. The probe is placed in a region with the radiation to be measured, and the opposite end of the optical fiber, from which radiation readings are measured, is placed in a location remote from the radiation source. When radiation impinges upon the electron trapping material, electrons in the material are raised to a higher state where they are trapped and stay indefinitely. When infrared light strikes the material, the stored electrons are released from their traps and, upon falling to a lower energy level, emit visible light which can be detected and measured. Thus, to measure the amount of ambient radiation, the electron trapping material is stimulated with infrared light from an infrared source at the opposite end of the optical fiber. This infrared stimulation releases trapped electrons and causes the emission of visible light, at least a portion of which is collected and directed back down the optical fiber to the visible light detector, where it is converted into an electrical signal and measured. The infrared stimulation is modulated to allow for measurement of background fluorescence during radiation exposure when infrared stimulation is not applied. This background fluorescence is then substracted from the gross signal under infrared stimulation to yield a signal which changes linearly with radiation input.