Abstract: An implantable dosimeter uses salt crystals such as NaCl or KCl, or other materials that vary in color as a function of incident, ionizing radiation. The color change of the salts may occur through the creation of F-centers, where electrons become trapped in crystal defects (e.g., halide vacancies) and absorb light at certain wavelengths. Vacancies in the salt crystals absorb photons at precise wavelengths. Thus, the change in color can be correlated to the integrated dose in an implantation site. The salt crystals may be optically coupled to optical fibers or the like for remote measurement of color using, e.g., a spectrometer and a computer system. In this manner, the dosage of ionizing radiation can be measured in vivo with a fault tolerant, passively integrating dosimeter.