A new radiochromic thin-film sensor using the blue tetrazolium chloride salt (BT) dissolved in polyvinyl alcohol (PVA) has been developed for deep ultraviolet, and gamma and xray dosimetry systems. The radiation-sensitive blue tetrazolium (BT2+), pale yellowish before irradiaton, registers a permanent purple high-resolution image upon irradiation. The distinctive coloration in terms of increase of optical density (net absorbance, ΔA) at 552 nm is measured spectrophotometrically and is proportional to the amount of radiation energy absorbed per unit mass (absorbed dose, D). The films have broad monitoring range, being able to detect radiation doses as low as 1 kGy and as high as 75 kGy. The radiochromic images show a gradual postirradiation increase in absorbance, especially during the first 24 hrs. There is also a small but predictable variation in sensitivity with temperature during irradiation. Furthermore, the responses to gamma radiation are found to be dependent on absorbed dose rate, but can be minimized by increasing the dye precursor concentration (BT2+).