A dosimeter is any device used to measure an individual's exposure to a hazardous environment, particularly when the hazard is cumulative over long intervals of time, or one's lifetime. This item is a radiation dosimeter, but other dosimeters, particularly sound dosimeters, also exist.
A radiation dosimeter is a pen-like device that measures the cumulative dose of radiation received by the device. It is usually clipped to one's clothing to measure one's actual exposure to radiation. Magnifying lenses (a low-power microscope) and an illumination lens allow one to directly read the dose by aiming the illumination lens at a light source and looking into the device.
Dosimeters must be periodically recharged. A dosimeter is usually read before it is charged, and the dose is logged, to chart one's exposure. In many organizations, possession of the recharger is limited to health physicists to prevent falsification of exposures.