A dosimeter is a type of quartz fiber electrometer uses to measure ionizing radiation doses. The quartz fiber electrometer (sometimes called the pen meter) is clipped into a pocket and monitors received radiation. It detects X-rays and gamma rays, both of which are ionizing radiation and damage human tissue.

Before the meter is given to the wearer, the inner electrode (1) is charged to between 100 and 200 volts. Electrostatic repulsion causes the flexible quartz fiber (2) to bend away from the electrode and zero reading is observed on the calibrated scale (3). The charging pin (4) springs back and the electrode is now insulated from the outside wall by air molecules in the chamber. High energy radiation entering the chamber through the walls of the meter (5) ionizes the air molecules and some of the charge leaks away (6) to the outer wall. This reduces the voltage difference and the fiber moves back progressively to its relaxed position due to the reduced charge (7). The system of magnifying lenses allows the position of the quartz fiber to be viewed, and the dose to be read off against a calibrated scale (8).