Dosimeters measure an individual's or an object's exposure to something in the environment particularly to a hazard inflicting cumulative impact over long periods of time, or over a lifetime. This article concentrates on the radiation dosimeter, which measures exposure to ionizing radiation. The radiation dosimeter is of fundamental importance in the disciplines of radiation dosimetry and health physics. Other types of dosimeters are sound dosimeters, ultraviolet dosimeters and electromagnetic field dosimeters.Ionizing radiation, such as X-rays, alpha rays, beta rays, and gamma rays, are undetectable by the human senses, therefore a measuring device is used to detect measure and record this, and in some cases give an alarm when a preset level is exceeded. Ionizing radiation damage to the body is cumulative, and is related to the total dose received, for which the SI unit is the Sievert. Therefore, workers exposed to radiation, such as radiographers, nuclear power plant workers, doctors using radiotherapy, those in laboratories using radionuclides, and some HAZMAT teams are required to wear dosimeters so their employers can keep a record of their exposure to verify that it is below legally prescribed limits.