Radiation intensity is the amount of energy passing through a given area that is
perpendicular to the direction of radiation travel in a given unit of time. One way to
measure the intensity of X-rays or gamma rays is to measure the amount of ionization
they cause in air. The amount of ionization in air produced by the radiation is called the
exposure. Exposure is expressed in terms of a scientific unit called a Roentgen (R). The
unit roentgen is equal to the amount of radiation that ionizes 1 cm3 of dry air (at 0°C
and standard atmospheric pressure) to one electrostatic unit of charge, of either sign.
Most portable radiation detection safety devices used by radiographers measure
exposure and present the reading in terms of Roentgens or Roentgens/hour, which is
known as the “dose rate”.