The term exposure refers to the concentration, in air, of x-rays at a specific point and is the ionization produced in a specific volume of air: E = Q / m where E is exposure, Q is the quantity of charge on the ions and m is the unit mass of air.
Exposure describes the ability of x-ray photons to ionize air and cannot be used for protons, neutrons or electrons. Exposure decreases with the square of the distance from an x-ray source (inverse square law). It was historically measured in roentgens (R), however the SI unit is the coulomb per kilogram (C kg) which lacks an eponym:
- 1 roentgen = 2.58×10C kg
It is typically measured with an ionization chamber.
Exposure can be converted to absorbed dose with a conversion factor called the F-factor. The F-factor takes into account the photon energy of the x-rays and the medium irradiated.