Roentgen (unit) this derivative quantity the roentgen equivalent physical (rep) to distinguish it from the ICR roentgen. The introduction of the roentgen measurement unit, which relied upon measuring the ionisation of air, replaced earlier less accurate practices that relied on timed exposure, film exposure, or fluorescence. This led the way to setting exposure limits,and the National Council on Radiation Protection and Measurements of the United States established the first formal dose limit in 1931 as 0.1 roentgen per day. The International X-ray and Radium Protection Committee, now known as the International Commission on Radiological Protection (ICRP) soon followed with a limit of
Roentgen equivalent man as the unit of equivalent dose in 1962 to measure the way different types of radiation distribute energy in tissue, and began recommending values of relative biological effectiveness (RBE) for various types of radiation. In practice, the unit of rem was used to denote that an RBE factor had been applied to a number which was originally in units of rad or roentgen. In 1977 the rem was redefined by the ICRP as 0.01 sievert or 0.01 J/kg, with the intention that the sievert would come to replace the rem. The International Committee for Weights and Measures (CIPM) adopted the