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
Ludwig Wilhelm Sachs "Geheimen Medizinalrat" (private medical advisor). Ludwig Wilhelm Sachs Ludwig Wilhelm Sachs (29 December 1787 in Groß-Glogau – 17 June 1848 in Königsberg) was a German physician. From 1807 studied medicine at the universities of Königsberg, Berlin and Göttingen, receiving his doctorate at the latter institution with the dissertation-thesis ""De Humorum corporis animalis vi vitali"" (1812). He later served as a physician in war-time hospitals in Königsberg, and in 1816 obtained his habilitation. In 1818 he became an associate professor at Königsberg, followed by a full professorship in 1826. In 1832 he was named director of the medical clinic, then in