Roentgen (unit) Roentgen (unit) The roentgen or röntgen () (symbol R) is a legacy unit of measurement for the exposure of X-rays and gamma rays. It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. In 1928 it was the first international measurement quantity for ionising radiation to be defined for radiation protection, and was an easily replicated method of measuring air ionization directly by using an ion chamber. It is named after the German physicist Wilhelm Röntgen, who discovered X-rays. Although relatively easy to measure, it had
Wilhelm Filehne at the medical polyclinic. In 1876 he became an associate professor of pharmacology at Erlangen, and ten years later, was appointed a full professor of the same subject at the University of Breslau. In 1911 he was succeeded at Breslau by Julius Pohl. In addition to work in the field of pharmacology, he made contributions in his research of optical illusions, being known for his experimentation with a phenomenon known as a Zöllner illusion. The so-called "Filehne illusion" is the illusory motion of a stationary background when smooth pursuit eye movements are made across the stationary background. Wilhelm Filehne Wilhelm