The work of Pohl's Institute became internationally known only shortly before the outbreak of WW II, when Pohl and his research assistant Hilsch were invited to a conference on "The conduction of electricity in solids" at Bristol in 1937. In 1946, the first review article on color centers appeared in the USA. This resulted in a 1951 invitation to the University of Illinois in Urbana, including visits to Bell Telephone Laboratories, the Naval Research Laboratory and other research institutes. On this occasion, Pohl once again encountered James Franck and they renewed their friendship which had been interrupted in 1933 by Franck's forced emigration. In 1956, the first International Color Center Conference took place at Argonne National Laboratory; it was repeated at three-year intervals in the following years until 1977.