O'Brien was born in
Denver, Colorado in 1898 to Michael Phillip and Lina Prime O' Brien. He attended the
Latin School of Chicago from 1909 to 1915, and continued at the
Sheffield Scientific School of
Yale University, where he earned a Ph.B. in electrical engineering in 1918 and a Ph.D. in physics in 1922. He also did course work at
Massachusetts Institute of Technology and
Harvard University. In 1922 he married Ethel Cornelia Dickerman and they had one son, Brian, Jr. After Ethel Cornelia died, he married a second time to Mary Nelson Firth in 1956. He was a research engineer at
Westinghouse Electric Co. from 1922 to 1923. During this period he developed, along with
Joseph Slepian, the auto-valve lightning arrester, which is still in use. In 1923 he moved to J. N. Adam Memorial hospital in
Perrysburg, New York, a
tuberculosis sanitarium run by the city of Buffalo's public health department. Prior to the use of antibiotics, the primary treatment for tuberculosis was fresh air and
light therapy. There was some evidence that
sun tanning did help in the remission of the disease, but Perrysburg—40 miles south of Buffalo—had very little sunshine in the winter. Therefore, O'Brien, as a physicist on staff, developed a
carbon arc with cored carbons that very closely matched the solar spectrum. With this development the patients could have sun therapy year-round. Due to a general interest in biological effects of solar radiation, he published some of the early work on the ozone layer and
erythema caused by the sun. O'Brien moved to the
University of Rochester in 1930 to hold the chair of physiological optics. Shortly thereafter he became the director of the
Institute of Optics. His continuing interest in the biological effects of solar radiation led to research in vitamin chemistry. The need for
vitamin D, especially in the diet of children, had been recognized for preventing
rickets. At that time there was no synthetic vitamin D, but the dehydrocholesterol in milk can be converted to vitamin D by radiation with ultraviolet light. The carbon arcs developed at Perrysburg were an ideal source of ultraviolet, but for proper irradiation, the milk had to be in a very thin film. ... A film of high enough flow volume for commercial application was produced, and vitamin D-fortified milk became widespread." ==War and peace: 1940 - 1953==