As a result of extensive measurements by the flicker and equality-of-brightness methods, mostly in America, a certain set of average luminosity data was adopted by the International Commission on Illumination in 1924. These I.C.I. luminosity factors were incorporated in the 1931 I.C.I. standard observer and coordinate system for colorimetry and were accepted by the International Committee on Weights and Measures in 1933. In the period from 1935 to 1938, however, the validity of these factors was challenged by several German investigators, who believed as a result of their measurements that a new set of standard values should be adopted based on results by the flicker photometer with pure cone vision (1.2° field and daylight adaptation). The German measurements, however, were not wholly consistent and were in conflict with similar recent measurements in England and America. At the 1939 meetings of the I.C.I., the Germans withdrew their opposition to the 1924 I.C.I. luminosity factors and the Commission formally reaffirmed their validity in the photometry of lights of different colors.
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