Units and standards of light maintained at the National Physical Laboratory, 1915¿60

Work on photometric standards at the National Physical Laboratory from 1915 to 1960 is surveyed. The three chief aspects of the work have been, first, the establishment of the present primary standard of light, second, a study of the problems arising from the colour difference between the primary standard and lamps in common use and the replacement of visual by physical methods of photometry, and third, the improvement of the methods of deriving standards of luminous flux from those of luminous intensity. The main decisions of the Comité International des Poids et Mesures are given, and so also is a summary of the results of the three international comparisons conducted by the Bureau International des Poids et Mesures in 1948, 1952 and 1956.