For over sixty years, the Bureau International des Poids et Mesures (BIPM) has maintained the results of international comparisons of luminous intensity and luminous flux as "world-mean" values, in the form of groups of lamps. It was recently decided that this is no longer a satisfactory and reliable means of maintaining the photometric units of the International System of Units (SI). To maintain the luminous-flux unit, an attempt was made at the BIPM to use the Absolute Integrating-Sphere Method developed at the National Institute of Standards and Technology (NIST). A new technique (ac/dc) employing a chopper for the external source, which allows simultaneous measurement of the internal and external sources, has been developed to overcome an unexpected problem with the characteristics of the BIPM sphere, in that the heat from the lamp affected the sphere coating. The ac/dc technique allows absolute calibration of the integrating sphere while the internal lamp is operating and being measured. Having implemented this technique and replaced the sphere coating, experiments on the derivation of the unit are in progress at the BIPM and the NIST, the preliminary results of which are reported.
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