Realization of the unit of luminous flux at the HUT using the absolute integrating-sphere method

A description of a detector-based realization of the unit of luminous flux (lumen) at the Helsinki University of Technology (HUT) is presented. The realization is based on the absolute integrating-sphere method developed at the National Institute of Standards and Technology (NIST), with some modifications. The measurement set-up consists of a 1.65 In integrating sphere.. two photometers, a precision aperture and an external luminous-flux source. The characterization and maintenance of the measurement system are described and the uncertainty budget of the realization is presented. The uncertainty analysis indicates a relative expanded uncertainty (k = 2) of 4.7 x 10(-3) for the realization. According to the results of an earlier bilateral comparison between the HUT and the NIST, the ratio of the measured luminous flux value of HUT to that of NIST was 1.0006 with an expanded uncertainty (k = 2) of 10 x 10(-3), including uncertainties due to realization of the units. Another indirect test measurement indicated a corresponding ratio of 0.9984 with the luminous flux measurements of BIPM with an expanded uncertainty (k = 2) of 11 x 10(-3), including uncertainties due to realization of the units.