In the absolute integrating-sphere method, the total luminous flux of a lamp inside an integrating sphere is determined by comparing it with a known flux introduced into the sphere from an external light source. As the measurement geometry of the lamps to be compared is different, the spatial non-uniformity of the sphere surface may affect the results. In order to evaluate this effect, the spatial response must be measured. Miniature incandescent lamps have been used as scanning-beam sources in previous realizations, but these lamps are not widely available. In the present realization of the luminous-flux unit by the Helsinki University of Technology (HUT), light-emitting diodes (LEDs) were used as the light source in scanning the spatial response. Preliminary results confirm the applicability of the LED scanner and indicate moderate deviations of about 1% from earlier luminous-flux calibrations.
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