Detector-Based Luminous Flux Calibration Using Absolute Integrating Sphere Method
The Absolute Integrating-sphere Method was developed and introduced by NIST for the realization of luminous flux unit using an integrating sphere rather than a goniophotometer. The total luminous flux of a lamp inside the sphere is calibrated against the known amount of flux introduced into the sphere from the external source through a calibrated aperture. The key element of this method is the correction for the spatial nonuniformity of the integrating sphere, which has been made possible by a technique using a scanning beam source. This method is to be applied directly to routine calibration measurements of luminous flux. A new luminous flux calibration facility with a 2.5 m integrating sphere has been recently completed at the at the NIST and will allow the calibration of test lamps with no need for luminous-flux standard lamps. Calibrations will be performed based on the illuminance measurement of the external source by standard photometers. This method brings the luminous-flux calibration a detector-based procedure, thereby eliminating the uncertainties associated with the use of standard lamps while achieving lower uncertainties by shortening the calibration chain. The characteristics of the new integrating sphere obtained by computer simulation and measurements are reported.