Realization of NIST 1995 Luminous Flux Scale using Integrating Sphere Method

A new NIST luminous flux scale has been realized using the integrating sphere method, on which theoretical and experimental studies were previously conducted and reported. The NIST 2 m integrating sphere was modified to add an opening and an additional baffle to apply this method. Flux from an external source (1000 W quartz halogen lamp) is introduced into the sphere through a limiting aperture and the opening. The flux entering the sphere is determined from the measured area of the aperture and the illuminance on the aperture plane measured with the NIST standard photometers that maintain the NIST illuminance scale. The total luminous flux of a group of standard lamps measured inside the sphere is deduced by comparison to the flux from the external source. Corrections are made for the spatial non-uniformity and the incident angle dependence of the sphere response, and the spectral mismatch of the integrating sphere photometer responsivity. The total luminous flux measured with the integrating sphere system is compared to the values obtained with a goniophotometer of PTB, Germany. The values obtained by the two methods agreed to within 0.2 %. The magnitude of the new NIST luminous flux unit is 1.1% larger than previously realized. The relative expanded uncertainty (k=2) of the NIST luminous flux scale is now 0.5%.