Establishment of 2π total spectral radiant flux scale with a broadband LED-based transfer standard source

The National Metrology Institute of Japan realized a total spectral radiant flux scale in the visible wavelength range (from 380 nm to 780 nm) for sphere-spectroradiometer calibration in 2 pi geometry. For a transfer standard in 2 pi geometry, a standard source based on a specially-designed broadband light-emitting diode (2 pi standard LED) was used. For the scale realization, we used the goniophotometer/spectroradiometer system. In the realization, an effect of a bandpass function of the array spectroradiometer was corrected by a matrix inversion approach. The relative expanded uncertainty (k = 2) for the scale is from 3.3% to 5.7% depending on the wavelength range. We also evaluated the contributions of the wavelength calibration and the bandpass function to the measurement of the total spectral radiant flux of typical white LEDs against the 2 pi standard LED. Our results demonstrate that the 2 pi standard LED achieves uncertainties comparable to those of a conventional standard lamp, within a practical wavelength range for LED measurements. These results show that the 2 pi standard LED is a useful transfer standard for the realization of the total spectral radiant flux scale and for evaluation of the total luminous flux of LEDs in 2 pi geometry.