Realization of the NIST Total Spectral Radiant Flux Scale

The total spectral radiant flux scale has been realized at National Institute of Standards and Technology using a goniospectroradiometer for the 360 nm to 800 nm region. The construction of the goniospectroradiometer and results of the scale realization as well as the uncertainty budget are presented. Introduction. There are increasing needs for total spectral radiant flux standards, which are required to calibrate integrating sphere systems employing a spectroradiometer. Such integrating sphere systems are increasingly used for measurement of color and total luminous flux of light sources such as discharge lamps and light-emitting diodes (LEDs) as well as for measurement of total radiant flux of ultraviolet (UV) sources. A realization of total spectral radiant flux in the visible region using a gonio-colorimeter was reported (Goodman, 1991), which was based on measurement of the correlated color temperature of a test lamp in many directions, and the average spectral distribution of the lamp was derived based on Planck’s equation. No further developments have been reported. Since then, new methods and technologies have become available to realize the scale more directly, possibly with lower uncertainties. To address the strong needs for total spectral flux standards in the U.S. (CORM, 2001), a project has started at National Institute of Standards and Technology (NIST) to realize the total spectral radiant flux scale from the UV to visible region. Two independent methods are employed to realize the scale, allowing cross-check of the realized scales. The first method uses a goniospectroradiometer designed and built at NIST. The second method uses the NIST 2.5 m integrating sphere, applying the principles of the Absolute Integrating Sphere (AIS) method, which is successfully used for the realization of the lumen since 1995 at NIST (Ohno, 1996). For the first phase of the project, NIST total spectral radiant flux scale has been realized in the 360 nm to 800 nm region using the goniospectroradiometer. This paper presents the principles of the methods of realization, the construction of the goniospectroradiometer, and the results of the realization of the scale as well as its uncertainty budget. Methods for Scale Realization Goniospectroradiometric method The spectral radiant intensity of a light source is measured in many directions (θ, φ) over 4π steradian using a goniospectroradiometer. The total spectral radiant flux Φλ(λ) of the light source is given by Φ λ(λ) = φ=0 2π ∫ I λ(λ ,θ ,φ) θ =0 π ∫ sinθ dθ dφ , (1) where I λ(λ ,θ ,φ) is the spectral radiant intensity distribution of the source on spherical coordinates (θ, φ). If the goniospectroradiometer measures spectral irradiance, the total spectral radiant flux Φλ(λ) of the light source is given by Φ λ(λ) = r 2 φ=0 2π ∫ Eλ(λ ,θ ,φ) θ =0 π ∫ sinθ dθ dφ , (2) where Eλ(λ ,θ ,φ) is the spectral irradiance distribution on the spherical surface with radius r around the light source being measured. The realization of the scale can be done more easily if the absolute scale is brought from the luminous flux unit. Then the spectral irradiance measurements can be done relatively. In such a method, the total spectral radiant flux of a lamp is given by Φ λ(λ) = kscale φ=0 2π ∫ S(λ ,θ ,φ) θ =0 π ∫ sinθ dθ dφ , (3)