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Total Spectral Radiant Flux Calibration
Figure 1

Figure 1. NIST goniospectroradiometer.		   The total spectral radiant flux scale between 360 nm and 830 nm has been realized using a goniospectroradiometer. Total spectral radiant flux standards are required to calibrate integrating spheres coupled to spectroradiometers. Such sphere systems are increasingly used in by the lighting and LED industries. We selected a 75 W halogen lamp equipped with an outer bulb and a screw base, for use as the first type of transfer standard for dissemination to industry. The uncertainty of the measurement is 1.5 % to 0.7 % (k = 2), depending on the wavelength. Other lamp types will be added as transfer standards, including miniature halogen lamps for use in small integrating spheres for LED measurements.

Introduction

Total spectral radiant flux (of a source) is the spatial integration of spectral radiant intensity over the entire solid angle or spectral irradiance over the entire surrounding area:

$$\Phi_{\ell,\lambda}(\lambda)=\int_\Omega I_\lambda(\lambda) \, {\rm d}\Omega, \quad {\rm or} \quad \Phi_{\ell,\lambda}(\lambda)=\int_A E_\lambda(\lambda) \, {\rm d}A \quad {\rm (unit : W/m)}$$

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). By using such a spectroradiometer sphere system, theoretically there are no spectral mismatch errors in measuring total luminous flux (lumen) of sources. The color of light sources are measured at the same time with the total luminous flux. The color of the source is measured as geometrical average in all directions.
Figure 2

Figure 2. Schematic of sphere-spectroradiometer and various types of lamps.		 Figure 3

Figure 3. Principle of spatial integration of luminous flux by goniophotometric method.

Approach - Relative goniospectroradiometric method

When the spectral irradiance of a test lamp is measured in many directions (q,f) over 4p using a goniometer equipped with a spectroradiometer – called goniospectroradiometer, the total spectral radiant flux is given by

$\Phi_\lambda(\lambda)=r^2\int_{\phi=0}^{2\pi}\int_{\theta=0}^\pi E_\lambda(\lambda,\theta,\phi)\sin\theta{\rm d}\theta{\rm d}\phi.$

The theory is simple, but it is not easy to obtain low uncertainty in absolute flux due to imperfection of the goniometer (dead angle, stray light, sampling errors, etc.).

The solution is to bring the absolute scale from the well-established unit of the lumen. Then the spectral measurements can be done relatively. In this method, the total spectral radiant flux of a lamp is given by

$\Phi_\lambda(\lambda)=k_{\rm scale}\int_{\phi=0}^{2\pi}\int_{\theta=0}^\pi S(\lambda,\theta,\phi)\sin\theta{\rm d}\theta{\rm d}\phi$

with

$k_{\rm scale}=\frac{\Phi_{\rm v}}{K_{\rm m}\int_{\lambda=0}^\infty V(\lambda)\int_{\phi=0}^{2\pi}\int_{\theta=0}^\pi S(\lambda,\theta,\phi)\sin\theta{\rm d}\theta{\rm d}\phi{\rm d}\lambda}$

where $S(\lambda,\theta,\phi)$ is the relative spatial spectral distribution of the source as given by

$S(\lambda,\theta,\phi)=k\cdot E_\lambda(\lambda,\theta,\phi)\quad (k:{\rm arbitrary constant).}$

With this method, the calibrated lamp has the total luminous flux scale as well as total spectral radiant flux scale, both of which are perfectly consistent with each other.

NIST Goniospectroradiometer

A goniospectroradiometer for this purpose has been developed at NIST. The rotation radius of the detector is 1.25 m. The spectral irradiance of the test lamp is measured with a CCD array spectroradiometer (350 nm to 1000 nm), employing a fiber optic input. The array spectroradiometer is calibrated by a spectral irradiance standard lamp (1000 W FEL type tungsten halogen lamp) traceable to the NIST spectral irradiance scale. The mechanism of the goniospectroradiometer is designed so that the dead angle due to the lamp rotation mechanism is ± 3° (covering 0° to 177° of q positioning), the angular resolution of both axes to be <0.01°, and uncertainty of the absolute angle to be <0.1°. The scan speed is 3 second per point for 10° × 10° scan, in which case 0.5 h per lamp.
Figure 4

Figure 4. Design of NIST goniospectroradiometer. 		Figure 5

Figure 5. Transfer standard lamps for total spectral radiant flux.

Transfer standards

Several different types of quartz-halogen lamps equipped with screw base are prepared. A 75 W halogen lamp equipped with an outer bulb will be used as the first type of transfer standard for dissemination to industry. We plan to prepare larger range of transfer standard lamps from 25 W to 200 W level.

Data example
Figure 6

Figure 6. An example of spatial distribution of correlated color temperature of a FEL lamp.		 Figure 7

Figure 7. An example of spatial radiant intensity distribution of a FEL lamp.

Calibration service

Calibration service is now available tentatively under NIST Test No. 37100S, where lamps submitted by the customer are calibrated. We are preparing to establish a new service to issue calibrated total spectral radiant flux standard lamps.

Reference

NIST Facility for Total Spectral Radiant Flux Calibration (868 kB) PDF,
Y. Ohno and Y. Zong, Proc. Symp. Metrology 2004, CENAM, Mexico, published in CD (2004).


Optical Sensor Group
For technical information or questions, contact:

Yuqin Zong
Phone: (301) 975-2332
Email: yzong@nist.gov
Yoshi Ohno
Phone: (301)-975-2321
Email: ohno@nist.gov

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Online: July 1999   -   Last updated: April 2007