Realization of the NIST total Spectral Radiant Flux Scale - PMOD/WRC
Realization of the NIST total Spectral Radiant Flux Scale - PMOD/WRC
Realization of the NIST total Spectral Radiant Flux Scale - PMOD/WRC
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where y test (λ) is <strong>the</strong> detector signal <strong>of</strong> <strong>the</strong><br />
spectroradiometer for <strong>the</strong> test lamp at wavelength λ, and<br />
y ref (λ) is that for <strong>the</strong> introduced flux at wavelength λ.<br />
k cor (λ) is a correction factor for spatial nonuniformity <strong>of</strong><br />
<strong>the</strong> integrating sphere system and <strong>the</strong> ratio <strong>of</strong> diffuse<br />
reflectance <strong>of</strong> <strong>the</strong> sphere coating at 0° and 45° incidence.<br />
Similar to <strong>the</strong> case with <strong>the</strong> goniospectroradiometric<br />
method, <strong>the</strong> measurement can be done for relative <strong>total</strong><br />
spectral radiant flux, with <strong>the</strong> absolute scale determined by<br />
<strong>the</strong> luminous flux unit.<br />
Instrumentation<br />
The mechanical design <strong>of</strong> <strong>the</strong> goniospectroradiometer<br />
developed at <strong>NIST</strong> is shown in Fig. 1. The detector arm<br />
and lamp holder are rotated by servo motors, and <strong>the</strong> actual<br />
angles are measured by rotary encoders. Measurements are<br />
taken, typically at 5° or 10° intervals, and <strong>the</strong> motor stops<br />
during measurement. The arm holding <strong>the</strong> lamp holder can<br />
also be rotated to change <strong>the</strong> burning position <strong>of</strong> <strong>the</strong> lamp.<br />
The spectral irradiance <strong>of</strong> <strong>the</strong> test lamp is measured with<br />
an array spectroradiometer employing a fiber optic input.<br />
The fiber bundle is connected to <strong>the</strong> spectroradiometer<br />
through a rotation coupler, which allows free rotation <strong>of</strong><br />
<strong>the</strong> arm without twisting <strong>the</strong> fiber. The constancy <strong>of</strong> <strong>the</strong><br />
spectral transmittance <strong>of</strong> <strong>the</strong> coupler was tested with a<br />
white LED mounted on <strong>the</strong> irradiance head. The variations<br />
in measured spectra depending on <strong>the</strong> arm angle was found<br />
to be less than 2 %, and is corrected in actual<br />
measurements. From <strong>the</strong> rotation coupler to <strong>the</strong> irradiance<br />
measuring head, a fiber bundle <strong>of</strong> 8 mm diameter is used.<br />
The fiber bundle on <strong>the</strong> spectroradiometer side is 5 mm<br />
diameter.<br />
The array spectroradiometer covering <strong>the</strong> UV and visible<br />
region is calibrated by a spectral irradiance standard lamp<br />
(1000 W FEL type quartz halogen lamp) traceable to <strong>the</strong><br />
<strong>NIST</strong> spectral irradiance scale [7]. The stray light <strong>of</strong> <strong>the</strong><br />
array spectrometer has been corrected (Zong, 2005). For<br />
<strong>the</strong> consistency with <strong>the</strong> <strong>total</strong> luminous flux unit, <strong>the</strong> <strong>total</strong><br />
spectral radiant flux scale has been realized using Eqs.<br />
Servo<br />
motor<br />
Stepping<br />
motor<br />
Irradiance<br />
head<br />
1.25 m<br />
φ<br />
Light<br />
trap<br />
Encoder<br />
Laser<br />
Servo<br />
motor<br />
Fiber<br />
bundle<br />
Figure 1. Construction <strong>of</strong> <strong>the</strong> <strong>NIST</strong><br />
goniospectroradiometer.<br />
θ<br />
Spectroradiometer<br />
Rotation<br />
coupling<br />
Figure 2. <strong>NIST</strong> 2.5 m integrating sphere configured<br />
for <strong>the</strong> <strong>total</strong> spectral radiant flux measurement.<br />
(3)-(5). The uncertainty budget is to be reported at<br />
presentation and <strong>the</strong> full paper.<br />
Figure 2 shows <strong>the</strong> arrangement <strong>of</strong> <strong>the</strong> <strong>NIST</strong> 2.5 m sphere<br />
configured for <strong>total</strong> spectral radiant flux measurement.<br />
The sphere is equipped with a high-sensitivity<br />
spectroradiometer employing a back-lit CCD array. The<br />
external source is a 1000 W FEL type quartz halogen lamp<br />
calibrated for spectral irradiance. The aperture is 50 mm in<br />
diameter. The spectroradiometer is an array<br />
spectroradiometer, which is corrected for stray light (Zong,<br />
2005). Linearity <strong>of</strong> <strong>the</strong> spectroradiometer has also been<br />
determined and corrected. The integrating sphere response<br />
has been mapped spectrally by rotating a beam scanner.<br />
Angular correction factor for <strong>the</strong> coating has also been<br />
measured spectrally. An experimental realization <strong>of</strong> <strong>total</strong><br />
spectral radiant flux scale in <strong>the</strong> near UV region (360 nm –<br />
450 nm) has been successfully made at <strong>NIST</strong> using <strong>the</strong> 2.5<br />
m integrating sphere to calibrate <strong>the</strong> <strong>total</strong> radiant flux <strong>of</strong><br />
deep blue and UV LEDs (Zong, 2004). Work is underway<br />
to realize <strong>the</strong> scale in <strong>the</strong> full visible region.<br />
Conclusion<br />
The <strong>total</strong> spectral radiant flux scale has been established at<br />
<strong>NIST</strong> using a goniospectroradiometer for <strong>the</strong> 360 nm to<br />
800 nm region. The AIS method, now under development,<br />
will enable realization <strong>of</strong> <strong>the</strong> scale with much simpler<br />
instrumentation and fast measurements.<br />
References<br />
Goodman T. M., et al, The Establishment <strong>of</strong> a New National<br />
<strong>Scale</strong> <strong>of</strong> <strong>Spectral</strong> Total <strong>Flux</strong>, Proc., 22 nd Session <strong>of</strong> CIE,<br />
Melbourne 1991, Vol. 1, Part 1, 50-53 (1991).<br />
CORM 7 th Report 2001–Pressing Problems and Projected<br />
National Needs in Optical Radiation Measurements, December<br />
2001.<br />
Ohno Y., <strong>Realization</strong> <strong>of</strong> <strong>NIST</strong> 1995 Luminous <strong>Flux</strong> <strong>Scale</strong> using<br />
Integrating Sphere Method, J. IES, 25-1, 13-22 (1996).<br />
Zong, Y., et al, A Simple Stray-light Correction Matrix for Array<br />
Spectrometers, to be presented at NEWRAD 2005<br />
Zong, Y., Miller, C.C., Lykke, K., Ohno, Y., Measurement <strong>of</strong><br />
Total <strong>Radiant</strong> <strong>Flux</strong> <strong>of</strong> UV LEDs, Proceeding <strong>of</strong> <strong>the</strong> CIE Expert<br />
Symposium on LED Light Sources, June 2004, Tokyo,<br />
107-110 (2004)