Here - PMOD/WRC
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Here - PMOD/WRC
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where y test (λ) is the detector signal of the<br />
spectroradiometer for the test lamp at wavelength λ, and<br />
y ref (λ) is that for the introduced flux at wavelength λ.<br />
k cor (λ) is a correction factor for spatial nonuniformity of<br />
the integrating sphere system and the ratio of diffuse<br />
reflectance of the sphere coating at 0° and 45° incidence.<br />
Similar to the case with the goniospectroradiometric<br />
method, the measurement can be done for relative total<br />
spectral radiant flux, with the absolute scale determined by<br />
the luminous flux unit.<br />
Instrumentation<br />
The mechanical design of the goniospectroradiometer<br />
developed at NIST is shown in Fig. 1. The detector arm<br />
and lamp holder are rotated by servo motors, and the actual<br />
angles are measured by rotary encoders. Measurements are<br />
taken, typically at 5° or 10° intervals, and the motor stops<br />
during measurement. The arm holding the lamp holder can<br />
also be rotated to change the burning position of the lamp.<br />
The spectral irradiance of the test lamp is measured with<br />
an array spectroradiometer employing a fiber optic input.<br />
The fiber bundle is connected to the spectroradiometer<br />
through a rotation coupler, which allows free rotation of<br />
the arm without twisting the fiber. The constancy of the<br />
spectral transmittance of the coupler was tested with a<br />
white LED mounted on the irradiance head. The variations<br />
in measured spectra depending on the arm angle was found<br />
to be less than 2 %, and is corrected in actual<br />
measurements. From the rotation coupler to the irradiance<br />
measuring head, a fiber bundle of 8 mm diameter is used.<br />
The fiber bundle on the spectroradiometer side is 5 mm<br />
diameter.<br />
The array spectroradiometer covering the UV and visible<br />
region is calibrated by a spectral irradiance standard lamp<br />
(1000 W FEL type quartz halogen lamp) traceable to the<br />
NIST spectral irradiance scale [7]. The stray light of the<br />
array spectrometer has been corrected (Zong, 2005). For<br />
the consistency with the total luminous flux unit, the total<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 of the NIST<br />
goniospectroradiometer.<br />
θ<br />
Spectroradiometer<br />
Rotation<br />
coupling<br />
Figure 2. NIST 2.5 m integrating sphere configured<br />
for the total spectral radiant flux measurement.<br />
(3)-(5). The uncertainty budget is to be reported at<br />
presentation and the full paper.<br />
Figure 2 shows the arrangement of the NIST 2.5 m sphere<br />
configured for total 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 of the 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 the coating has also been<br />
measured spectrally. An experimental realization of total<br />
spectral radiant flux scale in the near UV region (360 nm –<br />
450 nm) has been successfully made at NIST using the 2.5<br />
m integrating sphere to calibrate the total radiant flux of<br />
deep blue and UV LEDs (Zong, 2004). Work is underway<br />
to realize the scale in the full visible region.<br />
Conclusion<br />
The total spectral radiant flux scale has been established at<br />
NIST using a goniospectroradiometer for the 360 nm to<br />
800 nm region. The AIS method, now under development,<br />
will enable realization of the scale with much simpler<br />
instrumentation and fast measurements.<br />
References<br />
Goodman T. M., et al, The Establishment of a New National<br />
Scale of Spectral Total Flux, Proc., 22 nd Session of 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., Realization of NIST 1995 Luminous Flux Scale 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 of<br />
Total Radiant Flux of UV LEDs, Proceeding of the CIE Expert<br />
Symposium on LED Light Sources, June 2004, Tokyo,<br />
107-110 (2004)<br />
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