Here - PMOD/WRC

incorporate a monochromator to allow full spectral
measurements of multi-angle regular reflectance.
Measurements have shown good agreement with existing
near and mid infrared reflectance scales. This
complements the existing V-W measurements at 7° in the
200 nm to 2500 nm region, and V-only and multiangle
reflectance capabilities in the mid infrared [6]
Uncertainties
The uncertainty budget covering diffuse reflectance
measurements in the ultra violet to visible [2] has been
refined and extended to cover diffuse and regular
reflectance measurements up to 2.5 µm. The total
combined uncertainty varies depending upon the scattering
properties of the sample and the spectral region of interest.
In the UV signal to noise and reproducibility are dominant,
whilst in the visible these are less significant and geometry
becomes important. The non-uniformity of the detector
becomes a considerable source of uncertainty in the near
infrared. For 0°/45° radiance factor and total
hemispherical reflectance of a white Spectralon target, the
total combined uncertainty is better than ± 0.25% (k = 2) in
the visible.
Acknowledgments
This work was funded by the UK DTI National Measurement
Systems Directorate ’ s Programme on Optical Radiation
Metrology.
References
[1] Williams, D. C., Establishment of absolute diffuse reflectance
scales using the NPL Reference Reflecteometer, Analytica
Chimica Acta, 380, 165-172, Elsevier 1999
[2] Chunnilall, C. J., NPL Scales for Radiance Factor and Total
Diffuse Reflectance, Metrologia, 40, no. 1, S192 – S195, 2003
[3] Clarke, F. J. J. & Larkin, J. A. Measurement of total
reflectance, transmittance and emissivity over the thermal IR
spectrum. Infrared Physics 25, 359-367 (1985).
[4] Chunnilall C. J., Clarke F. J. J. & Shaw M. J., Diffuse
reflectance scales at NPL, Proc. SPIE 4826, 12-20, (2003).
[5] Morren, L., A study of the reflection factor of usual
photometric standards in the near infra-red, Lighting Research
and Technology 4, pp. 243-249, 1972.
[6] Clarke, F. J. J., Birch, J. R., Chunnilall, C. J. & Smart, M. P.
FTIR measurements, standards and accuracy. Vibrational
Spectroscopy 30, 25-29 (2002).
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