The United Kingdom scales for diffuse reflectance are realized using two primary instruments. In the 360 nm to 2.5 μm spectral region the National Reference Reflectometer (NRR) realizes absolute measurement of reflectance and radiance factor by goniometric measurements. Hemispherical reflectance scales are obtained through the spatial integration of these goniometric measurements. In the mid-infrared region (2.5 μm - 55 μm) the hemispherical reflectance scale is realized by the Absolute Hemispherical Reflectometer (AHR). This paper describes some of the uncertainties resulting from errors in aligning the NRR and non-ideality in sample topography, together with its use to carry out measurements in the 1 - 1.6 μm region. The AHR has previously been used with grating spectrometers, and has now been coupled to a Fourier transform spectrometer.
United Kingdom scales for diffuse reflectance in the ultra-violet to near-infrared region are realized at National Physical Laboratory using goniometric techniques, while the mid-infrared scale is based on measurements with a hemisphere reflectometer. Both of these scales have changed recently. In the ultra-violet to near-infrared region this took place on adoption of the goniometrically realized scale which replaced the previous scale traceable to NRC and PTB, while in the mid-infrared the scale change was based on the development of a new direct absolute technique which allowed a re-determination of the diffuse reflectance of BaSO4. Dissemination of the UV/vis/NIR scale will still in the main rely on integrating sphere techniques, and this requires the use of reference standards calibrated using the goniometric technique. The new scale is found to be 0.4 - 0.5 % higher than the previous scale in the visible region, while harmonization with the mid-infrared at 2.5 μm has made good progress, thereby ensuring an independent and continuous scale from the ultra-violet to the far-infrared.
To meet demands for increased traceability of fluorescence measurement a new two-monochromator reference spectrofluorimeter has been designed and built at NPL. This new instrument is capable of measuring the radiance factor and associated colorimetry of solid and liquid fluorescent materials at virtually any angle to the incident light beam. A 1 kW xenon light source and the use of reflective optics enable the device to make bispectral measurements from the ultraviolet to the near infrared. This paper describes the optical arrangement of the new system..
The National Physical Laboratory currently disseminates United Kingdom diffuse reflectance scales from 320 nm to 56 μm. The scale for hemispherical reflectance from 360 nm to 830 nm was previously traceable to the National Research Council of Canada and the Physikalisch-Technische Bundesanstalt, and its extension to 56 μm has relied partly on data published by Morren et al. (1972). Measurements of 0°/45° radiance factor from 360 nm to 830 nm were traceable to the Physikalisch-Technische Bundesanstalt. The National Physical Laboratory now realizes all United Kingdom diffuse reflectance scales independently as described here.