Recent progress at NPL in the development of mid-infrared spectrometric measurements

Abstract

ABSTRACT The National Physical Laboratory (NPL) realises and disseminates the UK spectrometric scales from 200 nm to 56 µm wavelength. Its mid-infrared (MIR) regular and hemispherical reflectance and transmittance scales, and transfer standards for the wavenumber and ordinate calibration of MIR spectrometers are realised from 2.5 µm to 56 µm using specially modified grating spectrometers. This paper will discuss a technique, recently developed at NP L, for the direct absolute realization of the hemispherical reflectance scale. This scale was previously based on a relative method using an absolutely calibrated mirror and published data for BaSO 4 . The new method has given an improved value for the pure BaSO 4 to underpin the relative method. It opens up new applications as it can be applied to types of sample not previously measurable e.g. foil-covered insulation, and this aspect is discussed. The various sources of uncertainties are considered and the existing standards and services are also described briefly to place the technique in context. Although developed on grating instruments, this new measurement capability can be realized on Fourier transform (FT) spectrometers. Progress is being made at NPL in transferring its other MIR measurement capabilities to FT instrumentation. This will safeguard NPL's future ability to provide these services to customers. Keywords: spectrometry, spectrophotometry, reflectance, transmittance, hemispherical reflectance, MIR spectrometers, mid-infrared, NPL