The NPL hemispherical reflectometer is used between 2.5 and 55 micrometres to measure spectral and total emissivities for the calculation of radiative energy transfer for materials with a uniform surface. However, materials used in energy saving and military applications are often non-uniform with random patchiness or with a repeating pattern or intrinsic structure. A new facility has been devised at NPL for the measurement of the spatial distribution of emissivity of non-uniform materials. A Rank Taylor Hobson TICM-II infrared thermal imager is used to study the variation of radiance across the surface of a sample maintained at a uniform near-ambient temperature. Reflected radiation from the sample is eliminated by covering most of the hemisphere of sample irradiation with a cold non-reflecting screen. A 2.7x telescope supplied by Rank Pullin Controls together with a new deep-focus special attachment, developed by NPL, allows samples to be viewed at distances from infinity down to 0.27 m. The resulting images are enhanced by digital filtering in a GEMS customised image processing system with a framestore holding 4 images of 512 x 512 pixels at 16-bit depth. This provides a highly flexible system for the measurement of the emissivity of any element of a sample from 100 mm down to a fraction of a millimetre across. A range of statistical, analytical and graphics functions is available from the GEMS software. Corrected emissivity maps are derived from comparison with uniform standard samples, measured on the NPL hemispherical reflectometer and residual non-uniformities are corrected using large NPL reference black body cavities.
J. A. Larkin,
F. J. J. Clarke,
"Npl Facilities For Measuring The Emissivity Of Materials For Near-Ambient Conditions", Proc. SPIE 0636, Thermal Imaging, (5 August 1986); doi: 10.1117/12.964199; https://doi.org/10.1117/12.964199