Determination of the surface temperature of different materials based on thermographic imaging is a difficult task as the thermal emission spectrum is both temperature and emissivity dependent. Without prior knowledge of the emissivity of the object under investigation, it makes up a temperature-emissivity underdetermined system. This work demonstrates the possibility of recognizing specific materials from hyperspectral thermal images (HSTI) in the wavelength range from 8–14 µm. The hyperspectral images were acquired using a microbolometer sensor array in combination with a scanning 1st order Fabry-Pérot interferometer acting as a bandpass filter. A logistic regression model was used to successfully differentiate between polyimide tape, sapphire, borosilicate glass, fused silica, and alumina ceramic at temperatures as low as 34.0 ± 0.05 °C. Each material was recognized with true positive rates above 94% calculated from individual pixel spectra. The surface temperature of the samples was subsequently predicted using pre-fitted partial least squares (PLS) models, which predicted all surface temperature values with a common root mean square error (RMSE) of 1.10 °C and thereby outperforming conventional thermography. This approach paves the way for a practical solution to the underdetermined temperature-emissivity system.
This study presents the first results of a new type of hyperspectral imager in the longwave thermal radiation range from 8.0-14.0 µm which is simpler than readily available Fourier transform infrared spectroscopy (FTIR) based imagers. Conventional thermography images the thermal radiation from hot objects, but an accurate determination of temperature is hampered by the often unknown emissivities of different materials present in the same image. This paper describes the setup and development of a hyperspectral thermal camera based on a low order scanning Fabry-Pérot interferometer (FPI) acting as a bandpass filter. A three-dimensional hyperspectral datacube (two spatial and one spectral dimension) was measured by imaging a high emissivity carbon nanotube coated surface (Vantablack), black painted aluminium, borosilicate glass, Kapton Tape and bare aluminium. A principal component analysis (PCA) of the hyperspectral thermal image (HSTI) clearly segregates the individual samples. The most distinguishable sample from the PCA is the borosilicate petri-dish of which the Si-O-Si bond in borosilicate glass was the most noticeable. Additionally, it was found that the relatively large 1024x768x70 datacube can be reduced to a much smaller cube of size 1024x768x5 containing 92 % of the variance in the original dataset. The possibility of discriminating between the samples by their spectroscopic signature was tested using a logistic regression classifier. The model was fitted to a chosen set of principal components obtained from a PCA of the original hyperspectral datacube. The model was used to predict all pixels in the original datacube resulting in estimates with very high true positive rate (TPR). The highest TPR was obtained for borosilicate glass with a value of 99 % correctly predicted pixels. The remaining TPRs were 94 % for black painted aluminium, 81 % for bare aluminium, 79 % for Kapton Tape, and 70 % for Vantablack. A standard thermographic image was acquired of the same objects where it was found that the samples were mutually indistinguishable in this image. This shows that the hyperspectral thermal image contains sample characteristics which are material related and therefore outperforms standard thermography in the amount of information contained in an image.
Reactions of chemisorbed reagents inside the crystalline molecular solid state are rare but offer unexploited methods for selective solvent‐free chemical synthesis. Here we show that the greenhouse gas precursor, nitric oxide (NO) is chemisorbed by crystals of the hexafluorophosphate salts of complexes containing dicobalt sites. On NO sorption a cascade of reactions results in the in‐crystal synthesis of nitrite and other gaseous NOx. Recrystallization enabled structural elucidation of the mixed valent {[(bpbp)Co2(μ‐(η1‐O : η1‐N)‐ONO)]2(bdc)}4+ (bpbp=2,6‐bis(N,N‐bis(2‐pyridylmethyl)aminomethyl)‐4‐tert‐butylphenolato, bdc=1,4‐benzenedicarboxylato) cation. Overlapping signals in the solid‐state EPR spectra confirm the CoIICoIII oxidation state and the presence of NO2 trapped inside the unrecrystallised solid products (br. g=4, triplet g=2 (340 mT), A(N)=73 MHz), despite three cycles of vacuum and N2 flushing. Consistently, νN−O bands appear in the Raman and IR spectra that are due to the coordinated nitrate and the trapped NO2 that were synthesized in‐crystal. The latter is expelled by heating the solid to 160 °C or by recrystallization. Dimetallic cooperativity is proposed for the NO transformations in these rare examples of selective, chemisorptive substrate reactions in the solid‐state.
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