Acquisition and Analysis of Hyperspectral Thermal Images for Sample Segregation

Abstract

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.