Effect and correction of the shift in spectral images for polychromatic thermography

This articles investigates the temperature errors due to chromatic aberration in multiwavelength thermography methods. The Chromatic aberration leads to a shift in the perspective projection of a point in the 3D space on the image formed at different wavelengths. This shift causes an error in the temperature field calculated by polychromatic methods, from the fusion for each pixel of radiance temperature images at different wavelengths. The temperature error can reach 40% on a sample with high spatial non-uniformities, due to wide variations of emissivity. This paper suggests an approach to correcting the chromatic aberration that is based not on equipment but on software, coupled with a calibration, using Digital Image Correlation (DIC). This experimental technique is a 2D optical method used in mechanical engineering, for inferring a deformation of a plane structure's surface from a displacement field calculated by correlation between pixels of two successive images. This paper applies the technique to achieve the field displacement induced between two images at two wavelengths by chromatic aberration. After applying this correcting displacement field, the temperature error decreases from 40% to 1% for the pixels located at the boundary of two areas with different emissivities.