Lens Calibration for Focus Shift Correction in Close-Range Multispectral Imaging

Abstract

Multispectral imaging has become a popular tool to reveal properties and structures in cultural heritage objects that are hidden to the human observer. One of the inherent problems of multispectral imaging applications is chromatic aberration. Due to an extended spectral range, the effect appears more pronounced than in conventional photography in the visible spectrum. This paper is concerned with longitudinal chromatic aberrations, i.e. shifts of the focal plane along the principal axis of the camera, as they are hard to correct in post-processing and should be avoided during acquisition. To this end, a calibration scheme to measure the wavelength- and distance-dependent focal shift behavior of a given camera/lens system is proposed, which allows for a mechanical compensation at acquisition time. The approach is demonstrated on a multispectral imaging system for historical manuscript analysis. We show that the application of this compensation approach enables the acquisition of in-focus images in non-visible wavelengths using a lens optimized for the visible spectrum only.