A method for characterizing illumination systems for hyperspectral imaging.

Near-infrared hyperspectral imaging is becoming a popular tool in various fields. In all imaging systems, proper illumination is crucial for attaining optimal image quality that is needed for the best performance of image analysis algorithms. In hyperspectral imaging, the acquired spectral signature has to be representative in all parts of the imaged object. Therefore, the whole object must be equally well illuminated-without shadows or specular reflections. As there are no restrictions imposed on the material and geometry of the object, the desired illumination of the object can only be achieved with completely diffuse illumination. In order to minimize shadows and specular reflections, the light illuminating the object must be spatially, angularly and spectrally uniform. The quality of illumination systems for hyperspectral imaging can therefore be assessed using spatial-intensity, spatial-spectral, angular-intensity and angular-spectral non-uniformity measures that are presented in this paper. Emphasis is given to the angular-intensity and angular-spectral non-uniformity measures, which are the most important contributions of this paper. The measures were defined on images of two reference targets-a flat, white diffuse reflectance target and a sphere grid target-acquired with an acousto-optic tunable filter (AOTF) based hyperspectral imaging system. The proposed measures were tested on a ring light and on a diffuse dome illumination system.