Katrašnik Jaka, Pernuš Franjo, Likar Boštjan
University of Ljubljana, Faculty of Electrical Engineering, Laboratory of Imaging Technologies, 1000 Ljubljana, Slovenia.
Opt Express. 2013 Feb 25;21(4):4841-53. doi: 10.1364/OE.21.004841.
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.
近红外高光谱成像正成为各个领域中一种流行的工具。在所有成像系统中，适当的照明对于获得图像分析算法最佳性能所需的最佳图像质量至关重要。在高光谱成像中，所获取的光谱特征必须在成像物体的所有部分都具有代表性。因此，整个物体必须被均匀照亮——没有阴影或镜面反射。由于对物体的材料和几何形状没有限制，只有通过完全漫射照明才能实现对物体的理想照明。为了最小化阴影和镜面反射，照射物体的光必须在空间、角度和光谱上均匀。因此，可以使用本文介绍的空间强度、空间光谱、角度强度和角度光谱不均匀性测量来评估高光谱成像照明系统的质量。重点是角度强度和角度光谱不均匀性测量，这是本文最重要的贡献。这些测量是在两个参考目标的图像上定义的——一个平坦的白色漫反射目标和一个球体网格目标——使用基于声光可调滤波器（AOTF）的高光谱成像系统获取。所提出的测量在环形灯和漫射穹顶照明系统上进行了测试。
