Abstract
We describe a spectral-imaging system with a liquid-crystal tunable filter and algorithms for recovering the spectral functions of the illuminant and surface reflectance from the image data. The filter works as a polarization-interference filter with birefringent elements and linear polarizers ; the center wavelength of filtration is electronically tunable with no moving parts. The camera system is composed of a CCD camera, a personal computer, the filter, and its rotating stage. The system characteristics of (1) the influence of room temperature, (2) the spectral-sensitivity functions, and (3) the setting of filter axis are shown in detail. We define the camera outputs by a vector equation and describe the spectral functions by a linear model. Then algorithms are presented for estimating the spectral functions from the image data. An effective image-processing algorithm is proposed for highlight extraction and region segmentation. Experimental estimation of the illuminant and spectral reflectances, in which 21 spectral images were measured in the range of 450-650 nm, showed that the efficiency in measurement and the accuracy in estimation are better than those with the previous method.
