Characterization of natural illuminants in forests and the use of digital video data to reconstruct illuminant spectra

We describe illumination spectra in forests and show that they can be accur ately recovered from recorded digital video images. Natural illuminant spec tra of 238 samples measured in temperate forests were characterized by prin cipal-component analysis. The spectra can be accurately approximated by the mean and the first two principal components. Compared with illumination un der open skies, the loci of forest illuminants are displaced toward the gre en region in the chromaticity plots, and unlike open sky illumination they cannot be characterized by correlated color temperature. We show that it is possible to recover illuminant spectra accurately from digital video image s by a linear least-squares-fit estimation technique. The use of digital vi deo data in spectral analysis provides a promising new approach to the stud ies of the spatial and temporal variation of illumination in natural scenes and the understanding of color vision in natural environments. (C) 2000 Op tical Society of America [S0740-3232(00)02410-8]. OCIS codes: 150.2950, 300 .6170, 120.5630, 040.1520, 040.7290, 330.1720.