A method of image processing. A band-averaged spectral radiance is measured using at least one optical filter upon scanning a plurality of original radiances. The measured band-averaged spectral radiance includes a measured in-band-averaged spectral radiance and a measured band-gap-averaged spectral radiance. A multispectral radiance vector is generated from the measured band-averaged spectral radiance. The multispectral radiance vector and an out-of-band correction transform matrix corresponding to the at least one optical filter are matrix-multiplied to generate a band-averaged spectral radiances image vector representing a plurality of recovered band-averaged spectral radiances. The plurality of recovered band-averaged spectral radiances is analyzed for a presence of a target.