A solution method is proposed for the illuminant estimation problem in the case of composite illuminants involving both spiky and continuous spectra. A spectral imaging system is utilized for image acquisition and analysis of scene reflections under spiky-spectrum illuminants. Fluorescent illuminants can be classified into three groups based on a set of wavelengths of the line spectra. Therefore we can infer the material classes of fluorescent light sources by estimating the wavelength positions of spikes on the illuminant spectrum. An estimation procedure is comprised of two major steps: First, the fluorescent illuminant class is determined from the multi-spectral images of a scene. Second, the spectral distributions of the illuminant components of fluorescent light and daylight are estimated based on an extended linear model. The feasibility of the proposed method is examined on natural scene reflections occurring under spiky fluorescent light and continuous daylight.