Raw data acquired by imaging devices are converted into digital images by post-processing algorithms. However, these algorithms are significantly affected by numerous illumination conditions. Particularly, in the case in which illumination conditions depend on canonical light sources, unwanted light sources that locally illuminate the scene or mixed light from several light sources are recognized as the spatially varying illumination conditions. These complex illumination conditions cause several artifacts by affecting the digital image acquisition process. For example, optical aberrations are generated by refraction of complex light, or illumination estimation is significantly affected by the different color temperatures of multiple light sources. To overcome these problems, this study proposes an algorithm that decomposes the complex illumination from several light sources. First, the spatially varying illumination condition is discussed, and the artifacts generated by the conditions, such as false colors and aberrations, are analyzed. Second, mixed light from several canonical light sources is decomposed based on the imaging devices and spectral information of the canonical light sources. The proposed method has low complexity and increased applicability. Furthermore, the improvement scheme based on the proposed method has a parallelized structure and can be easily applied to various types of algorithms, such as color constancy, deconvolution, denoising, and contrast enhancement. The algorithms employing the improvement scheme show the potential of the proposed method for solving the problems associated with multiple light sources.