Multispectral imaging has been widely used for high-fidelity color reproduction. For accurate color reproduction, a robust characterization process is essential. In this paper, we present a robust and convenient characterization method for a multispectral imaging system using the irradiance of the illuminant. We measure the irradiance of the illuminant instead of the reference of the color chart, thereby reduces tedious measuring tasks. Furthermore, our method can obtain the spectral reflectance of a scene, which enables realistic relighting and image composition under different illumination conditions. The experiment results demonstrate that our method outperforms the previous method by giving smaller errors in terms of the estimated spectral reflectance, and showing more realistic image composition.