Digital imaging continues its expansion into various applications. Spectral images are becoming more popular as one field of digital imaging. At the same time, the economic and security aspects in the utilization of images are finding increased emphasis. In this study, the authors apply a watermarking method for spectral images, whereby embedding is based on the three-dimensional wavelet transform. The authors study the influence of illumination to the watermarked images. The authors also define how to estimate an unknown illuminant. Experiments were performed on a dataset of 13 spectral images. These experiments indicate that the illumination in spectral images should be established before watermarking, owing to the properties of the illuminants. Using the proposed watermarking method, the embedded watermark is also robust towards lossy compression. Guidelines for the parameter selection for watermarking with the proposed approach are given.