Illuminant estimation in natural scenes includes the problem of estimating a spatial distribution of light sources by omnidirectional observations. So far most omnidirectional measuring systems used a mirrored ball as an optical tool. This kind of system, however, has several essential problems, such as low and nonuniform spatial resolution and a dead region due to imaging the camera itself, as reflected by the mirror. The present article realizes multiband spectral imaging with high spatial resolution for omnidirectional estimation of scene illuminant in a simple measuring system. To overcome the mirrored ball problems, we propose a multiband omnidirectional imaging system using a high-resolution trichromatic digital camera, a fisheye lens, and two commercial color filters. The spatial resolution of omnidirectional imaging systems is analyzed based on an optics model in detail. We describe a practical spectral imaging system. Use of the RGB camera with each of the two color filters allows two different sets of trichromatic spectral sensitivity functions via spectral multiplication, resulting in six spectral bands, after image capture with each color filter. An algorithm is developed based on statistical estimation theory for estimating illuminant spectra from noisy observations of the sensor outputs. The feasibility of the proposed method is examined from the viewpoints of spatial resolution and omnidirectional illuminant estimation.