Physically accurate spectral rendering generally brings further computational complexity to RGB rendering pipelines which precludes real-time rates. In this report, we show that by building on several physical a-prioris for the spectral phe- nomena in standard astronomical scenes, we achieve to obtain a model greatly reducing the complexity of the spectral aspect for volumetric rendering - by up to 83 factor in our examples. By projecting the total local contribution on a small op- timal basis, thus providing scalability for the method, our rendering scheme then makes it possible to render a volumetric scene with any number and spectra of lights in real-time and no significant visual discrepancies.