@article{spencer:hpm,
        author={Ben Spencer and Mark W. Jones},
        title={Hierarchical Photon Mapping},
        year={2009},
        journal={IEEE Transactions on Visualization and Computer Graphics},
        volume={15},
        number={1},
        month={Jan-Feb},
        pages={49--61},
        issn={1077-2626},
        doi = {http://doi.acm.org/10.1109/TVCG.2008.67}
}