Proceedings of effects of low-power light on biological systems IV [electronic resource] : 8-9 September 1998, Stockholm, Sweden / Giovanni F. Bottiroli, Tina I. Karu, Rachel Lubart, chairs/editors ; sponsored by EOS--The European Optical Society, SPIE--The International Society for Optical Engineering, [and] ELA--The European Laser Association ; cosponsored by SSL--The Scandinavian Society for Laser Therapy [and others]