Hell, S. W.; Willig, K.; Dyba, M.; Jakobs, S.; Kastrup, L.; Westphal, V.: Nanoscale resolution with focused light: STED and other RESOLFT microscopy concepts. In: Handbook of biological confocal microscopy, S. 571 - 579 (Hg. Pawley, J.B.). Springer, New York (2006)