Marker-free cell discrimination by holographic optical tweezers F. Schaal, M. Warber, S. Zwick, H. van der Kuip, T. Haist and W. Osten J. Eur. Opt. Soc.-Rapid Publ., 4 (2009) 09028 Published online: 03 June 2009 DOI: 10.2971/jeos.2009.09028