1. a Biological Imaging Center, Beckman Institute , California Institute of Technology , Pasadena, California, 91125 E-mail:
2. b Biological Imaging Center, Beckman Institute , California Institute of Technology , Pasadena, California, 91125 E-mail:
3. c Biological Imaging Center, Beckman Institute , California Institute of Technology , Pasadena, California, 91125
4. d Electrical and Computer Engineering , University of California , Santa Barbara, California, 93106 E-mail: