We propose the use of a nonlinear bacterior-hodopsin film to self-induce a
Zernike-type filter in robust optical phase-contrast systems. The device re
quires relatively low light intensity levels (as low as 200 nW/cm(2)) at wa
velengths around 633 nm and can contrast dynamical phase distributions. (C)
2001 Society of Photo-Optical Instrumentation Engineers.