The irradiation at 514.5 nm of a ferroelectric liquid crystal (FLC) film doped with a photochromic chiral thioindigo dye causes a change in the birefringence of the FLC film under a constant dc voltage. This birefringence photomodulation is used to create electric field controlled dynamic holograms. The diffraction efficiency of the hologram is found to depend resonantly on the applied dc voltage. The unwinding of the FLC helical structure into a planar alignment as a result of a photoinduced increase of the spontaneous polarization(Ps)and of the corresponding dielectric torque is thought to be at the origin of the observed birefringence photomodulation. ©1998 American Institute of Physics.