We propose a mathematical model for the movement in absorbing materials of
photorefractive holograms under feedback constraints. We use this model to
analyze the speed of a fringe-locked running hologram in photorefractive si
llenite crystals that usually exhibit a strong absorption effect. Fringe-lo
cked experiments permit us to compute the quantum efficiency for the photog
eneration of charge carriers in photorefractive crystals if the effect of b
ulk absorption and the effective value of the externally applied field are
adequately taken into consideration. A Bi12TiO20 sample was measured with t
he 532-nm laser wavelength, and a quantum efficiency of Phi = 0.37 was obta
ined. Disregarding absorption leads to large errors in Phi. (C) 2000 Optica
l Society of America [S0740-3224(00)00209-5].