Special features of the optical transmission of lithium niobate crystal samples with surface doping by photorefractive impurities at the wavelength of a helium-neon laser under short-wavelength incoherent background illumination are experimentally investigated and discussed. The possibility of optically controlled thermooptical modulation of the coherent light intensity with characteristic time of a few seconds is demonstrated for samples with surface doping by a combination of iron and copper ions. © 2006 Springer Science+Business Media, Inc.