A small photochromic grating is selectively and continuously detected from a much larger photorefractive modulation in Fe-doped LiNbO3 crystals in a two-wave mixing experiment. The recording setup is actively fixed to either the photochromic or the photorefractive grating. In the former case a photorefractive modulation largely exceeding the value required for 100% diffraction efficiency is achieved. In the latter case this modulation is limited to 100%. Experimental results are in good agreement with the theoretical model.11918371841