A light modulator (1) is proposed which has a photochromic layer (15), which can be activated optically by control light, for modulating signal light, and at least one optically transparent substrate (13) for the photochromic layer (15), in which the light modulator (1) has at least one filter layer (14), which reflects the control light in a wavelength-selective fashion for the purpose of retroreflecting control light which has penetrated the photochromic layer (15). The reflecting filter layer (14) ensures that the control light is utilized efficiently for the photochemical conversion (photoconversion) of the photochromic material, since the control light passes twice through the photochromic layer (15). The reflecting filter layer (14) has the function, furthermore, of separating the control light from the modulated signal light such that the modulated signal light can be evaluated without significant interference from the control light.