The pattern of ferroelectric domain orientations can be read out optically. The refractive index of the ferroelectric material at wavelengths from near- infrared through to near-ultraviolet is affected by the electric field within the material. A changing pattern of electric field domains within a ferroelectric substrate results in different regions of the substrate having different refractive indices. Under these conditions, the substrate behaves as a diffraction grating, allowing the pattern of domains to be inferred from the interference pattern present in the transmitted readout beam.