The homogeneity of trapping centers in photorefractive crystals has been investigated with a novel method. Phase gratings written in the crystal are read out with a beam that is focused by a cylindrical lens. The diffracted intensity distribution is measured while scanning the focal line of the readout beam over the crystal plate. In an iron-doped and reduced KNbO3 crystal we find simultaneous electron and hole conduction in different crystal areas. The effects of poling and electrochemical reduction treatment are studied in doped and undoped, and reduced and unreduced KNbO3 crystals.