A photon scanning tunneling microscope with a three-dimensional multiheight mode has been developed for the mapping of optical field distributions in integrated optical waveguide structures. The optical field is measured at different heights above the waveguide surface. The multiheight measurements also contain the optical information gathered with the commonly used constant gap measurements in addition to the topography of the waveguide surface. With the multiheight method, the decay length of the evanescent field is readily determined as function of the in-plane coordinate. Moreover, the evanescent light can be distinguished from scattered light.