The transport-of-intensity equation (TIE) describes a deterministic relation between the intensity distribution in different focal planes and the corresponding phase distribution. A Green's function solution of the TIE is used to retrieve the phase distribution of an object considering specific boundary conditions. This leads to an accurate reconstruction of the properties of phase objects, e.g. the refractive indices and thus the numerical aperture (NA) of optical fibers. The required intensity distributions are captured simultaneously by the use of a multi-camera microscope. The TIE is solved using a computer algorithm, which can be massively parallelized. This offers the application of general purpose computation on graphics processing units (GPGPU). Therefore real-time reconstruction of the phase distribution is possible.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.