Multidirectional interferometric data of an inhomogeneous phase object are obtained by double exposure reflection type holography. The evaluation of the interferograms for a computerized tomographic reconstruction of the local index of refraction by the convolution method is performed with subfringe resolution. For testing purposes the phase object is made by careful modeling of a convective heat flow in air. The reconstructed refractive index field is converted into a temperature distribution and is compared with independently measured temperature values, thus providing a strict examination of the quality of the tomograms.