An interferogram analysis technique for the measurement of convective heat transfer rates is described in which the local surface temperature gradient is measured directly from a wedge fringe field. It is shown that the local surface gradient for a two-dimensional temperature field can be obtained directly by measuring the angle of intersection of a wedge fringe with an isothermal surface. An uncertainty analysis is performed and it is shown that this "direct gradient method" is best suited for the measurement of relatively low temperature gradients. An expression for the optimum wedge fringe spacing and a correction term for refraction effects are also derived. Measurements of the free convective heat transfer from an isothermal vertical flat plate are made and compared to the exact boundary-layer solution.