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An interferometric procedure is described for measuring the local surface temperature gradient directly from a wedge (finite) fringe field. For a two-dimensional temperature field, it is shown that the local convective heat transfer coefficient can be obtained by measuring the angle of intersection of a fringe with an isothermal surface. Using an uncertainty analysis it is shown that this method gives the most accurate results for the measurement of relatively low gradients. A correction term for refraction effects and an expression for the optimum wedge fringe spacing are also derived. Measurements of the natural convection from a vertical flat plate are compared to the boundary layer solution.