We present an innovative method for quantitative and/or qualitative flow visualization by fringe projection through a test section with nonuniform refractive index distribution induced by convective heat transfer. Fringes are generated by a diffractive optical element (DOE). Information about the temperature gradient, encoded in the deformed fringe pattern, is demodulated by Fourier transform algorithms. The main advantages of the proposed procedure are flexibility in suiting to different heat transfer phenomena; full digital and automated data processing to provide both qualitative visualizations and quantitative maps of the temperature gradient field; full-field visualization; and low cost compared to interferometric equipment. The main disadvantages are reduced investigation area with low-cost TV cameras; sensitivity (i.e., smallest detectable temperature gradient); and lower accuracy than that of conventional interferometric and Schlieren techniques.