A modified schlieren optical system was designed and constructed for a quantitative investigation of free-convection boundary layer temperature profiles. The differential equations governing convective heat transfer belong to the most difficult class of theoretical physics and have been solved analytically for only a few simple cases. Interference photographs, interferograms, obtained using the schlieren interferometer provide a means of circumventing the mathematical problem for many conditions. The photographic results, checked by using thermocouples, were used in evaluating the temperature distribution, the key to the solution of these problems. This paper reports the application of a modified schlieren system in determining the geometric configuration of the thermal boundary layer for free convection from a flat plate at various orientations with the horizontal. Included is a description of the optical and photographic methods used.