A time-average technique was developed to measure the unsteady and turbulent free convection heat transfer in tall vertical enclosure using a Mach-Zehnder interferometer. The method used a digital high speed camera to obtain the time-averaged heat transfer rates. Optical heat transfer measurements were made in a differentially heated vertical cavity with isothermal walls. The cavity widths (distance between the plates) were L = 12.7, 32.3, 40, and 56.2 mm. The corresponding Rayleigh numbers were about 3X10[superscript] 3, 5 X 10⁴, 1 X 10⁵, 2.7. X 10⁵, respectively and the enclosure aspect ratio ranged from A=18 to 76. The test fluid was air and the temperature differential was about 15 K for all the measurements. Finite fringe interferograms were taken with a high speed camera. Interferograms of the fluctuating temperature field were captured for ten seconds at a frequency of 100Hz. These images were enhanced and processed using MATLAB to measure the local time-averaged heat transfer rate. This time-averaged heat flux was measured at many locations along the vertical cavity walls in order to obtain the spatial average. To validate the proposed technique, the average Nusselt number was compared to measured values and correlations from the literature. In both laminar and turbulent flow conditions, the current measurements compared well with the ElSherbiny correlation.