Flows having a variable fluid density can be visualized with optical methods. Recently, a background distortion technique, which can be considered a type of Schardin's schlieren #2, was proposed to study the refractive index variation in fluid and hence to measure the convective heat transfer. The background is a fringe pattern generated by a diffractive optical element (DOE). However, this system may exhibit poor performance when using air as a working fluid. Different strategies were proposed for improving sensitivity, such as modifying the experimental setup or increasing the frequency of the projected fringe pattern. In this paper the performance of the system is evaluated, by using different data processing methods, through a numerical simulation of free convection from a vertical plate in air. In particular, we found that the use of windowed Fourier transform method for data processing enhances both the accuracy and the sensitivity of the temperature gradient measurements. (C) 2012 Elsevier Ltd. All rights reserved.