In the last ten years liquid crystals have emerged as reliable temperature sensors for heat transfer research, and have been applied in a number of situations to visualize temperature distribution under complex flow fields. In this study, the true-colour image processing of the liquid crystal has been successfully applied to the study of heat transfer problems. A new procedure has been developed (this involves a colour scale representation of the local contours of local Nusselt number) to obtain an average Nusselt number using the Hue ratio coefficient (ratio of the number of pixels for the hue range considered to the total number of pixels of a certain area).