The present work reports the development and application of a lens-based rainbow schlieren deflectometry technique for non-intrusive investigation of heat transfer characteristics of dilute nanofluids. The experiments have been performed under natural convection regime. Buoyancy-driven convection has been set up in the vicinity of a heated vertical flat plate that is immersed in a quiescent ambient fluid medium with de-ionized water as the base fluid and Al2O3/water-based dilute nanofluids. The experiments have been carried out for three different volumetric concentrations of alumina nanoparticles i.e. 0.01%, 0.02% and 0.03%. The color variant of the classical schlieren technique namely the rainbow schlieren deflectometry has been employed to record the projection data of the temperature gradient field in the test cavity. The optical configuration of the imaging technique employs a specially-designed color filter for generating the desired schlieren effects. The recorded schlieren images are characterized by a gradation of hue distribution that is proportional to the strength of the temperature gradients prevailing in the test section. Results have been discussed in the form of two-dimensional rainbow schlieren images, whole field distribution of temperature gradients in the vicinity of the heated plate and local distribution of heat transfer coefficients as a function of the length of the plate for the varying concentrations of nanofluids. The profiles of local heat transfer coefficients along the length of the heated plate as obtained with nanofluids show a considerable difference when compared with that of the de-ionized water. Significant enhancement in the average values of heat transfer rates is achieved with nanofluids as the ambient fluid medium in comparison with the base fluid. The study clearly reveals the potential of rainbow schlieren deflectometry technique in the context of heat transfer measurements with nanofluids as the working medium. (C) 2016 Elsevier Ltd. All rights reserved.