Based on the above discussion, the following conclusions can be drawn.

1. In low temperature gradient experiments, all the three techniques correlate well with one another. Interferograms are limited by few fringes in air, and too many fringes in water.  The shadowgraph image does not show sufficient contrast for analysis. In this respect, the schlieren technique is most amenable to data reduction.
2. In high gradient experiments, both schlieren and shadowgraph yield clear images.  The interferograms are however corrupted by refraction errors. Schlieren and shadowgraph track the temporal response of the fluid medium in the form of the light intensity variation.
3. In high Rayleigh number experiments with water, the flow field is turbulent. Shadowgraph images are seen to be meaningful, as against interferograms and schlieren. The shadowgraph images reveal a considerable amount of physical information, including boundary-layers, plumes and time scales. It can be independently used to improve existing models of engineering turbulence.