The spatial coordinates of the fringes can be used to obtain the temperature profile. For schlieren and shadowgraph, the information regarding the thermal field is contained in the light intensity variation.  The respective thermal properties recovered are the local values of the first derivative and the Laplace operator applied to the temperature. These quantities have been plotted for the mid-plane of the cavity on the left side of Figure 5.19. The individual data points are specific to the mid-plane of the cavity, while the solid line indicates the overall trend.  The shaded circles of the left column indicate the gradients calculated from interferometric data (for schlieren) and from schlieren data (for shadowgraph).  A good overall match is a confirmation of the result that schlieren is a derivative of the interferometric field, and the shadowgraph in turn is the derivative of the schlieren. The appearance of dense fringes near the horizontal walls is indicative of high temperature gradients at these locations. This is brought in the schlieren image in the form of an increase in intensity as well as the data points. The central region is a zone of nearly constant temperature, where the gradients (and the light intensity values) are close to zero.  Thus, the schlieren images and interferograms correlate quite well with each other. They also correlate with the shadowgraph, once it is realized that in this approach, light is redistributed over the image. In a shadowgraph image, light from the region close to the cold top wall deflects towards the lower hot wall, where the light intensity shows a maximum.  Thus, high gradients are represented in a shadowgraph by regions of very low as well as very high light intensity. In the central core, the change in light intensity with respect to the initial setting is small. Thus the Laplacian operation of temperature in this region yields a practically zero value. The thermal lensing effect that distorts the shape of the cavity cross-section is most visible in the shadowgraph.

Quantitative analysis of the temperature field and wall heat transfer rates is reported for the lower range of cavity temperature differences (and hence Rayleigh number). Clear shadowgraph images could not be recorded for small temperature differences and have not been shown. At higher temperature differences, the field was seen to become unsteady. The discussion for larger cavity temperature differences is based on qualitative comparison of the three imaging techniques.