The present discussion is focussed towards fringe thinning and the associated errors. The thinning operations have been carried out with the filtered and enhanced interferograms. Three different approaches have been adopted in this regard: (a) search of the minimum intensity points within the dark band, (b) curve fitting through the centers of the dark band, and (c) freehand tracing of the fringe skeleton using a paintbrush option available in windows-95. These three methods are discussed below in detail.

The temperature corresponding to the individual fringes have been computed using the two known wall temperature and the temperature difference between two successive fringes. Refraction errors have been estimated to be quite small in present experiments. The information available about temperature at fringe locations has been transferred to a two-dimensional grid by using two-dimensional quardratic interpolation. Interpolation errorss have been found to be less than 0.1%. The major source of error betweenthe original interferograms and the data on the interpolated grid was found to be due to fringe thinning alone, other errors owing to filtering for example, being negligible.

The temperature available at this stage for each grid point is a line integral of the temperature field and constitutes the input to the tomographic algorithm. Since refraction errors are small in the present set of experiments, projection data on different horizontal planes can be taken to be independent. Hence, a sequential plane-by-plane reconstruction has been carried out to cover the three dimensions of the cavity. The present discussion on the influence of fringe thinning on tomographic reconstruction is based on two projection angles. Since the number of projections is limited, the algebraic reconstruction technique as against the transform methods has been employed. The use of two orthogonal projection angles is not a limitation because these can still be used to determine the overall features of the dependent variable [88]. Subbarao et al. [32] have evaluated the performance of the algebric reconstruction techniques for interferometric projection data of the temperature field. They have concluded that the multiplicative algebraic reconstruction technique (MART) is best suited in terms of errors and computer time. Hence, in the present work the multiplicative reconstruction technique has been used.