Evaluation of Interferograms

The thinned fringes essentially carry the information of the path integrated temperature field. Hence to extract temperature profiles and heat transfer rates from the interferogram, it is fringe skeleton rather that the fringe band that is needed. In the context of three-dimensional reconstruction of the temperature field, the line integral of the temperature field is required over a uniform grid so that topographic algorithms can be applied. Consequently the calculation of methodology adopted for the calculation of fringe temperature associated with the fringes is an important step in interferometry. The technique has been discussed in the context of an experiment with a differentially in this section. The technique has been discussed in the context of an experiment with a differentially with a differentially heated fluid layer.

Figure 4.31: Calculation of the fringe temperature from an idealized fringe skeleton

For definiteness, consider the fringe skeleton as shown in figure 4.31. The upper and lower walls as shown in the figure have known temperatures. It is possible that high temperature gradients near the wall produce a large number of thin fringes. Hence during the recording and processing of the interferogram a few near wall fringes could be lost. The loss of near wall fringes could be due to the finite resolution the CCD camera, and loss of signal information during filtering and other image processing operations. The first fringe seen in a thinned interferogram near the wall will thus be of arbitrary order. One cannot assign a temperature to the fringes directly from the wall temperature though the wall itself is an is an isotherm. Even when no near-wall fringe is lost, assigning a temperature to the first fringe is not straight-forward since the wall (though an isotherm) need not be a fringe i.e., a site for destructive interference. The following procedure has been adopted in the present work to derive temperature values at the fringes.