To locate the point of minimum intensity, eight directions of movement (1-8) are defined (fig.4.19).

Figure 4.19: Eight possible directions for movements.

The direction in which the minima should be searched is located by placing a template whose size is user-specified at the concerned pixel. While the directions of movement are defined on a template, the intensity minima are computer independently. The choice of size of the template for minimum intensity calculation is related to the fringe thickness. The near wall fringes in the present study were very thin owing to large heat flux. Hence, the choice of the template size was limited to the minimum possible, namely a square. Use of a bigger template is likely to interfere with neighboring fringes and is, hence, undesirable. However, a large template can be used when the fringe bands spread over several pixels. Use of a template as big as and results in an average direction of minimum intensity and tends to produce a smooth tracing. On the other hand, local unphysical variations can be bypassed by using a large template. Hence a template appears to be an optimal choice for interferometric fringes.

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