50


    The outlier criteria used here is based on comparing standard deviation. First, the mean u and standard deviation o7 of the data set are calculated for each column in an image. Then, all data points that lie outside the interval (p - 3u, p + 3u) are disregarded


according to Modified Tau Thompson criterion [Holman 2000]. Applying this to the sum of sequence of interface images, the result, shown in Figure 4-6, is obtained.


                                             30

                                           025
     200

                                             20
     400-                                  U)
     U)
   z                                       0
   .X                                       2 15
                                           .C:
     600                                   0
                                             10
                                           0
     800
                                              5

    1000                                      0           L         =
             200   400   600   800   1000      0    200   400   600   800  1000
                    X pixels                               Y pixels


Figure 4-6 Sum of sequence of edge images after outlier rejection and a column (at 188th
       X pixel) of sum of sequence of interface images without any outlier points.

    After the outliers are eliminated, the interface image and standard deviation values are calculated again. The outlier criterion does not significantly affect the mean of the data set; hence the average interface image is almost unchanged (Figure 4-7). Analysis of the two images in Figure 4-7 shows that the difference between the interfaces is not more than one pixel.