The flame length was found by using the averaged OH* chemiluminescence images. First, a line profile was drawn horizonally across the flame image somewhere above the high intensity area. If the maximum intensity of that line profile was less than 750, the line intensity profile would move down one pixel at a time until the maximum intensity reached 750.
Figure 1 shows where this point is in relation to the flame, and Figure 2 shows its corresponding intensity profile. The final flame length was calculated by finding the difference between the bottom of the flame (easily identifiable and consistent) and the top of the flame.