there are three amplitude states a non reversing fringe cycle will go through a sequence of four amplitude states eg positive mid band negative mid band if there are four amplitude states a non reversing fringe cycle will go through a sequence of six amplitude states eg 0 u2212 87 gt 87 0 u2212 87 u2212 87 u2212 0 lt u2212 87 u2212 87 u2212 0 a further case is shown in fig2 c in this situation a reversal is identified by the fringe amplitude dropping below 87 and not reaching 0 however it should be noted that noise on the fringe signal during a non reversing fringe cycle can lead to momentary changes of state and give a similar signature this form of reversal can also be recognised by the half period of the interval marked u201c long u201d being significantly greater than each of the adjacent half periods marked u201c short u201d in fig2 c theoretically the ratio of half periods is greater than 3 4 thus the preferred method is to identify this type of reversal by the method shown in fig2 b the fringe counting algorithm analyses each zero crossing period that is to say a period of half a fringe to identify reverses in scan count direction these processes include the following a zero crossing detection and fringe counting in the current direction b determining if the peak amplitude in a half fringe.