An analysis of the data using the standard deviation of the signals produces a pattern that corresponds directly to the pattern of the grating. The time series is divided into window lengths of data. The standard deviation of the first data point of the first window together with the first data point of the rest of the windows is calculated. A second standard deviation calculation is made with the second points of all the windows. The calculation proceeds until all the points in the windows have been used. All the standard deviation values are then averaged. The following equations are used in the calculations.