the fiber 39 s principal axis locations 1 through 4 in fig5 a represent one representative clockwise circular arc generated by rotating mirror 470 wherein the arc encompasses the fovea region 520 similarly fig5 a 1 maps in the time domain a one to one relationship of locations 1 through 4 in the circular arc to the electronic signal generated by the differential polarization output x u2212 y described earlier as can be seen in one revolution of the arc 510 about the fovea at angular frequency u03c9 the time domain differential polarization signal x u2212 y generates a 2 u03c9 frequency doubled component in contrast fig5 b depicts one representative clockwise circular arc generated by rotating mirror 470 wherein the patient 39 s eye is skewed off axis relative to the incoming optical beam 451 by a sufficient amount that the circular arc lies outside and does not encompass the fovea region 520 in this case the one to one mapping in the time domain generates a time domain differential polarization signal x u2212 y at the same angular frequency u03c9 as the rotating mirror 470 given this the differential polarization signal can be analyzed by frequency spectrum analysis to detect the presence or absence of the 2 u03c9 frequency doubled signal which can be used to determine if the patient 39 s eye is within a particular angular offset relative to the incoming optical beam 451 for example fast fourier transform techniques can be used to analyze the ratio of the 1 u03c9 to 2 u03c9 signal strength and a predetermined threshold for this ratio may be established in clinical trials to establish a pass fail u201c refer u201d.