Abstract: A technique is provided for analyzing the shape of the amplitude envelope of a diffraction pattern's first order side lobe resulting from passing coherent light through a fabric material. The light intensity of the lobe is scanned from one side of the lobe to the other by a linear photo-diode array to provide successive voltage signals whose respective voltage values are a function of the light intensity at successively spaced increments from one side of the lobe to the other. Each of these successive voltage signals are compared with a series of reference voltages of substantially less number than the number of spaced increments, each reference voltage having a value greater than the value of the preceding reference voltage in the series to define a voltage range encompassing the highest voltage in the series of voltage signals.