a substrate having a diffraction grating of a first periodicity formed thereon , a mask having a diffraction grating of a second periodicity formed thereon , the mask and substrate being positioned such that the respective mask and substrate gratings are generally parallel opposing one another on the mask and substrate , means for providing collimated coherent light directed so as to impinge on the mask and substrate gratings , and means for separately collecting , recombining , and detecting the intensity of at least a first given order of diffracted light beams as respectively diffracted by the mask and substrate gratings .