A diffraction grating device (100) has both reflecting (108) and transmitting (106) gratings inter leaved to allow both transmission diffraction patterns and reflection diffraction patterns to be formed by means of the same device (102). This makes it possible, for example, to produce a diffraction pattern of one older with a beam (134) directed to be diffracted by the reflecting diffraction grating or with a beam (132) directed to be diffracted by the transmitting diffraction grating when either beam is incident alone. It also makes it possible to generate a diffraction pattern of a different order when both beams are incident simultaneously, which reduces effective order of the grating.