A diffraction grating generates even-order, odd-order, and 0^th-order exit-pupil images. The even-order exit-pupil images have brightness levels within a first range and the odd-order exit-pupil images have brightness levels within a second range that is different from the first range. In one example, the even-order exit-pupil images are virtually invisible, i.e., missing, the odd-order exit-pupil images have the same or approximately the same intensities, and the 0^th-order exit-pupil image has an intensity greater than the respective intensities of the odd-order exit-pupil images.