various techniques have existed for see-through hmds. most of these techniques can be summarized into two main families: "curved mirror" (or curved combiner) based and "waveguide" or "light-guide" based. the curved mirror technique has been used by vuzix in their star 1200 product, by olympus, and by laster technologies. various waveguide techniques have existed for some time. these techniques include diffraction optics, holographic optics, polarized optics, and reflective optics: