The holographic technique is quite close to the diffraction grating technique described above with the exception that a holographic element is used to diffract the light [3]. Holograms work by reflecting certain wavelengths of light. In this way, the incident light is reflected at a certain angle with regard to the hologram. Holograms are intrinsically limited when used in a waveguide due to the fact that the reflected light loses intensity with angular variation.
Only limited angles are possible in order not to lose too much light and to keep good image uniformity. Therefore, this technique is intrinsically limited in FOV. This technique is also plagued by color issues known as the “rainbow effect”. Holographic elements reflect only one wavelength of light so for full color, three holograms are necessary; one that reflects Red, Green, and Blue respectively. This not only adds cost but since the three holograms need to be “sandwiched” together, each wavelength of the light is slightly diffracted by the other color hologram adding color “cross-talk” in the image. Therefore, the eye sees some color non-uniformity or color bleeding when viewing the virtual image. Some of this color non-uniformity can be corrected electronically but there are limits to this as the human eye is extremely sensitive to this phenomenon. The holographic technique is used in the Sony and Konica Minolta as shown in figure below.
