Norihito Nakazawa

Application of DuPont photopolymer films to automotive holographic display

Automotive holographic head-up display (HUD) systems employing DuPont holographic photopolymer films are presented. Holographic materials for automotive application are exposed to severe environmental conditions and are required high performance. This paper describes the improvement of DuPont photopolymer films for the automotive use, critical technical issues such as optical design, external color and stray light. The holographic HUD combiner embedded in a windshield of an automobile has peculiar problems called external color. Diffraction light from holographic combiner makes its external color tone stimulative. We have introduced RGB three color recording and color simulation in order to improve the external color. A moderate external color tone was realized by the optimization in terms of wavelengths and diffraction efficiencies of the combiner hologram. The stray light called flare arises from a reflection by glass surface of windshield. We have developed two techniques to avoid the flare. First is a diffuser type trap beam guard hologram which reduces the intensity of the flare. Second is the optimization of the design of hologram so that the incident direction of flare is lower than the horizon line. As an example of automotive display a stand-alone type holographic HUD system attached on the dashboard of an automobile is demonstrated, which provides useful driving information such as route guidance. The display has a very simple optical system that consists of only a holographic combiner and a vacuum fluorescent display. Its thin body is only 35 mm high and does not obstruct drivers view. The display gives high contrast and wide image.

IDT-SEM observation of fringe pattern in volume hologram and its application to multicolor recorded HOEs

Photopolymer films developed by DuPont have excellent features as volume hologram recording material including high resolution, high diffraction efficiency, full color recording capability and fabrication with all dry processing. We have prepared several types of HOEs by using the films to implement designed function such as high power, multi-color recording and controlling of the diffraction bandwidth. To accomplish them, direct and precise knowledge of the fringe pattern such as slant angle and fringe space was strongly required. We have developed a new technique for the observation of the fringe pattern by scanning electron microscope (SEM) with an aid of Iodine dying pre-treatment (IDT-SEM). It is thought that Iodine is preferentially adsorbed at double bondings in unpolymerized monomers, which leads to a presumption that Iodine distribution detected by SEM accurately reproduce the refractive index modulation in volume hologram. This has been confirmed experimentally by the observation that fast Fourier transform analyses of SEM images for single, two and three color recorded HOEs are good agreement with optical spectra of the HOEs. The SEM image provides us the spatial information of both fringe space and slant angle with good resolution. This technique has been further applicable to other type of HOEs.