Research On Optical Key Technologies Of Head-Mounted Display With An Exit Pupil Expander

A head-mounted display(HMD) is a miniaturized near-eye visual optical system with a combination of micro-display and imaging eyepiece. It is widely used in fields of virtual reality and augmented reality as it meets the human eyes’ needs of obtaining real-time optical display of information in special environments. To enhance the system performance of a military HMD, such as contrast adjustment range, displays brightness and resolution, the reaserchers proposed a solution of the so called retinal scanning display(RSD) based on the laser micro-scanning display technology. In recent years, in order to adapt to the urgent portable needs of the civilian HMD systems, new design ideas of a HMD based on the optical planar waveguide has emerged. These new protypes of HMD are unique in that: the use of the exit pupil expander which effectively expanding a small exit pupil in two dimensions so that meet the visual requirements of the human eyes, while improving system performance or achieving portability of the whole system. In this paper, key technologies of exit pupil expansion in retinal scan display(RSD) and the HMD with semi-transparent films array waveguide are studied theoretically, which reveals the physical laws of realizing the exit pupil expansion. The effective ways to extend exit pupil has been obtained. The main work can be divided into two parts.(1) Through extensive research study on foreign RSD status, the in-depth analysis of system design ideas and the key technical difficulties are analyzed in detail. The physical mechanisms of exit pupil limits are described on the perspective of optical system design. Two typical exit pupil expanders of binary grating and microlens array are theoretically studied by using a method of diffraction integral transformation. It provides a theoretical support for the design of exit pupil expanders. The exit pupil expanding laws of hexagonal arrangement of both a single lens array and a double lens arrays are comparatively studied by using methods of optical modeling and simulations. Double lens arrays are found as an effective way to extend the color display RSD pupil. Taking an exit pupil diameter of 15 mm for example, the validity of the exit pupil expansion is verified. The subjective evaluation model of pupil expansion is proposed which provides an effective method for the evaluation of the expanded exit pupil. Micro-lens surface error models are established where the errors are introduced by the typical microlens array processing methods. The influences on the uniformity of exit pupil expansion by different processing error are studied, which provids a scientific guidance for the design and development of the microlens array.(2) The unique advantages of the planar optical waveguide in a portable HMD design are discussed. The realization of the eyepiece’ eixt pupil extension has been summarized. The study is focus on the exit pupil expansion program based on semi-transparent films array waveguide. The structure constraints of the planar waveguide are theoretically deduced using light refraction and reflection model, which has important theoretical significance to an initial optical design. The mothod of using two perpendicular arrayed planar waveguide to achieve an effective two-dimensional expansion of the eyepiece’ exit pupil is proposed. Using this method meets the optical design demand in exit pupil. Meanwile, the volume of the eyepiece is noteblly reduced. A small exit pupil(about 2.4mm) eyepiece optical system is designed. The eyepiece and planar waveguide system model is simulated in optical software. The quality of the final image and the pupil irradiance distribution of system are studied. The design examples show that it is possible to use only spherical lens to achieve an exit pupil size of 7mm×12mm and field of 15°×20°. The weight of optical components is just about 36 g. The distortion is less than 0.13% and the MTF at 40 lp / mm of each field of view is greater than 0.58. The research laid the foundation for the further development and design of semi-transparent films array waveguide HMD.