Magic Leap Patent | Apparatus for optical see-through head mounted display with mutual occlusion and opaqueness control capability
Publication Number: 10175491
Publication Date: 2019-01-08
Applicants: Magic Leap
The present invention comprises a compact optical see-through head-mounted display capable of combining, a see-through image path with a virtual image path such that the opaqueness of the see-through image path can be modulated and the virtual image occludes parts of the see-through image and vice versa.
Over the past decades, Augmented Reality (AR) technology has been applied in many application fields, such as medical and military training, engineering design and prototyping, tele-manipulation and tele-presence, and personal entertainment systems. See-through Head-Mounted Displays (ST-HMD) are one of the enabling technologies of an augmented reality system for merging virtual views with a physical scene. There are two types of ST-HMDs: optical and video (J. Rolland and H. Fuchs, “Optical versus video see-through head mounted. displays,” In Fundamentals of Wearable Computers and Augmented Reality, pp. 113-157, 2001.). The major drawbacks of the video see-through approach include: degradation of the image quality of the see-through view; image lag due to processing of the incoming video stream; potentially loss of the see-through view due to hardware/software malfunction. In contrast, the optical see-through HMD (OST-HMD) provides a direct view of the real world through a beamsplitter and thus has minimal affects to the view of the real world. It is highly (preferred in demanding applications where a user’s awareness to the live environment is paramount.
Developing optical see-through HMDs, however, confronts complicated technical challenges. One of the critical issues lies in that the virtual views in an OST-HMD appear “ghost-like” and are floating in the real world due to the lack of the occlusion capability. FIG. 1 shows a comparison illustration of the augmented view seen through a typical OST-HWID (FIG. 1a) and the augmented view seen through an occlusion capable OST-HMD (OCOST-HMD) system (FIG. 1b), In the figure, a virtual car model is superimposed on a solid platform which represents a real object. Without proper occlusion management as shown in FIG. 1a, in a typical AR view, the car is mixed with the platform and it is difficult to distinguish the depth relationship of the car and the platform. On the contrary, with proper occlusion management as shown in FIG. 1b, the car blocks a portion of the platform and it can be clearly identified that the car seats on the top of the platform. Adding occlusion capability to the AR display enables realistically merging virtual objects into the real environment. Such occlusion-enabled capability may generate transformative impacts on AR display technology and is very appealing for many augmented-reality based applications.
An OCOST-HMD system typically comprises of two key sub-systems. The first is an eyepiece optics that allows a user to see a magnified image displayed on a microdisplay; and the second is a relay optics that collects and modulates the light from an external scene in the real world, which enables the opaqueness and occlusion control on the external scene when presenting to the viewers. The key challenges of creating truly portable and lightweight OCOST-HMD system lies in addressing three cornerstone issues: (1) an optical scheme that allows the integration of the two subsystems without adding significant weight and volume to the system. (2) a proper optical method that maintains the parity of the coordinate system of the external scene; (3) an optical design method that enables the design of these optical subsystems with an elegant form factor, which has been a persisting dream for HMD developers. Several occlusion-capable optical ST-HMD concepts have been developed (U.S. Pat. No. 7,639,208 B1 Kiyokawa, K., Kurata, Y., and Ohno, H., “An Optical See-through Display for Mutual Occlusion with a Real-time Stereo Vision System,” Elsevier Computer & Graphics, Special Issue on “Mixed Realities–Beyond. Conventions,” Vol, 25, No. 5, pp. 2765-779, 2001. K. Kiyokawa, M, Billinghurst, B. Campbell, E. Woods, “An Occlusion-Capable Optical See-through Head Mount Display for Supporting Co-located Collaboration,” ISMAR 2003, pp, 133-141). For example, Kiyokawa et. al. developed ELMO series occlusion displays using conventional lenses, prisms and minors. Not only because of the number of elements being used, but also more importantly due to the rotationally symmetric nature of the optical systems, the existing occlusion-capable OST-HMDs have a helmet-like, bulky form factor. They have been used exclusively in laboratory environments due to the heavy weight and cumbersome design. The cumbersome, helmet-like form factor prevents the acceptance of the technology for many demanding and emerging applications.
This invention concerns an optical see-through head mounted display (OST-HMD) device with opaqueness control and mutual occlusion capability, The display system typically comprises of a virtual view path for viewing a displayed virtual image and a see-through path for viewing an external scene in the real world. In the present invention, the virtual view path includes a miniature image display unit for supplying virtual image content and an eyepiece through which a user views a magnified virtual image. The see-through path comprises of an objective optics to directly capture the light from the external scene and firm at least one intermediate image, a spatial light modular (SLM) placed at or near an intermediate image plane in the see-through path to control and modulate the opaqueness of the see-through view, and an eyepiece optics through which the modulated see-through view is seen by the viewer. In the see-through path, the objective optics and eyepiece together act as a relay optics for passing the light from the real world to viewer’s eye. To achieve a compact form factor and reduce the viewpoint offset, the see-through path is folded into two layers through several reflective surfaces, a front layer accepting the incoming light from an external scene and a back layer coupling the light captured by the front layer into a viewer’s eye. The see-through path is merged with the virtual image path by a beamsplitter so that the same the eyepiece is shared by both paths for viewing displayed virtual content and the modulated see-through image. The microdisplay and the SLM are optically conjugate to each other through the beamsplitter, which makes the pixel level occlusion manipulation possible. In the present invention, the eyepiece, the objective optics, or both may be rotationally symmetric lenses or non-rotationally symmetric freeform optics. In one of its significant aspects, the present invention may utilize freeform optical technology in eyepiece optics, objective optics or both to achieve a compact and lightweight OCOST-HMD design.
The reflective surfaces for folding the optical paths may be planar mirrors, spherical, aspherical, or freeform surfaces with optical power. In another significant aspect of the present invention, some of the reflective surfaces may utilize freeform optical technology. Some of the reflective surfaces may also be strategically designed to be an integral part of the eyepiece or objective optics where the reflective surfaces not only facilitate the folding of the optical path for achieving compact display design but also contribute optical power and correct optical aberrations. In an exemplary configuration, the present invention may use a one-reflection or multi-reflection freeform prism as an eyepiece or objective optics where the prism is a single optical element comprises of refractive surfaces and one or more than one reflective surfaces for folding the optical path and correcting aberrations.
In another significant aspect of the present invention, the objective optics in the see-through path forms at least one accessible intermediate image, near which an SLM is placed to provide opaqueness control and see-through modulation. In the present invention, either a reflection-type SLM or a transmission-type SLM may be used for modulating the see-through view for occlusion control. A longer back focal distance for the objective optics is required for a reflection-type SLM than a transmission-type SLM. A reflection-type SLM may have the advantage of higher light efficiency than a transmission-type SLM.
In another significant aspect of the present invention, the see-through path may form an odd or even number of intermediate images. In the case of an odd number of intermediate images, an optical method is provided to invert and/or revert the see-through view in the see-through path. For example, depending on the number of reflections involved in the see-through path, examples of the possible methods include, but not limited to, inserting an additional reflection or reflections, utilizing a roof mirror surface, or inserting an erection prism or lens. In the case of an even number of intermediate images, no image erection element is needed if there is no parity change in the see-through view. For instance, multiple-reflection freeform prism structure (typical more than 2) may be utilized as eyepiece or objective optics, or both, which allow folding the see-through optical path inside the objective and/or eyepiece prism multiple times and form intermediate image(s) inside the prisms which eliminates the necessity of using an erection roof reflective surface. The potential advantage of eliminating the erection prism is that the approach may lead to a more compact design.