1. Technical Field
The present invention relates to a virtual image display device that provides an image formed by an image display element or the like to an observer, particularly to a virtual image display device that is suitable for a head-mounted display mounted on the observer's head.
2. Related Art
A variety of optical systems are proposed, which are incorporated into a virtual image display device such as a head mount display (hereafter, referred to as HMD) that is mounted on an observer's head (refer to Japanese Patent No. 2746697, Japanese Patent No. 3787399, Japanese Patent No. 4218553, and Japanese Patent No. 4819532).
Regarding the virtual image display device such as the HMD, it is desirable to widen an angle of view and to reduce a weight of the device. Particularly, in order to improve a wear, it is important to make the device thin in thickness in a viewing axis direction of the observer and to make the center of gravity of the device closer to the observer.
In addition, if a view of the observer is entirely covered such that the observer can see only the image light, the observer cannot see the outside state and may have a feeling of anxiety. Furthermore, a variety of new applications of the virtual reality can be generated by making it possible to see the outside and the image in overlapping. For this reason, a display that displays the image light in overlapping without interfering with the view of the outside is desired.
Furthermore, in order to improve the wear on the observer and to make an external appearance of the device better, it is generally desirable to take on the form of glasses in general, and to place the image display device beside the face, not over the eyes.
In order to separate the image display device from the position of the eyes of the observer such that the size of the optical system becomes small and the view is not interfered, it is preferable to use a relay optical system in which an intermediate image is formed after forming an image of the display image light in the optical system once, and in which the enlarged intermediate image is displayed.
For example, in Japanese Patent No. 2746697, as a relay optical system, an optical system is proposed, in which an intermediate image is formed inside of a light guide plate using the light guide plate of plane-parallel shape in which the end surface has a parabolic mirror and a projection lens. However, in a case of the optical system in Japanese Patent No. 2746697, since the projection lens is large, it is difficult to reduce the size and weight.
In Japanese Patent No. 3787399, as a relay optical system, an optical system that includes a light guide prism having an emission reflection surface of curved surface and a projection lens, is proposed. However, in the optical system in Japanese Patent No. 3787399, there is no consideration for the outside viewing by the observer. In order for the observer to see the outside using this optical system, it is needed to paste a compensation prism on the reflective surface spread over the entire prism and to provide a half mirror on the bonded surface. Then, the image light is reflected twice on the half mirror surface, and thus the image becomes very dark.
In Japanese Patent No. 4218553, as a relay optical system, an optical system that includes a projection lens, a concave mirror, and a light guide plate is proposed. In the optical system, the reflection efficiency is increased by combining a wavelength plate and a polarization half mirror. However, in order for the observer to view the outside using the optical system in Japanese Patent No. 4218553, it is needed to paste a compensation lens together with the concave mirror, and accordingly, the system becomes thick as a whole.
In Japanese Patent No. 4819532, as a relay optical system, an optical system in which the overall length is shortened to be compact by bending the light path is proposed. However, in a case of the optical system in Japanese Patent No. 4819532, a convex portion of the prism center and a projection lens interfere with the view. In addition, the image light is folded back after transmitting through the half mirror, and is reflected at the half mirror, and is incident on the eyes. Therefore, the observed image becomes dark.
Moreover, in a method of forming a virtual image using a light guide prism, for example, bending of the light path on a numbers of different curved surfaces by forming a light guide prism by connecting a plurality of prism portions or forming a light guide prism by dividing a light guide prism into a plurality of prism portions, can be considered. In this case, the degree of freedom is increased in each curved surface, and an improvement in image forming performance is expected. In contrast, depending on the optical design, a use environment, and the like, there is a concern that the external light is reflected in an unintended direction, and a ghost light can be generated in the cuts and the joints between the curved surfaces.