The present invention relates to an image display apparatus such as a head mounted display (HMD), and especially to an image display apparatus which forms images in mutually different viewing angle areas in order to display a combined image of the images joined to each other.
Observation optical systems used in HMD are desired to be compact and lightweight. The observation optical systems are also desired to be capable of displaying an image with a wide viewing angle to increase realism in the displayed image.
Among observation optical systems for the HMD, a so-called ocular optical system type observation optical system is comparatively easy to be miniaturized and provided with a wide viewing angle. However, such an ocular optical system type observation optical system requires, for providing a wide viewing angle with one image-forming element, a large-size image-forming element having a large image-forming area where an original image is formed.
In contrast thereto, a so-called tiling technology is proposed which enables displaying of an image with a wide viewing angle by using plural small-size image-forming elements having a small image-forming area. The tiling technology joins plural enlarged images to each other which are formed by light fluxes from plural original images formed on the plural image-forming elements to enable the displaying of one enlarged combined image. Such observation optical systems (HMD) for displaying an image with a wide viewing angle by the tiling technology are disclosed in Japanese Patent Laid-Open No. 09-166759 and Japanese Patent Laid-Open No. 11-326820.
Japanese Patent Laid-Open No. 09-166759 discloses an observation optical system in which two optical units are provided for one eye, each optical unit including an image-forming element and a three-surface prism having an entrance surface, an internal total reflection/exit surface and a reflection surface, and the two optical units are disposed at upper and lower positions so as not to be overlapped with each other in a right and left direction. In this observation optical system, two enlarged images are formed by the two optical units at right and left positions, and thereby one enlarged combined image with a horizontally wide viewing angle is displayed. When a cross section where a light flux (optical path) is folded by at least twice reflections by an optical element such as the three-surface prism is defined as a local meridional cross section, the “right and left (horizontal) direction” in Japanese Patent Laid-Open No. 09-166759 corresponds to a direction along a local sagittal cross section orthogonal to the local meridional cross section.
Japanese Patent Laid-Open No. 09-166759 also discloses an example in which the two optical units disposed at the upper and lower positions are relatively moved so as to be overlapped with each other in a visual axis direction of an observer. A region where the two optical units are overlapped with each other allows observation of an outside optical image (so-called optical see-through observation). In this example, enlarged images formed by the two optical units are overlapped with each other at a central region in the right and left direction to form one enlarged combined image.
Japanese Patent Laid-Open No. 11-326820 discloses an observation optical system including two division optical systems which form two enlarged images, the two division optical systems using one prism having a symmetric shape in a local meridional cross section. In this observation optical system, a light flux from an original image formed on one image-forming element enters the prism through an entrance surface thereof, is totally reflected by an internal total reflection/exit surface thereof, is then reflected by a concave mirror surface, and finally exits from the prism through the internal total reflection/exit surface to proceed to an exit pupil. A light flux from an original image formed on the other image-forming element proceeds to the same exit pupil in a same manner as that of the light flux from the one image-forming element. The two concave mirror surfaces that cause the light fluxes from the two image-forming elements to proceed to the same exit pupil are closely adjacent (joined) to each other.
However, the latter observation optical system disclosed in Japanese Patent Laid-Open No. 09-166759 does not contribute to miniaturization of the observation optical system (in other words, of the HMD) in a direction of the local meridional cross section (vertical direction) in which the observation optical system is decentered, because its viewing angle is not divided in the direction of the local meridional cross section while the viewing angle is divided in a direction of the local sagittal cross section (horizontal direction) to obtain a wide viewing angle. Each of the observation optical systems disclosed in Japanese Patent Laid-Open No. 09-166759 aims to enable the see-through observation through the overlapped region of the two optical units. Therefore, in the local meridional cross section, the viewing angles of the two optical units are the same, and parts thereof are overlapped with each other. Therefore, image division cannot be performed in the local meridional cross section, which is not effective for reducing the thickness or vertical size of the observation optical system.
Moreover, the former observation optical system disclosed in Japanese Patent Laid-Open No. 09-166759 and the observation optical system disclosed in Japanese Patent Laid-Open No. 11-326820 form a joined portion of the two images formed by the light fluxes from the two original images (corresponding to a joined portion of the two concave mirror surfaces) in the enlarged combined image. The joined portion of the two concave mirror surfaces generates light scattering and flare, which makes the joined portion of the images noticeable.