1. Field of the Invention
The present invention relates to a projection type image display apparatus that projects an image formed by a reflection type image forming device.
2. Description of the Related Art
A projection type image display apparatus where a digital micromirror device (DMD) modulates illumination light from an illumination optical system, a projection optical system enlarges and projects the modulated image light onto a screen is known. DMD is a microdisplay manufactured by Texas Instruments Incorporated. DMD has an image forming surface which is constituted so that micromirrors which can be set at any one of two rotational angles according to an input picture signal are arranged two-dimensionally. The micromirror which is set at one of the two angles reflects illumination light in a direction led to the projection optical system (hereinafter, such reflected light is called as projection light or image light). The micromirror which is set at the other angle reflects the illumination light in a direction which is not led to a screen (hereinafter, such reflected light is called as off light or unnecessary light).
The off light causes heat generation in the device. Further, when the off light enters the projection optical system and becomes harmful light which is imaged on the screen, deterioration in image quality such as deterioration in contrast is caused. For this reason, various handlings of the off light are proposed. For example, U.S. Pat. No. 6,560,048 B1 and Japanese Patent Application Laid-Open No. 2001-166255 disclose projection type image display apparatuss which are constituted so that off light is totally reflected by a prism provided to the projection optical system in a direction different from that of projection light and is absorbed by a light absorbing member. Further, U.S. Pat. No. 6,886,947 B2 discloses a projection type image display apparatus which is constituted so that illumination light is emitted from an approximately front of DMD, and off light is absorbed by a light absorbing member.
A system, which emits illumination light emitted from below by an illumination optical system to an upper part by a reflection type image forming device, is called as a bottom-light type system. The bottom-light type projection type image display apparatus, in which a center principal ray of the illumination light to enter the reflection type image forming device is on the same plane as a center principal ray of the projection light, can project an image onto a large-scale screen with a comparatively simple and inexpensive constitution. In the bottom-light type device where the center principal ray of the illumination light to enter the reflection type image forming device is on the same plane as the center principal ray of the projection light from the image forming device to the screen, off light advances towards the projection optical system similarly to the projection light. For this reason, the off light easily enters the projection optical system. The constitution relating to the handling of the off light, however, is not proposed for such a projection type image display apparatus.