Recently, there is an increasing demand for rear projection televisions. The rear projection television has the feature of flat television represented by LCD (Liquid Crystal Display) direct-view television and PDP (Plasma Display Panel). Compared to these televisions, the rear projection television provides a system that enlarges and projects an image display element as large as approximately one inch. Consequently, the rear projection television is available at low costs. The rear projection television is relatively easily compliant with demands for high image quality of full HD (high definition) signal levels represented by BS (Broadcasting Satellite) digital broadcasting and terrestrial digital broadcasting.
On the other hand, the rear projection television is mounted with a light source, an illumination optical system, and a projection optical system. The rear projection television uses a large-diameter rear mirror to project images onto a screen. FIG. 62 shows a well-known projection method so that an optical axis a travels through the center of an image display element b and a screen c. According to such method, it is principally very difficult to decrease a depth d and a size e of the bottom of a display at the same time.
In addition, there is available an optical system that obliquely projects images onto the screen (hereafter referred to as oblique projection) so as to enlarge projected images and thin the rear projection television (to decrease the depth).
For example, JP-A No. 100312/1993 proposes the projection optical system that uses a wide-angle lens having a large field angle. The projection optical system is provided with an image display element and a screen shifted from the projection optical system's optical axis and uses the end of the field angle for oblique projection.
As proposed in JP-A No. 80418/1993, the first projection optical system intermediately forms image light based on a light bulb. The second projection optical system enlarges and projects the image on a screen. Optical axes of the respective projection optical systems are appropriately tilted for oblique projection. A recently proposed projection optical system uses a reflector for oblique projection so as to prevent chromatic aberration from occurring in accordance with tendency toward wide angles.
The projection optical system shown in domestic re-publication of PCT international publication for patent applications WO01/006295 mainly uses a refracting optical system g and an optical system composed of a convex reflector f to implement oblique projection. FIG. 63 outlines the optical system shown in domestic re-publication of PCT international publication for patent applications WO01/006295.
JP-A No. 40326/2002 discloses the projection optical system having no chromatic aberration using four reflectors i (concave), j (convex), k (convex), and l (convex) arranged in order from an image display panel h (see FIG. 64).
The optical systems shown in these patent gazettes are rotationally symmetric about the optical axis and perform oblique projection using part of the super-wide angle optical system.
JP-A No. 255462/2001 proposes the oblique projection optical system using an off-axial optical system. The projection optical system uses the off-axial optical system to correct trapezoidal distortion. In addition, the projection optical system forms an intermediate image between multiple reflectors m and reflector n to limit the reflector size (see FIG. 65).
As shown in domestic re-publication of PCT international publication for patent applications WO01/006295, the projection optical system is composed of combination of the convex reflector f and the refracting optical system g. Compared to a configuration using only the refracting optical system, the proposed projection optical system facilitates removal of the chromatic aberration and enlargement of angles. On the contrary, one reflector is used for most part of the divergence. The refractive power needs to be decreased to some extent so as to appropriately correct the distortion and the image surface aberration. The projection optical system inevitably tends to enlarge the convex reflector f and increase the sag amount. This not only makes the manufacture of convex reflectors difficult, but also heightens a display bottom portion (see reference symbol e in FIG. 62). In addition, applying this projection optical system to a rear projection television increases the depth (see reference symbol d in FIG. 62).
The projection optical system shown in JP-A No. 40326/2002 has no refracting optical system and principally causes no chromatic aberration. Since no absorption occurs due to a lens, it is possible to provide an optical system capable of high brightness. Since only reflectors are used to configure the system, however, the system tends to be very susceptive to the surface accuracy and the assembly accuracy of each reflector and increase manufacturing costs. Since multiple reflectors i through l are vertically arranged, the bottom of the display (see reference symbol e in FIG. 62) heightens. Increasing a projection angle for a lower profile enlarges the reflectors, especially the last reflector l, making the manufacture of reflectors difficult. Applying this projection optical system to a rear projection television further enlarges the bottom of the display.
The projection optical system described in JP-A No. 255462/2001 uses an intermediate imaging surface and has an advantage of widening the angle and decreasing the reflector. On the contrary, the reflector is decentered to cause decentering aberration. The remaining decentered reflectors are used to correct the decentering aberration. At least three reflectors are needed. Applying this projection optical system to a rear projection television still leaves a difficulty in miniaturizing the bottom of the display. Further, the reflectors are not rotationally symmetric, but are configured to form a freely curved surface. The projection optical system tends to be very susceptive to the surface accuracy and the assembly accuracy, make the manufacture difficult, and increase manufacturing costs.
The present invention has been made in consideration of the foregoing. It is therefore an object of the present invention to provide a projection optical system that is appropriate for maintaining high resolution with low distortion, miniaturizing a reflector, decreasing the number of reflectors, and decreasing the depth and the bottom (or top) of a display used for a rear projection television, for example.