Abstract:
There is provided a compact, strong and high-performance catoptric optical system. This catoptric optical system has plural optical curved reflective surfaces. A gap between a pair of reflective surfaces, through which light incident upon said catoptric projection optical system passes, is the same gap between a pair of reflective surfaces, through which light exited from the catoptric projection optical system passes.

Description:
This application claims a benefit of priority based on Japanese Patent Application No. 2003-102805, filed on Apr. 7, 2003, which is hereby incorporated by reference herein in its entirety as if fully set forth herein. 
   BACKGROUND OF THE INVENTION  
   The present invention relates to a reflection type of catoptric optical system, and is suitable, for example, for a projection optical system that projects light from an imaging device, such as a liquid crystal panel and a digital micromirror device (“DMD”) manufactured by Texas Instruments Incorporated, onto an object surface, such as a screen. 
   Prior art has proposed to shorten an optical path length in an optical system for a compact catoptric system that includes plural optical reflective surfaces. 
   Japanese Patent Application, Publication No. 2001-222063 has proposed a compact rear projection optical system that uses three curved mirrors to properly extend a screen diagonal length relative to an optical path length, and enables two plane mirrors to easily return or turn around the light. 
   Japanese Patent Application, Publication No. 2001-221949 has proposed a thin and compact oblique incidence optical system that uses four reflective surfaces to properly adjust a ratio between a distance from a final reflective surface to a screen and a distance from a final reflective surface to a lens system, maintains an optical path length for turnaround of the light, and arranges a return mirror. 
   Another proposed technology has arranged a curved mirror on an optical path to prevent an expansion of the optical path in the optical system for a compact structure. 
   For example, Japanese Patent Application, Publication No. 2001-242381 sets a spherical mirror just before a screen in addition to a three- or four-aspheric mirror system to produce a rear projection monitor catoptric imaging optical system that includes totally four or five mirrors. 
   Japanese Patent Application, Publication No. 07-013157 (corresponding to U.S. Pat. No. 5,477,394) has proposed a projection optical system that introduces light from a first focal point in an ellipsoidal mirror into a second focal point, accords the second focal point of the ellipsoidal mirror with a focal point of a parabolic mirror, and collimates light introduced into the focal point of the inclined parabolic mirror, obliquely projecting the collimated light onto a screen. 
   However, when plural optical surfaces are provided as in Japanese Patent Applications, Publication Nos. 2001-222063, 2001-221949 and 2000-104095, light&#39;s different entrance and exit gates need plural openings for the light to pass through, and thus more and longer spacing among mirrors by the openings, disadvantageously enlarging the optical system. 
   The projection optical system disclosed in Japanese Patent Application, Publication No. 07-013157 generates aberration since rays other than a principal ray do not pass through the first focal point in the ellipsoidal mirror. The same ray generates aberration at the second focal point in the ellipsoidal mirror, and an enlarged image incident upon the parabolic mirror increases the aberration on the screen, remarkably deteriorating imaging performance. 
   The conventional optical system that uses plural optical reflective surfaces requires a retainer mechanism that has at least two clearances for light to pass through, and thus has a difficulty in maintaining strength. 
   BRIEF SUMMARY OF THE INVENTION  
   Accordingly, it is an exemplified object of the present invention to provide a smaller, stronger and higher-performance catoptric optical system than the prior art. 
   A catoptric optical system of one aspect according to the present invention includes plural reflective surfaces, each of which is a curved surface, wherein a gap between a pair of reflective surfaces, through which light incident upon the catoptric projection optical system passes, is the same gap between a pair of reflective surfaces, through which light exited from the catoptric projection optical system passes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an optical-path diagram in a catoptric optical system of a first embodiment according to the present invention. 
       FIG. 2  is an overview of a projection optical system that includes the catoptric optical system of the first embodiment. 
       FIG. 3  shows distortion on an image surface in the projection optical system of the first embodiment. 
       FIG. 4  is an optical-path diagram in a catoptric optical system of a second embodiment according to the present invention. 
       FIG. 5  is an overview of a projection optical system that includes the catoptric optical system of the second embodiment. 
       FIG. 6  shows distortion on an image surface in the projection optical system of the second embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     FIG. 1  is an optical-path diagram in a catoptric optical system of a first embodiment according to the present invention.  FIG. 2  is an overview of a projection optical system in a projector that includes the catoptric optical system of the first embodiment.  FIG. 3  shows distortion on an image surface in this projection optical system.  FIG. 2  omits an illumination optical system for illuminating a display device, such as a LED panel. 
   In  FIGS. 1 and 2 ,  100  is a display device that modulates and emits incident light,  101  is a refractive optical system (dioptric optical system) as part of a projection optical system,  102  is a catoptric optical system as part of the projection optical system,  103  is an area (enclosed by a dotted line) in which an opening that allows the light to pass through it is located in a gap between a pair of optical reflective surfaces,  104  schematically shows a retainer mechanism, and  105  is a screen, onto which an image formed by the display device  100  is projected by the projection optical system. The refractive optical system  101  includes plural refractive surfaces having a rotationally symmetrical shape around a common axis. The catoptric optical system  102  includes plural reflective surfaces having a rotationally asymmetrical shape that does not have a common axis. 
   The light from the display device  100  is properly refracted by the refractive optical system  101 , and then incident upon the catoptric optical system  102 . The catoptric optical system  102  includes four optical reflective surfaces M 1  to M 4  each having a rotationally asymmetrical surface that does not have a rotational symmetrical axis. The incident light passes through the gap between the optical reflective surfaces M 2  and M 3  among four surfaces. The light incident upon the catoptric optical system  102  is sequentially reflected on the optical reflective surfaces M 1 , M 2 , M 3  and M 4  in this order totally four times and exited from the catoptric optical system  102 . An optical path (between the optical reflective surfaces M 1  and M 4 ) intersects once inside the catoptric optical system  102 . The light that is about to enter the optical reflective surface M 1  as a first reflective surface intersects twice with the optical path between the optical reflective surfaces M 1  and M 4 . The light that has been exited from the optical reflective surface M 4  as a final reflective surface intersects twice with the optical path between the optical reflective surfaces M 1  and M 4 . The light incident upon the optical reflective surface M 1  intersects with the light that has been reflected on the optical reflective surface M 4 . The light exited from the catoptric optical system  102  passes through an opening in the gap  103  between a pair of optical reflective surfaces M 2  and M 3 . In other words, the incident light passes through the same gap. Therefore, the retainer mechanism  104  for holding the catoptric optical element  102  may have only one opening, and can maintain its strength. A position and size of the opening are determined so as not to shield the light that passes through the catoptric optical system. 
   This projection optical system forms a pupil image in the gap  103  between the pair of optical reflective surfaces M 2  and M 3 . Therefore, the gap between the optical reflective surfaces can narrow a section in an exiting direction form the optical reflective surface  102 . In general, a stop (located in the refractive optical system  101  in this embodiment) as light shielding means in an optical system serves as vignetting that shields a marginal ray in an off-axis ray, and the light is constricted between the front and back of the stop. The instant embodiment forms a constricted pupil image conjugate with the stop in the area in the gap  103  between the pair of optical reflective surfaces M 2  and M 3 , and shortens a surface separation in this area between the pair of optical reflective surfaces M 2  and M 3 , making the catoptric optical system  102  compact. An actual image of a pupil when formed just before the catoptric optical system would be able to make the incident light diameter small. This configuration is preferable for a compact area in the gap  103  between the pair or optical reflective surfaces M 2  and M 3 . 
   When the light exited from the catoptric optical system  102  forms an image conjugate with the light shielding means near a position possibly closest to the catoptric optical system  102 , or when the image conjugate with the light shielding means is provided at the gap  103 , narrower exit light can be obtained and an area in the gap  103  between the pair of optical reflective surfaces M 2  and M 3  can be made small. When the area in the gap  103  between the pair of optical reflective surfaces M 2  and M 3  can be made small, the catoptric optical system  102  that has an opening in the area can be made small and the retainer mechanism  104  may reduce the number of openings to one to pass the light, maintaining the strength. 
   Although the instant embodiment introduces the light from the refractive optical system  101  to the catoptric optical system  102 , the inventive catoptric optical system can receive the light emitted from the display device  100  directly or from another catoptric optical element. 
     FIG. 4  is an optical-path diagram in a catoptric optical system of a second embodiment according to the present invention.  FIG. 5  is an overview of a projection optical system in a projector that includes the catoptric optical system of the second embodiment.  FIG. 6  shows distortion on an image surface in this projection optical system. A description of the reference numerals is similar to that for the first embodiment, and thus omitted. 
   The catoptric optical system  102  in the instant embodiment includes three optical reflective surfaces M 1  to M 3 , and light passes through a gap between two of them, i.e., the optical reflective surfaces M 1  and M 2 . The incident light is sequentially reflected on the optical reflective surfaces M 1 , M 2  and M 3  in this order totally three times in the catoptric optical system  102 , and then exited from it. An optical path between the optical reflective surfaces M 1  and M 3  inside the catoptric optical system  102  (although not shown in the instant embodiment) intersects once with the light that has been reflected on the reflective optical surface M 3  as a final reflective surface. The light that is about to enter the optical reflective surface M 1  intersects with the light that has been exited from the optical reflective surface M 3 . The light exited from the catoptric optical system  102  passes through an opening in the same gap  103  between the pair of optical reflective surfaces M 2  and M 3 . In other words, the incident light passes through the same gap. Therefore, the retainer mechanism for holding the catoptric optical element  102  may have only one opening, and the optical system can be made small.