US006072635A United States Patent (19) 11 Patent Number: 6,072,635 Hashizume et al. (45) Date of Patent: Jun. 6, 2000

54) DICHROIC PRISM AND PROJECTION FOREIGN PATENT DOCUMENTS DISPLAY APPARATUS 7-294.845 11/1995 Japan. 75 Inventors: Toshiaki Hashizume, Okaya; Akitaka Primary Examiner Ricky Mack Yajima, Tatsuno-machi, both of Japan Attorney, Agent, or Firm-Oliff & Berridge, PLC 73 Assignee: Seiko Epson Corporation, Tokyo, 57 ABSTRACT Japan The invention provides a dichroic prism capable of reducing displacements of projection pixels of caused by 21 Appl. No.: 09/112,132 chromatic aberration of magnification. A dichroic prism is formed in the shape of a quadrangular prism as a whole by 22 Filed: Jul. 9, 1998 joining four rectangular prisms together. A reflecting 30 Foreign Application Priority Data dichroic plane and a reflecting dichroic plane intersect to form Substantially an X shape along junction Surfaces of Jul. 15, 1997 JP Japan ...... 9-1900OS the prisms. The red reflecting dichroic plane is convex shaped by partly changing the thickness of an adhesive layer 51 Int. Cl." ...... G02B 27/12: G02B 27/14 for connecting the rectangular prisms together. Accordingly, 52 U.S. Cl...... 359/640; 359/634 Since a red beam can be guided to a projection optical System 58 Field of Search ...... 359/634, 637, while being enlarged, it is possible to reduce a projection 359/640 image of the red beam to be projected onto a projection plane Via the projection optical System. This makes it 56) References Cited possible to reduce relative displacements of projection pix U.S. PATENT DOCUMENTS els of beams caused by chromatic aberration of mag nification. 5,098,183 3/1992 Sonehara et al...... 353/31 5,221,998 6/1993 Sugahara ...... 359/831 18 Claims, 11 Drawing Sheets

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