1.  Field of the Invention
The present invention relates to a method and a combining prism for improving illumination efficiency in an optical projection system.  In detail, the present invention relates to a method for designing a thin-film optical coating for blue reflecting (or light splitting thin-film coating) and a thin-film optical coating for red reflecting inside an X-cube of an optical projection system and a combining prism produced according to the method. 
2.  Description of the Related Art
Conventional three-piece digital light processing (DLP) projection systems use Philips Prisms for the light splitter and combining systems.  The DLP projection system disclosed in US 20050057729 uses blue-reflecting and red-reflecting dichroic mirrors as the splitter system to split the three primary colours and an X-cube is disposed after the splitter system as the three primary colours combining system, as shown in FIG.  1.  In order to use the X-cube as the combining element for a liquid crystal display (LCD) projection system and to meet the requirements of liquid crystal, the application of a polarised incident light is necessary.  Therefore, a conventional X-cube has film optical coatings for blue-reflecting and red-reflecting lights formed therein, and such film optical coatings have bigger wavelength drift for each of the P and S polarised lights when the incident angle is 45°.  For example, FIG.  2 shows a spectrum of a conventional X-cube, where P represents the P-polarised light and S represents the S-polarised light, respectively. 
Generally, for dichroic mirrors used in a three primary colours splitter system, the thin-film optical coatings for blue-reflecting and red-reflecting lights are usually coated on a mirror.  With an incident light of 45°, the wavelength drift of the P and S polarised lights is about 15 to 25 nm.  FIGS.  3 and 4 represent the wavelength drift of the spectrum for each of the P and S polarised blue-reflecting and red-reflecting lights for conventional blue-reflecting and red-reflecting dichroic mirrors, where P represents the P-polarised lights and S represents the S-polarised lights, respectively.  The thin-film optical coatings for blue-reflecting and red-reflecting dichroic inside the X-cube are generally coated on prisms, which are subsequently assembled to form an X-cube, as shown in FIG.  5.  When the incident light angle is 45°, the wavelength drift of the P and S polarised lights is about 40 to 60 nm, which is different from that in the splitter system.  Therefore, the split-polarised lights cannot be effectively combined in the X-Cube combining system.  When projected by a projection lens, problems such as bad efficiency of light source, over heated X-cube, and slightly greenish projected white screen, and forming ghost images often arise as a result. 
In response to the above problems, U.S. Pat.  Nos.  3,303,278, 5,321,499, 5,826,959, 6,019,474 and 6,238,051 have proposed some suggestions including varying the shape of the prisms and using a smaller incident angle to obtain a smaller wavelength drift of the P and S polarised lights.  However, the proposed methods have the following defects:    1.  Since the prisms are not squarely cut, the problem of assembling precision arises during manufacturing and assembling;    2.  The size of such prisms is bigger than that of a right angle prism which in turn, will increase the overall weight of the projection system; and    3.  To meet the requirements of a prism with perpendicular incident, since the three primary lights R, G, B are not perpendicular to one another due to the non-right angle prisms, it is necessary that the incident light path is configured to have an inclined angle, which increases the error of light path calibration and cost of light engine assembling.