Patent ID: 7887192

Claim:
A method for improving illumination efficiency in optical projection system, comprising the steps of: providing a splitter system; providing a light combining system using an X-cube and disposed after the splitter system; and choosing two materials with one refractive index close to the other to form thin-film optical coatings over dichroic mirrors for blue-reflecting and red-reflecting inside the X-cube, respectively, wherein the ratio of high refractive index to low refractive index ( n H n L ) , is between 1.296 and 1.111, and the wavelength drift of the spectrum for the P and S polarizsed blue-reflecting and red-reflecting lights in the X-cube is the same as that in the splitter system; and increasing the number of layers of the thin-film optical coatings of the dichroic mirror for blue-reflecting to that from 50 to 90 and increasing the number of layers of the thin-film optical coatings of the dichroic mirror for red-reflecting to that from 70 to 110 such that the wavelength drift of the spectrum for the P and S polarised blue-reflecting and red-reflecting lights in the X-cube is the same as that in the splitter system; wherein the X-cube is squarely cut such that the plane of the thin-film optical coating of the dichroic mirror for blue-reflecting is orthogonal to that of the dichroic mirror for red- reflecting; wherein the incident light angle of the thin-film optical coatings of the dichroic mirrors for blue-reflecting and red-reflecting is 45° ; and wherein the thin-film optical coating of the dichroic mirror for blue-reflecting with the high refractive index is titanium oxide (TiO 2 ), the thin-film optical coating of the dichroic mirror for blue-reflecting with the low refractive index is a mixed film of aluminium oxide and tantalum oxide (Al 2 O 3 +Ta 2 O 5 ), the thin-film optical coating of the dichroic mirror for red-reflecting with the high refractive index is tantalum oxide (Ta 2 O 5 ), and the thin-film optical coating of the dichroic mirror for red-reflecting with the low refractive index is a mixed film of aluminium oxide and tantalum oxide (Al 2 O 3 +Ta 2 O 5 ).