Patent ID: 6661579
Filing Date: 2003-12-09
Classification: G02B

Abstract:
A beam splitter for a digital camera for capturing an image of an object using an image capturing element, said beam splitter splitting light passed through a photographing lens of said camera into a beam directed to said image capturing element and a beam directed to a finder optical system of said camera, said beam splitter comprising:at least one optical element provided with a beam splitting surface that is inclined with respect to an optical axis of said photographing lens; a multi-layer film formed on said beam splitting surface, said multi-layer film including a plurality of layers of dielectric materials, wherein said at least one optical element comprises first and second right-angle prisms, inclined surfaces of said first and second prisms being adhered to each other with said multi-layer film sandwiched therebetween, the inclined surface of said second prism being said beam splitting surface, wherein said multi-layer film is formed such that said beam splitting surface exhibits a substantially constant reflectivity for light whose wavelength is within a range broader than one wavelength range of the visible light in both a lower wavelength direction and a higher wavelength direction, and which light is incident on said beam splitting surface at a predetermined incident angle, wherein said multi-layer film includes at least two types of dielectric layers respectively having a relatively low refractive index and a relatively high refractive index, said two type of dielectric layers being alternately layered, wherein said low refractive index is within a range from 1.30 to 1.66, and wherein said high refractive index is within a range from 1.90 to 2.50, wherein said multi-layer film includes six layers L1-L6 respectively having the low refractive indexes and seven layers H1-H7 respectively having the high refractive indexes; wherein said layers L1-L6 and H1-H7 are arranged, from the light incident side, in the order of H1, L1, H2, L2, H3, L3, H4, L4, H5, L5, H6, L6 and H7; and wherein a quarter-wavelength optical thickness (QWOT) of each of said layers H1-H7 and L1-L6 is defined as follows: 143 nm1<175 nm; 172 nm1<211 nm; 1304 nm2<1594 nm; 182 nm2<223 nm; 204 nm3<249 nm; 463 nm3<565 nm; 928 nm4<1134 nm; 450 nm4<550 nm; 835 nm5<1021 nm; 464 nm5<567 nm; 498 nm6<609 nm; 179 nm6<218 nm; and 480 nm7<586 nm.