Beam splitting for camera using a multilayer film

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 nm<QWOT of H1<175 nm;
• 172 nm<QWOT of L1<211 nm;
• 1304 nm<QWOT of H2<1594 nm;
• 182 nm<QWOT of L2<223 nm;
• 204 nm<QWOT of H3<249 nm;
• 463 nm<QWOT of L3<565 nm;
• 928 nm<QWOT of H4<1134 nm;
• 450 nm<QWOT of L4<550 nm;
• 835 nm<QWOT of H5<1021 nm;
• 464 nm<QWOT of L5<567 nm;
• 498 nm<QWOT of H6<609 nm;
• 179 nm<QWOT of L6<218 nm;
• and
• 480 nm<QWOT of H7<586 nm.