CROSS-REFERENCE TO RELATED APPLICATIONS
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This application is a Divisional of co-pending U.S. application Ser. No. 12/470,216 filed on May 21, 2009, which claims priority under 35 USC 119 from Japanese Patent Application No. 2008-134224 filed May 22, 2008 and Japanese Patent Application No. 2008-134225 filed May 22, 2008; the entire contents of which are incorporated herein by reference.
1. Technical Field
The present invention relates to a reflection reducing film which is formed on one face of an optical member such as a lens, a filter and the like to exhibit a reflection reducing effect with respect to light having a predetermined wavelength band, and an optical member and an optical system which have the reflection reducing film.
2. Related Art
Generally, an image pickup apparatus such as a photographic camera or a TV broadcasting camera has a number of optical members such as lenses, prisms, filters and the like disposed on an optical path thereof. When light is incident on surfaces of the respective optical members, a portion of the light becomes reflection light. Herein, when the number of the optical members increases, a total amount of the reflection light increases and thus defects, for example, flare and ghost occur in images of a broadcasting camera. In addition, since reflectances on the surfaces of the optical members have distribution with respect to a wavelength of the incident light and wavelength dependences of various reflectances are shown in accordance with constituent materials for the respective optical member, a chromaticity balance deteriorates, and thus it is necessary to adjust a white balance in an entire image pickup apparatus.
Accordingly, a surface of each optical member may be provided with a reflection reducing film (also referred to as the antireflection film). The reflection reducing film is a multilayer film in which dielectric films having different refractive indices are combined, and a configuration thereof is disclosed in, for example, the following Non-Patent Document 1. In the Non-Patent Document 1 (“Light•Thin Film Manual”, The Optronics Co., Ltd, Oct. 9, 1989, P 246-247), an antireflection film having a 5-layer structure is disclosed and an attempt has been made to achieve a low reflectance over a wider band.
In addition to the Non-Patent Document 1, Patent Document 1 (JP-A-2002-267801) discloses an antireflection film having a 9-layer structure.
For example, the antireflection film disclosed in the above Non-Patent Document 1 exhibits relatively excellent low reflectance characteristics when being provided on an optical substrate having a refractive index lower than 1.70 at the d-line. However, it is found that there is a tendency in which flatness in reflectance distribution is lost as described later and a reflectance is thus increased at a specific wavelength when the antireflection film is provided on an optical substrate having a high refractive index higher than 1.75.
Tables 1 and 2 show basic data (constituent materials, refractive indices and optical film thicknesses) of antireflection films (Related examples 1 and 2) having a laminate structure corresponding to the antireflection film disclosed in Non-Patent Document 1. And FIG. 45 shows reflectance distributions of the antireflection films having the basic data of Tables 1 and 2.
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