Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2001-07-17
2002-10-22
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S901000, C501S903000
Reexamination Certificate
active
06468935
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical glass having optical constants which are a refractive index (nd) having a range of 1.75 to 1.87 and an Abbe number (&ngr;d) having a range of 21 to 28, wherein a light transmittance is excellent to a short wavelength band. In particular, the present invention relates to an optical glass having an excellent light transmittance to a short wavelength band, and optical constants which are a refractive index (nd) having a range of 1.80 to 1.87 and an Abbe number (&ngr;d) having a range of 21 to 26, and having a photoelastic constant (&bgr;) having an absolute value of not more than 0.2×10
−5
nm/cm/Pa in case of e-line (wavelength 546.07 nm), the optical glass being especially suitable for being used for a spatial light modulation element for carrying out a polarization modulation or for a polarization optical system, such as a beam splitter or the like.
BACKGROUND OF THE INVENTION
In recent years, an optical system utilizing a polarization, that is, a polarization optical system is used in various fields, such as a liquid crystal projector or the like. For example, a spatial light modulation element for spatially modulating a polarization, a polarizing beam splitter for separating a light into S-polarization and P-polarization and the like are used for a liquid crystal projector or the like. In these polarization optical systems, it is desired to control the properties of polarization at a high accuracy.
In optical parts, such as a substrate, a prism for a polarization optical system, or the like, when a material having an optical anisotropy is used in a part which is required to keep the properties of polarization, a phase difference (optical path difference) between a transmitted principal ray and an extraordinary ray perpendicular to the principal ray is changed as compared with the difference before a principal ray is transmitted through the material. Because the properties of polarization cannot be kept, it is necessary that a material having an optical isotropy should be used for these parts.
A glass in which strains are sufficiently removed, in particular, an optical glass has an optical isotropy. Further, the durability, the strength and the light transmittance of the optical glass are more excellent than those of the other materials. There are various types of optical glasses having various optical constants. The optical glasses are more selective in an optical design. Therefore, the optical glasses have been used for a polarization optical system before. In particular, S-BSL7 (which is a trade name of the optical glass manufactured by Kabushiki Kaisha Ohara) is inexpensive and has a good durability. Further, the dispersion thereof is small. Therefore, the S-BSL7 is often used in a polarization optical system.
However, even though the above-mentioned optical glass having an optical isotropy according to an earlier development was used in an optical part of a polarization optical system, the optical glass has an optical anisotropy caused by the photoelastic effect when mechanical stress and thermal stress are applied to these parts, that is, has a double refraction property. As a result, there was a problem that it is difficult to obtain desired properties of polarization. The above mechanical stress is caused, for example, by joining a material having a coefficient of thermal expansion which is different from that of the glass, to the glass. The above thermal stress is caused, for example, by generating heat from peripheral devices, or by generating heat from the glass itself because of absorption of energy of a transmitted light. An amount of the double refraction which the glass induces by applying these stresses to the glass, can be represented by using the optical path difference. When &dgr; (nm) is the optical path difference, d (cm) is the thickness of the glass and F (Pa) is the stress, the following equation (1) holds. The equation (1) means that the more the optical path difference increases, the more the double refraction increases.
&dgr;=&bgr;.
d.F
(1)
In the equation (1), the proportional constant (&bgr;) is called photoelastic constant. The values thereof vary in type of glass. As shown in the equation (1), when the stress (F) applied to the glass and the thickness (d) of the glass are constant, the smaller the absolute value of the photoelastic constant (&bgr;) of the glass is, the shorter the optical path difference (&dgr;) is, that is, the smaller the double refraction is. In the above S-BSL7 (which is a trade name of the optical glass manufactured by Kabushiki Kaisha Ohara), the value of the &bgr; is 2.79×10
−5
nm/cm/Pa in case of e-line (wavelength 546.07 nm), and is large. In order to control the properties of polarization at a high accuracy in the polarization optical system as described above, the glass is required to have a small absolute value of the photoelastic constant (&bgr;).
In an earlier development, as a glass having a small photoelastic constant &bgr;, PBH53 (which is a trade name of the optical glass manufactured by Kabushiki Kaisha Ohara) having a composition of SiO
2
—PbO system wherein the refractive index (nd) is about 1.847 and the Abbe number (&ngr;d) is 23.9, and equivalent glasses manufactured by other companies, have been known. These glasses have sufficiently small photoelastic constants (&bgr;) of less than 0.1×10
−5
nm/cm/Pa in case of e-line (wavelength 546.07 nm). However, in a short wavelength band of not more than 450 nm, the light transmittance cannot be sufficiently obtained. Because a difference between the intensities of three separated lights, blue (B), green (G) and red (R) which are used in a liquid crystal projector or the like, is caused, it is necessary that the intensities of the lights are adjusted in accordance with the blue light (B) having a low intensity. As a result, there is a problem that an amount of light to be projected from a liquid crystal projector or the like, is not sufficient.
An object of the present invention is to provide an optical glass having optical constants which are a refractive index (nd) having a range of 1.75 to 1.87 and an Abbe number (&ngr;d) having a range of 21 to 28, wherein a light transmittance is excellent to a short wavelength band, in consideration of the above circumstances of the earlier development. In particular, an object of the invention is to provide an optical glass having optical constants which are a refractive index (nd) having a range of 1.80 to 1.87 and an Abbe number (&ngr;d) having a range of 21 to 26, having an excellent light transmittance to a short wavelength band, and having a photoelastic constant (&bgr;) having a small absolute value.
DISCLOSURE OF THE INVENTION
In order to accomplish the object, the inventors have examined and researched an optical glass. As a result, the inventors have found that an optical glass having optical constants which are a refractive index (nd) having a range of 1.75 to 1.87 and an Abbe number (&ngr;d) having a range of 21 to 28, wherein a light transmittance is excellent to a short wavelength band as compared with that of an optical glass according to an earlier development, can be obtained by adding the component TeO
2
to a glass having a composition system including SiO
2
and PbO. Further, the inventors have found that an optical glass having optical constants which are a refractive index (nd) having a range of 1.80 to 1.87 and an Abbe number (&ngr;d) having a range of 21 to 26, wherein a light transmittance is excellent to a short wavelength band, and an absolute value of a photoelastic constant (&bgr;) is small, can be obtained. Then, the present invention has been accomplished.
That is, in order to accomplish an object of providing an optical glass having optical constants which are a refractive index (nd) having a range of 1.75 to 1.87 and an Abbe number (&ngr;d) having a range of 21 to 28, wherein a light transmittance is excellent to a short wavelength band, in accordance with the first aspect of
Mori Yoshio
Nakahara Muneo
Kabushiki Kaisha Ohara
Sample David
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