Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2002-05-21
2004-06-29
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S076000, C501S901000, C501S903000
Reexamination Certificate
active
06756334
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical glass suitable for using for an optical element, such as a lens, a prism or the like, or a substrate. In particular, it relates to an optical glass having a high refractive index, which is suitable for using for an optical element in a polarization optical system, or for a light polarization control element, such as a polarization beam splitter (hereinafter it is called PBS), a spatial light modulation element (hereinafter it is called SLM), a polarization conversion element or the like.
2. Description of Related Art
An optical system utilizing polarization, that is, a polarization optical system, is utilized for various optical instruments, such as a liquid crystal projector or the like. For example, recently, in a projection device, such as the liquid crystal projector or the like, has been made to have high luminance. However, there is a problem that a transmittance of an optical glass used in each portion in the optical system of the projection device deteriorates with time,
When a high luminance light having large irradiance is irradiated to an optical glass, for example, an undesirable coloring phenomenon, such as solarization, is often caused, so that the transmittance of the glass deteriorates. Solarization generally points the coloring phenomenon of a glass caused by irradiating a light with a wavelength in an ultraviolet region. However, the coloring phenomenon caused by irradiating a light is reported not only as solarization but also as various coloring phenomenon including the coloring phenomenon of a glass caused by two-photon absorption process with visible light caused when a light with a wavelength in a visible range is irradiated to the glass. In particular, when a light with a wavelength in a visible range is irradiated to a borosilicate glass including lead, it is known that coloring of the glass is easily caused by two-photon absorption. As described above, even though there are various wavelengths of the irradiating light that induce the coloring phenomenon, or various mechanism for causing the coloring phenomenon, deterioration of transmittance of the glass is caused as a result of the coloring phenomenon of the glass caused by irradiating a light.
In order to prevent these various coloring phenomenon, means such as incorporating a filter for cutting a light with a harmful wavelength, which becomes a cause of deterioration of luminance (output) of a light source or a cause of the coloring phenomenon, into the optical system or the like, are considered. However, in the optical system that high luminance is required (for example, an optical system of a high luminance liquid crystal projector), those means are not desirable since those are directly related to deterioration of an amount of an output light.
Further, in the projection device such as the liquid crystal projector or the like, in order to project a high luminance light without loosing its color rendering, the optical glass for using for the optical system is required to have an excellent transmittance to a ray with a wavelength from the long wavelength side to the purple region of the visible range.
Moreover, generally, the optical system of the projection device, such as the liquid crystal projector or the like, is a polarization optical system. It is required to control properties of polarization in high accuracy. Among optical parts of the polarization optical system, when a material having an optical anisotropy is used for a member, such as a prism of a light polarization control element, a substrate or the like, which is required to keep the properties of polarization of the PBS, SLM or the like, 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. Since the properties of polarization cannot be kept, it is necessary that a material having an optical isotropy should be used for these parts.
Even when optical glasses in earlier technology having an optical isotropy annealed sufficiently and removed strain are used for the optical parts which are required to keep the properties of polarization in the polarization optical system, an optical anisotropy caused by photoelastic effect, that is, a double refraction property, is seen if an absolute value of a photoelastic constant of the glass is large when a mechanical stress or thermal stress is added to those glasses. As a result, there is a problem that it becomes difficult to obtain desired properties of polarization.
The above-described mechanical stress is caused, for example, by joining a material having different coefficient of thermal expansion from a glass to the glass. Further, the above-described thermal stress is caused, for example, by generating heat from peripheral devices, or by generating heat from the glass itself caused by absorption of energy of a transmitted light.
An amount of double refraction which the glass induces by applying these stresses to the glass, can be represented by using the optical path difference. When a (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)
The proportional constant (&bgr;) in the equation (1) 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 property is.
In earlier technology, as a material of an optical part in a polarization optical system, mainly, a borosilicate glass, such as an S-BSL7 (a trade name of the optical glass manufactured by Kabushiki Kaisha Ohara.), or other equivalent glasses manufactured by other companies, for example, a BK7 (a trademark of the optical glass manufactured by Schott Glas), or the like, is used since it has an excellent transmittance to a ray with a wavelength from the long wavelength side to the purple region in the visible range, and it is inexpensive and easy to be acquired. However, these optical glasses have a large absolute value of photoelastic constant (&bgr;). For example, in the S-BSL7, the refractive index (nd) is 1.52 and the value of &bgr; at e-line (wavelength of 546.07 nm) is 2.79×10
−5
nm/cm/Pa. In order to control the polarization properties at higher accuracy in the polarization optical system, as described above, an optical glass having a smaller absolute value of the photoelastic constant (&bgr;) is required strongly. Further, an optical glass having a higher refractive index is needed from viewpoint of optical design.
As an optical glass having a high refractive index, which has a small absolute value of a photoelastic constant (&bgr;), a glass including a large amount of lead has been known since the beginning of the 20
th
century. A typical glass of SiO
2
-PbO system manufactured and sold as a glass including a high amount of lead at present, for example, a PBH53 (a trade name of the optical glass manufactured by Kabushiki Kaisha Ohara.), an SF57 (a trade name of the optical glass manufactured by Schott Glas) which has the same refractive index as the PBH53, or the like, is intended to be used in the polarization optical system as a material for controlling the optical properties in higher accuracy, from the above-described reason. For example, in the PBH53, the refractive index (nd) is 1.85 and the photoelastic constant (&bgr;) at e-line (wavelength of 546.07 nm) is less than 0.1×10
−5
nm/cm/Pa. The PBH53 has a high refractive index, and a sufficiently small abs
Ishioka Junko
Mori Yoshio
Ogino Michiko
Onozawa Masahiro
Bolden Elizabeth A.
Group Karl
Kabushiki Kaisha Ohara
Oliff & Berridg,e PLC
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