JP7555747B2 - Optical Glass and Optical Elements - Google Patents

Description

The present invention relates to optical glass and optical elements.

Optical elements incorporated in in-vehicle optical devices, and in heat-generating optical devices such as projectors, copy machines, laser printers, and broadcasting equipment, are used in environments with large temperature changes. If optical characteristics such as the refractive index change due to temperature changes, this affects the imaging characteristics of the optical system.

Here, it is known that by combining an optical element with a negative temperature coefficient of the relative refractive index (dn/dT) and an optical element with a positive temperature coefficient, the effect of temperature changes on the imaging characteristics of the optical system can be reduced.

The temperature coefficient of the relative refractive index (dn/dT) represents the change in refractive index with respect to temperature change. For optical elements whose refractive index decreases as the temperature increases, the temperature coefficient of the relative refractive index is negative. Conversely, for optical elements whose refractive index increases as the temperature increases, the temperature coefficient of the relative refractive index is positive.

In addition, if the glass melting temperature and forming temperature are high, not only is productivity poor, but the glass melting equipment (e.g., crucible, molten glass stirring equipment, etc.) used in the melting process is corroded, making it less economical. Therefore, there is a demand for glass with a low liquidus temperature LT, i.e., a low glass melting temperature and forming temperature.

Patent Document 1 discloses optical glass with a negative temperature coefficient of relative refractive index (dn/dT). However, it was found that the glass in Patent Document 1 has a high liquidus temperature LT, and is therefore inferior in productivity and economic efficiency.

In addition, the average linear thermal expansion coefficient of the optical element is important in optical design. When combining a low refractive index, low dispersion glass material with a high refractive index, high dispersion glass material, the smaller the difference in the average linear thermal expansion coefficient of the glass materials, the better the bonding. For example, a low refractive index, low dispersion glass material containing fluorine usually has a large average linear thermal expansion coefficient. Therefore, the high refractive index, high dispersion glass material to be combined with it is required to have a high average linear thermal expansion coefficient as well. The optical glass disclosed in Patent Document 2 has a high refractive index, but low dispersion and a small average linear thermal expansion coefficient. Therefore, there is a demand for optical glass that is high refractive index, high dispersion, and has a large average linear thermal expansion coefficient.

JP 2019-1697 A JP 2007-106611 A

Therefore, the present invention aims to provide an optical glass that has a low temperature coefficient of the relative refractive index (dn/dT) due to temperature change and a large average linear thermal expansion coefficient, as well as an optical element made of the optical glass.

The gist of the present invention is as follows.
(1) The content of _P2O5 is 25 _to 50 mass%;
The Nb ₂ O ₅ content is 14 to 40% by mass,
the total content of TiO ₂ , Nb ₂ O ₅ , WO ₃ , Bi ₂ O ₃ and Ta ₂ O ₅ [TiO ₂ +Nb ₂ O ₅ +WO ₃ +Bi ₂ O ₃ +Ta ₂ O ₅ ] is 35 to 60 mass %,
the mass ratio of the content of _TiO2 to the total content of _TiO2 , _{Nb2O5 , WO3 , Bi2O3 and Ta2O5} [ _TiO2 /( _TiO2 + _Nb2O5 + _WO3 + _{Bi2O3 + Ta2O5 )} ] is 0.25 or more _;
the mass ratio of the content of B ₂ O ₃ to the content of P ₂ O ₅ [B ₂ O ₃ /P ₂ O ₅ ] is 0.05 to 0.39;
The total content of Li ₂ O, Na ₂ O, K ₂ O and Cs ₂ O [Li ₂ O + Na ₂ O + K ₂ O + Cs ₂ O] is 10 mass% or more;
An optical glass having a mass ratio of the Na ₂ O content to the K ₂ O content [Na ₂ O/K ₂ O] of 1.50 or more.

(2) The optical glass described in (1) above, which is substantially free of F.

(3) The optical glass according to (1) or (2), which has an average linear thermal expansion coefficient α from 100 to 300° C. of 100×10 ⁻⁷ to 200×10 ⁻⁷ ° C. ⁻¹ .

(4) An optical glass according ^{to any one of (1) to (3), which has a temperature coefficient of relative refractive index dn/dT at the wavelength (633 nm) of a He-Ne laser of -0.1x10-6 to -13.0x10-6°C-1 in the} range of 20 to 40°C.

(5) An optical element made of the optical glass described in any one of (1) to (4) above.

The present invention provides optical glass that has a low temperature coefficient of the relative refractive index (dn/dT) due to temperature change and a large average linear thermal expansion coefficient, as well as optical elements made of said optical glass.

In the present invention and this specification, the glass composition of optical glass is expressed on an oxide basis unless otherwise specified. Here, "glass composition on an oxide basis" refers to a glass composition obtained by converting the glass raw materials into oxides present in the optical glass after all of them are decomposed during melting, and each glass component is conventionally expressed as _SiO2 , _TiO2 , etc. The content and total content of glass components are on a mass basis unless otherwise specified, and "%" means "mass%".

The content of glass components can be quantified by known methods, such as inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). In this specification and the present invention, a content of 0% of a component means that the component is substantially not contained, and it is acceptable for the component to be present at an unavoidable impurity level.

In this specification, the thermal stability and devitrification resistance of glass both refer to the resistance to crystal precipitation in the glass. In particular, thermal stability refers to the resistance to crystal precipitation when molten glass solidifies, and devitrification resistance refers to the resistance to crystal precipitation when solidified glass is reheated, such as during reheat pressing.

In addition, in this specification, unless otherwise specified, the refractive index refers to the refractive index nd at the helium d line (wavelength 587.56 nm).

The Abbe number νd is used as a value representing a property related to dispersion, and is expressed by the following formula: where nF is the refractive index at the F line (wavelength 486.13 nm) of blue hydrogen, and nC is the refractive index at the C line (656.27 nm) of red hydrogen.
νd=(nd-1)/nF-nC...(1)

The optical glass according to this embodiment will now be described in detail.
The optical glass according to this embodiment is
The _content of _P2O5 is 25-50%,
The Nb ₂ O ₅ content is 14-40%,
the total content of TiO ₂ , Nb ₂ O ₅ , WO ₃ , Bi ₂ O ₃ and Ta ₂ O ₅ [TiO ₂ +Nb ₂ O ₅ +WO ₃ +Bi ₂ O ₃ +Ta ₂ O ₅ ] is 35-60%;
the mass ratio of the content of _TiO2 to the total content of _TiO2 , _{Nb2O5 , WO3 , Bi2O3 and Ta2O5} [ _TiO2 /( _TiO2 + _Nb2O5 + _WO3 + _{Bi2O3 + Ta2O5 )} ] is 0.25 or more _;
the mass ratio of the content of B ₂ O ₃ to the content of P ₂ O ₅ [B ₂ O ₃ /P ₂ O ₅ ] is 0.05 to 0.39;
The total content of Li ₂ O, Na ₂ O, K ₂ O and Cs ₂ O [Li ₂ O + Na ₂ O + K ₂ O + Cs ₂ O] is 10% or more;
The mass ratio of the Na ₂ O content to the K ₂ O content [Na ₂ O/K ₂ O] is 1.50 or more.

In the optical glass according to this embodiment, the P ₂ O ₅ content is 25 to 50%. The lower limit of the P ₂ O ₅ content is preferably 26%, and more preferably 26.5% and 26.7%, in that order. The upper limit of the P ₂ O ₅ content is preferably 42%, and more preferably 40%, 38%, 37%, and 36%, in that order.

_P2O5 is _{a glass network-forming component and is an essential component for containing a large amount of highly dispersible components in the glass. By setting the content of P2O5 within the} above range, an optical glass having high thermal stability and desired optical constants can be obtained.

In the optical glass according to this embodiment, the content of Nb ₂ O ₅ is 14 to 40%. The lower limit of the content of Nb ₂ O ₅ is preferably 16%, and more preferably 17%, 18%, 19%, and 20% in that order. The upper limit of the content of Nb ₂ O ₅ is preferably 38%, and more preferably 36%, 34%, and 32% in that order.

Nb ₂ O ₅ is a component that contributes to high refractive index and high dispersion. Therefore, by setting the content of Nb ₂ O ₅ within the above range, an optical glass having the desired optical constants can be obtained. On the other hand, if the content of Nb ₂ O ₅ is too high, the coloring of the glass may be intensified.

In the optical glass according to this embodiment, the total content of TiO ₂ , Nb ₂ O ₅ , WO ₃ , Bi ₂ O ₃ and Ta ₂ O ₅ [TiO ₂ +Nb ₂ O ₅ +WO ₃ +Bi ₂ O ₃ +Ta ₂ O ₅ ] is 35 to 60%. The lower limit of this total content is preferably 36%, and more preferably 37%, 38%, and 39% in that order. The upper limit of this total content is preferably 55%, and more preferably 50%, 48%, 47%, and 46% in that order.

TiO ₂ , Nb ₂ O ₅ , WO ₃ , Bi ₂ O ₃ and Ta ₂ O ₅ are components that contribute to high dispersion of glass. Therefore, by setting the total content [TiO ₂ + Nb ₂ O ₅ + WO ₃ + Bi ₂ O ₃ + Ta ₂ O ₅ ] within the above range, optical glass with desired optical constants can be obtained. In addition, the thermal stability of the glass can also be improved. On the other hand, if the total content is too high, there is a risk that optical glass with desired optical constants cannot be obtained, and the thermal stability of the glass may decrease, and the coloring of the glass may be intensified.

In the optical glass according to this embodiment, the mass ratio of the content of _TiO2 to the total content of _TiO2 , _Nb2O5 , _{WO3 , Bi2O3} and _Ta2O5 [ _TiO2 /(TiO2+ _Nb2O5 + _WO3 + _Bi2O3 + _{Ta2O5 )} ] is _0.25 or more. The lower limit of this mass ratio is preferably _0.26 , and more preferably _0.27 , 0.28, 0.29, and 0.30 in this order. The upper limit of _this mass ratio is preferably 0.65, and more preferably 0.60, _0.58 , 0.56, 0.54, 0.52, 0.50, and 0.48 in this order.

Among the components that increase the refractive index, _TiO2 is a component that has a particularly large effect of increasing dispersion. Therefore, _{from the} viewpoint of obtaining the desired optical constants, it is preferable that the mass ratio [ _TiO2 / _{( TiO2} + _Nb2O5 + _WO3 + _Bi2O3 + _Ta2O5 )] is in the above range.

In the optical glass according to this embodiment, the mass ratio of the content of B ₂ O ₃ to the content of P ₂ O ₅ [B ₂ O ₃ /P ₂ O ₅ ] is 0.05 to 0.39. The upper limit of this mass ratio is preferably 0.30, and more preferably 0.25, 0.22, 0.20, 0.19, and 0.18 in that order. The lower limit of this mass ratio is preferably 0.06, and more preferably 0.07, 0.08, and 0.09 in that order.

By setting the mass ratio [B ₂ O ₃ /P ₂ O ₅ ] within the above range, it becomes easier to obtain an optical glass that has desired optical constants, high thermal stability, and high resistance to devitrification.

In the optical glass according to this embodiment, the total content of _Li2O , _Na2O , _K2O and _Cs2O [ _Li2O + _Na2O + _K2O + _Cs2O ] is 10% or more. The lower limit of the total content is preferably 12%, more preferably 14%, 16%, and 17% in that order. The upper limit of the total content is preferably 35%, more preferably 30%, 28%, 26%, and 25% in that order.

_Li2O , _Na2O , _K2O and _Cs2O all have the function of improving the thermal stability of glass. However, if the content of these elements is large, the chemical durability and weather resistance may be reduced. Therefore, the total content of _Li2O , _Na2O , _K2O and _Cs2O [ _Li2O + _Na2O + _K2O + _Cs2O ] is preferably within the above range.

In the optical glass according to this embodiment, the mass ratio of the content of _Na2O to the content of _K2O [ _Na2O / _K2O ] is 1.50 or more. The lower limit of this mass ratio is preferably 1.70, more preferably 1.90, 2.10, and 2.30 in that order. The upper limit of this mass ratio is preferably 10.0, more preferably 8.50, 7.50, 7.00, and 6.50 in that order.

Na ₂ O and K ₂ O are components that contribute to lowering the specific gravity of glass, improving the melting property of glass, and increasing the average linear thermal expansion coefficient. However, if the content of K ₂ O increases, the thermal stability, devitrification resistance, chemical durability, and weather resistance decrease. Therefore, it is preferable that the mass ratio [Na ₂ O/K ₂ O] is within the above range.

The following are non-limiting examples of the contents and ratios of glass components other than those described above in the optical glass according to this embodiment.

In the optical glass according to this embodiment, the upper limit of the B ₂ O ₃ content is preferably 10%, and more preferably 9%, 8%, 7% and 6%, in that order.

_B2O3 is a glass network forming component and has the function of improving the thermal stability of _glass . On the other hand, if the content of _B2O3 is high, _{the devitrification resistance tends to decrease. Therefore, the content of B2O3 is} preferably within the above range.

In the optical glass according to this embodiment, the upper limit of the mass ratio of the content of _B2O3 to the total content of _{P2O5 and B2O3} [ _B2O3 /( _P2O5 + _{B2O3 )} ] is preferably 0.18, more preferably _{0.17 ,} 0.16, and 0.15 in that order. The lower limit of this mass ratio is preferably 0, more preferably 0.01 _, 0.03 _, and 0.05 in that order.

From the viewpoint of improving the thermal stability and devitrification resistance of the glass, the mass ratio [B ₂ O ₃ /(P ₂ O ₅ +B ₂ O ₃ )] is preferably in the above range.

In the optical glass according to this embodiment, the content of Al ₂ O ₃ is preferably 3% or less, more preferably 2% or less, and further more preferably 1% or less. The content of Al ₂ O ₃ may be 0%.

Al ₂ O ₃ is a glass component that has the function of improving the chemical durability and weather resistance of glass, and can be considered as a network forming component. On the other hand, if the content of Al ₂ O ₃ increases, the devitrification resistance of the glass decreases. In addition, problems such as an increase in the glass transition temperature Tg and a decrease in thermal stability are likely to occur. In order to avoid such problems, it is preferable that the upper limit of the content of Al ₂ O ₃ is within the above range.

In the optical glass according to this embodiment, the upper limit of the _SiO2 content is preferably 5%, and more preferably 3%, 2%, and 1% in that order. The _SiO2 content may be 0%.

In addition, a quartz glass melting device such as a quartz glass crucible may be used to melt glass. In this case, a small amount of SiO ₂ is dissolved from the melting device into the glass melt, so even if the glass raw material does not contain SiO _{2, the produced glass contains a small amount of SiO 2.} The amount of SiO ₂ mixed into the glass from the quartz glass melting device depends on the melting conditions, but is, for example, about 0.5 to 1 mass% of the total content of all glass components. The content ratio of glass components other than SiO ₂ remains constant, and the amount of SiO ₂ increases by about _0.5 to 1 mass%. The above amount increases or decreases depending on the melting conditions. Since optical properties such as refractive index and Abbe number change depending on the content of SiO ₂ , the content of glass components other than SiO ₂ is finely adjusted to obtain optical glass with desired optical properties.

_SiO2 is a glass network forming component, and has the functions of improving the thermal stability, chemical durability, and weather resistance of glass, increasing the viscosity of molten glass, and making it easier to mold the molten glass. On the other hand, if the _SiO2 content is high, the devitrification resistance of the glass tends to decrease. Therefore, the upper limit of the _SiO2 content is preferably within the above range.

_In the optical glass according to this embodiment, the upper limit of the total content of _P2O5 , _{B2O3 and SiO2} [ _P2O5 + _B2O3 + _SiO2 ] is preferably 50%, more preferably 45 _% , ₄₃ %, 42% and 41% in that order. The lower limit of the total content is preferably 25%, more preferably 27%, 28% and 29% in that order.

By setting the total content [P ₂ O ₅ +B ₂ O ₃ +SiO ₂ ] within the above range, an optical glass having high thermal stability and desired optical constants can be obtained.

In the optical glass according to this embodiment, the lower limit of the mass ratio [( _P2O5 + _B2O3 )/( _P2O5 + _B2O3 + _SiO2 + _{Al2O3 )} ] of the total content of _{P2O5 and B2O3} to the total content of _{P2O5 , B2O3 , SiO2 ,} and _Al2O3 is preferably 0.80, and more preferably _0.90 , 0.93, 0.96, and 0.98 in _this order _. The upper limit of the _mass ratio is preferably 1.00. The mass ratio _may be 1.00.

From the viewpoint of obtaining a glass having high thermal stability and desired optical constants, the mass ratio [(P ₂ O ₅ +B ₂ O ₃ )/(P ₂ O ₅ +B ₂ O ₃ +SiO ₂ +Al ₂ O ₃ )] is preferably in the above range.

In the optical glass according to this embodiment, the lower limit of the _TiO2 content is preferably 10%, more preferably 11%, 12%, and 13%, in that order. The upper limit of the _TiO2 content is preferably 50%, more preferably 40%, 35%, 30%, 28%, 26%, 23%, and 21%, in that order.

_TiO2 contributes greatly to high dispersion. On the other hand, _TiO2 tends to increase the coloring of the glass and may deteriorate the meltability. Therefore, the content of _TiO2 is preferably within the above range.

In the optical glass according to this embodiment, the upper limit of the _WO3 content is preferably 15%, and more preferably 10%, 5%, 3%, 2%, and 1% in that order. The lower _{the WO3} content, the more preferable, and the lower limit is preferably 0%. The _WO3 content may be 0%.

By setting the content of _WO3 within the above range, it is possible to increase the transmittance, suppress an increase in the specific gravity of the glass, and reduce the temperature coefficient of the relative refractive index (dn/dT).

In the optical glass according to this embodiment, the upper limit of the _Bi2O3 content is preferably 15%, and more preferably 10%, 7%, 5%, and 3% in _that order. The lower limit of _{the Bi2O3 content} is preferably 0%.

_Bi2O3 has the function of improving the thermal stability of glass by containing an _{appropriate amount. On the other hand, if the content of Bi2O3 is increased, the coloring of the glass increases. Therefore, the content of Bi2O3 is preferably within the} above range.

In the optical glass according to this embodiment, the upper limit of the mass ratio of the _TiO2 content to the total content of _P2O5 and _B2O3 [ _TiO2 /( _P2O5 + _{B2O3 )} ] is preferably _0.70 , more preferably _0.66 , 0.64, _0.62 , and 0.60 in that order. The lower limit of this mass ratio is preferably 0.25, more preferably 0.27, 0.29, and 0.31 in that order.

By setting the mass ratio [TiO ₂ /(P ₂ O ₅ +B ₂ O ₃ )] within the above range, it becomes easier to obtain an optical glass having desired optical constants and high thermal stability.

In the optical glass according to this embodiment, the upper limit of the mass ratio of the _TiO2 content to _{the P2O5} content [ _TiO2 / _P2O5 ] is preferably 0.70, more preferably 0.66 _, 0.64, and 0.62 in that order. The lower limit of this mass ratio is preferably 0.25, more preferably 0.28, 0.31, and 0.34 in that order.

By setting the mass ratio [TiO ₂ /P ₂ O ₅ ] within the above range, it becomes easier to obtain an optical glass having desired optical constants and high thermal stability.

In the optical glass according to this embodiment, the lower limit of the total content of _TiO2 and _{Nb2O5 [ TiO2} + _{Nb2O5 ]} is preferably 35.0%, more preferably 37.0%, 39.0%, and 40.0% in that order. The upper limit of the total content is preferably 65.0%, more preferably 60.0%, 55.0%, 50.0%, 48.0%, and 46.0% in that order.

From the viewpoint of obtaining glass that has a high refractive index and high dispersion and is excellent in thermal stability, the total content [TiO ₂ +Nb ₂ O ₅ ] is preferably in the above range.

In the optical glass according to this embodiment, the lower limit of the mass ratio [ _Nb2O5 /( _Nb2O5 + _{WO3 )} ] of the content of _Nb2O5 to the total content of _Nb2O5 and _WO3 is preferably 0.70, more preferably _0.80 , _{0.90 ,} and 0.95 in that order. The upper limit of the mass ratio is preferably 1.00. The mass ratio may be 1.00.

From the viewpoint of obtaining a glass which has a high refractive index and high dispersion and in which an increase in the temperature coefficient of the relative refractive index (dn/dT) is suppressed, the mass ratio [Nb ₂ O ₅ /(Nb ₂ O ₅ +WO ₃ )] is preferably in the above range.

In the optical glass according to this embodiment, the lower limit of the mass ratio [ _TiO2 /( _TiO2 + _WO3 )] of the content of _TiO2 to the total content of _TiO2 and _WO3 is preferably 0.70, more preferably 0.80, 0.90, and 0.95 in that order. The upper limit of the mass ratio is preferably 1.00. The mass ratio may be 1.00.

From the viewpoint of obtaining a glass which is highly dispersed and in which an increase in the temperature coefficient of the relative refractive index (dn/dT) is suppressed, the mass ratio [TiO ₂ /(TiO ₂ +WO ₃ )] is preferably in the above range.

In the optical glass according to this embodiment, the lower limit of the mass ratio [( _TiO2 + _{Nb2O5 )} /( _{TiO2 +Nb2O5 + WO3} + _{Bi2O3 )} ] of the total content of _TiO2 and _Nb2O5 to the total content of _{TiO2, Nb2O5} , _WO3 , and _Bi2O3 is preferably 0.70, more preferably 0.80 _{, 0.90} , and _0.95 in that order. The upper limit of the mass ratio is preferably 1.00. The mass ratio may be 1.00.

From the viewpoint of obtaining a glass having a high refractive index and high dispersion and in which an increase in the temperature _coefficient of the relative refractive index (dn/ _dT ) is suppressed, the mass ratio [( _TiO2 + _Nb2O5 )/( _TiO2 + _Nb2O5 + _WO3 + _Bi2O3 )] is preferably in the above range _.

In the optical glass according to this embodiment, the upper limit of the _Ta2O5 content is preferably 10%, and more preferably 7 _% , 5%, and 3% in that order. The lower limit of _{the Ta2O5 content} is preferably 0%. _{The Ta2O5} content may be 0%.

Ta ₂ O ₅ is a glass component that improves the thermal stability and devitrification resistance of glass. On the other hand, Ta ₂ O ₅ increases the refractive index and makes the glass highly dispersible. In addition, when the content of Ta ₂ O ₅ increases, the thermal stability of the glass decreases, and when the glass is melted, the glass raw material is likely to remain unmelted. Therefore, the content of Ta ₂ O ₅ is preferably within the above range. Furthermore, Ta ₂ O ₅ is an extremely expensive component compared to other glass components, and when the content of Ta ₂ O ₅ increases, the production cost of the glass increases. Furthermore, since Ta ₂ O ₅ has a large molecular weight compared to other glass components, it increases the specific gravity of the glass, and as a result, increases the weight of the optical element.

In the optical glass according to this embodiment, the upper limit of the Li ₂ O content is preferably 5%, and more preferably 3%, 2%, and 1% in that order. The lower limit of the _{Li 2} O content is preferably 0%. The _{Li 2} O content may be 0%.

Li ₂ O is a component that contributes to lowering the specific gravity of glass, improves the meltability of glass, and increases the average linear thermal expansion coefficient. On the other hand, if the content of Li ₂ O increases, the devitrification resistance decreases. Therefore, it is preferable that the content of Li ₂ O is within the above range.

In the optical glass according to this embodiment, the lower limit of the _Na2O content is preferably 6%, more preferably 10%, 12%, and 13%, in that order. The upper limit of the _Na2O content is preferably 30%, more preferably 22%, 20%, 19%, and 18%, in that order.

Na ₂ O is a component that contributes to lowering the specific gravity of glass, improves the meltability of glass, and increases the average linear thermal expansion coefficient. On the other hand, if the content of Na ₂ O increases, the devitrification resistance decreases. Therefore, it is preferable that the content of Na ₂ O is within the above range.

In the optical glass according to this embodiment, the lower limit of the K ₂ O content is preferably 1%, more preferably 2%, 3%, and 4%, in that order. The upper limit of the K ₂ O content is preferably 13%, more preferably 12%, 11%, and 10%, in that order.

K ₂ O is a component that contributes to lowering the specific gravity of glass and improves the thermal stability of glass. It also works to increase the average linear thermal expansion coefficient. On the other hand, if the content of K ₂ O increases, the thermal stability, devitrification resistance, chemical durability, and weather resistance decrease. Therefore, the content of K ₂ O is preferably within the above range.

In the optical glass according to this embodiment, the upper limit of the total content of _Li2O , _Na2O and _K2O [ _Li2O + _Na2O + _K2O ] is preferably 35%, more preferably 30%, 28%, 26% and 25% in that order. The lower limit of the total content is preferably 10%, more preferably 14%, 15%, 16% and 17% in that order.

Li ₂ O, Na ₂ O and K ₂ O all have the function of improving the thermal stability of glass. However, if their contents are too high, chemical durability and weather resistance may be reduced. Therefore, the total content of Li ₂ O, Na ₂ O and K ₂ O [Li ₂ O + Na ₂ O + K ₂ O] is preferably within the above range.

In the optical glass according to this embodiment, the upper limit of the _Cs2O content is preferably 5%, and more preferably 3%, 2%, and 1% in that order. The lower limit of the _Cs2O content is preferably 0%. The _Cs2O content may be 0%.

Cs ₂ O has the function of improving the melting property of glass, but if the content is too high, the thermal stability and refractive index nd of the glass decrease, and the volatilization of glass components during melting increases, making it impossible to obtain the desired glass. Therefore, the content of Cs ₂ O is preferably within the above range.

In the optical glass according to this embodiment, the lower limit of the mass ratio of the content of _Na2O to the total content of _Li2O , _Na2O , _{K2O and Cs2O} [ _Na2O /(Li2O+ _Na2O + _K2O + _Cs2O )] is preferably 0.20, more preferably 0.50, 0.55, 0.60 and 0.65 in that order. The upper limit of the mass ratio is preferably 0.98, more preferably 0.95, 0.92, 0.90 and 0.88 in that order.

From the viewpoint of obtaining glass having excellent resistance to devitrification and thermal stability, the mass ratio [Na ₂ O/(Li ₂ O+Na ₂ O+K ₂ O+Cs ₂ O)] is preferably in the above range.

In the optical glass according to this embodiment, the upper limit of the mass ratio [( _P2O5 + _B2O3 + _SiO2 )/( _Li2O + _Na2O + _K2O + _{Cs2O )} ] of the total content of _{P2O5 , B2O3 and SiO2} to the total content of _Li2O , _Na2O , _K2O and _Cs2O is preferably 2.50, more preferably 2.40, 2.35, 2.30, 2.27, _2.25 in this order. The lower limit of the mass ratio is preferably 1.20, more preferably 1.30, 1.35, 1.38, 1.40 in this order.

By setting the mass ratio [( _P2O5 + _B2O3 + _SiO2 )/( _Li2O + _Na2O + _K2O + _Cs2O )] in the above range, an optical glass having _high thermal stability, a low temperature coefficient of relative refractive index (dn/ _dT ), and a large average linear thermal expansion coefficient can be obtained.

In the optical glass according to this embodiment, the MgO content is preferably 5% or less, more preferably 3% or less, and even more preferably 1% or less. The lower limit of the MgO content is preferably 0%. The MgO content may be 0%.

In the optical glass according to this embodiment, the CaO content is preferably 5% or less, more preferably 3% or less, and even more preferably 1% or less. The lower limit of the CaO content is preferably 0%. The CaO content may be 0%.

In the optical glass according to this embodiment, the content of SrO is preferably 6% or less, more preferably 5% or less, 3% or less, and 1% or less, in that order. The lower limit of the content of SrO is preferably 0%.

In the optical glass according to this embodiment, the content of BaO is preferably 8% or less, more preferably 5% or less, 3% or less, and 1% or less, in that order. The lower limit of the content of BaO is preferably 0%.

In the optical glass according to this embodiment, the upper limit of the total content of MgO, CaO, SrO, and BaO [MgO+CaO+SrO+BaO] is preferably 8.0%, and more preferably 5.0%, 4.0%, 3.0%, 1.5%, 1.0%, and 0.5%, in that order. The lower limit of the total content is preferably 0%. The total content may be 0%.

MgO, CaO, SrO, and BaO are all glass components that work to improve the thermal stability and devitrification resistance of glass. However, if the content of these glass components is too high, the high dispersibility is lost and the thermal stability and devitrification resistance of the glass are reduced. Furthermore, if the BaO content is too high, the specific gravity of the glass increases. Therefore, it is preferable that the individual contents and the total content of these glass components are within the above ranges.

In the optical glass according to this embodiment, the upper limit of the ZnO content is preferably 10%, and more preferably 6%, 4%, and 3%, in that order. The lower the ZnO content, the more preferable, and the lower limit is preferably 0%. The ZnO content may be 0%.

ZnO is a glass component that improves the thermal stability of glass. However, if the ZnO content is too high, the specific gravity of the glass increases. In addition, the temperature coefficient of the relative refractive index (dn/dT) increases. For this reason, it is preferable that the ZnO content be within the above range.

In the optical glass according to this embodiment, the lower limit of the mass ratio of the content of _Na2O to the total content of _Na2O and ZnO [ _Na2O /( _Na2O +ZnO)] is preferably 0.50, and more preferably 0.60, 0.70, and 0.90 in that order. The upper limit of the mass ratio is preferably 1.00. The mass ratio may be 1.00.

From the viewpoint of suppressing an increase in the specific gravity of the glass and suppressing an increase in the temperature coefficient of the relative refractive index (dn/dT), it is preferable that the mass ratio [Na ₂ O/(Na ₂ O+ZnO)] is in the above range.

In the optical glass according to this embodiment, the lower limit of the mass ratio [ _TiO2 /( _TiO2 +ZnO)] of the content of _TiO2 to the total content of _TiO2 and ZnO is preferably 0.70, more preferably 0.80, 0.90, and 0.95 in that order. The upper limit of the mass ratio is preferably 1.00. The mass ratio may be 1.00.

From the viewpoint of obtaining a glass which is highly dispersed and in which an increase in the temperature coefficient of the relative refractive index (dn/dT) is suppressed, the mass ratio [TiO ₂ /(TiO ₂ +ZnO)] is preferably in the above range.

In the optical glass according to this embodiment, the lower limit of the mass ratio [( _TiO2 + _{Nb2O5 )} /( _TiO2 + _Nb2O5 + _WO3 + _Bi2O3 +ZnO ₎ ] of the total content of _TiO2 and _Nb2O5 to the total content of TiO2 _{, Nb2O5 , WO3, Bi2O3 and} ZnO is preferably 0.70, _and more preferably _0.80 , 0.90, and 0.95 in this order. The upper limit of the mass ratio is preferably 1.00. The mass ratio may be 1.00.

From the viewpoint of obtaining a glass having a high refractive index and high dispersion and in which an increase in the temperature _coefficient of the relative refractive index (dn/ _dT ) is suppressed, the mass ratio [( _TiO2 + _Nb2O5 )/( _TiO2 + _Nb2O5 + _WO3 + _Bi2O3 + _ZnO )] is preferably in the above range.

In the optical glass according to this embodiment, it is preferable that the mass ratio of the total content of _{TiO2 , Nb2O5} , _WO3 , _Bi2O3 and _Ta2O5 to the total content of _{P2O5 , B2O3} , _{SiO2 , Al2O3} , _Li2O , _{Na2O , K2O} and _Cs2O [ _{( TiO2} + _{Nb2O5 + WO3} + _Bi2O3 + _{Ta2O5 )} /( _{P2O5 + B2O3} + _SiO2 + _{Al2O3 + Li2O} + _{Na2O +K2O + Cs2O} )] is _1.10 or _less . The upper limit of the mass ratio is preferably 1.00, and more preferably 0.95, 0.90, 0.85, 0.82, and 0.80 in that order. The lower limit of the mass ratio is more preferably 0.50, and more preferably 0.60, 0.65, 0.68, and 0.70 in that order.

By setting the mass ratio [( _TiO2 + _Nb2O5 + _WO3 + _{Bi2O3 + Ta2O5 ) /(P2O5 + B2O3 + SiO2 + Al2O3 + Li2O + Na2O + K2O + Cs2O ) ] within the} above range, _{it becomes} easier to obtain an optical glass having _{the desired} optical constants.

In the optical glass according to this embodiment, the content of _ZrO2 is preferably 5% or less, more preferably 3% or less, and further more preferably 1% or less. The lower limit of the content of _ZrO2 is preferably 0%.

_ZrO2 is a glass component that improves the thermal stability and devitrification resistance of glass. However, if the content of _ZrO2 is too high, the thermal stability tends to decrease. Therefore, the content of _ZrO2 is preferably within the above range.

In the optical glass according to this embodiment, the upper limit of the Sc ₂ O ₃ content is preferably 2%. The lower limit of the Sc ₂ O ₃ content is preferably 0%.

In the optical glass according to this embodiment, the upper limit of the _HfO2 content is preferably 2%. The lower limit of the _HfO2 content is preferably 0%.

Both Sc ₂ O ₃ and HfO ₂ have the function of increasing the refractive index nd, and are expensive components, so the content of each of Sc ₂ O ₃ and HfO ₂ is preferably within the above range.

In the optical glass according to this embodiment, the upper limit of _{the Lu2O3} content is preferably 2%. The lower limit of _{the Lu2O3} content is preferably 0%.

_Lu2O3 has the function of increasing the _refractive index nd. In addition, since it has a large molecular weight, it is also a glass component that increases the specific gravity of the glass. Therefore, the content of _Lu2O3 is preferably _within the above range.

In the optical glass according to this embodiment, the upper limit of the _GeO2 content is preferably 2%. The lower limit of the _GeO2 content is preferably 0%.

_GeO2 has the function of increasing the refractive index nd, and is an extremely expensive component among commonly used glass components. Therefore, from the viewpoint of reducing the manufacturing cost of the glass, it is preferable that the content of _GeO2 is within the above range.

In the optical glass according to this embodiment, the upper limit of the _La2O3 content is preferably 2%. The lower limit of _{the La2O3} content is preferably 0%. _{The La2O3 content may} be 0%.

If the content of La ₂ O ₃ is large, the thermal stability and devitrification resistance of the glass are reduced, and the glass is easily devitrified during production. Therefore, from the viewpoint of suppressing the deterioration of the thermal stability and devitrification resistance, the content of La ₂ O ₃ is preferably within the above range.

In the optical glass according to this embodiment, the upper limit of the Gd ₂ O ₃ content is preferably 2%. The lower limit of the Gd ₂ O ₃ content is preferably 0%.

If the content of Gd ₂ O ₃ is too high, the thermal stability and devitrification resistance of the glass are reduced, and the glass is easily devitrified during production. If the content of Gd ₂ O ₃ is too high, the specific gravity of the glass increases, which is not preferable. Therefore, from the viewpoint of suppressing the increase in specific gravity while maintaining the thermal stability and devitrification resistance of the glass well, the content of Gd ₂ O ₃ is preferably within the above range.

In the optical glass according to this embodiment, the upper limit of the Y ₂ O ₃ content is preferably 2%. The lower limit of the Y ₂ O ₃ content is preferably 0%. The Y ₂ O ₃ content may be 0%.

If the content of Y ₂ O ₃ is too high, the thermal stability and resistance to devitrification of the glass are reduced. Therefore, from the viewpoint of suppressing the deterioration of the thermal stability and resistance to devitrification, the content of Y ₂ O ₃ is preferably within the above range.

In the optical glass according to this embodiment, the upper limit of the Yb ₂ O ₃ content is preferably 2%. The lower limit of the Yb ₂ O ₃ content is preferably 0%.

_Yb2O3 has _a larger molecular weight than _La2O3 , _{Gd2O3 , and Y2O3} , and therefore increases the specific gravity of the glass. When the specific gravity of the glass increases, the mass of the optical element increases. For example, when _a lens with a large mass is incorporated into an autofocus type imaging lens, the power required to drive the lens during _autofocus increases, and the battery is consumed rapidly. Therefore, it is desirable to reduce the content of _Yb2O3 to suppress the increase in the specific gravity of the glass.

Moreover, if the content of Yb ₂ O ₃ is too high, the thermal stability and devitrification resistance of the glass are reduced. From the viewpoint of preventing a decrease in the thermal stability of the glass and suppressing an increase in the specific gravity, the content of Yb ₂ O ₃ is preferably within the above range.

The optical glass according to this embodiment mainly _comprises the above-mentioned glass components, namely, the essential components being _P2O5 , _{Nb2O5 , B2O3} , _TiO2 , _Na2O , and _K2O , and the optional components _{being Al2O3 , SiO2, WO3, Bi2O3, Ta2O5, Li2O , Cs2O , MgO , CaO} , SrO, BaO _, ZnO _{, ZrO2 , Sc2O3} , _{HfO2 , Lu2O3 , GeO2 , La2O3} , _Gd2O3 , _Y2O3 , and _{Yb2O .} The total content of _the above glass components is preferably 95% or more, more preferably 98% or more, further preferably 99% or more, and even more preferably 99.5% or more.

In the optical glass according to this embodiment, the upper limit of the _TeO2 content is preferably 2%. The lower limit of the _TeO2 content is preferably 0%.

Since _TeO2 is toxic, it is preferable to reduce the content of _TeO2 . Therefore, the content of _TeO2 is preferably within the above range.

In the optical glass according to this embodiment, the fluorine (F) content is preferably 3% or less, with the upper limit being 1%, 0.5%, and 0.3%, in that order. The lower the F content, the more preferable, and the lower limit is preferably 0%. The F content may be 0%. Also, preferably, the glass is substantially free of fluorine (F).

By keeping the F content within the above range, it is possible to suppress volatilization during melting of the glass, and to suppress fluctuations in the refractive index and striae.

The optical glass according to this embodiment is preferably basically composed of the above glass components, but may contain other components as long as they do not impede the effects of the present invention. Furthermore, the present invention does not exclude the inclusion of unavoidable impurities.

<Other component composition>
Pb, As, Cd, Tl, Be, and Se are all toxic, so it is preferable that the optical glass according to this embodiment does not contain these elements as glass components.

U, Th, and Ra are all radioactive elements. Therefore, it is preferable that the optical glass according to this embodiment does not contain these elements as glass components.

V, Cr, Mn, Fe, Co, Ni, Cu, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, and Tm increase the coloring of the glass and can be sources of fluorescence. For this reason, it is preferable that the optical glass according to this embodiment does not contain these elements as glass components.

Sb (Sb ₂ O ₃ ) and Ce (CeO ₂ ) are elements that can be added arbitrarily and function as fining agents. Of these, Sb (Sb ₂ O ₃ ) is a fining agent with a large fining effect. However, Sb (Sb ₂ O ₃ ) is highly oxidizing, and if the amount of Sb (Sb ₂ O ₃ ) added is increased, the coloring of the glass increases due to light absorption by Sb ions, which is not preferable. In addition, when melting glass, if Sb is present in the melt, the elution of platinum constituting the glass melting crucible into the melt is promoted, and the platinum concentration in the glass increases. If platinum exists as an ion in the glass, the coloring of the glass increases due to light absorption. In addition, if platinum exists as a solid in the glass, it becomes a source of light scattering, which reduces the quality of the glass. Ce (CeO ₂ ) has a smaller fining effect than Sb (Sb ₂ O ₃ ). Adding a large amount of Ce (CeO ₂ ) intensifies the coloring of the glass. Therefore, when adding a fining agent, it is preferable to add Sb (Sb ₂ O ₃ ) while paying attention to the amount added.

The content of Sb ₂ O ₃ is expressed as an external percentage. That is, when the total content of all glass components other than Sb ₂ O ₃ and CeO ₂ is 100 mass%, the content of Sb ₂ O ₃ is preferably less than 1 mass%, more preferably less than 0.1 mass%. Furthermore, it is preferable in this order to be less than 0.05 mass%, less than 0.03 mass%, less than 0.02 mass%, and less than 0.01%. The content of _{Sb 2} O ₃ may be 0 mass%.

The content of _CeO2 is also expressed as an external percentage. That is, when _the total content of all glass components other than _CeO2 and _Sb2O3 is 100 mass%, the content of _CeO2 is preferably less than 2 mass%, more preferably less than 1 mass%, even more preferably less than 0.5 mass%, and even more preferably less than 0.1 mass%. The content of _CeO2 may be 0 mass%. By setting the content of _CeO2 in the above range, the clarity of the glass can be improved.

(Glass properties)
Next, the characteristics of the optical glass according to this embodiment will be described.

<Refractive index nd>
In the optical glass according to this embodiment, the refractive index nd is preferably 1.63 to 1.80. The lower limit of the refractive index nd may be 1.65, 1.67, 1.69, 1.71 or 1.73, and the upper limit of the refractive index nd may be 1.79, 1.78 or 1.77.

The refractive index nd can be adjusted to a desired value by appropriately adjusting the content of each glass component. Components that relatively increase the refractive index nd (high refractive index components) are _Nb2O5 , _TiO2 , _WO3 , _Bi2O3 , _Ta2O5 , _ZrO2 , _La2O3 , _etc. On the other hand _, components that relatively decrease the refractive index nd ₍ low refractive index components) are _P2O5 , _{SiO2 , B2O3 , Li2O} , _Na2O , _{K2O ,} etc.

<Abbe number νd>
In the optical glass according to this embodiment, the Abbe number vd is preferably 20 to 30. The lower limit of the Abbe number vd may be 22, 22.5, 23, or 23.2, and the upper limit of the Abbe number vd may be 28, 26, or 25.

The Abbe number vd can be adjusted to a desired value by appropriately adjusting the content of each glass component. Components that relatively lower the Abbe number vd, i.e., high dispersion components _, are _Nb2O5 , _{TiO2 , WO3} , _Bi2O3 , _Ta2O5 , ZrO2, etc. On the other hand, components that relatively increase the Abbe number _vd , i.e., low dispersion components, are _P2O5 , _SiO2 , _B2O3 , _Li2O , _Na2O , _K2O , _La2O3 , _BaO , CaO, _{SrO , etc.}

<Average coefficient of linear thermal expansion α>
In the optical glass according to this embodiment, the lower limit of the average linear thermal expansion coefficient α at 100 to 300° C. is preferably 100×10 ⁻⁷ ° C. ⁻¹ , and more preferably 102×10 ⁻⁷ ° C. ⁻¹ , 104×10 ⁻⁷ ° C. ⁻¹ , 106×10 ⁻⁷ ° C. ⁻¹ , and 108×10 ⁻⁷ ° C. ⁻¹ in that order. The upper limit of the average linear thermal expansion coefficient α is more preferably 200×10 ⁻⁷ ° C. ⁻¹ , and more preferably 190×10 ⁻⁷ ° C. ⁻¹ , 180×10 ⁻⁷ ° C. ⁻¹ , 170×10 ⁻⁷ ° C. ⁻¹ , 160×10 ⁻⁷ ° C. ⁻¹ , 150×10 ⁻⁷ ° C. ⁻¹ , and 145×10 ⁻⁷ ° C. ⁻¹ in that order.

By setting the average linear expansion coefficient α between 100 and 300°C in the above range, it is possible to suppress the change in refractive index that accompanies the thermal expansion of the glass, i.e., the increase in the temperature coefficient of the relative refractive index dn/dT.

The average linear expansion coefficient α is measured based on the provisions of JOGIS08-2003, except that the sample is a round bar having a length of 20 mm±0.5 mm and a diameter of 5 mm±0.5 mm, and is heated at a constant rate of 4°C per minute while a load of 98 mN is applied to the sample, and the temperature and the elongation of the sample are measured.
In this specification, the average linear expansion coefficient α is expressed in units of [° C. ⁻¹ ], but even when the unit [K ⁻¹ ] is used, the numerical value of the average linear expansion coefficient α is the same.

<Temperature coefficient of relative refractive index dn/dT>
In the optical glass according to this embodiment, the temperature coefficient of the relative refractive index dn/dT at the wavelength (633 nm) of a He—Ne laser is in the range of 20 to 40° C., and is preferably −1.0×10 ⁻⁶ to −13.0×10 ⁻⁶ ° C. ⁻¹ , and more preferably −1.0×10 ⁻⁶ to −10.0×10 ⁻⁶ ° C. ⁻¹ , −1.3×10 ⁻⁶ to −9.0×10 ⁻⁶ ° C. ⁻¹ , −1.3×10 ⁻⁶ to −8.0×10 ⁻⁶ ° C. ⁻¹ , −1.5×10 ⁻⁶ to −7.0×10 ⁻⁶ ° C. ⁻¹ , and −1.6×10 ⁻⁶ to −6.5×10 ⁻⁶ ° C. ⁻¹ in that order.

By setting dn/dT within the above range and combining it with an optical element with a positive dn/dT, the variation in the refractive index is reduced even in an environment where the temperature of the optical element varies greatly, making it possible to achieve the desired optical characteristics with high precision over a wider temperature range.

The temperature coefficient of the relative refractive index dn/dT is measured based on the interferometric method of JOGIS18-2008.
In this specification, the temperature coefficient dn/dT is expressed in units of [° C. ⁻¹ ], but even if the unit is [K ⁻¹ ], the value of the temperature coefficient dn/dT will be the same.

<Glass transition temperature Tg>
The glass transition temperature Tg of the optical glass according to this embodiment is preferably 600° C. or less, and more preferably 590° C. or less, 580° C. or less, 570° C. or less, and 560° C. or less in that order.

By ensuring that the upper limit of the glass transition temperature Tg falls within the above range, it is possible to suppress increases in the molding temperature and annealing temperature of the glass, thereby reducing thermal damage to the press molding equipment and annealing equipment. In addition, by ensuring that the lower limit of the glass transition temperature Tg falls within the above range, it becomes easier to maintain good thermal stability of the glass while maintaining the desired Abbe number and refractive index.

<Specific gravity of glass>
In the optical glass according to this embodiment, the specific gravity is preferably 3.40 or less, and more preferably 3.30 or less, and then 3.20 or less. If the specific gravity of the glass can be reduced, the weight of the lens can be reduced. As a result, the power consumption of the autofocus drive of the camera lens in which the lens is mounted can be reduced.

<Light transmittance of glass>
The light transmittance of the optical glass according to this embodiment can be evaluated by the coloring degree λ5.
The spectral transmittance of a glass sample having a thickness of 10.0 mm±0.1 mm is measured in the wavelength range of 200 to 700 nm, and the wavelength at which the external transmittance becomes 5% is defined as λ5.

The λ5 of the optical glass according to this embodiment is preferably 400 nm or less, more preferably 390 nm or less, and even more preferably 385 nm or less.

By using optical glass with a shorter λ5 wavelength, it is possible to provide optical elements that enable optimal color reproduction.

(Optical glass manufacturing)
The optical glass according to the embodiment of the present invention may be produced by blending glass raw materials to obtain the above-mentioned predetermined composition, and by using the blended glass raw materials according to a known glass manufacturing method. For example, a plurality of compounds may be blended and thoroughly mixed to obtain a batch raw material, and the batch raw material may be placed in a quartz crucible or a platinum crucible to be roughly melted (rough melt). The molten material obtained by the rough melting is quenched and crushed to produce cullet. The cullet is then placed in a platinum crucible, heated and remelted (remelt) to obtain a molten glass, which is then clarified and homogenized, molded, and slowly cooled to obtain an optical glass. A known method may be applied to the molding and slowly cooling of the molten glass.

The compounds used to prepare the batch raw materials are not particularly limited as long as the desired glass components can be introduced into the glass in the desired content, but examples of such compounds include oxides, carbonates, nitrates, hydroxides, fluorides, etc.

(Manufacturing of optical elements, etc.)
To manufacture an optical element using the optical glass according to the embodiment of the present invention, a known method may be applied. For example, a glass raw material is melted to form a molten glass, and the molten glass is poured into a mold and molded into a plate shape to manufacture a glass material made of the optical glass according to the present invention. The obtained glass material is appropriately cut, ground, and polished to manufacture cut pieces having a size and shape suitable for press molding. The cut pieces are heated and softened, and press molded (reheat pressed) by a known method to manufacture an optical element blank that is similar to the shape of the optical element. The optical element blank is annealed, and then ground and polished by a known method to manufacture an optical element.

The optically functional surfaces of the fabricated optical elements may be coated with anti-reflection films, total reflection films, etc., depending on the intended use.

Examples of optical elements include various lenses such as spherical lenses, prisms, and diffraction gratings.

The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

(Example)
[Preparation of glass samples]
Compound raw materials corresponding to each component, i.e., raw materials such as phosphates, carbonates, and oxides, were weighed and thoroughly mixed to prepare blended raw materials so as to obtain glass having the compositions of Samples No. 1 to 8 shown in Table 1. The blended raw materials were placed in a platinum crucible, heated to 900 to 1,350°C in an air atmosphere to melt, and homogenized and clarified by stirring to obtain a molten glass. The molten glass was cast into a mold to be shaped, and slowly cooled to obtain a block-shaped glass sample.
Alternatively, the raw materials may be charged into a quartz glass crucible, melted, transferred to a platinum crucible, and further heated to melt, homogenized by stirring, and the molten glass obtained by clarification may be poured into a mold and shaped, followed by slow cooling.

[Evaluation of Glass Samples]
The glass composition, refractive index nd, Abbe number vd, λ5, glass transition temperature Tg, average linear expansion coefficient α, temperature coefficient of relative refractive index dn/dT, and specific gravity of the obtained glass samples were measured by the methods described below. The results are shown in Table 1.

[1] Glass composition The content of each glass component of the obtained glass samples was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES). In addition, the F content was 0% in all glass samples No. 1 to 8 shown in Table 1.

[2] Specific Gravity: Measured in accordance with the Japan Optical Glass Industry Association standard JOGIS-05.

[3] Refractive index nd and Abbe number vd
The measurement was performed based on the Japan Optical Glass Industry Association standard JOGIS-01.

[4] λ5
A glass sample was processed to have a thickness of 10 mm and parallel optically polished flat surfaces, and the spectral transmittance was measured in the wavelength range from 280 nm to 700 nm. The intensity of a light beam perpendicularly incident on one of the optically polished flat surfaces was defined as intensity A, and the intensity of a light beam emerging from the other flat surface was defined as intensity B, and the spectral transmittance B/A was calculated. The wavelength at which the spectral transmittance was 5% was defined as λ5. The spectral transmittance also includes the reflection loss of the light beam on the sample surface.

[5] Glass transition temperature Tg
The glass transition temperature Tg was measured using a thermomechanical analyzer (TMA) (TMA-4000S, manufactured by Mac Science) at a heating rate of 4° C./min.

[6] Measurement of temperature coefficient of relative refractive index dn/dT The obtained glass samples were measured based on the interference method of JOGIS18-2008. A He-Ne laser with a wavelength of 633 nm was used as the light source, and measurements were made continuously at temperatures ranging from -70 to 150°C. Among the measurement results, dn/dT values in the range of 20°C to 40°C are shown in Table 1.

[7] Measurement of average linear expansion coefficient α The average linear expansion coefficient α from 100 to 300°C was measured in accordance with the provisions of JOGIS08-2003. The sample was a round bar with a length of 20 mm±0.5 mm and a diameter of 5 mm±0.5 mm, and was heated at a constant rate of 4°C per minute while a load of 98 mN was applied to the sample, and the temperature and the elongation of the sample were measured.

Example 2
The glass sample obtained in Example 1 was cut and ground to prepare cut pieces. The cut pieces were press molded by a reheat press to prepare optical element blanks. The optical element blanks were precisely annealed to precisely adjust the refractive index to a required refractive index, and then ground and polished by a known method to obtain various lenses such as a biconvex lens, a biconcave lens, a plano-convex lens, a plano-concave lens, a concave meniscus lens, and a convex meniscus lens.

The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is indicated by the claims rather than the above description, and it is intended to include all modifications within the meaning and scope of the claims.

For example, by adjusting the composition as described in the specification to the glass compositions exemplified above, an optical glass according to one aspect of the present invention can be produced.
Furthermore, it is of course possible to arbitrarily combine two or more of the items described in the specification as examples or preferred ranges.

Claims (5)

The content of _P2O5 is 25 to 50 mass% _;
The content of _Nb2O5 is 19 to 40 mass% _;
the total content of TiO ₂ , Nb ₂ O ₅ , WO ₃ , Bi ₂ O ₃ and Ta ₂ O ₅ [TiO ₂ +Nb ₂ O ₅ +WO ₃ +Bi ₂ O ₃ +Ta ₂ O ₅ ] is 35 to 50 mass %,
the mass ratio of the content of _TiO2 to the total content of _TiO2 , _{Nb2O5 , WO3 , Bi2O3 and Ta2O5} [ _TiO2 /( _TiO2 + _Nb2O5 + _WO3 + _{Bi2O3 + Ta2O5 )} ] is 0.25 or more _;
the mass ratio of the content of B ₂ O ₃ to the content of P ₂ O ₅ [B ₂ O ₃ /P ₂ O ₅ ] is 0.05 to 0.39;
The total content of _Li2O , _Na2O , _K2O and _Cs2O [ _Li2O + _Na2O + _K2O + _Cs2O ] is 14 mass% or more;
the mass ratio of the Na ₂ O content to the K ₂ O content [Na ₂ O/K ₂ O] is 1.50 or more;
An optical glass having a refractive index nd of 1.80 or less . The optical glass according to claim 1, which is substantially free of F. 3. The optical glass according to claim 1, wherein the average linear thermal expansion coefficient α at 100 to 300° C. is 100×10 ⁻⁷ to 200×10 ⁻⁷ ° C. ⁻¹ . 4. The optical glass according to claim 1, wherein the temperature coefficient dn/dT of the relative refractive index at the wavelength (633 nm) of a He-Ne laser is -0.1×10 ^-6 to -13.0×10 ^-6 ° C. ^-1 in the range of 20 to 40°C. An optical element comprising the optical glass according to any one of claims 1 to 4.