JP5275674B2 - Optical glass composition, preform and optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical glass composition that has a refractive index nd falling within a high refractive index range from 1.88 to 1.92 and an Abbe number &nu;d falling within a range from 33 to 37 and has excellent thermal characteristics. <P>SOLUTION: The optical glass composition contains, by weight, 1.0-12.0% SiO<SB>2</SB>, 8.0-18.0% B<SB>2</SB>O<SB>3</SB>, 0-6.0% ZnO, 1.0-10.0% ZrO<SB>2</SB>, 25.0-47.0% La<SB>2</SB>O<SB>3</SB>, 0.5-4.0% Li<SB>2</SB>O, 0-16.0% Nb<SB>2</SB>O<SB>5</SB>and 0-7.0% TiO<SB>2</SB>, 0-15.0% Ta<SB>2</SB>O<SB>5</SB>, 1.0% or more of Nb<SB>2</SB>O<SB>5</SB>+TiO<SB>2</SB>+Ta<SB>2</SB>O<SB>5</SB>, 0-26.0% Gd<SB>2</SB>O<SB>3</SB>, and 0-15.0% WO<SB>3</SB>, and has nd of 1.88-1.92, &nu;d of 33-37, and a surrender point of 700&deg;C or lower. A preform and an optical element are formed from the optical glass composition. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to an optical glass composition, a preform, and an optical element. In particular, the present invention relates to an optical glass composition suitable as a material for an optical element such as a lens element included in a photographing lens system of a digital still camera or a digital video camera (hereinafter simply referred to as a digital camera), and press molding the optical element. The present invention relates to a preform for manufacturing according to the above, and an optical element thereof.

  In recent years, various types of digital cameras have been proposed ranging from popular types to high-end types according to consumer needs. Regardless of whether the digital camera is a widespread type or a high-end type, there is a strong demand for thinning it in order to improve portability. In order to reduce the thickness of a digital camera, it is essential to reduce the thickness of a taking lens system that occupies a relatively large volume.

  A reduction in the number of lens elements is effective in reducing the thickness of the taking lens system. However, in recent years, the optical performance required for photographing lens systems has been improved, so that the number of lens elements can be drastically reduced. Therefore, to reduce the thickness of the photographing lens system, it is necessary to reduce the thickness of each lens element included in the photographing lens system.

In order to reduce the thickness of the lens element, it is effective to increase the refractive index of the glass material constituting the lens element. Examples of such a glass material having a high refractive index include optical glasses described in Patent Documents 1 and 2.
JP 2005-239476 A JP 2005-179142 A

  The imaging lens system needs to perform aberration correction. Usually, aberration is corrected by variously combining the optical constants such as the refractive index (nd) and dispersion (νd: Abbe number) of the lens element and the shape of the lens element. Therefore, aberration correction cannot be performed sufficiently by simply using many glasses having a high refractive index for the purpose of reducing the thickness.

  Among the image pickup lens systems, the zoom lens system performs zooming by changing the interval between a plurality of lens groups, and therefore it is necessary to perform certain aberration correction such as chromatic aberration correction for each lens group. For this reason, the minimum number of lenses in the lens group is two, that is, a positive power lens element and a negative power lens element. However, the optical constants such as the refractive index (nd) and dispersion (νd: Abbe number) of the two lens elements have certain constraints when considering aberration correction.

  Patent Document 1 discloses an optical glass having a high refractive index exceeding 2.00. When such an optical glass is used for a lens element having a positive power among the lens groups of the zoom lens system, the effect of reducing the thickness of the lens group is high. However, in a high refractive index region exceeding 2.00, the Abbe number (νd) tends to be small. For this reason, an optical glass having a relatively high refractive index and a large Abbe number is required for a lens element having negative power. Specifically, the lens element having negative power has a refractive index (nd) with respect to d-line of 1.88 to 1.92 and an Abbe number (νd) with respect to d-line of 33 to 37. Optical glass is required. Furthermore, in addition to these optical constant limiting conditions, optical glass is also required to have excellent thermal properties in heat treatment when manufacturing optical elements. However, in these Patent Documents 1 and 2, the refractive index (nd) with respect to the d-line is 1.88 or more and 1.92 or less, the Abbe number (νd) with respect to the d-line is in the range of 33 to 37, and the heat An optical glass having excellent characteristics is not described.

  In view of the above problems, the present invention is in a high refractive index region where the refractive index (nd) for d-line is 1.88 or more and 1.92 or less, and the Abbe number (νd) for d-line is 33 or more and 37 or less. Furthermore, it is an object of the present invention to provide an optical glass composition excellent in thermal characteristics, and a preform and an optical element comprising the optical glass composition.

  The above object is achieved by the following optical glass composition, preform, and optical element.

That is, the present invention
(1) In weight% display
The SiO ₂ 1.0% 12.0% or less,
B ₂ O ₃ is 8.0% or more and 18.0% or less,
ZnO is 2.0 % or more and 6.0% or less,
ZrO ₂ is 1.0% or more and 10.0% or less,
La ₂ O ₃ is 25.0% or more and 47.0% or less,
Li ₂ O is 1.5 % or more and 4.0% or less,
Nb ₂ O ₅ is 0% or more and 16.0% or less,
TiO ₂ is 0% or more and 7.0% or less,
Ta ₂ O ₅ is 0% or more and 6.5 % or less,
Nb ₂ O ₅ + TiO ₂ + Ta ₂ O ₅ is 1.0% or more,
Containing 0 to 26.0% of Gd ₂ O ₃ and 0 to 15.0% of WO ₃ ;
Substantially free of Y ₂ O ₃ ,
an optical glass composition having a refractive index (nd) for d-line of 1.88 or more and 1.92 or less, an Abbe number (νd) for d-line of 33 or more and 37 or less, and a yield point of 700 ° C. or less,
(2) A preform comprising the optical glass composition described in the above item (1) and softened by heating and used for at least press molding, and (3) the optical glass composition described in the above item (1) The present invention relates to an optical element.

  According to the present invention, the Abbe number (νd) for the d line is in the range of 33 to 37, while the refractive index (nd) for the d line is in the high refractive index region of 1.88 to 1.92. Moreover, an optical glass composition having a yield point of 700 ° C. or lower can be provided.

  According to the present invention, the Abbe number (νd) for the d-line is in the range of 33 to 37, while the refractive index (nd) for the d-line is in a high refractive index region of 1.88 to 1.92. In addition, it is possible to provide a preform for producing an optical element made of an optical glass composition having a yield point of 700 ° C. or less by press molding.

  Furthermore, according to the present invention, the Abbe number (νd) for the d-line is in the range of 33 to 37 while the refractive index (nd) for the d-line is in the high refractive index region of 1.88 to 1.92. In addition, an optical element made of an optical glass composition having a yield point of 700 ° C. or lower can be provided.

(Embodiment 1)
First, the optical glass composition according to Embodiment 1 of the present invention will be described in detail. The optical glass composition has the following composition.

That is, the optical glass composition according to the first embodiment is expressed in wt%, and SiO ₂ is 1.0% or more and 12.0% or less, B ₂ O ₃ is 8.0% or more and 18.0% or less, ZnO is 2.0 % to 6.0%, ZrO ₂ is 1.0% to 10.0%, La ₂ O ₃ is 25.0% to 47.0%, and Li ₂ O is 1.5 %. % To 4.0%, Nb ₂ O ₅ to 0% to 16.0%, TiO ₂ to 0% to 7.0%, Ta ₂ O ₅ to 0% to 6.5 %, Nb _{2 O 5 + TiO 2 + Ta 2} O 5 1.0% or more, _Gd 2 _{O 3} less 26.0% 0%, and WO ₃ below viewing containing 15.0% 0% or more, substantially, _{Y 2} O ₃ does not contain. From the optical glass composition, the refractive index (nd) for d-line is 1.88 to 1.92, the Abbe number (νd) for d-line is in the range of 33 to 37, and the yield point is 700 ° C. The following more stable optical glass can be obtained.

  Next, each component constituting the optical glass composition will be described in detail. Hereinafter, the content of each component is expressed by weight%.

SiO ₂ functions as a network forming component and is an essential component that improves devitrification resistance. However, when the amount of SiO ₂ exceeds 12.0%, the solubility is deteriorated and it is difficult to produce stably, and the liquidus temperature rises, making the production difficult. On the contrary, when the amount of SiO ₂ is less than 1.0%, the devitrification resistance deteriorates and the glass becomes unstable. A preferred amount of SiO ₂ is 1.0% or more and 10.5% or less, and a more preferred amount is 3.0% or more and 8.5% or less.

B ₂ O ₃ functions as a network forming component and expresses the effect of lowering the temperature range for ensuring meltability and viscous flow. However, if the amount of B ₂ O ₃ exceeds 18.0%, the refractive index is too low. Conversely, if the amount of B ₂ O ₃ is less than 8.0%, the temperature range for securing the meltability and fluidity becomes too high. A preferable amount of B ₂ O ₃ is 8.5% or more and 18.0% or less, and a more preferable amount is 9.5% or more and 16.0% or less.

ZnO has the effect of improving devitrification resistance and lowering the temperature of viscous flow. When the amount of ZnO is out of the range of 2.0 % or more and 6.0% or less, it is difficult to adjust the refractive index (nd) and the Abbe number (νd) to a desired range.

ZrO ₂ improves the refractive index and also exhibits the effect of improving devitrification resistance. When the amount of ZrO ₂ is less than 1.0%, the effect of improving the refractive index is reduced. On the contrary, when the amount of ZrO ₂ exceeds 10.0%, the devitrification resistance is lowered and the solubility is also deteriorated. A preferable amount of ZrO ₂ is 2.0% or more and 9.0% or less.

La ₂ O ₃ is one of the most important components for improving the refractive index and controlling the Abbe number. When the amount of La ₂ O ₃ is less than 25.0%, it is difficult to adjust the Abbe number to a desired range. On the other hand, when the amount of La ₂ O ₃ exceeds 47.0%, devitrification resistance is lowered, so that it becomes unstable as glass, and manufacturing becomes difficult. A preferable amount of La ₂ O ₃ is 26.5% or more and 45.0% or less.

Li ₂ O exhibits the effect of lowering the glass transition temperature (hereinafter referred to as Tg) and improving the meltability. However, when a large amount of Li ₂ O is used, the devitrification resistance and the refractive index are remarkably lowered, so the amount of Li ₂ O is 4.0% or less, preferably 3.0% or less. Note that lowering the Tg, To more fully exhibit the effect of improving the melting property, the amount of the Li 2 _O and on a 1.5% or. Further, alkali metal oxides other than the Li _{2 O} R 2 _O (wherein, R is at least one of Na and K) in the case of using, because there is a possibility that devitrification resistance and refractive index is remarkably lowered The R ₂ O is preferably not used.

Nb ₂ O ₅ is one of components that improve the refractive index and control the Abbe number, like La ₂ O ₃ . Further, by replacing the La ₂ O _3, also express the effect of improving resistance to devitrification. However, if a large amount of Nb ₂ O ₅ is used, it becomes difficult to adjust the Abbe number to a desired range. Therefore, the amount of Nb ₂ O ₅ is 0% or more and 16.0% or less, preferably 0% or more. It is 14.5% or less. In order to fully develop the effect of improving the refractive index and devitrification resistance, the amount of Nb ₂ O ₅ is preferably 0.5% or more.

TiO ₂ controls the refractive index and Abbe number and also exhibits the effect of improving devitrification resistance. However, if the TiO ₂ is used in a large amount, it becomes difficult to adjust the Abbe number to a desired range. Therefore, the amount of TiO ₂ is 0% to 7.0%, preferably 0% to 5.0%. It is as follows. In order to fully develop the effect of improving the refractive index and devitrification resistance, the amount of TiO ₂ is preferably 0.5% or more.

Similar to La ₂ O ₃ , Ta ₂ O ₅ is one of components that improve the refractive index and control the Abbe number. However, the use of the _Ta 2 _{O 5} in a large amount, because the melting property becomes difficult to produce reduced, the amount of _Ta 2 _{O 5} is a hereinafter 6.5% 0% or more. In order to fully develop the effect of improving the refractive index, the amount of Ta ₂ O ₅ is preferably 0.5% or more.

In order to improve the high devitrification property of La ₂ O ₃ and adjust the refractive index and the Abbe number to a desired range, at least one of Nb ₂ O ₅ , TiO ₂ and Ta ₂ O ₅ is used. Is preferably used. Specifically, the total amount of Nb ₂ O ₅ , TiO ₂ and Ta ₂ O ₅ (Nb ₂ O ₅ + TiO ₂ + Ta ₂ O ₅ ) is 1.0% or more, preferably 10.0% or more. adjust. If the total amount is too large, it may be difficult to adjust the Abbe number to a desired range, or the meltability may be lowered and production may be difficult. Therefore, the total amount is 30.0%. It is preferable to adjust so that it may become the following.

Gd ₂ O ₃ is one of components that improve the refractive index and control the Abbe number, like La ₂ O ₃ . However, if the Gd ₂ O ₃ is used in a large amount, the devitrification resistance is lowered, which makes the glass unstable and difficult to produce. Therefore, the amount of Gd ₂ O ₃ is 0% or more and 26.0%. Hereinafter, it is preferably 0% or more and 20.0% or less. In order to fully develop the effect of improving the refractive index, the amount of Gd ₂ O ₃ is preferably 0.5% or more.

WO ₃ is a component for improving the high devitrification property of La ₂ O ₃ and adjusting the refractive index and the Abbe number to a desired range. However, when a large amount of WO ₃ is used, the transmittance in the blue region is deteriorated, so the amount of WO ₃ is 0% or more and 15.0% or less, preferably 0% or more and 13.0% or less. In addition, in order to improve the high devitrification property of La ₂ O ₃ and to fully exhibit the effect of adjusting the refractive index and the Abbe number to a desired range, the amount of WO ₃ is set to 0.5% or more. It is preferable to do.

  BaO is a component that improves manufacturability, but is not essential because it tends to deteriorate devitrification resistance. Therefore, the BaO can be used in an amount of 0% or more and 14.0% or less. The alkaline earth metal oxide R′O containing BaO (where R ′ is at least one of Ba, Sr, Ca and Mg) tends to deteriorate the devitrification resistance when used in a large amount. Therefore, it is preferable not to use it. Therefore, when R′O is used, the total content is 0% or more and 14% or less.

Y ₂ O ₃ and Yb ₂ O ₃ are components that improve the refractive index and control the Abbe number, similarly to La ₂ O ₃ . However, if these Y ₂ O ₃ and Yb ₂ O ₃ are used in a large amount, the devitrification resistance is lowered, so that the glass becomes unstable and the production becomes difficult. Therefore, _{substantially,} Y 2 _{O 3} is not used, in the case of using the Y _b 2 _{O 3} is replaced with _La 2 _{O 3} is 10% or less than 0%.

GeO ₂ can be replaced with SiO ₂ and functions as a network forming component, so it can be added up to 12.0%. However, when a large amount of GeO ₂ is used, the devitrification resistance may be lowered. Therefore, the amount of GeO ₂ is preferably 12.0% or less.

Al ₂ O ₃ can be used to adjust the refractive index, but the amount is preferably 0% or more and 10% or less. Ga ₂ O ₃ and In ₂ O ₃ can also be used in amounts up to about 10% in order to adjust the refractive index. However, it is desirable not to use them in large quantities because the use of these materials may deteriorate the devitrification resistance.

In addition to the above components, Sb ₂ O ₃ and SnO ₂ that are generally used as fining agents can be used. The amounts of Sb ₂ O ₃ and SnO ₂ are each preferably 0% or more and 2% or less. However, it is preferable not to use As ₂ O ₃ having a strong action as a clarifying agent because it is toxic.

  In addition, it is desirable not to use Pb and its compounds, radioactive materials such as U and Th, etc. from the viewpoint of safety, such as Te, Se, Cd. Moreover, it is desirable not to use substances that cause coloring such as Cu, Cr, V, Fe, Ni, and Co.

  By adjusting each component to have the ratio as described above, the refractive index (nd) for the d line is 1.88 or more and 1.92 or less, and the Abbe number (νd) for the d line is 33 or more and 37 or less. In addition, an optical glass composition having a yield point of 700 ° C. or lower can be obtained.

  In general, the liquid phase temperature of the optical glass is preferably low from the viewpoint of meltability, workability (droplet properties), crystallinity, and the like when the glass is handled in a softened and melted state. However, with respect to the optical glass composition according to the first embodiment, it is not an essential condition to lower the liquidus temperature, and even an optical glass having a high liquidus temperature is processed without impairing various properties of the glass. It is desirable to select a temperature range in which

(Embodiment 2)
Next, the preform and the manufacturing method thereof according to Embodiment 2 of the present invention will be described in detail. The preform is softened by heating and used for at least press molding. The weight and shape of the preform are appropriately determined according to the size and shape of the final press-formed product. The preform according to the second embodiment is made of the optical glass composition according to the first embodiment, and can be obtained without impairing various characteristics of the optical glass composition according to the first embodiment.

  A preform made of the optical glass composition according to Embodiment 1 will be described below. The preform means a glass preform that is heated and used for precision press molding. Preforms are roughly classified into gob preforms that are manufactured by molding a glass material from a molten state, and polishing preforms that are manufactured by physically polishing a glass material. The optical glass composition according to Embodiment 1 is applicable to both gob preforms and polishing preforms.

  Hereinafter, the preform manufacturing method will be described. First, a glass raw material (each of the above components) for obtaining the optical glass composition according to Embodiment 1 above is weighed, prepared, and subjected to steps such as dissolution, defoaming, clarification, homogenization, and the like so that no foreign matter is inherent. A new molten glass. Next, when the molten glass is allowed to flow out from an outflow pipe (hereinafter referred to as a nozzle) made of platinum alloy or the like, the temperature condition in the vicinity of the nozzle is set strictly within a range in which the glass is not devitrified. The molten glass that flows out is cast into a receiving mold having a receiving surface such as a flat shape, a concave shape, or a convex shape, or a mold having an enclosure around each of the flat surface, the concave surface, and the convex surface, and is formed into a desired shape. Hereinafter, a suitable molding method will be exemplified.

  The first molding method is an example of a gob preform manufacturing method. First, on a plurality of receiving molds arranged below the nozzle, a molten glass lump having a weight matching the final molded product or a desired weight added for secondary processing of the final molded product is dropped. Next, it cools, shaping | molding a glass lump, and obtains a gob preform.

  The second molding method is another example of a gob preform manufacturing method. The second molding method is suitable for producing a relatively large weight preform. First, the tip of the molten glass flowing out from the nozzle is brought into contact with the mold surface, and the molten glass is cut by quickly pulling the receiving mold away from the molten glass every time the desired weight is reached. Molding is performed. Next, if necessary, press molding is performed to cool the molten glass lump, and a desired shape is given to obtain a gob preform.

  The third molding method is an example of a method for producing a polishing preform. First, a desired shape is imparted to the glass lump by the same method as the first and second molding methods. At this time, the glass lump is added with a weight necessary for finishing all the surfaces including the optical functional surface of the final product (for example, a lens) by machining. Next, this glass lump is cut and polished by machining to obtain a polished preform.

  Any of the preforms obtained by the first and second molding methods can be used for precision press molding as it is as a press molding preform. In the third molding method, a polishing preform can be produced. In order to prevent the preform from being damaged at the time of handling, for example, a three-dimensional cooling method, an optimum cooling rate, an annealing treatment, and the like can be selected according to the shape and weight.

  As described above, by using the optical glass composition according to Embodiment 1, a press-molding preform and a polishing preform having a desired weight can be obtained. In the case of a press-molding preform, it is preferable to adjust the surface roughness of the receiving mold surface or form a release film for the purpose of releasing during molding. In the case of a polishing preform, it is preferable to apply HBN (boron-based release agent) in order to further facilitate the release.

(Embodiment 3)
Next, an optical element according to Embodiment 3 of the present invention will be described. The optical element according to Embodiment 3 has an optical constant and thermal characteristics determined by the composition of the optical glass composition according to Embodiment 1 described above. That is, the refractive index (nd) for the d-line is 1.88 to 1.92, the Abbe number (νd) for the d-line is 33 to 37, and the yield point is 700 ° C. or less. The optical element also has a characteristic that the amount of light absorption due to coloring is small in the visible region. The optical element using the optical glass composition according to Embodiment 1 is an optical element suitable for an optical system such as a digital camera, a video camera, or a mobile device.

  Examples of the optical element according to the third embodiment include a prism, a diffraction grating, and the like in addition to a spherical lens, an aspherical lens, a microlens, and the like. Furthermore, an optical element bonded to an optical element formed of a different optical material including a glass material can also be exemplified.

  Next, a method for manufacturing the optical element according to Embodiment 3 will be described. The optical element according to the third embodiment can be manufactured by putting the preform according to the second embodiment into a mold, softening it by heating, press-molding, and further polishing as necessary.

  The press molding method for obtaining the optical element can be roughly classified into two methods. The method is selected according to the means for forming an optical functional surface for entering and exiting light.

  The first means is a means called precision press molding. The mold surface of the press mold is precisely processed in a reverse shape with respect to the optical function surface of the optical element that is the final molded product, and if necessary, release to prevent fusion between the preform and the mold A film is formed, and the shape of the molding surface is precisely transferred to a press molding preform softened by heating by press molding. According to this means, it is not necessary to grind and polish the optical functional surface, and the optical element can be manufactured only by press molding. The press molding is performed in an inert atmosphere such as nitrogen gas. However, when a press-molding preform with a weight added to the final molded product is used, an amount corresponding to the weight added can be centered by grinding for a lens, for example.

  The second means is means for press-molding using a polishing preform that approximates the shape of the optical element that is the final molded product and is larger than the optical element. The molded press-molded product includes an optical functional surface, and the surface of the optical element is formed by machining. In press-molded products, it is necessary to minimize residual distortion in order to withstand machining and prevent glass breakage, and in order to set the required optical constant, a suitable annealing treatment is necessary. . With this means, press molding in the atmosphere is possible, and use of the release agent is also possible.

  Note that, regardless of which of the first and second means is selected, the refractive index (nd) and Abbe number (νd) of the obtained optical element slightly change depending on the thermal history in the manufacturing process. . When producing an optical element having a precisely defined optical constant, in consideration of changes in the refractive index (nd) and Abbe number (νd), adjustment of components of the optical glass composition, It is possible to appropriately select heat history adjustment, adjustment to incorporate the change amount into the optical design, if necessary. As described above, an optical element that has a desired optical constant and excellent transmittance and is particularly suitable as an optical component of a device on which a solid-state imaging device or the like is mounted can be obtained.

  Next, embodiments of the present invention will be described in more detail with reference to the following examples. However, the embodiments are not limited to these examples.

The operation in each example and comparative example is as follows. First, a raw material mixture composed of a predetermined amount of each oxide and carbonate was put in a platinum crucible, and the raw material mixture was melted while being stirred intermittently at 1350 to 1450 ° C. for 1 hour. Next, the melt is poured into a preheated mold, held for 1 hour in an electric furnace set higher than the expected Tg, and then cooled in a furnace at a cooling rate of 30 ° C./hour to optical glass block. Got. Then, using a polished sample cut out from the optical glass block, refractive index (nd), dispersion (νd: Abbe number), glass transition temperature (Tg), glass yield point (At), and linear expansion coefficient (α) were measured. did. The compositions (component ratios) of the glass in Examples and Comparative Examples are shown in the following tables. In the present specification, Examples 1, 7, 19, 20, 24, 25, and 36 to 40 are reference examples.

In each table, the following points are added.
(1) The ratio of each component in the composition column in each table is represented by weight% calculated from the batch raw material.
(2) nd and νd are the refractive index and Abbe number at room temperature.
(3) Tg and At are a glass transition temperature and a glass yield point obtained from a thermal expansion curve obtained by heating at a rate of 5 ° C./min using a thermal expansion measuring device.
(4) α is a linear expansion coefficient of 100 to 300 ° C. obtained from the thermal expansion curve obtained by the measurement of (3) above.

  As is apparent from Tables 1 to 5, the optical glass compositions in the examples are in the high refractive index region where the refractive index (nd) for the d-line is 1.88 or more and 1.92 or less, but the Abbe for the d-line. It can be seen that the number (νd) is in the range of 33 to 37, and the yield point is 700 ° C. or less.

  The optical glass composition of the present invention is suitable as a material for an optical element such as a lens element included in a photographing lens system of a digital camera. Further, it can be used for an optical pickup optical system lens element used in an optical head device, an illumination optical system used in a projection projector, a projection optical system lens element, or the like, and the performance of these devices can be improved.

Claims (3)

In weight% display
The SiO ₂ 1.0% 12.0% or less,
B ₂ O ₃ is 8.0% or more and 18.0% or less,
ZnO is 2.0 % or more and 6.0% or less,
ZrO ₂ is 1.0% or more and 10.0% or less,
La ₂ O ₃ is 25.0% or more and 47.0% or less,
Li ₂ O is 1.5 % or more and 4.0% or less,
Nb ₂ O ₅ is 0% or more and 16.0% or less,
TiO ₂ is 0% or more and 7.0% or less,
Ta ₂ O ₅ is 0% or more and 6.5 % or less,
Nb ₂ O ₅ + TiO ₂ + Ta ₂ O ₅ is 1.0% or more,
Containing 0 to 26.0% of Gd ₂ O ₃ and 0 to 15.0% of WO ₃ ;
Substantially free of Y ₂ O ₃ ,
An optical glass composition having a refractive index (nd) for a d-line of 1.88 or more and 1.92 or less, an Abbe number (νd) for a d-line of 33 or more and 37 or less, and a yield point of 700 ° C. or less.   A preform comprising the optical glass composition according to claim 1 and softened by heating and used for at least press molding.   An optical element comprising the optical glass composition according to claim 1.