Patent Publication Number: US-2021179479-A1

Title: Optical glass and optical component

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a bypass continuation of International Patent Application No. PCT/JP2019/033490, filed on Aug. 27, 2019, which claims priority to Japanese Patent Application Nos. 2018-163582 filed on Aug. 31, 2018 and 2018-218577 filed on Nov. 21, 2018. The contents of these applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an optical glass and an optical component. 
     BACKGROUND ART 
     Glass for use in wearable appliances, such as spectacles equipped with a projector, spectacle- or goggle-type displays, virtual-reality or augmented-reality display devices, and virtual-image display devices, is required to have a high refractive index from the standpoints of an increase in image display angle, increases in luminance and contrast, an improvement in lightguide property, easiness of processing for producing diffraction gratings, etc. Meanwhile, small-sized imaging glass lenses with a wide imaging angle of view have conventionally been used in applications such as in-vehicle cameras and visual sensors for robots, and such imaging glass lenses are required to have a high refractive index so as to be smaller and capable of wide-range imaging. 
     Optical glass for use in such applications is required to have a low density in order to make the user have a comfortable wear feeling or to enable the vehicle or robot, which is required to be reduced in weight, to have a reduced overall weight. Moreover, it is important for the optical glass to be less apt to suffer a surface deterioration or alteration due to acid rain, in view of outdoor uses, and due to chemicals such as detergents used for cleaning or waxes. 
     With respect to glass lenses for mounting on vehicles, an attempt has, for example, been made to attain increases in refractive index and strength and improvements in acid resistance and water resistance by using a lens glass material for in-vehicle cameras which has predetermined acid resistance (see, for example, Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A-2013-256446 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, a composition for attaining a high refractive index has hitherto been frequently obtained using heavy-metal oxides as glass components. Because of this, high-refractive index glass typically has a high density. 
     Meanwhile, wearable appliances sometimes employ glass having a plate shape, and such glass is often produced by forming methods having high production efficiency, such as a float process, a fusion process, and a rolling-out process. Important for efficient production is a relationship between the temperature during the production and the viscosity of the glass. 
     In the case of use as an optical component, visible-light transmittance is also an important parameter. In the case of high-refractive index glass, melting at a high temperature may result in a decrease in visible-light transmittance especially on the shorter-wavelength side, while a steep viscosity curve makes it difficult to control viscosity in production. 
     The present invention has been achieved in order to solve the problems described above, and an object thereof is to provide an optical glass which has a high refractive index and a low density and has satisfactory production properties. 
     Solution to Problem 
     An optical glass of the present invention has: a refractive index (n d ) of 1.81 to 2.15, a density of 6.0 g/cm 3  or less; a temperature T 1 , at which a viscosity of the glass is 10 1  dPa·s, of 900° C. to 1,200° C.; a devitrification temperature of 1,300° C. or lower; and a content of SiO 2  of 5% to 44% in mol % based on oxides. 
     An optical component of the present invention includes the optical glass of the present invention, in which the optical glass has a plate shape. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING 
       The FIGURE is a cross-sectional view for illustrating a warpage of an optical glass. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the optical glass and optical component of the present invention are described below. 
     The optical glass of the present invention has predetermined values of refractive index (n d ), density (d), and melting characteristics as shown above. These properties are explained below in order. 
     The optical glass of the present invention has a high refractive index (n d ) in the range of 1.81 to 2.15. Since the refractive index (n d ) is 1.81 or higher, the optical glass of the present invention is suitable as an optical glass for use in wearable appliances from the standpoints of an increase in image display angle, increases in luminance and contrast, an improvement in lightguide property, easiness of processing for producing diffraction gratings, etc. Furthermore, the optical glass of the present invention is suitable for use as a small imaging glass lens with a wide imaging angle for use in applications such as in-vehicle cameras and visual sensors for robots, since the imaging glass lens can have a smaller size and be capable of wide-range imaging. The refractive index (n d ) thereof is preferably 1.85 or higher, more preferably 1.88 or higher, still more preferably 1.91 or higher, still more preferably 1.94 or higher, still more preferably 1.97 or higher, still more preferably 1.99 or higher, still more preferably 2.00 or higher. 
     Meanwhile, glass having a refractive index (n d ) exceeding 2.15 tends to have an increased density and a high devitrification temperature. Especially in the case where it is important for the optical glass to have a low density, the refractive index (n d ) is preferably 2.10 or less, more preferably 2.06 or less, still more preferably 2.03 or less, still more preferably 2.01 or less, still more preferably 1.98 or less, still more preferably 1.95 or less, still more preferably 1.94 or less, yet still more preferably 1.92 or less. 
     The optical glass of the present invention has a density (d) of 6.0 g/cm 3  or less. Since the optical glass of the present invention has a density within the aforementioned range, this optical glass, when used in a wearable appliance, can make the user have a comfortable wear feeling and, when used in in-vehicle cameras, visual sensors for robots, or the like, can reduce overall weight of devices. The density (d) is preferably 5.6 g/cm 3  or less, more preferably 5.2 g/cm 3  or less, still more preferably 4.8 g/cm 3  or less, still more preferably 4.6 g/cm 3  or less, still more preferably 4.4 g/cm 3  or less, still more preferably 4.2 g/cm 3  or less. 
     Meanwhile, from the standpoint of rendering the surfaces of the optical glass of the present invention less apt to suffer scratches, the density (d) is preferably 3.3 g/cm 3  or higher. The density (d) is more preferably 3.6 g/cm 3  or higher, still more preferably 3.9 g/cm 3  or higher, still more preferably 4.2 g/cm 3  or higher, still more preferably 4.4 g/cm 3  or higher, yet still more preferably 4.7 g/cm 3  or higher. 
     The optical glass of the present invention has a temperature T 1 , at which a viscosity of the glass is 10 1  dPa·s, being in the range of 900° C. to 1,200° C. T 1  is a reference temperature for melting characteristics. Glass having too high T 1  needs to be melted at high temperatures. In the case where such a glass is a high-refractive index glass, the glass may have a reduced visible-light transmittance especially on the shorter-wavelength side. The T 1  of the optical glass of the present invention is preferably 1,180° C. or lower, more preferably 1,150° C. or lower, still more preferably 1,130° C. or lower, yet still more preferably 1,110° C. or lower. 
     Meanwhile, too low T 1  results in a steep viscosity curve to cause a problem that viscosity control in production is difficult. Since the optical glass of the present invention has T 1  within the aforementioned range, this optical glass can have satisfactory producibility. The T 1  is preferably 950° C. or higher, more preferably 1,000° C. or higher, still more preferably 1,050° C. or higher, still more preferably 1,080° C. or higher, yet still more preferably 1,100° C. or higher, especially preferably 1,120° C. or higher. 
     The optical glass of the present invention has a devitrification temperature of 1,300° C. or lower. Since the optical glass has such property, the glass can be inhibited from being devitrified when forming the glass, and thus the formability is satisfactory. The devitrification temperature is preferably 1,275° C. or lower, more preferably 1,250° C. or lower, still more preferably 1,225° C. or lower, still more preferably 1,200° C. or lower, still more preferably 1,175° C. or lower, still more preferably 1,150° C. or lower, still more preferably 1,125° C. or lower, still more preferably 1,100° C. or lower, still more preferably 1,075° C. or lower, especially preferably 1,050° C. or lower. The devitrification temperature is a lowest temperature at which no crystals of 1 μm or larger in terms of major-side of major-axis dimension are observed in the surface and inside of the glass when the glass having been heated and melted is allowed to cool naturally. 
     The optical glass of the present invention preferably has a glass transition point (Tg) of 600° C. or higher. In the case where the optical glass of the present invention has Tg of 600° C. or higher, the glass can be inhibited from suffering a deformation such as deflection, in processing. The Tg is more preferably 630° C. or higher, still more preferably 660° C. or higher, still more preferably 690° C. or higher, still more preferably 720° C. or higher, especially preferably 750° C. or higher. In the case where the optical glass is formed by press forming, redraw forming, etc., Tg is preferably 800° C. or lower. Tg is more preferably 760° C. or lower, still more preferably 720° C. or lower, yet still more preferably 680° C. or lower, especially preferably 640° C. or lower. Tg can be measured, for example, by a thermal expansion method. 
     It is preferable that the optical glass of the present invention, at the devitrification temperature, has a viscosity (devitrification viscosity) η which, when expressed in dPa·s unit, satisfies log η=0.4 or larger. In the case where the optical glass has such property, the glass can be inhibited from being devitrified when formed and has satisfactory formability. The viscosity thereof at the devitrification temperature more preferably satisfies log η=0.5 or larger, still more preferably satisfies log η=0.6 or larger, yet still more preferably satisfies log η=0.7 or larger. Especially preferably, log η is 0.8 or larger. 
     It is preferable that the optical glass of the present invention has an Abbe number (v d ) of 60 or less. In the case where the optical glass of the present invention is used as a glass plate such as a lightguide plate, the low v d  of this optical glass within the aforementioned range not only facilitates optical designs for wearable appliances but also makes improvements in chromatic aberration easy. This optical glass hence enables high-resolution pictures or images to be reproduced. The v d  is more preferably 50 or less, still more preferably 40 or less, still more preferably 38 or less, still more preferably 35 or less, yet still more preferably 32 or less, especially preferably 30 or less. 
     Meanwhile, it is preferable that the optical glass of the present invention has an Abbe number (v d ) of 15 or higher. Specifically, in the case where the optical glass of the present invention is used as a glass plate such as a lightguide plate, the high v d  of this optical glass within that range makes it easy to obtain refractive index matching to a resin to be applied to the surface. The v d  thereof is more preferably 18 or higher, still more preferably 21 or higher, still more preferably 23 or higher, still more preferably 25 or higher, yet still more preferably 27 or higher, especially preferably 29 or higher. 
     It is preferable that the optical glass of the present invention has a coefficient of thermal expansion (a) in 50 to 350° C. of 50 to 150 (×10 −7 /K). In the case where the optical glass of the present invention has an a within that range, satisfactory expansion matching to peripheral members is achieved. In terms of the lower limit, the a is preferably 60 (×10 −7 /K) or higher, more preferably 70 (×10 −7 /K) or higher, still more preferably 80 (×10 −7 /K) or higher, especially preferably 90 (×10 −7 /K) or higher. 
     In addition, in the case where the optical glass of the present invention has an a within that range, this optical glass is less apt to crack when cooled and, hence, an increased cooling rate can be used. As a result, the difference (Tf−Tg) between the fictive temperature (Tf) and glass transition temperature (Tg) of the optical glass can be 0° C. or larger, and this enables the optical glass to have a less dense structure so that even when an impact is given to the optical glass, the structure of the glass becomes dense so that the optical glass absorbs the impact more. As a result, the optical glass itself has improved strength and can be inhibited from being damaged due to falling, etc. In terms of the upper limit, the a is preferably 120 (×10 −7 /K) or less, more preferably 110 (×10 −7 /K) or less, still more preferably 100 (×10 −7 /K) or less, especially preferably 95 (×10 −7 /K) or less. 
     It is preferable that the optical glass of the present invention is a glass plate having a thickness of 0.01 mm to 2 mm. In the case where the thickness thereof is 0.01 mm or larger, this optical glass can be inhibited from being damaged in handling or processing. In addition, this optical glass can be inhibited from being bent by its own weight. The thickness thereof is more preferably 0.1 mm or larger, still more preferably 0.3 mm or larger, yet still more preferably 0.5 mm or larger. Meanwhile, in the case where the thickness thereof is 2 mm or less, optical elements employing this optical glass can be lightweight. The thickness thereof is more preferably 1.5 mm or less, still more preferably 1.0 mm or less, yet still more preferably 0.8 mm or less. 
     In the case where the optical glass of the present invention is a glass plate, it is preferable that one main surface thereof has an area of 8 cm 2  or larger. In the case where the area thereof is 8 cm 2  or larger, a large number of optical elements can be disposed on the glass plate, resulting in an improvement in production efficiency. The area thereof is more preferably 30 cm 2  or larger, still more preferably 170 cm 2  or larger, yet still more preferably 300 cm 2  or larger, especially preferably 1,000 cm 2  or larger. Meanwhile, in the case where the area thereof is 6,500 cm 2  or less, the glass plate is easy to handle and can be inhibited from being damaged in handling or processing. The area thereof is more preferably 4,500 cm 2  or less, still more preferably 4,000 cm 2  or less, yet still more preferably 3,000 cm 2  or less, especially preferably 2,000 cm 2  or less. 
     In the case where the optical glass of the present invention is a glass plate, it is preferable that one main surface thereof has an LTV (local thickness variation) for an area of 25 cm 2  of 2 μm or less. In the case where the one main surface has a flatness within that range, not only nanostructures of a desired shape can be formed on the main surface using imprinting, etc. but also desired lightguide properties can be obtained. In particular, in the case where the glass plate is a lightguide, a ghost phenomenon or strain due to a difference in optical path length can be prevented. The LTV thereof is more preferably 1.8 μm or less, still more preferably 1.6 μm or less, yet still more preferably 1.4 μm or less, especially preferably 1.2 μm or less. 
     It is preferable that the optical glass of the present invention, when assuming that the optical glass is a disk-shaped glass plate having a diameter of 8 inches, has a warpage of 50 μm or less. In the case where the warpage of the glass plate is 50 μm or less, not only nanostructures of a desired shape can be formed on one main surface thereof using imprinting, etc. but also desired lightguide properties are obtained. In the case where a plurality of lightguides are to be obtained, lightguides of satisfactory quality are stably obtained. The warpage of the glass substrate is more preferably 40 μm or less, still more preferably 30 μm or less, especially preferably 20 μm or less. 
     When assuming that the optical glass is a disk-shaped glass plate having a diameter of 6 inches, the warpage thereof is preferably 30 μm or less. In the case where the warpage of the glass plate is 30 μm or less, not only nanostructures of a desired shape can be formed on one main surface thereof using imprinting, etc. but also desired lightguide properties are obtained. In the case where a plurality of lightguides are to be obtained, lightguides of satisfactory quality are stably obtained. The warpage of the glass plate is more preferably 20 μm or less, still more preferably 15 μm or less, especially preferably 10 μm or less. 
     When assuming that the optical glass is a square glass plate in which each side has a length of 6 inches, the warpage thereof is preferably 100 μm or less. In the case where the warpage of the glass plate is 100 μm or less, not only nanostructures of a desired shape can be formed on one main surface thereof using imprinting, etc. but also desired lightguide properties are obtained. In the case where a plurality of lightguides are to be obtained, lightguides of satisfactory quality are stably obtained. The warpage of the glass plate is more preferably 70 μm or less, still more preferably 50 μm or less, still more preferably 35 μm or less, especially preferably 20 μm or less. 
     The FIGURE is a cross-sectional view of an optical glass of the present invention, which is denoted by glass plate G 1 . “Warpage” is defined as follows. In any cross-section passing through the center of one main surface G 1 F of the glass plate G 1  and perpendicular to the main surface G 1 F of the glass plate G 1 , a base line G 1 D and a center line G 1 C for the glass plate G 1  are determined and the difference C between a maximum value B and a minimum value A of the vertical-direction distance between the base line G 1 D and the center line G 1 C is taken as the warpage. 
     The intersection line where the perpendicular cross-section meets the one main surface G 1 F of the glass plate G 1  is referred to as bottom line G 1 A. The intersection line where the perpendicular cross-section meets the other main surface G 1 G of the glass plate G 1  is referred to as top line G 1 B. The center line G 1 C is a line which connects thickness-direction center points of the glass plate G 1 . The center line G 1 C is calculated by determining midpoints, along the direction of laser irradiation which will be described later, between the bottom line G 1 A and the top line G 1 B. 
     A base line G 1 D is determined in the following manner. First, a bottom line G 1 A is calculated by a measuring method in which any influence of the weight of the glass plate G 1  itself is eliminated. From the bottom line G 1 A, a straight line is determined by the least squares method. The determined straight line is the base line G 1 D. As the method in which any influence of the weight of the glass plate G 1  itself is eliminated, a common method is used. 
     For example, one main surface G 1 F of the glass plate G 1  is supported at three points. The glass plate G 1  is irradiated with laser light using a laser displacement meter to measure the heights of the one main surface G 1 F and the other main surface G 1 G of the glass plate G 1  from a base plane. 
     Next, the glass plate G 1  is reversed and supported at three points located on the other main surface G 1 G which face the three points at which said one main surface G 1 F was supported, and the heights of said one main surface G 1 F and the other main surface G 1 G of the glass plate G 1  from a base plane are measured. 
     The heights of each measurement point measured respectively before and after the reversal are averaged, thereby eliminating the influence of the weight of the glass plate G 1  itself. For example, heights of the one main surface G 1 F are measured before reversal in the manner shown above. The glass plate G 1  is reversed, and heights of the other main surface G 1 G are then measured in positions corresponding to the measurement points on said one main surface G 1 F. Likewise, heights of the other main surface G 1 G are measured before reversal. After the glass plate G 1  is reversed, heights of said one main surface G 1 F are measured in positions corresponding to the measurement points on the other main surface GIG. 
     The warpage is measured, for example, with a laser displacement meter. 
     In the optical glass of the present invention, it is preferable that one main surface thereof has a surface roughness Ra of 2 nm or less. In the case where the one main surface has an Ra within that range, not only nanostructures of a desired shape can be formed on the main surface using imprinting, etc. but also desired lightguide properties are obtained. In particular, in the case where the glass plate is a lightguide, irregular reflection can be inhibited from occurring at the interfaces and a ghost phenomenon or strain can be prevented. The Ra thereof is more preferably 1.7 nm or less, still more preferably 1.4 nm or less, yet still more preferably 1.2 nm or less, especially preferably 1 nm or less. The surface roughness Ra is the arithmetic average roughness defined in JIS B0601 (year 2001). In this description, the surface roughness Ra is a value determined through an examination of a 10 μm×10 μm area with an atomic force microscope (AFM). 
     [Glass Components] 
     One embodiment of content ranges for components that can be contained in the optical glass of the present invention is described in detail below. In this description, the content of each component is indicated in mol % based on oxides unless otherwise noted. The expression “containing substantially no X” used for the optical glass of the present invention means that the glass does not contain X except the X included in unavoidable impurities. The content of unavoidable impurities is 0.1% or less in the present invention. 
     A composition of the optical glass according to this embodiment which satisfies the properties described above is, for example, one that includes, in mol % based on oxides, at least one oxide selected from the group consisting of TiO 2 , Ta 2 O 5 , WO 3 , Nb 2 O 5 , ZrO 2 , and Ln 2 O 3  (Ln is at least one element selected from the group consisting of Y, La, Gd, Yb, and Lu), which are high-refractive index components, in an amount of 30% to 80%, and has a total content of SiO 2  and B 2 O 3 , which are glass framework components, of 20% to 70% and that, in a case where the glass includes one or more alkaline-earth metal components (MgO, CaO, SrO, and BaO), a proportion of BaO in the alkaline metal components is 0.5 or less. 
     The components of glass composition A which satisfies this requirement are explained below in detail. The optical glass of the present invention is not limited to the compositions of the following embodiments so long as the optical glass has the properties described above. 
     &lt;Glass Composition A&gt; 
     SiO 2  is a glass-forming component and is a component which imparts high strength and crack resistance to the glass and improves the stability and chemical durability of the glass. The content of SiO 2  is preferably 5% or higher and 44% or less. In the case where the content of SiO 2  is 5% or higher, the temperature T 1  at which the glass viscosity is 10 1  dPa·s can be regulated to a value within the preferred range. The content of SiO 2  is preferably 7% or higher, more preferably 9% or higher, still more preferably 10% or higher, especially preferably 11% or higher. Meanwhile, in the case where the content of SiO 2  is 44% or less, components for obtaining a high refractive index can be contained in a larger amount. The content of SiO 2  is more preferably 38% or less, more preferably 30% or less, still more preferably 20% or less, still more preferably 15% or less, especially preferably 12% or less. 
     B 2 O 3  is a component which lowers the Tg, improves mechanical properties, including strength and crack resistance, of the glass, and lowers the devitrification temperature. However, too high B 2 O 3  contents are prone to result in a decrease in refractive index. Consequently, the content of B 2 O 3  is preferably 0% or higher and 40% or less. The content of B 2 O 3  is more preferably 35% or less, still more preferably 30% or less, still more preferably 25% or less, still more preferably 23% or less, especially preferably 22% or less. Meanwhile, the content of B 2 O 3  is more preferably 5% or higher, still more preferably 12% or higher, still more preferably 18% or higher, especially preferably 20% or higher. 
     SiO 2  and B 2 O 3  are glass-forming components and are components which improve the stability of the glass. In the case where the total content of SiO 2  and B 2 O 3  is high, the glass has a lowered devitrification temperature and is easy to produce. Consequently, the total content of SiO 2  and B 2 O 3  is 20% or higher. The total content thereof is preferably 25% or higher, more preferably 28% or higher, still more preferably 30% or higher, especially preferably 32% or higher. Meanwhile, by reducing the total content of SiO 2  and B 2 O 3 , an improvement in refractive index can be attained. Hence, in the case where an especially high refractive index is required, the total content thereof is preferably 70% or less, more preferably 50% or less, still more preferably 40% or less, still more preferably 35% or less, still more preferably 33% or less, especially preferably 32% or less. 
     In the case where B 2 O 3  is contained, the glass is prone to be devitrified if the ratio of the content of SiO 2  to the content of B 2 O 3 , SiO 2 /B 2 O 3 , is high. Consequently, in the case where B 2 O 3  is contained, SiO 2 /B 2 O 3  is preferably 5.0 or less, more preferably 4.0 or less, still more preferably 3.0 or less, still more preferably 2.0 or less, still more preferably 1.0 or less, still more preferably 0.8 or less, especially preferably 0.6 or less. 
     TiO 2 , Ta 2 O 5 , WO 3 , Nb 2 O 5 , ZrO 2 , and Ln 2 O 3  (Ln is at least one element selected from the group consisting of Y, La, Gd, Yb, and Lu) are high-refractive index components which heighten the refractive index of the glass. The total content of these components is preferably 30% to 80%. In the case where an especially high refractive index is required, the content thereof is preferably 40% or higher, more preferably 55% or higher, still more preferably 60% or higher, yet still more preferably 65% or higher, especially preferably 67% or higher. Meanwhile, in the case where the total content of the high-refractive index components exceeds 80%, the glass is prone to be devitrified. For use in applications where lower surface roughness Ra is required, the content of these components is more preferably 70% or less, still more preferably 60% or less, still more preferably 50% or less, especially preferably 45% or less. 
     The total content of alkali metal components (Li 2 O+Na 2 O+K 2 O) is 0% or higher and 10% or less. By increasing the content of these alkali metal components, the Tg can be lowered. However, in the case where Li 2 O+Na 2 O+K 2 O is too high, the glass is prone to have too low a T 1  and a steep viscosity curve, resulting in reduced producibility. Meanwhile, in the case where Li 2 O+Na 2 O+K 2 O is too low, the glass is prone to have too high a T 1  and requires higher temperatures for melting. As a result, the TiO 2  and Nb 2 O 5  among the high-refractive index components are prone to be reduced and this may cause coloring. Consequently, in the case where one or more of Li 2 O, Na 2 O, and K 2 O are contained, Li 2 O+Na 2 O+K 2 O is preferably 0.5% or higher and 10% or less. Li 2 O+Na 2 O+K 2 O is more preferably 1% or higher, still more preferably 2% or higher, still more preferably 4% or higher, especially preferably 5% or higher. Meanwhile, Li 2 O+Na 2 O+K 2 O is preferably 6% or less, more preferably 3% or less, still more preferably 2% or less, especially preferably 1% or less. The expression “Li 2 O+Na 2 O+K 2 O” indicates the total content of at least one alkali metal oxide component selected from the group consisting of Li 2 O, Na 2 O, and K 2 O. Hereinafter, each set of components connected by “+” likewise indicates the total content of at least one component selected from the group consisting of the components connected by “+”. 
     The content of Li 2 O is 0% to 10%. In the case where Li 2 O is contained, the content thereof is 0.2% or higher and 10% or less. The inclusion of Li 2 O can improve the strength (Kc) and crack resistance (CIL). In the case where the optical glass of the present invention contains Li 2 O, the content thereof is preferably 1% or higher, more preferably 2% or higher, still more preferably 4% or higher, especially preferably 5% or higher. Meanwhile, too high Li 2 O contents are prone to result in devitrification. Especially in the case where quality regarding devitrification is required, the content of Li 2 O is preferably 8% or less, more preferably 6% or less, still more preferably 4% or less, still more preferably 2% or less, especially preferably 1% or less. 
     In the case where the optical glass according to this embodiment is to be chemically strengthened, the content of Li 2 O is preferably 3.0% or higher, more preferably 6.0% or higher, still more preferably 9.0% or higher, especially preferably 11.0% or higher. 
     Na 2 O is a component which inhibits devitrification and lowers the Tg, and the content thereof is 0% or higher and 10% or less. The inclusion of Na 2 O produces an excellent devitrification-inhibiting effect. In the case where the optical glass of the present invention contains Na 2 O, the content thereof is preferably 1% or higher, more preferably 2% or higher, still more preferably 3% or higher, especially preferably 4% or higher. Meanwhile, too high Na 2 O contents are prone to reduce the strength and crack resistance. The content of Na 2 O, in the case where strength is especially required, is preferably 7% or less, more preferably 4% or less, still more preferably 2% or less, especially preferably 1% or less. 
     Tg tends to be lower as the total content of Li 2 O and Na 2 O increases. Consequently, the total content of Li 2 O and Na 2 O is preferably 0% or higher and 10% or less. The total content thereof is more preferably 6% or less, still more preferably 4% or less, still more preferably 2% or less, especially preferably 1% or less. 
     K 2 O is a component which inhibits devitrification and lowers the Tg, and the content of K 2 O is 0% or higher and 10% or less. The inclusion of K 2 O produces an excellent devitrification-inhibiting effect. In the case where the optical glass of the present invention contains K 2 O, the content of K 2 O is preferably 1% or higher, more preferably 2% or higher, still more preferably 3% or higher, especially preferably 4% or higher. Meanwhile, too high K 2 O contents are prone to reduce the strength and crack resistance. The content of K 2 O, in the case where strength is especially required, is preferably 7% or less, more preferably 4% or less, still more preferably 2% or less, especially preferably 1% or less. 
     MgO is a component which improves glass meltability, inhibits devitrification, and is useful in regulating optical constants of the glass, e.g., Abbe number and refractive index. Meanwhile, too high MgO contents rather promote devitrification. Consequently, the content of MgO is preferably 0% or higher and 10% or less. The content of MgO is more preferably 8% or less, especially preferably 6% or less. Meanwhile, the content of MgO is preferably 0.3% or higher, more preferably 0.5% or higher, still more preferably 1% or higher. 
     CaO is a component which inhibits devitrification, but too high CaO contents are prone to reduce the crack resistance. Consequently, the content of CaO is preferably 0% or higher and 25% or less. The content of CaO is more preferably 20% or less, still more preferably 10% or less, especially preferably 6% or less. Meanwhile, the content of CaO is more preferably 0.3% or higher, still more preferably 0.5% or higher, especially preferably 1% or higher. 
     SrO is a component which improves glass meltability, inhibits devitrification, and is useful in regulating optical constants of the glass. Meanwhile, too high SrO contents rather promote devitrification. Consequently, the content of SrO is preferably 0% or higher and 20% or less. The content of SrO is more preferably 15% or less, still more preferably 8% or less, especially preferably 4% or less. Meanwhile, the content of SrO is more preferably 0.3% or higher, still more preferably 0.5% or higher, especially preferably 1% or higher. 
     In the case where the total content of MgO, CaO, and SrO is too high, the glass is prone to be devitrified. Consequently, the total content of MgO, CaO, and SrO is preferably 30% or less. The total content thereof is more preferably 20% or less, still more preferably 12% or less, still more preferably 10% or less, still more preferably 5% or less, especially preferably 2% or less. 
     BaO is a component which inhibits devitrification, but too high BaO contents are prone to result in an increase in density. Consequently, in the case where BaO is contained, the content thereof is preferably 0% or higher and 30% or less. The content of BaO is more preferably 25% or less, still more preferably 15% or less, still more preferably 8% or less, especially preferably 4% or less. Meanwhile, the content of BaO is more preferably 0.3% or higher, still more preferably 0.5% or higher, especially preferably 1% or higher. 
     In the case where the glass contains alkaline-earth metal components (MgO+CaO+SrO+BaO), a reduction in specific gravity can be attained by regulating the ratio of the content of BaO to the total content of the alkaline-earth metal components (BaO/(MgO+CaO+SrO+BaO)) to 0.5 or less. The ratio is preferably 0.4 or less, more preferably 0.3 or less, still more preferably 0.2 or less, especially preferably 0.1 or less. By heightening the proportion of BaO to the alkaline metal components, the devitrification temperature can be lowered. For use in applications where lower surface roughnesses Ra are required, that ratio is preferably 0.1 or larger, more preferably 0.2 or larger, still more preferably 0.3 or larger, especially preferably 0.4 or larger. 
     In the case where the total content of the alkali metal components (Li 2 O+Na 2 O+K 2 O) and the alkaline-earth metal components (MgO+CaO+SrO+BaO) is too high, the glass is prone to have a lowered Tg. Consequently, the total content of the alkali metal components and the alkaline-earth metal components is preferably 30% or less. The total content thereof is more preferably 16% or less, still more preferably 12% or less, still more preferably 10% or less, still more preferably 5% or less, especially preferably 2% or less. 
     Al 2 O 3  is a component which improves the chemical durability, but too high Al 2 O 3  contents render the glass prone to be devitrified. Consequently, the content of Al 2 O 3  is preferably 0% or higher and 5% or less. The content of Al 2 O 3  is more preferably 3% or less, especially preferably 2% or less. Meanwhile, the content of Al 2 O 3  is more preferably 0.3% or higher, still more preferably 0.5% or higher, especially preferably 1% or higher. 
     TiO 2  is a component which heightens the refractive index of the glass to enhance dispersion by the glass, and the content thereof is 0% or higher and 50% or less. In the case where TiO 2  is contained, the content thereof is preferably 10% or higher, more preferably 20% or higher, still more preferably 25% or higher, still more preferably 28% or higher, still more preferably 30% or higher, especially preferably 32% or higher. Meanwhile, too high TiO 2  contents are prone to cause coloring and result in a decrease in transmittance. Consequently, especially in the case where transmittance is required, the content of TiO 2  is preferably 50% or less, more preferably 40% or less, still more preferably 37% or less, still more preferably 35% or less, still more preferably 34% or less, still more preferably 33% or less, especially preferably 32% or less. 
     In the case where B 2 O 3  is contained, the glass requires higher temperatures for melting if the ratio of the content of TiO 2  to the content of B 2 O 3 , TiO 2 /B 2 O 3 , is high. In such cases, the titanium is prone to be reduced and the glass is prone to take a color and have a reduced transmittance. Consequently, in the case where B 2 O 3  is contained, TiO 2 /B 2 O 3  is preferably 5.0 or less, more preferably 4.0 or less, still more preferably 3.0 or less, still more preferably 2.0 or less, still more preferably 1.8 or less, still more preferably 1.7 or less, still more preferably 1.6 or less, especially preferably 1.5 or less. 
     WO 3 , when added, inhibits the glass from being devitrified. However, too large addition amounts of WO 3  rather render the glass prone to be devitrified. Consequently, the content of WO 3  is preferably 0% to 10%. The content of WO 3  is more preferably 6% or less, still more preferably 2% or less, still more preferably 1.5% or less, still more preferably 1.0% or less, especially preferably 0.5% or less. The addition of WO 3  can improve the refractive index of the glass. Consequently, in the case where an especially high refractive index is required, the content of WO 3  is more preferably 0.3% or higher, still more preferably 0.5% or higher, especially preferably 1% or higher. 
     Nb 2 O 5  is a component which heightens the refractive index of the glass and reduces the Abbe number (v d ) thereof. The content of Nb 2 O 5  is 0% or higher and 35% or less. The content of Nb 2 O 5  is more preferably 1% or higher, still more preferably 2% or higher, still more preferably 2.5% or higher, still more preferably 3% or higher, still more preferably 4% or higher, still more preferably 5% or higher, especially preferably 6% or higher. 
     Meanwhile, too high Nb 2 O 5  contents render the glass prone to be devitrified. Consequently, for use in applications where lower surface roughnesses Ra are required, the content of Nb 2 O 5  is preferably 20% or less, more preferably 15% or less, still more preferably 10% or less, still more preferably 6% or less, still more preferably 4% or less, still more preferably 3% or less, still more preferably 2% or less, especially preferably 1% or less. 
     In the case where the total content of TiO 2 , WO 3 , and Nb 2 O 5  is too low, the glass has a reduced refractive index. Consequently, the total content of TiO 2 , WO 3 , and Nb 2 O 5  is preferably 10% or higher and 50% or less. The total content thereof is more preferably 15% or higher, still more preferably 20% or higher, still more preferably 25% or higher, especially preferably 30% or higher. 
     Y 2 O 3  is a component which heightens the refractive index of the glass and with which the T 1  of the glass can be regulated to a value within the preferred range. The content of Y 2 O 3  is 0% or higher and 7% or less. The content of Y 2 O 3  is preferably 1% or higher, more preferably 2% or higher, still more preferably 2.5% or higher, still more preferably 3% or higher, still more preferably 3.5% or higher, still more preferably 4% or higher, especially preferably 5% or higher. Meanwhile, too high Y 2 O 3  contents render the glass prone to be devitrified. Consequently, for use in applications where a lower surface roughness Ra is required, the content of Y 2 O 3  is preferably 5% or less, more preferably 4% or less, still more preferably 3.5% or less, especially preferably 3% or less. 
     ZrO 2  is a component which heightens the refractive index of the glass and enhances the chemical durability of the glass, and the content thereof is 0% or higher and 20% or less. The inclusion of ZrO 2  can improve the crack resistance. In the case where ZrO 2  is contained, the content thereof is preferably 1% or higher, more preferably 3% or higher, still more preferably 5% or higher, still more preferably 6% or higher, especially preferably 6.5% or higher. Meanwhile, too high ZrO 2  contents render the glass prone to be devitrified. Consequently, for use in applications where a lower surface roughness Ra is required, the content of ZrO 2  is more preferably 15% or less, still more preferably 10% or less, still more preferably 8% or less, especially preferably 7% or less. 
     ZnO is a component which improves mechanical properties, including strength and crack resistance, of the glass, and the content thereof is 0% or higher and 15% or less. In the case where ZnO is contained, the content of ZnO is more preferably 0.3% or higher, still more preferably 0.5% or higher, especially preferably 1% or higher. Meanwhile, too high ZnO contents render the glass prone to be devitrified. Consequently, for use in applications where a lower surface roughness Ra is required, the content of ZnO is more preferably 10% or less, still more preferably 5% or less, still more preferably 2% or less, still more preferably 1% or less, especially preferably 0.5% or less. 
     La 2 O 3  is a component which improves the refractive index of the glass, and the content thereof is 0% or higher and 35% or less. In the case where La 2 O 3  is contained, the content of La 2 O 3  is preferably 10% or higher, more preferably 15% or higher, still more preferably 16% or higher, still more preferably 18% or higher, especially preferably 20% or higher. Meanwhile, too high La 2 O 3  contents result in a decrease in mechanical property and an increase in devitrification temperature. Consequently, in the case where mechanical properties and producibility are important, the content of La 2 O 3  is preferably 30% or less, more preferably 25% or less, still more preferably 22% or less, still more preferably 20% or less, still more preferably 19% or less, still more preferably 18% or less, especially preferably 17% or less. 
     Gd 2 O 3  is a component which improves the refractive index of the glass, and the content thereof is 0% or higher and 15% or less. In the case where Gd 2 O 3  is contained, the content of Gd 2 O 3  is preferably 1% or higher, more preferably 2% or higher, still more preferably 3% or higher, still more preferably 4% or higher, especially preferably 5% or higher. Meanwhile, too high Gd 2 O 3  contents result in a decrease in mechanical property and an increase in devitrification temperature. Consequently, in the case where mechanical properties and producibility are important, the content of Gd 2 O 3  is preferably 10% or less, more preferably 7% or less, still more preferably 5% or less, still more preferably 4% or less, still more preferably 3% or less, still more preferably 2% or less, especially preferably 1% or less. 
     In the case where the ratio of the total content of ZrO 2 , Ta 2 O 5 , and Nb 2 O 5  to the total content of SiO 2  and B 2 O 3  [(ZrO 2 +Ta 2 O 5 +Nb 2 O 5 )/(SiO 2 +B 2 O 3 )] is too large, the glass is prone to have reduced devitrification viscosity. Consequently, (ZrO 2 +Ta 2 O 5 +Nb 2 O 5 )/(SiO 2 +B 2 O 3 ) is preferably 1.0 or less, more preferably 0.8 or less, still more preferably 0.6 or less, still more preferably 0.4 or less, especially preferably 0.35 or less. 
     In the case where the ratio of the total content of Nb 2 O 5 , TiO 2 , WO 3 , and Ta 2 O 5  to the total content of La 2 O 3 , Gd 2 O 3 , Y 2 O 3 , and Yb 2 O 3  [(Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 )] is too large, the glass is prone to have a color and a reduced transmittance. Consequently, (Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 ) is preferably 10.0 or less, more preferably 8.0 or less, still more preferably 6.0 or less, still more preferably 5.0 or less, still more preferably 4.5 or less, still more preferably 4.0 or less, still more preferably 3.0 or less, still more preferably 2.5 or less, still more preferably 2.0 or less, still more preferably 1.5 or less, especially preferably 1.3 or less. Meanwhile, in the case where (Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 ) is too small, the glass is prone to have a lowered Tg. Consequently, for use in applications where high heat resistance is required, (Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 ) is preferably 0.5 or larger, more preferably 0.7 or larger, still more preferably 0.9 or larger, still more preferably 1.1 or larger, still more preferably 1.2 or larger, still more preferably 1.3 or larger, still more preferably 1.4 or larger, still more preferably 2.0 or larger, still more preferably 3.0 or larger, especially preferably 3.5 or larger. 
     As 2 O 3  is a harmful chemical substance, and use thereof hence tends to be avoided in recent years and requires any measure for environmental preservation. It is hence preferable, in the case where influence on the environment is important, that the glass contains substantially no As 2 O 3  except for unavoidable inclusion. 
     It is preferable that the optical glass according to this embodiment contains at least one of Sb 2 O 3  and SnO 2 . Although being not essential components, these can be added for the purposes of regulating refractive index characteristics, improving meltability, coloring inhibition, improving transmittance, refining, improving chemical durability, etc. In the case where the optical glass contains one or both of those components, the total content thereof is preferably 10% or less, more preferably 5% or less, still more preferably 3% or less, especially preferably 1% or less. 
     It is preferable that the optical glass according to this embodiment contains V 2 O 5 . Although not essential, V 2 O 5  can be added for the purposes of improving transmittance, improving refining properties, etc. In the case where V 2 O 5  is contained, the content thereof is preferably 5% or less, more preferably 3% or less, still more preferably 1% or less, especially preferably 0.5% or less. 
     It is preferable that the optical glass according to this embodiment contains F. Although not essential, F can be added for the purposes of improving meltability, improving transmittance, improving refining properties, etc. In the case where F is contained, the content of F is preferably 5% or less, more preferably 3% or less. 
     In producing the optical glass according to this embodiment, it is preferable that a melting step for heating and melting raw materials for glass in a melting vessel to obtain a molten glass is conducted so that an operation for heightening the water content in the molten glass is performed during the melting step. The operation for heightening the water content in the glass is not limited, and may be, for example, a treatment in which water vapor is added to the atmosphere for melting or a treatment in which a gas including water vapor is bubbled into the melt. Although the operation for heightening the water content is not essential, this operation can be performed for the purposes of improving transmittance, improving refining properties, etc. 
     In the case where the optical glass according to this embodiment contains an alkali metal oxide such as Li 2 O or Na 2 O, the glass can be chemically strengthened by replacing Li ions with Na or K ions or replacing Na ions with K ions. That is, the strength of this optical glass can be improved by conducting a chemical strengthening treatment. 
     &lt;Glass Composition A1&gt; 
     A glass composition that is the glass composition A described above in which the content of alkaline-earth metal components is 5% or less is referred to as glass composition A1, and components of the glass composition are explained below. Explanations of some of the components of the glass composition A1 are omitted here because these are the same as those of the corresponding components of the glass composition A. 
     SiO 2  is a glass-forming component and is a component which imparts high strength and crack resistance to the glass and improves the stability and chemical durability of the glass. The content of SiO 2  is preferably 5% or higher and 30% or less. In the case where the content of SiO 2  is 5% or higher, the temperature T 1  at which the glass viscosity is 10 1  dPa·s can be regulated to a value within the preferred range. The content of SiO 2  is preferably 7% or higher, more preferably 9% or higher, still more preferably 10% or higher, especially preferably 11% or higher. Meanwhile, in the case where the content of SiO 2  is 30% or less, components for obtaining a high refractive index can be contained in a larger amount. The content of SiO 2  is more preferably 25% or less, more preferably 20% or less, still more preferably 15% or less, still more preferably 13% or less, especially preferably 12% or less. 
     B 2 O 3  is a component which lowers the Tg, improves mechanical properties such as strength and crack resistance, of the glass, and lowers the devitrification temperature. However, too high B 2 O 3  contents are prone to result in a decrease in refractive index. Consequently, the content of B 2 O 3  is preferably 5% or higher and 40% or less. The content of B 2 O 3  is more preferably 35% or less, still more preferably 30% or less, still more preferably 25% or less, still more preferably 23% or less, especially preferably 22% or less. Meanwhile, the content of B 2 O 3  is more preferably 10% or higher, still more preferably 15% or higher, still more preferably 18% or higher, especially preferably 20% or higher. 
     SiO 2  and B 2 O 3  are glass-forming components and are components which improve the stability of the glass, and the total content of SiO 2  and B 2 O 3  is 20% or higher and 45% or less. In the case where the total content of SiO 2  and B 2 O 3  is high, the glass has a lowered devitrification temperature and is easy to produce. Consequently, the total content of SiO 2  and B 2 O 3  is 20% or higher. The total content thereof is preferably 25% or higher, more preferably 28% or higher, still more preferably 30% or higher, especially preferably 32% or higher. Meanwhile, by reducing the total content of SiO 2  and B 2 O 3 , an improvement in refractive index can be attained. Hence, in the case where an especially high refractive index is required, the total content thereof is preferably 45% or less, more preferably 40% or less, still more preferably 35% or less, still more preferably 33% or less, especially preferably 32% or less. 
     In the case where the ratio of the content of SiO 2  to the content of B 2 O 3 , SiO 2 /B 2 O 3 , is too high, the glass is prone to be devitrified. Consequently, SiO 2 /B 2 O 3  is preferably 1.4 or less, more preferably 1.2 or less, still more preferably 1.0 or less, still more preferably 0.8 or less, especially preferably 0.6 or less. 
     TiO 2 , Ta 2 O 5 , WO 3 , Nb 2 O 5 , ZrO 2 , and Ln 2 O 3  (Ln is at least one element selected from the group consisting of Y, La, Gd, Yb, and Lu) are high-refractive index components which heighten the refractive index of the glass. The total content of these components is preferably 45% or higher and 80% or less. In the case where an especially high refractive index is required, the total content of these components is preferably 50% or higher, more preferably 55% or higher, still more preferably 60% or higher, yet still more preferably 65% or higher, especially preferably 67% or higher. Meanwhile, in the case where the total content of the high-refractive index components exceeds 80%, the glass is prone to be devitrified. For use in applications where lower surface roughnesses Ra are required, the content of these components is more preferably 75% or less, still more preferably 70% or less, especially preferably 68% or less. 
     MgO is a component which improves glass meltability, inhibits devitrification, and is useful in regulating optical constants of the glass, e.g., Abbe number and refractive index. Meanwhile, too high MgO contents rather promote devitrification. Consequently, the content of MgO is preferably 0% or higher and 5% or less. The content of MgO is more preferably 4% or less, especially preferably 2% or less. Meanwhile, the content of MgO is preferably 0.3% or higher, more preferably 0.5% or higher, still more preferably 1% or higher. 
     CaO is a component which inhibits devitrification, but too high CaO contents are prone to reduce the crack resistance. Consequently, the content of CaO is preferably 0% or higher and 5% or less. The content of CaO is more preferably 4% or less, still more preferably 2% or less, especially preferably 1% or less. Meanwhile, the content of CaO is more preferably 0.3% or higher, still more preferably 0.5% or higher, especially preferably 1% or higher. 
     SrO is a component which improves glass meltability, inhibits devitrification, and is useful in regulating optical constants of the glass. Meanwhile, too high SrO contents promote, rather than inhibit, devitrification. Consequently, the content of SrO is preferably 0% or higher and 5% or less. The content of SrO is more preferably 15% or less, still more preferably 4% or less, especially preferably 2% or less. Meanwhile, the content of SrO is preferably 0.3% or higher, more preferably 0.5% or higher, still more preferably 1% or higher. 
     In the case where the total content of MgO, CaO, and SrO is too high, the glass is prone to be devitrified. Consequently, the total content of MgO, CaO, and SrO is preferably 5% or less. The total content thereof MgO, CaO, and SrO is more preferably 4% or less, still more preferably 3% or less, still more preferably 2% or less, still more preferably 1% or less, especially preferably 0.5% or less. 
     BaO is a component which inhibits devitrification, but too high BaO contents are prone to result in an increase in density. Consequently, in the case where BaO is contained, the content of BaO is preferably 0% or higher and 5% or less. The content of BaO is more preferably 4% or less, still more preferably 3% or less, still more preferably 2% or less, especially preferably 1% or less. Meanwhile, the content of BaO is more preferably 0.3% or higher, still more preferably 0.5% or higher, especially preferably 1% or higher. 
     In the case where the glass contains alkaline-earth metal components (MgO+CaO+SrO+BaO), a reduction in specific gravity can be attained by regulating the ratio of the content of BaO to the total content of the alkaline-earth metal components (BaO/(MgO+CaO+SrO+BaO)) to 0.5 or less. The ratio (BaO/(MgO+CaO+SrO+BaO)) is preferably 0.4 or less, more preferably 0.3 or less, still more preferably 0.2 or less, especially preferably 0.1 or less. By heightening the proportion of BaO to the alkaline metal components, the devitrification temperature can be lowered. For use in applications where a lower surface roughness Ra is required, the ratio is preferably 0.1 or larger, more preferably 0.2 or larger, still more preferably 0.3 or larger, especially preferably 0.4 or larger. 
     In the case where the total content of the alkali metal components (Li 2 O+Na 2 O+K 2 O) and the alkaline-earth metal components (MgO+CaO+SrO+BaO) is too high, the glass is prone to have low Tg. Consequently, the total content of the alkali metal components and the alkaline-earth metal components is preferably 15% or less. The total content of the alkali metal components and the alkaline-earth metal components is more preferably 12% or less, still more preferably 10% or less, still more preferably 5% or less, still more preferably 3% or less, especially preferably 2% or less. 
     In the case where the ratio of the content of TiO 2  to the content of B 2 O 3 , TiO 2 /B 2 O 3 , is too high, the glass requires higher temperature for melting. In such cases, the titanium is prone to be reduced and the glass is prone to take a color and have a reduced transmittance. Consequently, TiO 2 /B 2 O 3  is preferably 2.0 or less, more preferably 1.8 or less, still more preferably 1.7 or less, still more preferably 1.6 or less, especially preferably 1.5 or less. 
     The glass composition A1 gives an optical glass having a high refractive index (n d ) in the range of 1.92 to 2.15. The refractive index (n d ) is 1.92 or higher. The optical glass is suitable for use as an optical glass for wearable appliances from the standpoints of an increase in image display angle, increases in luminance and contrast, an improvement in lightguide property, processability for producing diffraction gratings, etc. The optical glass is suitable also as a small-sized imaging glass lens with a wide imaging angle of view for use in applications such as in-vehicle cameras and visual sensors for robots, the lens being smaller and capable of wide-range imaging. The refractive index (n d ) thereof is preferably 1.95 or higher, more preferably 1.97 or higher, still more preferably 1.98 or higher, still more preferably 1.99 or higher, still more preferably 2.00 or higher. 
     Meanwhile, in the case where the refractive index (n d ) exceeds 2.15, the glass tends to have a high density and a high devitrification temperature. Especially in the case where it is important for the optical glass to have a low density, the refractive index (n d ) thereof is preferably 2.10 or less, more preferably 2.06 or less, still more preferably 2.03 or less, still more preferably 2.01 or less, still more preferably 1.98 or less, still more preferably 1.95 or less, still more preferably 1.94 or less, yet still more preferably 1.93 or less. 
     The glass composition A1 gives an optical glass having a density (d) of 4.0 g/cm 3  to 6.0 g/cm 3 . Since the optical glass has a density within that range, a wear feeling of a user can be made comfortable when used in wearable appliances, and the overall apparatus weight can be reduced when used in in-vehicle cameras, visual sensors for robots, etc. The density (d) thereof is preferably 5.8 g/cm 3  or less, more preferably 5.6 g/cm 3  or less, still more preferably 5.4 g/cm 3  or less, still more preferably 5.2 g/cm 3  or less, still more preferably 5.1 g/cm 3  or less, still more preferably 5.0 g/cm 3  or less. 
     Meanwhile, from the standpoint of making the surfaces of the optical glass less apt to receive scratches, the density (d) thereof is preferably 4.0 g/cm 3  or higher, more preferably 4.3 g/cm 3  or higher, still more preferably 4.6 g/cm 3  or higher, still more preferably 4.7 g/cm 3  or higher, especially preferably 4.8 g/cm 3  or higher. 
     &lt;Glass Composition A2&gt; 
     A glass composition that is the glass composition A described above in which the content of alkaline-earth metal components is higher than 5% but not higher than 50% and the B 2 O 3  content is less than 15% is referred to as glass composition A2, and components of the glass composition are explained below. Explanations of some of the components of the glass composition A2 are omitted here because these are the same as those of the corresponding components of the glass composition A. 
     SiO 2  is a glass-forming component and is a component which imparts high strength and crack resistance to the glass and improves the stability and chemical durability of the glass. The content of SiO 2  is preferably 5% or higher and 44% or less. In the case where the content of SiO 2  is 5% or higher, the temperature T 1  at which the glass viscosity is 10 1  dPa·s can be regulated to a value within the preferred range. The content of SiO 2  is preferably 10% or higher, more preferably 15% or higher, still more preferably 20% or higher, still more preferably 24% or higher, still more preferably 28% or higher, especially preferably 30% or higher. Meanwhile, in the case where the content of SiO 2  is 44% or less, components for obtaining a high refractive index can be contained. The content of SiO 2  is more preferably 37% or less, more preferably 35.5% or less, still more preferably 34% or less, still more preferably 33% or less, especially preferably 31% or less. 
     B 2 O 3  is a glass-forming component and is optional. B 2 O 3  is a component which lowers the Tg, improves mechanical properties, including strength and crack resistance, of the glass, and lowers the devitrification temperature. From the standpoint of a balance between refractive index and mechanical strength, the content of B 2 O 3  is preferably 0% or higher but less than 15%. The content of B 2 O 3  is more preferably 14% or less, still more preferably 13% or less, still more preferably 12% or less, still more preferably 11% or less, especially preferably 10% or less. Meanwhile, the content of B 2 O 3  is more preferably 1% or higher, still more preferably 3% or higher, still more preferably 5% or higher, especially preferably 7% or higher. 
     SiO 2  and B 2 O 3  are glass-forming components and are components which improve the stability of the glass, and the total content of SiO 2  and B 2 O 3  is 30% or higher and 70% or less. In the case where the total content of SiO 2  and B 2 O 3  is high, the glass has a lowered devitrification temperature and is easy to produce. Consequently, the total content of SiO 2  and B 2 O 3  is 30% or higher. The total content of SiO 2  and B 2 O 3  is preferably 32% or higher, more preferably 34% or higher, still more preferably 36% or higher, especially preferably 37% or higher. Meanwhile, by reducing the total content of SiO 2  and B 2 O 3 , an improvement in refractive index can be attained. Hence, in the case where an especially high refractive index is required, the total content of SiO 2  and B 2 O 3  is preferably 70% or less, more preferably 60% or less, still more preferably 50% or less, still more preferably 40% or less, especially preferably 35% or less. 
     In the case where B 2 O 3  is contained, the glass is prone to be devitrified if the ratio of the content of SiO 2  to the content of B 2 O 3 , SiO 2 /B 2 O 3 , is high. Consequently, in the case where B 2 O 3  is contained, SiO 2 /B 2 O 3  is preferably 5.0 or less, more preferably 4.5 or less, still more preferably 4.0 or less, still more preferably 3.5 or less, especially preferably 3.0 or less. 
     TiO 2 , Ta 2 O 5 , WO 3 , Nb 2 O 5 , ZrO 2 , and Ln 2 O 3  (Ln is at least one element selected from the group consisting of Y, La, Gd, Yb, and Lu) are high-refractive index components which heighten the refractive index of the glass. The total content of these components is preferably 30% to 55%. In the case where an especially high refractive index is required, the total content of these components is preferably 33% or higher, more preferably 35% or higher, still more preferably 36% or higher, yet still more preferably 37% or higher, especially preferably 38% or higher. Meanwhile, in the case where the total content of these high-refractive index components is too high, the glass is prone to be devitrified. For use in applications where a lower surface roughness Ra is required, the content of these components is more preferably 50% or less, still more preferably 45% or less, still more preferably 40% or less, especially preferably 35% or less. 
     The total content of alkali metal components (Li 2 O+Na 2 O+K 2 O) is 0% or higher and 10% or less. By increasing the content of these alkali metal components, the Tg can be lowered. However, in the case where Li 2 O+Na 2 O+K 2 O is too high, T 1  is prone to be low and a viscosity curve is steep, resulting in reduced producibility. Meanwhile, in the case where Li 2 O+Na 2 O+K 2 O is too low, T 1  is prone to be high and high temperature is required for melting, and this may cause coloring. Consequently, in the case where one or more of Li 2 O, Na 2 O, and K 2 O are contained, Li 2 O+Na 2 O+K 2 O is preferably 0.5% or higher and 10% or less. Li 2 O+Na 2 O+K 2 O is more preferably 1% or higher, still more preferably 1.5% or higher, still more preferably 2% or higher, especially preferably 3% or higher. Meanwhile, Li 2 O+Na 2 O+K 2 O is preferably 6% or less, more preferably 4% or less, still more preferably 3% or less, especially preferably 2% or less. 
     The content of Li 2 O is 0% or higher and 10% or less. In the case where Li 2 O is contained, the content thereof is 0.2% or higher and 10% or less. The inclusion of Li 2 O can improve the strength (Kc) and crack resistance (CIL). In the case where the optical glass of the present invention contains Li 2 O, the content of Li 2 O is preferably 0.5% or higher, more preferably 1% or higher, still more preferably 1.5% or higher, especially preferably 2% or higher. Meanwhile, too high Li 2 O contents are prone to result in devitrification. Especially in the case where devitrification is problematic, the content of Li 2 O is preferably 8% or less, more preferably 6% or less, still more preferably 4% or less, especially preferably 2% or less. 
     In the case where the optical glass according to this embodiment is to be chemically strengthened, the content of Li 2 O is preferably 3.0% or higher, more preferably 6.0% or higher, still more preferably 9.0% or higher, especially preferably 11.0% or higher. 
     CaO is a component which inhibits devitrification, but too high CaO contents are prone to reduce the crack resistance. Consequently, the content of CaO is preferably 0% or higher and 25% or less. The content of CaO is more preferably 20% or less, still more preferably 15% or less, still more preferably 10% or less, still more preferably 8% or less, still more preferably 7% or less, especially preferably 6.5% or less. Meanwhile, the content of CaO is more preferably 2% or higher, still more preferably 4% or higher, still more preferably 5% or higher, especially preferably 6% or higher. 
     SrO is a component which improves glass meltability, inhibits devitrification, and is useful in regulating optical constants of the glass. Meanwhile, too high SrO contents rather promote devitrification. Consequently, the content of SrO is preferably 0% or higher and 20% or less. The content of SrO is more preferably 15% or less, still more preferably 10% or less, still more preferably 9% or less, still more preferably 8% or less, especially preferably 7% or less. Meanwhile, the content of SrO is more preferably 2% or higher, still more preferably 4% or higher, especially preferably 6% or higher. 
     In the case where the total content of MgO, CaO, and SrO is too high, the glass is prone to be devitrified. Consequently, the total content of MgO, CaO, and SrO is preferably 30% or less. The total content thereof is more preferably 25% or less, still more preferably 18% or less, still more preferably 16% or less, still more preferably 15% or less, especially preferably 14% or less. 
     BaO is a component which inhibits devitrification, but too high BaO contents are prone to result in an increase in density. Consequently, in the case where BaO is contained, the content of BaO is preferably 0% or higher and 30% or less. The content of BaO is more preferably 20% or less, still more preferably 15% or less, still more preferably 11% or less, still more preferably 9% or less, especially preferably 8% or less. Meanwhile, the content of BaO is more preferably 2% or higher, still more preferably 4% or higher, especially preferably 6% or higher. 
     The total content of alkaline-earth metal components (MgO+CaO+SrO+BaO) is 5% or higher and 50% or less. The total content of these components is preferably 50% or less such that the glass can be inhibited from being devitrified. The total content of these components is more preferably 40% or less, still more preferably 30% or less, still more preferably 27% or less, still more preferably 25% or less, still more preferably 23% or less, still more preferably 21% or less, especially preferably 20% or less. Since the total content of these components is 5% or higher, the glass can have improved meltability; such total contents are hence preferred. The total content thereof is more preferably 10% or higher, still more preferably 13% or higher, still more preferably 16% or higher, still more preferably 18% or higher, especially preferably 19% or higher. 
     By regulating the ratio of the content of BaO to the total content of the alkaline-earth metal components (MgO+CaO+SrO+BaO), BaO/(MgO+CaO+SrO+BaO), to 0.5 or less, the specific gravity can be reduced. The ratio is preferably 0.45 or less, more preferably 0.42 or less, still more preferably 0.40 or less, especially preferably 0.35 or less. By heightening the proportion of BaO to the alkaline-earth metal components, the devitrification temperature can be lowered and the producibility can be improved. In the case where producibility is especially important, that ratio is preferably 0.1 or larger, more preferably 0.2 or larger, still more preferably 0.3 or larger, especially preferably 0.35 or larger. 
     In the case where the total content of the alkali metal components (Li 2 O+Na 2 O+K 2 O) and the alkaline-earth metal components (MgO+CaO+SrO+BaO) is too high, Tg of the glass is prone to be low. Consequently, the total content of the alkali metal components and the alkaline-earth metal components is preferably 30% or less. The total content of these components is more preferably 16% or less, still more preferably 14% or less, still more preferably 13% or less, still more preferably 12% or less, especially preferably 11.5% or less. 
     TiO 2  is a component which heightens the refractive index of the glass to enhance dispersion by the glass, and the content of TiO 2  is 0% or higher and 50% or less. In the case where TiO 2  is contained, the content of TiO 2  is preferably 10% or higher, more preferably 15% or higher, still more preferably 17% or higher, still more preferably 19% or higher, still more preferably 20% or higher, especially preferably 22% or higher. Meanwhile, too high TiO 2  contents are prone to cause coloring and result in a decrease in transmittance. Consequently, especially in the case where transmittance is required, the content of TiO 2  is preferably 50% or less, more preferably 40% or less, still more preferably 30% or less, still more preferably 25% or less, still more preferably 23% or less, still more preferably 22% or less, especially preferably 21% or less. 
     In the case where B 2 O 3  is contained, the glass requires higher temperatures for melting if the ratio of the content of TiO 2  to the content of B 2 O 3 , TiO 2 /B 2 O 3 , is high. In such cases, the titanium is prone to be reduced and the glass is prone to take a color and have a reduced transmittance. Consequently, in the case where B 2 O 3  is contained, TiO 2 /B 2 O 3  is preferably 5.0 or less, more preferably 4.5 or less, still more preferably 4.0 or less, still more preferably 3.5 or less, still more preferably 3.0 or less, still more preferably 2.8 or less, especially preferably 2.7 or less. 
     Nb 2 O 5  is a component which heightens the refractive index of the glass and reduces the Abbe number (v d ) thereof. The content of Nb 2 O 5  is 0% or higher and 35% or less. The content of Nb 2 O 5  is preferably 2% or higher, more preferably 4% or higher, still more preferably 5% or higher, still more preferably 6% or higher, still more preferably 7% or higher, still more preferably 8% or higher, especially preferably 10% or higher. 
     Meanwhile, too high Nb 2 O 5  contents render the glass prone to be devitrified. Consequently, for use in applications where a lower surface roughness Ra is required, the content of Nb 2 O 5  is preferably 20% or less, more preferably 10% or less, still more preferably 8% or less, especially preferably 7% or less. 
     In the case where the total content of TiO 2 , WO 3 , and Nb 2 O 5  is too low, the glass has a reduced refractive index. Consequently, the total content of TiO 2 , WO 3 , and Nb 2 O 5  is preferably 10% or higher and 50% or less. The total content of TiO 2 , WO 3 , and Nb 2 O 5  is more preferably 14% or higher, still more preferably 18% or higher, still more preferably 22% or higher, especially preferably 26% or higher. Meanwhile, in the case where the total content of TiO 2 , WO 3 , and Nb 2 O 5  is too high, the glass is prone to be devitrified. Consequently, for use in applications where a lower surface roughness Ra is required, the total content of TiO 2 , WO 3 , and Nb 2 O 5  is preferably 40% or less, more preferably 35% or less, still more preferably 30% or less, especially preferably 28% or less. 
     ZrO 2  is a component which heightens the refractive index of the glass and enhances the chemical durability of the glass, and the content of ZrO 2  is 0% or higher and 20% or less. The inclusion of ZrO 2  can improve the crack resistance. In the case where ZrO 2  is contained, the content of ZrO 2  is more preferably 1% or higher, still more preferably 2% or higher, still more preferably 3% or higher, especially preferably 4% or higher. Meanwhile, too high ZrO 2  contents render the glass prone to be devitrified. Consequently, in the case where producibility is especially important, the content of ZrO 2  is more preferably 15% or less, still more preferably 10% or less, still more preferably 6% or less, especially preferably 5% or less. 
     ZnO is a component which improves mechanical properties, including strength and crack resistance, of the glass, and the content of ZnO is 0% or higher and 15% or less. In the case where ZnO is contained, the content of ZnO is more preferably 0.3% or higher, still more preferably 0.5% or higher, especially preferably 1% or higher. Meanwhile, too high ZnO contents render the glass prone to be devitrified. Consequently, the content of ZnO is more preferably 10% or less, still more preferably 5% or less, still more preferably 2% or less, still more preferably 1% or less, especially preferably 0.5% or less. 
     La 2 O 3  is a component which improves the refractive index of the glass, and the content of La 2 O 3  is 0% or higher and 35% or less. In the case where La 2 O 3  is contained, the content of La 2 O 3  is preferably 2% or higher, more preferably 4% or higher, still more preferably 5% or higher, still more preferably 6% or higher, especially preferably 7% or higher. Meanwhile, too high La 2 O 3  contents result in a decrease in mechanical property and an increase in devitrification temperature. Consequently, in the case where mechanical properties and producibility are important, the content of La 2 O 3  is preferably 30% or less, more preferably 25% or less, still more preferably 20% or less, still more preferably 15% or less, still more preferably 10% or less, still more preferably 9% or less, especially preferably 8% or less. 
     In the case where the ratio of the total content of Nb 2 O 5 , TiO 2 , WO 3 , and Ta 2 O 5  to the total content of La 2 O 3 , Gd 2 O 3 , Y 2 O 3 , and Yb 2 O 3  [(Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 )] is too large, the glass is prone to have a color and a reduced transmittance. Consequently, (Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 ) is preferably 10.0 or less, more preferably 8.0 or less, still more preferably 6.0 or less, still more preferably 5.0 or less, still more preferably 4.5 or less, especially preferably 4.0 or less. Meanwhile, in the case where (Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 ) is too small, the glass is prone to have a lowered Tg. Consequently, for use in applications where high heat resistance is required, (Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 ) is preferably 0.5 or larger, more preferably 1.0 or larger, still more preferably 2.0 or larger, still more preferably 3.0 or larger, especially preferably 3.5 or larger. 
     The glass composition A2 gives an optical glass having a high refractive index (n d ) in the range of 1.81 to 1.96. The refractive index (n d ) is 1.81 or higher. The optical glass is suitable for use as an optical glass for wearable appliances from the standpoints of an increase in image display angle, increases in luminance and contrast, an improvement in lightguide property, easiness of processing for producing diffraction gratings, etc. The optical glass is suitable also as a small-sized imaging glass lens with a wide imaging angle of view for use in applications such as in-vehicle cameras and visual sensors for robots, the lens being smaller and capable of wide-range imaging. The refractive index (n d ) of the glass is preferably 1.84 or higher, more preferably 1.86 or higher, still more preferably 1.87 or higher, still more preferably 1.88 or higher, still more preferably 1.89 or higher, especially preferably 1.90 or higher. 
     Meanwhile, in the case where the refractive index (n d ) of the glass exceeds 1.96, the glass tends to have an increased density and an elevated devitrification temperature. Especially in the case where it is important for the optical glass to have a low density, the refractive index (n d ) of the glass is preferably 1.94 or less, more preferably 1.93 or less, still more preferably 1.92 or less, still more preferably 1.91 or less, still more preferably 1.90 or less, still more preferably 1.89 or less, still more preferably 1.88 or less, yet still more preferably 1.87 or less. 
     The glass composition A2 gives an optical glass having a density (d) of 3.3 g/cm 3  to 5.4 g/cm 3 . In the case where the optical glass has a density within the range, the user can have a comfortable wear feeling when the glass is used in wearable appliances and the overall apparatus weight can be reduced when the glass is used in in-vehicle cameras, visual sensors for robots, etc. The density (d) of the glass is preferably 5.2 g/cm 3  or less, more preferably 5.0 g/cm 3  or less, still more preferably 4.8 g/cm 3  or less, still more preferably 4.6 g/cm 3  or less, still more preferably 4.4 g/cm 3  or less, still more preferably 4.2 g/cm 3  or less. 
     Meanwhile, from the standpoint of making the surfaces of the optical glass less apt to be scratched, the density (d) of the glass is preferably 3.6 g/cm 3  or higher. The density (d) of the glass is more preferably 3.8 g/cm 3  or higher, still more preferably 4.0 g/cm 3  or higher, still more preferably 4.2 g/cm 3  or higher, especially preferably 4.3 g/cm 3  or higher. 
     Furthermore, this optical glass has a devitrification temperature of 1,300° C. or lower. The glass having this property can be inhibited from being devitrified when formed such that the glass has satisfactory formability. The devitrification temperature of the glass is more preferably 1,275° C. or lower, still more preferably 1,240° C. or lower, yet still more preferably 1,225° C. or lower, yet still more preferably 1,200° C. or lower, yet still more preferably 1,175° C. or lower, yet still more preferably 1,150° C. or lower, yet still more preferably 1,125° C. or lower, yet still more preferably 1,100° C. or lower, yet still more preferably 1,075° C. or lower, especially preferably 1,050° C. or lower. The devitrification temperature is a lowest temperature at which no crystals of 1 μm or larger in terms of major-side or major-axis dimension are observed in the surface and inside of the glass when the glass which has been heated and melted is allowed to cool naturally. 
     &lt;Glass Composition A3&gt; 
     A glass composition that is the glass composition A described above in which the content of alkaline-earth metal components is higher than 5% but not higher than 50% and the B 2 O 3  content is 15% or higher is referred to as glass composition A3, and components of the glass composition are explained below. Explanations of some of the components of the glass composition A3 are omitted here because these are the same as those of the corresponding components of the glass composition A. 
     SiO 2  is a glass-forming component and is a component which imparts high strength and crack resistance to the glass and improves the stability and chemical durability of the glass. The content of SiO 2  is preferably 5% or higher and 44% or less. In the case where the content of SiO 2  is 5% or higher, the temperature T 1  at which the glass viscosity is 10 1  dPa·s can be regulated to a value within the preferred range. The content of SiO 2  is preferably 5% or higher, more preferably 10% or higher, still more preferably 12% or higher, still more preferably 13% or higher, still more preferably 14% or higher, especially preferably 15% or higher. Meanwhile, in the case where the content of SiO 2  is 44% or less, components for obtaining a high refractive index can be contained. The content of SiO 2  is more preferably 37% or less, more preferably 30% or less, still more preferably 23% or less, still more preferably 20% or less, especially preferably 17% or less. 
     B 2 O 3  is a glass-forming component and is essential. B 2 O 3  is a component which lowers the Tg, improves mechanical properties, including strength and crack resistance, of the glass, and lowers the devitrification temperature. However, too high B 2 O 3  contents are prone to result in a decrease in refractive index. Consequently, the content of B 2 O 3  is preferably 15% or higher and 40% or less. The content of B 2 O 3  is more preferably 35% or less, still more preferably 32% or less, still more preferably 29% or less, still more preferably 27% or less, especially preferably 26% or less. Meanwhile, the content of B 2 O 3  is more preferably 18% or higher, still more preferably 21% or higher, still more preferably 23% or higher, especially preferably 24% or higher. 
     SiO 2  and B 2 O 3  are glass-forming components and are components which improve the stability of the glass, and the total content of SiO 2  and B 2 O 3  is 30% or higher and 70% or less. In the case where the total content of SiO 2  and B 2 O 3  is high, the glass has a lowered devitrification temperature and is easy to produce. Consequently, the total content of SiO 2  and B 2 O 3  is 30% or higher, preferably 32% or higher, more preferably 34% or higher, still more preferably 36% or higher, especially preferably 39% or higher. Meanwhile, by reducing the total content of SiO 2  and B 2 O 3 , an improvement in refractive index can be attained. Hence, in the case where an especially high refractive index is required, the total content of SiO 2  and B 2 O 3  is preferably 70% or less, more preferably 60% or less, still more preferably 50% or less, still more preferably 45% or less, especially preferably 42% or less. 
     In the case where B 2 O 3  is contained, the glass is prone to be devitrified if the ratio of the content of SiO 2  to the content of B 2 O 3 , SiO 2 /B 2 O 3 , is high. Consequently, in the case where B 2 O 3  is contained, SiO 2 /B 2 O 3  is preferably 5.0 or less, more preferably 3.0 or less, still more preferably 2.0 or less, still more preferably 1.5 or less, especially preferably 1.0 or less. 
     TiO 2 , Ta 2 O 5 , WO 3 , Nb 2 O 5 , ZrO 2 , and Ln 2 O 3  (Ln is at least one element selected from the group consisting of Y, La, Gd, Yb, and Lu) are high-refractive index components which heighten the refractive index of the glass. The total content of these components is preferably 30% or higher and 55% or less. In the case where an especially high refractive index is required, the total content of these components is preferably 33% or higher, more preferably 35% or higher, still more preferably 36% or higher, yet still more preferably 37% or higher, especially preferably 38% or higher. Meanwhile, in the case where the total content of these high-refractive index components is too high, the glass is prone to be devitrified. For use in applications where lower surface roughnesses Ra are required, the content of these components is more preferably 50% or less, still more preferably 45% or less, still more preferably 40% or less, especially preferably 35% or less. 
     The total content of alkali metal components (Li 2 O+Na 2 O+K 2 O) is 0% or higher and 10% or less. By increasing the content of these alkali metal components, the Tg can be lowered. However, in the case where Li 2 O+Na 2 O+K 2 O is too high, T 1  is prone to be low and the glass has a steep viscosity curve, resulting in reduced producibility. Meanwhile, in the case where Li 2 O+Na 2 O+K 2 O is too low, T 1  is prone to be high and the glass requires higher temperatures for melting, and this may cause coloring. Consequently, in the case when one or more of Li 2 O, Na 2 O, and K 2 O are contained, Li 2 O+Na 2 O+K 2 O is preferably 0.5% or higher and 10% or less. Li 2 O+Na 2 O+K 2 O is more preferably 1% or higher, still more preferably 1.5% or higher, still more preferably 2% or higher, especially preferably 3% or higher. Meanwhile, Li 2 O+Na 2 O+K 2 O is preferably 6% or less, more preferably 4% or less, still more preferably 3% or less, especially preferably 2% or less. 
     The content of Li 2 O is 0% or higher and 10% or less. In the case where Li 2 O is contained, the content of Li 2 O is 0.2% or higher and 10% or less. The inclusion of Li 2 O can improve the strength (Kc) and crack resistance (CIL). In the case where the optical glass of the present invention contains Li 2 O, the content of Li 2 O is preferably 0.5% or higher, more preferably 1% or higher, still more preferably 1.5% or higher, especially preferably 2% or higher. Meanwhile, too high Li 2 O contents are prone to result in devitrification. Especially in the case where devitrification is problematic, the content of Li 2 O is preferably 6% or less, more preferably 3% or less, still more preferably 1% or less, especially preferably 0.1% or less. 
     In the case where the optical glass according to this embodiment is to be chemically strengthened, the content of Li 2 O is preferably 3.0% or higher, more preferably 6.0% or higher, still more preferably 9.0% or higher, especially preferably 11.0% or higher. 
     CaO is a component which inhibits devitrification, but too high CaO contents are prone to reduce the crack resistance. Consequently, the content of CaO is preferably 0% or higher and 25% or less. The content of CaO is more preferably 20% or less, still more preferably 17% or less, still more preferably 14% or less, still more preferably 13% or less, still more preferably 12% or less, especially preferably 11.5% or less. Meanwhile, the content of CaO is more preferably 4% or higher, still more preferably 8% or higher, still more preferably 10% or higher, especially preferably 11% or higher. 
     SrO is a component which improves glass meltability, inhibits devitrification, and is useful in regulating optical constants of the glass. Meanwhile, too high SrO contents rather promote devitrification. Consequently, the content of SrO is preferably 0% or higher and 20% or less. The content of SrO is more preferably 15% or less, still more preferably 12% or less, still more preferably 10% or less, still more preferably 9% or less, especially preferably 8% or less. Meanwhile, the content of SrO is more preferably 2% or higher, still more preferably 5% or higher, especially preferably 7% or higher. 
     In the case where the total content of MgO, CaO, and SrO is too high, the glass is prone to be devitrified. Consequently, the total content of MgO, CaO, and SrO is preferably 30% or less. The total content of these components is more preferably 25% or less, still more preferably 22% or less, still more preferably 21% or less, still more preferably 20% or less, especially preferably 19.5% or less. 
     BaO is a component which inhibits devitrification, but too high BaO contents are prone to result in an increase in density. Consequently, in the case where BaO is contained, the content of BaO is preferably 0% or higher and 30% or less. The content of BaO is more preferably 20% or less, still more preferably 15% or less, still more preferably 11% or less, still more preferably 9% or less, especially preferably 8% or less. Meanwhile, the content of BaO is more preferably 2% or higher, still more preferably 5% or higher, especially preferably 7% or higher. 
     The total content of alkaline-earth metal components (MgO+CaO+SrO+BaO) is 5% or higher and 50% or less. The total content of these components is preferably 50% or less so that the glass can be inhibited from being devitrified. The total content of these components is more preferably 40% or less, still more preferably 35% or less, still more preferably 32% or less, still more preferably 30% or less, still more preferably 29% or less, still more preferably 28% or less, especially preferably 20% or less. The total content of these components is preferably 5% or higher so that the glass can have improved meltability. The total content of these components is more preferably 10% or higher, still more preferably 15% or higher, still more preferably 20% or higher, still more preferably 25% or higher, especially preferably 26% or higher. 
     By regulating the ratio of the content of BaO to the total content of the alkaline-earth metal components (MgO+CaO+SrO+BaO), (BaO/(MgO+CaO+SrO+BaO)), to 0.5 or less, a reduction in specific gravity can be attained. The ratio is preferably 0.45 or less, more preferably 0.42 or less, still more preferably 0.40 or less, especially preferably 0.35 or less. By heightening the proportion of BaO to the alkaline-earth metal components, the glass can be made to have a lowered devitrification temperature and improved producibility. In the case where producibility is especially important, the ratio is preferably 0.1 or larger, more preferably 0.2 or larger, still more preferably 0.25 or larger, especially preferably 0.3 or larger. 
     In the case where the total content of the alkali metal components (Li 2 O+Na 2 O+K 2 O) and the alkaline-earth metal components (MgO+CaO+SrO+BaO) is too high, the glass is prone to have a lowered Tg. Consequently, the total content of the alkali metal components and the alkaline-earth metal components is preferably 30% or less, more preferably 29% or less, still more preferably 28% or less, especially preferably 27.5% or less. 
     TiO 2  is a component which heightens the refractive index of the glass to enhance dispersion by the glass, and the content of TiO 2  is 0% or higher and 50% or less. In the case where TiO 2  is contained, the content of TiO 2  is preferably 10% or higher, more preferably 15% or higher, still more preferably 17% or higher, still more preferably 19% or higher, still more preferably 20% or higher, especially preferably 20.5% or higher. Meanwhile, too high TiO 2  contents are prone to cause coloring and result in a decrease in transmittance. Consequently, especially in the case where transmittance is required, the content of TiO 2  is preferably 50% or less, more preferably 40% or less, still more preferably 30% or less, still more preferably 25% or less, still more preferably 23% or less, still more preferably 22% or less, especially preferably 21% or less. 
     In the case where B 2 O 3  is contained, the glass requires higher temperatures for melting if the ratio of the content of TiO 2  to the content of B 2 O 3 , TiO 2 /B 2 O 3 , is high. In such cases, the titanium is prone to be reduced and the glass is prone to take a color and have a reduced transmittance. Consequently, in the case where B 2 O 3  is contained, TiO 2 /B 2 O 3  is preferably 5.0 or less, more preferably 4.0 or less, still more preferably 3.0 or less, still more preferably 2.0 or less, still more preferably 1.5 or less, still more preferably 1.2 or less, especially preferably 1.0 or less. 
     Nb 2 O 5  is a component which heightens the refractive index of the glass and reduces the Abbe number (v d ) of the glass. The content of Nb 2 O 5  is 0% or higher and 35% or less. The content of Nb 2 O 5  is preferably 0.5% or higher, more preferably 1.0% or higher, still more preferably 1.5% or higher, still more preferably 2.0% or higher, especially preferably 2.5% or higher. 
     Meanwhile, too high Nb 2 O 5  contents render the glass prone to be devitrified. Consequently, for use in applications where a lower surface roughness Ra is required, the content of Nb 2 O 5  is preferably 20% or less, more preferably 10% or less, still more preferably 5% or less, especially preferably 3% or less. 
     In the case where the total content of TiO 2 , WO 3 , and Nb 2 O 5  is too low, the glass has a reduced refractive index. Consequently, the total content of TiO 2 , WO 3 , and Nb 2 O 5  is preferably 10% or higher and 50% or less. The total content of these components is more preferably 14% or higher, still more preferably 18% or higher, still more preferably 22% or higher, especially preferably 23% or higher. Meanwhile, in the case where the total content of TiO 2 , WO 3 , and Nb 2 O 5  is too high, the glass is prone to be devitrified. Consequently, for use in applications where a lower surface roughness Ra is required, the total content of these components is preferably 40% or less, more preferably 35% or less, still more preferably 30% or less, especially preferably 25% or less. 
     ZrO 2  is a component which heightens the refractive index of the glass and enhances the chemical durability of the glass, and the content of ZrO 2  is 0% or higher and 20% or less. The inclusion of ZrO 2  can improve the crack resistance. In the case where ZrO 2  is contained, the content of ZrO 2  is more preferably 1% or higher, still more preferably 2% or higher, still more preferably 3% or higher, especially preferably 4% or higher. Meanwhile, too high ZrO 2  contents render the glass prone to be devitrified. Consequently, in the case where producibility is especially important, the content of ZrO 2  is more preferably 15% or less, still more preferably 10% or less, still more preferably 6% or less, especially preferably 5% or less. 
     ZnO is a component which improves mechanical properties, including strength and crack resistance, of the glass, and the content of ZnO is 0% or higher and 15% or less. In the case where ZnO is contained, the content of ZnO is more preferably 0.3% or higher, still more preferably 0.5% or higher, especially preferably 1% or higher. Meanwhile, too high ZnO contents render the glass prone to be devitrified. Consequently, the content of ZnO is more preferably 10% or less, still more preferably 5% or less, still more preferably 2% or less, still more preferably 1% or less, especially preferably 0.5% or less. 
     La 2 O 3  is a component which improves the refractive index of the glass, and the content of La 2 O 3  is 0% or higher and 35% or less. In the case where La 2 O 3  is contained, the content of La 2 O 3  is preferably 2% or higher, more preferably 3% or higher, still more preferably 4% or higher, still more preferably 4.5% or higher, especially preferably 5% or higher. Meanwhile, too high La 2 O 3  contents result in a decrease in mechanical property and an increase in devitrification temperature. Consequently, in the case where mechanical properties and producibility are important, the content of La 2 O 3  is preferably 30% or less, more preferably 25% or less, still more preferably 20% or less, still more preferably 15% or less, still more preferably 10% or less, still more preferably 7% or less, especially preferably 6% or less. 
     In the case where the ratio of the total content of Nb 2 O 5 , TiO 2 , WO 3 , and Ta 2 O 5  to the total content of La 2 O 3 , Gd 2 O 3 , Y 2 O 3 , and Yb 2 O 3  [(Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 )] is too large, the glass is prone to have a color and a reduced transmittance. Consequently, (Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 ) is preferably 10.0 or less, more preferably 8.0 or less, still more preferably 7.0 or less, still more preferably 6.0 or less, still more preferably 5.5 or less, especially preferably 5.0 or less. Meanwhile, in the case where (Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 ) is too small, the glass is prone to have a lowered Tg. Consequently, for use in applications where high heat resistance is required, (Nb 2 O 5 +TiO 2 +WO 3 +Ta 2 O 5 )/(La 2 O 3 +Gd 2 O 3 +Y 2 O 3 +Yb 2 O 3 ) is preferably 0.5 or larger, more preferably 1.0 or larger, still more preferably 2.0 or larger, still more preferably 3.0 or larger, especially preferably 4.0 or larger. 
     The glass composition A3 gives an optical glass having a high refractive index (n d ) in the range of 1.81 to 1.96. The refractive index (n d ) is 1.81 or higher. The optical glass is suitable for use as an optical glass for wearable appliances from the standpoints of an increase in image display angle, increases in luminance and contrast, an improvement in lightguide property, easiness of processing for producing diffraction gratings, etc. The optical glass is suitable also as a small-sized imaging glass lens with a wide imaging angle of view for use in applications such as in-vehicle cameras and visual sensors for robots, the lens being smaller and capable of wide-range imaging. The refractive index (n d ) is preferably 1.820 or higher, more preferably 1.830 or higher, still more preferably 1.835 or higher, still more preferably 1.840 or higher, still more preferably 1.845 or higher, especially preferably 1.850 or higher. 
     Meanwhile, in the case where the refractive index (n d ) exceeds 1.96, the glass tends to have an increased density and an elevated devitrification temperature. Especially in the case where it is important for the optical glass to have a low density, the refractive index (n d ) of the glass is preferably 1.92 or less, more preferably 1.90 or less, still more preferably 1.89 or less, still more preferably 1.88 or less, still more preferably 1.87 or less, still more preferably 1.86 or less, still more preferably 1.855 or less, yet still more preferably 1.853 or less. 
     The glass composition A3 gives an optical glass having a density (d) of 3.3 g/cm 3  to 5.4 g/cm 3 . Since the optical glass has a density within that range, the user can have a comfortable wear feeling when the glass is used in wearable appliances and the overall apparatus weight can be reduced when the glass is used in in-vehicle cameras, visual sensors for robots, etc. The density (d) of the glass is preferably 5.2 g/cm 3  or less, more preferably 5.0 g/cm 3  or less, still more preferably 4.6 g/cm 3  or less, still more preferably 4.2 g/cm 3  or less, still more preferably 4.1 g/cm 3  or less, still more preferably 4.0 g/cm 3  or less. 
     Meanwhile, from the standpoint of making the surfaces of the optical glass less apt to be scratched, the density (d) of the glass is preferably 3.6 g/cm 3  or higher. The density (d) of the glass is more preferably 3.7 g/cm 3  or higher, still more preferably 3.8 g/cm 3  or higher, still more preferably 3.9 g/cm 3  or higher, especially preferably 3.95 g/cm 3  or higher. 
     Furthermore, the optical glass has a devitrification temperature of 1,300° C. or lower. The glass having this property can be inhibited from being devitrified when formed and has satisfactory formability. The devitrification temperature of the glass is more preferably 1,275° C. or lower, still more preferably 1,240° C. or lower, yet still more preferably 1,225° C. or lower, yet still more preferably 1,200° C. or lower, yet still more preferably 1,175° C. or lower, yet still more preferably 1,150° C. or lower, yet still more preferably 1,100° C. or lower, yet still more preferably 1,050° C. or lower, yet still more preferably 1,025° C. or lower, especially preferably 1,020° C. or lower. The devitrification temperature is a lowest temperature at which no crystals of 1 μm or larger in terms of major-side or major-axis dimension are observed in the surface and inside of the glass when the glass which has been heated and melted is allowed to cool naturally. 
     [Methods for producing Optical Glass and Formed glass Objects] 
     An optical glass of the present invention is produced, for example, in the following manner. First, given amounts of raw materials are weighed out so as to result in any of the predetermined glass compositions shown above, and are evenly mixed. The obtained mixture is charged into a platinum crucible, quartz crucible, or alumina crucible and roughly melted. Thereafter, this mixture is introduced into a gold crucible, platinum crucible, platinum alloy crucible, reinforced platinum crucible, or iridium crucible, melted by heating at a temperature in the range of 1,200° C. to 1,400° C. for 2 to 10 hours, homogenized by defoaming, stirring, etc., and then poured into a mold and gradually cooled. Thus, the optical glass of the present invention is obtained. 
     This optical glass can be obtained also as a glass plate by forming the molten glass into a plate shape by a forming method such as a float process, a fusion process, or a roll-out process. Alternatively, the molten glass may be temporarily formed into a block shape and then formed into a glass plate by a redrawing process, etc. Furthermore, a method such as, for example, reheating press forming or precision press forming can be used to produce a formed glass object. Specifically, use can be made of: a method in which a lens preform for mold press forming is produced from the optical glass and this lens preform is subjected to reheating press forming and then polishing to produce a formed glass object; or a method in which a lens preform produced through, for example, polishing is subjected to precision press forming to produce a formed glass object. Methods for producing formed glass objects are not limited to those shown above. 
     The number of residual bubbles present in the thus-produced optical glass of the present invention per kg is preferably 10 bubbles per kg or less, more preferably 7 bubbles per kg or less, still more preferably 5 bubbles per kg or less, especially preferably 3 bubbles per kg or less. In the case where glass plates are to be produced by the methods shown above, bubble-free glass plates can be efficiently formed so long as the number of residual bubbles is 10 or less per kg. In the case where the diameter of a minimum circle in which a residual bubble is included is taken as the size of the residual bubble, the size of each residual bubble is preferably 80 μm or less, more preferably 60 μm or less, still more preferably 40 μm or less, especially preferably 20 μm or less. 
     Furthermore, in the case where the diameter is regarded as the longitudinal-direction length L 1  of the residual bubble and where the length of a straight line which corresponds to the maximum dimension of the residual bubble in the direction perpendicular to the diameter is regarded as the transverse-direction length L 2  of the residual bubble, in terms of the shape of the residual bubble, the aspect ratio L 2 /L 1  is preferably 0.90 or larger, more preferably 0.92 or larger, still more preferably 0.95 or larger. So long as the L 2 /L 1  is 0.90 or larger, the shape of the residual bubbles is close to a complete circle (complete sphere). Even if such residual bubbles are contained, these bubbles are less apt to reduce the strength of the glass as compared with elliptic residual bubbles, and cracks originating from residual bubbles can be inhibited from occurring when glass plates are produced from the glass. In addition, such residual bubbles in a glass substrate have the effect of being less causative of the anisotropic scattering of light which has entered the glass plate, as compared with elliptic residual bubbles. The size and shape of residual bubbles are obtained from values measured with a laser microscope (VK-X100, manufactured by Keyence Corp.). 
     Optical members such as the glass plates and formed glass objects thus produced are useful as or in various optical elements. In particular, such optical members are suitable as (1) lightguides, filters, lenses, and the like for use in wearable appliances, e.g., spectacles equipped with a projector, spectacle- or goggle-type displays, virtual-reality or augmented-reality display devices, and virtual-image display devices, and (2) lenses, cover glasses, and the like for use in in-vehicle cameras and visual sensors for robots. The optical members are suitable for use even in applications where the optical members are exposed to severe environments, as in in-vehicle cameras. Furthermore, the optical members are suitable also in applications such as glass substrates for organic EL, substrates for wafer level lens arrays, substrates for lens units, substrates for lens formation by etching, and optical waveguides. 
     The optical glass according to this embodiment described above has a high refractive index and a low density, has satisfactory producibility, and is suitable for use as an optical glass for wearable appliances and for mounting on vehicles and robots. Moreover, an optical component obtained by coating a main surface of this optical glass with an antireflection film constituted of a multilayered dielectric film including 4 to 10 layers formed by alternately depositing a low-refractive index film of, for example, SiO 2 , and a high-refractive index film of, for example, TiO 2  is also suitable for use in wearable appliances and for mounting on vehicles and robots. 
     Examples 
     Raw materials were weighed out so as to result in each of the chemical compositions (in mol % based on oxides) shown in Tables 1 to 9. As the raw materials, high-purity raw materials for ordinary optical glasses, such as oxides, hydroxides, carbonates, nitrates, fluorides, hydroxides, and metaphosphoric acid compounds, which corresponded to the respective components were selected and used. 
     The weighed raw materials were evenly mixed, and the mixture was put into a platinum crucible having a capacity of about 300 mL and melted, refined, and stirred at about 1,300° C. for about 2 hours. Thereafter, the melt was held at 1,300° C. for 0.5 hours, poured into a rectangular mold having dimensions of 50 mm (length)×100 mm (width) preheated at about 650° C., and then gradually cooled at about 1° C./min. Thus, samples of Examples 1 to 85 were obtained, in which Examples 1 to 80 are examples according to the present invention and Examples 81 to 85 are comparative examples. 
     [Evaluation] 
     The samples obtained above were examined for refractive index (n d ), density (d), devitrification temperature, and viscosity (temperature T 1  at which the glass has a viscosity of 10 1  dPa·s) in the following manners. The results obtained are shown together in Tables 1 to 9. 
     Refractive index (n d ): Each sample glass was processed into a triangular prism shape which had a thickness of 10 mm and in which each side had a length of 30 mm, and this prism was examined with a refractometer (device name, KPR-2000; manufactured by Kalnew Corp.). 
     Density (d): Measurement was made in accordance with JIS Z8807 (1976; a measuring method of weighing performed in liquid). 
     Devitrification temperature: About 5 g of each sample was put on a platinum dish, held for 1 hour at temperatures ranging from 1,000° C. to 1,400° C. at intervals of 5° C., and then allowed to cool naturally. Thereafter, the sample was observed with a microscope to confirm whether precipitation of crystals occurred or not. The minimum temperature which had not resulted in the precipitation of crystals of 1 μm or larger in terms of major-side or major-axis dimension was taken as the devitrification temperature. 
     Temperature T 1 : The glass was examined for viscosity with a rotational viscometer in accordance with the method provided for in ASTM C 965-96, and the temperature T 1  (° C.) at which the glass had a viscosity of 10 1  dPa·s was measured. 
     Glass transition point (Tg): A value measured with a differential dilatometer (TMA); the value was determined in accordance with JIS R3101-3 (year 2001). 
     Young&#39;s modulus (E): A plate-shaped sample having dimensions of 20 mm×20 mm×1 mm was examined for Young&#39;s modulus (unit: GPa) with an ultrasonic precision thickness meter (MODEL 38DL PLUS, manufactured by OLYMPUS Co., Ltd.). 
     LTV: A glass substrate was examined for thickness by examining a plate-shaped sample thereof having dimensions of 50 mm×50 mm×1 mm at intervals of 3 mm with a non-contact laser displacement meter (NANOMETRO, manufactured by KURODA Precision Industries). The LTV was calculated therefrom. 
     Warpage: The heights of the two main surfaces of a glass substrate were measured by examining a disk-shaped sample having dimensions of 8 inches (diameter)×1 mm and a disk-shaped sample having dimensions of 6 inches (diameter)×1 mm with the non-contact laser displacement meter (NANOMETRO, manufactured by KURODA Precision Industries) at intervals of 3 mm. The warpage was calculated by the method explained above with reference to the FIGURE. 
     Surface roughness (Ra): A value determined through an examination of a 10 μm×10 μm area of a plate-shaped sample having dimensions of 20 mm×20 mm×1 mm with an atomic force microscope (AFM) (manufactured by Oxford Instruments). 
     Abbe number (ν d ): The sample used in the refractive index measurement is used, and the Abbe number is calculated using ν d =(n d −1)/(n F −n C ), where n d , n F , n C  are refractive indexes for helium d line, hydrogen F line, and hydrogen C line, respectively. These refractive indexes also were measured with the refractometer. 
     Coefficient of thermal expansion (α): Using a differential dilatometer (TMA), coefficients of thermal expansion were measured at temperatures in the range of 50° C. to 350° C. An average coefficient of thermal expansion for the range of 50° C. to 350° C. was determined in accordance with JIS R3102 (1995). 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
               
               
                   
                 ple 1 
                 ple 2 
                 ple 3 
                 ple 4 
                 ple 5 
                 ple 6 
                 ple 7 
                 ple 8 
                 ple 9 
                 ple 10 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 P 2 O 5   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 SiO 2   
                 26.0 
                 22.0 
                 23.0 
                 26.0 
                 26.0 
                 26.0 
                 30.0 
                 30.0 
                 30.0 
                 30.0 
               
               
                 B 2 O 3   
                 14.0 
                 18.0 
                 14.0 
                 14.0 
                 10.0 
                 10.0 
                 10.0 
                 10.0 
                 10.0 
                 10.0 
               
               
                 MgO 
               
               
                 CaO 
                 16.0 
                 16.0 
                 18.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 7.0 
                 5.0 
                 7.0 
               
               
                 SrO 
                   
                   
                   
                   
                   
                 3.0 
                   
                 7.0 
                 7.0 
                 7.0 
               
               
                 BaO 
                 7.0 
                 7.0 
                 7.0 
                 7.0 
                 11.0 
                 5.0 
                 7.0 
                 9.0 
                 7.0 
                 11.0 
               
               
                 Li 2 O 
                 2.0 
                 2.0 
                 3.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
               
               
                 Na 2 O 
               
               
                 K 2 O 
               
               
                 Al 2 O 3   
               
               
                 Y 2 O 3   
               
               
                 TiO 2   
                 19.0 
                 19.0 
                 15.0 
                 19.0 
                 19.0 
                 19.0 
                 19.0 
                 19.0 
                 19.0 
                 19.0 
               
               
                 Ta 2 O 5   
                   
                   
                   
                   
                 2.0 
               
               
                 WO 3   
               
               
                 Nb 2 O 5   
                 5.0 
                 5.0 
                 6.0 
                 5.0 
                 3.0 
                 5.0 
                 5.0 
                 5.0 
                 5.0 
                 5.0 
               
               
                 La 2 O 3   
                 9.0 
                 9.0 
                 12.0 
                 5.0 
                 9.0 
                 9.0 
                 9.0 
                 9.0 
                 9.0 
                 9.0 
               
               
                 ZrO 2   
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
               
               
                 Bi 2 O 3   
               
               
                 ZnO 
                   
                   
                   
                   
                   
                 3.0 
                   
                   
                 4.0 
               
               
                 Gd 2 O 3   
                   
                   
                   
                 4.0 
               
               
                 Sum 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 SiO 2  + Bi 2 O 3   
                 40.0 
                 40.0 
                 37.0 
                 40.0 
                 36.0 
                 36.0 
                 40.0 
                 40.0 
                 40.0 
                 40.0 
               
               
                 TiO 2  + Ta 2 O 5  + WO 3  + Nb 2 O 5  + 
                 33.0 
                 33.0 
                 33.0 
                 33.0 
                 35.0 
                 35.0 
                 35.0 
                 35.0 
                 35.0 
                 35.0 
               
               
                 ZrO 2  + Ln 2 O 3   
               
               
                 BaO/(MgO + CaO + SrO + BaO) 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.4 
                 0.2 
                 0.3 
                 0.4 
                 0.4 
                 0.4 
               
               
                 SiO 2 /B 2 O 3   
                 1.9 
                 1.2 
                 1.6 
                 1.9 
                 2.6 
                 2.6 
                 3.0 
                 3.0 
                 3.0 
                 3.0 
               
               
                 Li 2 O + Na 2 O + K 2 O 
                 2.0 
                 2.0 
                 3.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 0.0 
               
               
                 Li 2 O + Na 2 O 
                 2.0 
                 2.0 
                 3.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 0.0 
               
               
                 MgO + CaO + SrO 
                 16.0 
                 16.0 
                 18.0 
                 16.0 
                 16.0 
                 19.0 
                 16.0 
                 14.0 
                 12.0 
                 14.0 
               
               
                 Li 2 O + Na 2 O + K 2 O + MgO + CaO + 
                 25.0 
                 25.0 
                 28.0 
                 25.0 
                 29.0 
                 26.0 
                 25.0 
                 25.0 
                 21.0 
                 25.0 
               
               
                 SrO + BaO 
               
               
                 TiO 2 /B 2 O 3   
                 1.4 
                 1.1 
                 1.1 
                 1.4 
                 1.9 
                 1.9 
                 1.9 
                 1.9 
                 1.9 
                 1.9 
               
               
                 TiO 2  + WO 3  + Nb 2 O 5   
                 24.0 
                 24.0 
                 21.0 
                 24.0 
                 22.0 
                 24.0 
                 24.0 
                 24.0 
                 24.0 
                 24.0 
               
               
                 (ZrO 2  + Ta 2 O 5  + Nb 2 O 5 )/(SiO 2  + B 2 O 3 ) 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
               
               
                 (Nb 2 O 5  + TiO 2  + WO 3  + Ta 2 O 5 )/ 
                 2.7 
                 2.7 
                 1.8 
                 2.7 
                 2.7 
                 2.7 
                 2.7 
                 2.7 
                 2.7 
                 2.7 
               
               
                 (La 2 O 3  + Gd 2 O 3  + Y 2 O 3  + Yb 2 O 3 ) 
               
               
                 MgO + CaO + SrO + BaO 
                 23.00 
                 23.00 
                 25.00 
                 23.00 
                 27.00 
                 24.00 
                 23.00 
                 23.00 
                 19.00 
                 25.00 
               
               
                 Refractive index (nd) 
                 1.88 
                 1.88 
                 1.89 
                 1.88 
                 1.89 
                 1.89 
                 1.88 
                 1.88 
                 1.89 
                 1.88 
               
               
                 Density (d) [g/cm 3 ] 
                 4.1 
                 4.1 
                 4.3 
                 4.2 
                 4.3 
                 4.2 
                 4.2 
                 4.3 
                 4.3 
                 4.3 
               
               
                 Devitrification temperature [° C.] 
                 1125 
                 1125 
                 1150 
                 1150 
                 1150 
                 1150 
                 1150 
                 1150 
                 1150 
                 1150 
               
               
                 Temperature T1 [° C.] 
                 1086 
                 1057 
                 1080 
                 1098 
                 1103 
                 1097 
                 1123 
                 1123 
                 1116 
                 1144 
               
               
                 Devitrification viscosity 
                 0.7 
                 0.4 
                 0.3 
                 0.5 
                 0.6 
                 0.5 
                 0.8 
                 0.8 
                 0.7 
                 1.0 
               
               
                 Light transmittance (T360) [%] 
               
               
                 Glass transition point (Tg) [° C.] 
                 653 
                 645 
                 647 
                 664 
                 650 
                 654 
                 662 
                 660 
                 658 
                 671 
               
               
                 Young&#39;s modulus (E) [Gpa] 
               
               
                 Abbe number (νd) 
                 30 
                 30 
                 32 
                 30 
                 30 
                 30 
                 30 
                 30 
                 30 
                 30 
               
               
                 Coefficient of thermal expansion α 
                 87 
                 87 
                 95 
                 87 
                 93 
                 88 
                 87 
                 89 
                 86 
                 88 
               
               
                 (50-350° C.)[×10 −7 /K] 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
               
               
                   
                 ple 11 
                 ple 12 
                 ple 13 
                 ple 14 
                 ple 15 
                 ple 16 
                 ple 17 
                 ple 18 
                 ple 19 
                 ple 20 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 P 2 O 5   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 SiO 2   
                 30.0 
                 30.0 
                 30.0 
                 30.0 
                 30.0 
                 30.0 
                 30.0 
                 27.0 
                 30.0 
                 30.0 
               
               
                 B 2 O 3   
                 10.0 
                 10.0 
                 10.0 
                 10.0 
                 5.0 
                 8.0 
                 8.0 
                 10.0 
                 7.2 
                 8.0 
               
               
                 MgO 
               
               
                 CaO 
                 7.0 
                 7.0 
                 7.0 
                 14.2 
                 17.0 
                 6.4 
                 6.4 
                 7.4 
                 6.4 
                 6.4 
               
               
                 SrO 
                 7.0 
                 7.0 
                 7.0 
                   
                   
                 7.6 
                 7.6 
                 7.7 
                 7.6 
                 7.6 
               
               
                 BaO 
                 9.0 
                 9.0 
                 9.0 
                 5.8 
                 8.0 
                 9.0 
                 8.8 
                 8.9 
                 9.0 
                 8.8 
               
               
                 Li 2 O 
                 2.0 
                 2.0 
                 2.0 
                 5.0 
                 5.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
               
               
                 Na 2 O 
               
               
                 K 2 O 
               
               
                 Al 2 O 3   
               
               
                 Y 2 O 3   
               
               
                 TiO 2   
                 19.0 
                 21.0 
                 21.0 
                 21.0 
                 21.0 
                 22.0 
                 20.9 
                 20.5 
                 21.2 
                 20.9 
               
               
                 Ta 2 O 5   
               
               
                 WO 3   
               
               
                 Nb 2 O 5   
                 3.0 
                 5.0 
                 3.0 
                 5.0 
                 5.0 
                 5.5 
                 6.3 
                 6.5 
                 6.0 
                 5.8 
               
               
                 La 2 O 3   
                 9.0 
                 7.0 
                 9.0 
                 7.0 
                 7.0 
                 7.5 
                 6.8 
                 6.0 
                 6.1 
                 6.8 
               
               
                 ZrO 2   
                 4.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 3.2 
                 4.0 
                 3.0 
                 3.7 
               
               
                 Bi 2 O 3   
               
               
                 ZnO 
               
               
                 Gd 2 O 3   
                   
                   
                   
                   
                   
                   
                   
                   
                 1.5 
               
               
                 Sum 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 SiO 2  + Bi 2 O 3   
                 40.0 
                 40.0 
                 40.0 
                 40.0 
                 35.0 
                 38.0 
                 38.0 
                 37.0 
                 37.2 
                 38.0 
               
               
                 TiO 2  + Ta 2 O 5  + WO 3  + Nb 2 O 5  + 
                 35.0 
                 35.0 
                 35.0 
                 35.0 
                 35.0 
                 37.0 
                 37.2 
                 37.0 
                 37.8 
                 37.2 
               
               
                 ZrO 2  + Ln 2 O 3   
               
               
                 BaO/(MgO + CaO + SrO + BaO) 
                 0.4 
                 0.4 
                 0.4 
                 0.3 
                 0.3 
                 0.4 
                 0.4 
                 0.4 
                 0.4 
                 0.4 
               
               
                 SiO 2 /B 2 O 3   
                 3.0 
                 3.0 
                 3.0 
                 3.0 
                 6.0 
                 3.8 
                 3.8 
                 2.7 
                 4.2 
                 3.8 
               
               
                 Li 2 O + Na 2 O + K 2 O 
                 2.0 
                 2.0 
                 2.0 
                 5.0 
                 5.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
               
               
                 Li 2 O + Na 2 O 
                 2.0 
                 2.0 
                 2.0 
                 5.0 
                 5.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
               
               
                 MgO + CaO + SrO 
                 14.0 
                 14.0 
                 14.0 
                 14.2 
                 17.0 
                 14.0 
                 14.0 
                 15.1 
                 14.0 
                 14.0 
               
               
                 Li 2 O + Na 2 O + K 2 O + MgO + CaO + 
                 25.0 
                 25.0 
                 25.0 
                 25.0 
                 30.0 
                 25.0 
                 24.8 
                 26.0 
                 25.0 
                 24.8 
               
               
                 SrO + BaO 
               
               
                 TiO 2 /B 2 O 3   
                 1.9 
                 2.1 
                 2.1 
                 2.1 
                 4.2 
                 2.8 
                 2.6 
                 2.1 
                 2.9 
                 2.6 
               
               
                 TiO 2  + WO 3  + Nb 2 O 5   
                 22.0 
                 26.0 
                 24.0 
                 26.0 
                 26.0 
                 27.5 
                 27.2 
                 27.0 
                 27.2 
                 26.7 
               
               
                 (ZrO 2  + Ta 2 O 5  + Nb 2 O 5 )/(SiO 2  + B 2 O 3 ) 
                 0.2 
                 0.2 
                 0.1 
                 0.2 
                 0.2 
                 0.2 
                 0.3 
                 0.3 
                 0.2 
                 0.3 
               
               
                 (Nb 2 O 5  + TiO 2  + WO 3  + Ta 2 O 5 )/ 
                 2.4 
                 3.7 
                 2.7 
                 3.7 
                 3.7 
                 3.7 
                 4.0 
                 4.5 
                 3.6 
                 3.9 
               
               
                 (La 2 O 3  + Gd 2 O 3  + Y 2 O 3  + Yb 2 O 3 ) 
               
               
                 MgO + CaO + SrO + BaO 
                 23.00 
                 23.00 
                 23.00 
                 20.00 
                 25.00 
                 23.00 
                 22.80 
                 24.00 
                 23.00 
                 22.80 
               
               
                 Refractive index (nd) 
                 1.87 
                 1.88 
                 1.87 
                 1.89 
                 1.89 
                 1.89 
                 1.90 
                 1.90 
                 1.91 
                 1.90 
               
               
                 Density (d) [g/cm 3 ] 
                 4.3 
                 4.2 
                 4.3 
                 4.0 
                 4.1 
                 4.2 
                 4.2 
                 4.2 
                 4.2 
                 4.2 
               
               
                 Devitrification temperature [° C.] 
                 1150 
                 1100 
                 1150 
                 1135 
                 1135 
                 1180 
                 1150 
                 1165 
                 1160 
                 1160 
               
               
                 Temperature T1 [° C.] 
                 1137 
                 1113 
                 1126 
                 1092 
                 1110 
                 1127 
                 1129 
                 1109 
                 1137 
                 1136 
               
               
                 Devitrification viscosity 
                 0.9 
                 1.1 
                 0.8 
                 0.6 
                 0.8 
                 0.6 
                 0.8 
                 0.5 
                 0.8 
                 0.8 
               
               
                 Light transmittance (T360) [%] 
               
               
                 Glass transition point (Tg) [° C.] 
                 658 
                 658 
                 654 
                 640 
                 639 
                 663 
                 667 
                 662 
                 672 
                 667 
               
               
                 Young&#39;s modulus (E) [Gpa] 
               
               
                 Abbe number (νd) 
                 31 
                 29 
                 31 
                 32 
                 32 
                 30 
                 35 
                 28 
                 27 
                 30 
               
               
                 Coefficient of thermal expansion α 
                 91 
                 86 
                 91 
                 87 
                 92 
                 88 
                 86 
                 85 
                 88 
                 86 
               
               
                 (50-350° C.)[×10 −7 /K] 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
               
               
                   
                 ple 21 
                 ple 22 
                 ple 23 
                 ple 24 
                 ple 25 
                 ple 26 
                 ple 27 
                 ple 28 
                 ple 29 
                 ple 30 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 P 2 O 5   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 SiO 2   
                 11.2 
                 11.2 
                 11.2 
                 11.2 
                 11.2 
                 11.2 
                 11.2 
                 11.2 
                 9.0 
                 10.4 
               
               
                 B 2 O 3   
                 21.2 
                 22.2 
                 21.2 
                 20.9 
                 20.9 
                 20.9 
                 20.9 
                 20.9 
                 23.0 
                 19.7 
               
               
                 MgO 
               
               
                 CaO 
               
               
                 SrO 
               
               
                 BaO 
               
               
                 Li 2 O 
                 3.7 
                 3.7 
                 3.7 
                   
                   
                   
                   
                   
                   
                 2.0 
               
               
                 Na 2 O 
               
               
                 K 2 O 
               
               
                 Al 2 O 3   
               
               
                 Y 2 O 3   
                   
                   
                   
                 5.8 
                 3.5 
                 3.4 
                 6.0 
                 3.4 
                 5.8 
                 3.4 
               
               
                 TiO 2   
                 29.5 
                 27.0 
                 29.5 
                 31.1 
                 31.1 
                 31.1 
                 26.1 
                 33.2 
                 31.1 
                 33.2 
               
               
                 Ta 2 O 5   
               
               
                 WO 3   
               
               
                 Nb 2 O 5   
                 3.0 
                 6.0 
                 4.5 
                 2.6 
                 2.6 
                 2.6 
                 3.3 
                 2.6 
                 2.6 
                 2.6 
               
               
                 La 2 O 3   
                 19.6 
                 18.6 
                 18.6 
                 17.6 
                 19.2 
                 20.6 
                 24.3 
                 18.4 
                 21.6 
                 18.4 
               
               
                 ZrO 2   
                 6.5 
                 6.5 
                 6.5 
                 6.9 
                 6.9 
                 6.9 
                 7.0 
                 6.9 
                 6.9 
                 6.9 
               
               
                 Bi 2 O 3   
               
               
                 ZnO 
               
               
                 Gd 2 O 3   
                 5.3 
                 4.8 
                 4.8 
                 4.0 
                 4.6 
                 3.4 
                 1.2 
                 3.4 
                   
                 3.4 
               
               
                 Sum 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 SiO 2  + Bi 2 O 3   
                 32.4 
                 33.4 
                 32.4 
                 32.1 
                 32.1 
                 32.1 
                 32.1 
                 32.1 
                 32.0 
                 30.1 
               
               
                 TiO 2  + Ta 2 O 5  + WO 3  + Nb 2 O 5  + 
                 63.9 
                 62.9 
                 63.9 
                 67.9 
                 67.9 
                 67.9 
                 67.9 
                 67.9 
                 68.0 
                 67.9 
               
               
                 ZrO 2  + Ln 2 O 3   
               
               
                 BaO/(MgO + CaO + SrO + BaO) 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 SiO 2 /B 2 O 3   
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.4 
                 0.5 
               
               
                 Li 2 O + Na 2 O + K 2 O 
                 3.7 
                 3.7 
                 3.7 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 2.0 
               
               
                 Li 2 O + Na 2 O 
                 3.7 
                 3.7 
                 3.7 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 2.0 
               
               
                 MgO + CaO + SrO 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Li 2 O + Na 2 O + K 2 O + MgO + CaO + 
                 3.7 
                 3.7 
                 3.7 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 2.0 
               
               
                 SrO + BaO 
               
               
                 TiO 2 /B 2 O 3   
                 1.4 
                 1.2 
                 1.4 
                 1.5 
                 1.5 
                 1.5 
                 1.2 
                 1.6 
                 1.4 
                 1.7 
               
               
                 TiO 2  + WO 3  + Nb 2 O 5   
                 32.5 
                 33.0 
                 34.0 
                 33.6 
                 33.7 
                 33.7 
                 29.4 
                 35.8 
                 33.7 
                 35.8 
               
               
                 (ZrO 2  + Ta 2 O 5  + Nb 2 O 5 )/(SiO 2  + B 2 O 3 ) 
                 0.3 
                 0.4 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
               
               
                 (Nb 2 O 5  + TiO 2  + WO 3  + Ta 2 O 5 )/ 
                 1.3 
                 1.4 
                 1.5 
                 1.2 
                 1.2 
                 1.2 
                 0.9 
                 1.4 
                 1.2 
                 1.4 
               
               
                 (La 2 O 3  + Gd 2 O 3  + Y 2 O 3  + Yb 2 O 3 ) 
               
               
                 MgO + CaO + SrO + BaO 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 Refractive index (nd) 
                 1.99 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
                 2.01 
                 1.99 
                 2.01 
                 2.00 
                 2.01 
               
               
                 Density (d) [g/cm 3 ] 
                 5.0 
                 4.9 
                 4.9 
                 5.0 
                 5.0 
                 5.0 
                 5.1 
                 4.9 
                 4.9 
                 4.9 
               
               
                 Devitrification temperature [° C.] 
                 1140 
                 1200 
                 1175 
                 1200 
                 1200 
                 1160 
                 1200 
                 1140 
                 1160 
                 1200 
               
               
                 Temperature T1 [° C.] 
                 1082 
                 1075 
                 1070 
                 1121 
                 1115 
                 1118 
                 1137 
                 1109 
                 1102 
                 1100 
               
               
                 Devitrification viscosity 
                 0.7 
                 0.4 
                 0.4 
                 0.4 
                 0.5 
                 0.7 
                 0.5 
                 0.8 
                 0.6 
                 0.4 
               
               
                 Light transmittance (T360) [%] 
               
               
                 Glass transition point (Tg) [° C.] 
                 694 
                 698 
                 695 
                 732 
                 730 
                 727 
                 733 
                 721 
                 717 
                 710 
               
               
                 Young&#39;s modulus (E) [Gpa] 
               
               
                 Abbe number (νd) 
                 28 
                 28 
                 28 
                 28 
                 28 
                 28 
                 30 
                 27 
                 28 
                 27 
               
               
                 Coefficient of thermal expansion α 
                 106 
                 101 
                 103 
                 92 
                 97 
                 97 
                 98 
                 94 
                 92 
                 98 
               
               
                 (50-350° C.)[×10 −7 /K] 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
               
               
                   
                 ple 31 
                 ple 32 
                 ple 33 
                 ple 34 
                 ple 35 
                 ple 36 
                 ple 37 
                 ple 38 
                 ple 39 
                 ple 40 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 P 2 O 5   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 SiO 2   
                 10.4 
                 11.2 
                 11.2 
                 11.2 
                 11.2 
                 11.2 
                 11.2 
                 11.2 
                 11.2 
                 11.2 
               
               
                 B 2 O 3   
                 19.7 
                 20.9 
                 20.9 
                 20.9 
                 20.9 
                 20.9 
                 20.9 
                 20.9 
                 20.9 
                 20.9 
               
               
                 MgO 
               
               
                 CaO 
                 2.0 
               
               
                 SrO 
               
               
                 BaO 
               
               
                 Li 2 O 
               
               
                 Na 2 O 
               
               
                 K 2 O 
               
               
                 Al 2 O 3   
               
               
                 Y 2 O 3   
                 3.4 
                 4.0 
                 3.4 
                 4.0 
                 3.4 
                 4.4 
                 4.2 
                 3.4 
                 3.4 
                 3.7 
               
               
                 TiO 2   
                 33.2 
                 31.8 
                 32.0 
                 33.2 
                 33.2 
                 32.3 
                 33.2 
                 33.2 
                 31.7 
                 32.9 
               
               
                 Ta 2 O 5   
               
               
                 WO 3   
               
               
                 Nb 2 O 5   
                 2.6 
                 3.4 
                 3.8 
                 3.4 
                 3.8 
                 2.6 
                 2.6 
                 2.6 
                 2.6 
                 2.6 
               
               
                 La 2 O 3   
                 18.4 
                 18.4 
                 18.4 
                 17.0 
                 17.2 
                 18.4 
                 17.7 
                 17.8 
                 18.4 
                 18.2 
               
               
                 ZrO 2   
                 6.9 
                 6.9 
                 6.9 
                 6.9 
                 6.9 
                 6.9 
                 6.9 
                 6.9 
                 8.4 
                 7.2 
               
               
                 Bi 2 O 3   
               
               
                 ZnO 
               
               
                 Gd 2 O 3   
                 3.4 
                 3.4 
                 3.4 
                 3.4 
                 3.4 
                 3.3 
                 3.3 
                 4.0 
                 3.4 
                 3.3 
               
               
                 Sum 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 SiO 2  + Bi 2 O 3   
                 30.1 
                 32.1 
                 32.1 
                 32.1 
                 32.1 
                 32.1 
                 32.1 
                 32.1 
                 32.1 
                 32.1 
               
               
                 TiO 2  + Ta 2 O 5  + WO 3  + Nb 2 O 5  + 
                 67.9 
                 67.9 
                 67.9 
                 67.9 
                 67.9 
                 67.9 
                 67.9 
                 67.9 
                 67.9 
                 67.9 
               
               
                 ZrO 2  + Ln 2 O 3   
               
               
                 BaO/(MgO + CaO + SrO + BaO) 
                 0.0 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 SiO 2 /B 2 O 3   
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
               
               
                 Li 2 O + Na 2 O + K 2 O 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Li 2 O + Na 2 O 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 MgO + CaO + SrO 
                 2.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Li 2 O + Na 2 O + K 2 O + MgO + CaO + 
                 2.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 SrO + BaO 
               
               
                 TiO 2 /B 2 O 3   
                 1.7 
                 1.5 
                 1.5 
                 1.6 
                 1.6 
                 1.5 
                 1.6 
                 1.6 
                 1.5 
                 1.6 
               
               
                 TiO 2  + WO 3  + Nb 2 O 5   
                 35.8 
                 35.2 
                 35.8 
                 36.6 
                 37.0 
                 34.9 
                 35.8 
                 35.8 
                 34.3 
                 35.5 
               
               
                 (ZrO 2  + Ta 2 O 5  + Nb 2 O 5 )/(SiO 2  + B 2 O 3 ) 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
               
               
                 (Nb 2 O 5  + TiO 2  + WO 3  + Ta 2 O 5 )/ 
                 1.4 
                 1.4 
                 1.4 
                 1.5 
                 1.5 
                 1.3 
                 1.4 
                 1.4 
                 1.4 
                 1.4 
               
               
                 (La 2 O 3  + Gd 2 O 3  + Y 2 O 3  + Yb 2 O 3 ) 
               
               
                 MgO + CaO + SrO + BaO 
                 2.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 Refractive index (nd) 
                 2.02 
                 2.01 
                 2.01 
                 2.01 
                 2.01 
                 2.01 
                 2.01 
                 2.01 
                 2.01 
                 2.01 
               
               
                 Density (d) [g/cm 3 ] 
                 5.0 
                 4.9 
                 4.9 
                 4.9 
                 4.9 
                 4.9 
                 4.9 
                 4.9 
                 4.9 
                 4.9 
               
               
                 Devitrification temperature [° C.] 
                 1200 
                 1200 
                 1200 
                 1200 
                 1200 
                 1150 
                 1150 
                 1150 
                 1200 
                 1140 
               
               
                 Temperature T1 [° C.] 
                 1105 
                 1105 
                 1107 
                 1096 
                 1098 
                 1110 
                 1105 
                 1105 
                 1102 
                 1101 
               
               
                 Devitrification viscosity 
                 0.4 
                 0.5 
                 0.5 
                 0.4 
                 0.4 
                 0.7 
                 0.6 
                 0.8 
                 0.4 
                 0.7 
               
               
                 Light transmittance (T360) [%] 
               
               
                 Glass transition point (Tg) [° C.] 
                 723 
                 717 
                 717 
                 716 
                 715 
                 716 
                 714 
                 715 
                 716 
                 714 
               
               
                 Young&#39;s modulus (E) [Gpa] 
               
               
                 Abbe number (νd) 
                 27 
                 28 
                 27 
                 27 
                 27 
                 28 
                 27 
                 27 
                 28 
                 27 
               
               
                 Coefficient of thermal expansion α 
                 96 
                 93 
                 93 
                 91 
                 91 
                 93 
                 92 
                 94 
                 94 
                 93 
               
               
                 (50-350° C.)[×10 −7 /K] 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
               
               
                   
                 ple 41 
                 ple 42 
                 ple 43 
                 ple 44 
                 ple 45 
                 ple 46 
                 ple 47 
                 ple 48 
                 ple 49 
                 ple 50 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 P 2 O 5   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 SiO 2   
                 11.2 
                 12.2 
                 12.2 
                 14.0 
                 14.0 
                 14.0 
                 14.0 
                 14.0 
                 14.0 
                 14.0 
               
               
                 B 2 O 3   
                 21.2 
                 20.9 
                 20.9 
                 22.0 
                 22.0 
                 22.0 
                 22.0 
                 22.0 
                 22.0 
                 22.0 
               
               
                 MgO 
               
               
                 CaO 
                   
                   
                   
                 11.0 
                 11.0 
                 11.0 
                 11.0 
                 11.5 
                 14.5 
                 12.5 
               
               
                 SrO 
                   
                   
                   
                 11.0 
                 11.0 
                 11.0 
                 11.0 
                 11.0 
                 8.7 
                 9.0 
               
               
                 BaO 
                   
                   
                   
                 8.0 
                 8.0 
                 8.0 
                 8.0 
                 8.7 
                 8.0 
                 10.0 
               
               
                 Li 2 O 
                   
                   
                   
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
               
               
                 Na 2 O 
               
               
                 K 2 O 
               
               
                 Al 2 O 3   
               
               
                 Y 2 O 3   
                 3.7 
                 3.7 
                 3.3 
               
               
                 TiO 2   
                 32.9 
                 32.6 
                 32.6 
                 15.0 
                 18.0 
                 21.0 
                 15.0 
                 20.0 
                 20.0 
                 21.0 
               
               
                 Ta 2 O 5   
               
               
                 WO 3   
               
               
                 Nb 2 O 5   
                 2.6 
                 2.6 
                 2.6 
                 6.0 
                 3.0 
                 3.0 
                 3.0 
                 2.8 
                 2.8 
                 2.7 
               
               
                 La 2 O 3   
                 18.2 
                 18.2 
                 18.2 
                 8.0 
                 8.0 
                 5.0 
                 8.0 
                 5.0 
                 5.0 
                 3.6 
               
               
                 ZrO 2   
                 6.9 
                 6.5 
                 6.9 
                 3.0 
                 3.0 
                 3.0 
                 6.0 
                 3.0 
                 3.0 
                 3.2 
               
               
                 Bi 2 O 3   
               
               
                 ZnO 
               
               
                 Gd 2 O 3   
                 3.3 
                 3.3 
                 3.3 
               
               
                 Sum 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 SiO 2  + Bi 2 O 3   
                 32.4 
                 33.1 
                 33.1 
                 36.0 
                 36.0 
                 36.0 
                 36.0 
                 36.0 
                 36.0 
                 36.0 
               
               
                 TiO 2  + Ta 2 O 5  + WO 3  + Nb 2 O 5  + 
                 67.6 
                 66.9 
                 66.9 
                 32.0 
                 32.0 
                 32.0 
                 32.0 
                 30.8 
                 30.8 
                 30.5 
               
               
                 ZrO 2  + Ln 2 O 3   
               
               
                 BaO/(MgO + CaO + SrO + BaO) 
                 — 
                 — 
                 — 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
               
               
                 SiO 2 /B 2 O 3   
                 0.5 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
               
               
                 Li 2 O + Na 2 O + K 2 O 
                 0.0 
                 0.0 
                 0.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
               
               
                 Li 2 O + Na 2 O 
                 0.0 
                 0.0 
                 0.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
               
               
                 MgO + CaO + SrO 
                 0.0 
                 0.0 
                 0.0 
                 22.0 
                 22.0 
                 22.0 
                 22.0 
                 22.5 
                 23.2 
                 21.5 
               
               
                 Li 2 O + Na 2 O + K 2 O + MgO + CaO + 
                 0.0 
                 0.0 
                 0.0 
                 32.0 
                 32.0 
                 32.0 
                 32.0 
                 33.2 
                 33.2 
                 33.5 
               
               
                 SrO + BaO 
               
               
                 TiO 2 /B 2 O 3   
                 1.6 
                 1.6 
                 1.6 
                 0.7 
                 0.8 
                 1.0 
                 0.7 
                 0.9 
                 0.9 
                 1.0 
               
               
                 TiO 2  + WO 3  + Nb 2 O 5   
                 35.5 
                 35.2 
                 35.2 
                 21.0 
                 21.0 
                 24.0 
                 18.0 
                 22.8 
                 22.8 
                 23.7 
               
               
                 (ZrO 2  + Ta 2 O 5  + Nb 2 O 5 )/(SiO 2  + B 2 O 3 ) 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.2 
                 0.2 
                 0.3 
                 0.2 
                 0.2 
                 0.2 
               
               
                 (Nb 2 O 5  + TiO 2  + WO 3  + Ta 2 O 5 )/ 
                 1.4 
                 1.4 
                 1.4 
                 2.6 
                 2.6 
                 4.8 
                 2.3 
                 4.6 
                 4.6 
                 6.6 
               
               
                 (La 2 O 3  + Gd 2 O 3  + Y 2 O 3  + Yb 2 O 3 ) 
               
               
                 MgO + CaO + SrO + BaO 
                 0.00 
                 0.00 
                 0.00 
                 30.00 
                 30.00 
                 30.00 
                 30.00 
                 31.20 
                 31.20 
                 31.50 
               
               
                 Refractive index (nd) 
                 2.01 
                 2.00 
                 2.00 
                 1.87 
                 1.86 
                 1.86 
                 1.85 
                 1.85 
                 1.85 
                 1.85 
               
               
                 Density (d) [g/cm 3 ] 
                 4.9 
                 4.9 
                 4.9 
                 4.2 
                 4.2 
                 4.0 
                 4.2 
                 4.0 
                 4.0 
                 4.0 
               
               
                 Devitrification temperature [° C.] 
                 1140 
                 1140 
                 1140 
                 1100 
                 1125 
                 1050 
                 1100 
                 1075 
                 1050 
                 1025 
               
               
                 Temperature T1 [° C.] 
                 1104 
                 1106 
                 1098 
                 1020 
                 1020 
                 1006 
                 1043 
                 1010 
                 1008 
                 1011 
               
               
                 Devitrification viscosity 
                 0.8 
                 0.8 
                 0.7 
                 0.5 
                 0.4 
                 0.7 
                 0.6 
                 0.5 
                 0.7 
                 1.1 
               
               
                 Light transmittance (T360) [%] 
               
               
                 Glass transition point (Tg) [° C.] 
                 713 
                 713 
                 713 
                 628 
                 619 
                 615 
                 625 
                 612 
                 613 
                 609 
               
               
                 Young&#39;s modulus (E) [Gpa] 
               
               
                 Abbe number (νd) 
                 27 
                 28 
                 28 
                 31.4 
                 32.0 
                 30.0 
                 33.2 
                 30.7 
                 30.7 
                 30.2 
               
               
                 Coefficient of thermal expansion α 
                 93 
                 93 
                 93 
                 92 
                 94 
                 89 
                 94 
                 90 
                 90 
                 89 
               
               
                 (50-350° C.)[×10 −7 /K] 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
               
               
                   
                 ple 51 
                 ple 52 
                 ple 53 
                 ple 54 
                 ple 55 
                 ple 56 
                 ple 57 
                 ple 58 
                 ple 59 
                 ple 60 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 P 2 O 5   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 SiO 2   
                 15.0 
                 14.0 
                 15.0 
                 15.0 
                 15.0 
                 15.0 
                 15.8 
                 15.8 
                 12.8 
                 12.8 
               
               
                 B 2 O 3   
                 26.0 
                 22.0 
                 25.0 
                 24.0 
                 25.0 
                 25.0 
                 21.0 
                 21.0 
                 24.7 
                 21.7 
               
               
                 MgO 
               
               
                 CaO 
                 10.5 
                 12.0 
                 10.5 
                 10.5 
                 10.4 
                 11.4 
                 3.0 
                   
                   
                 3.0 
               
               
                 SrO 
                 7.0 
                 7.0 
                 7.0 
                 7.0 
                 6.8 
                 7.8 
               
               
                 BaO 
                 8.0 
                 12.0 
                 8.0 
                 8.0 
                 7.9 
                 7.9 
               
               
                 Li 2 O 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                   
                   
                 3.0 
               
               
                 Na 2 O 
               
               
                 K 2 O 
               
               
                 Al 2 O 3   
               
               
                 Y 2 O 3   
                   
                   
                   
                 3.0 
                   
                   
                 3.5 
                 3.5 
                 3.5 
                 3.5 
               
               
                 TiO 2   
                 22.0 
                 21.5 
                 20.5 
                 21.0 
                 20.9 
                 20.9 
                 25.0 
                 25.0 
                 25.0 
                 25.0 
               
               
                 Ta 2 O 5   
               
               
                 WO 3   
                   
                   
                   
                   
                   
                   
                 0.2 
                 0.2 
                 0.2 
                 0.2 
               
               
                 Nb 2 O 5   
                 2.7 
                 2.7 
                 2.7 
                 2.7 
                 2.7 
                 2.7 
                 4.0 
                 4.0 
                 4.3 
                 4.3 
               
               
                 La 2 O 3   
                 3.6 
                 3.6 
                 5.1 
                 3.6 
                 5.1 
                 5.1 
                 20.5 
                 20.5 
                 22.5 
                 22.5 
               
               
                 ZrO 2   
                 3.2 
                 3.2 
                 4.2 
                 3.2 
                 4.2 
                 4.2 
                 7.0 
                 7.0 
                 7.0 
                 7.0 
               
               
                 Bi 2 O 3   
               
               
                 ZnO 
               
               
                 Gd 2 O 3   
                   
               
               
                 Sum 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 SiO 2  + Bi 2 O 3   
                 41.0 
                 36.0 
                 40.0 
                 39.0 
                 40.0 
                 40.0 
                 36.8 
                 36.8 
                 37.5 
                 34.5 
               
               
                 TiO 2  + Ta 2 O 5  + WO 3  + Nb 2 O 5  + 
                 31.5 
                 31.0 
                 32.5 
                 33.5 
                 32.9 
                 32.9 
                 60.2 
                 60.2 
                 62.5 
                 62.5 
               
               
                 ZrO 2  + Ln 2 O 3   
               
               
                 BaO/(MgO + CaO + SrO + BaO) 
                 0.3 
                 0.4 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.0 
                 — 
                 — 
                 0.0 
               
               
                 SiO 2 /B 2 O 3   
                 0.6 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
                 0.8 
                 0.8 
                 0.5 
                 0.6 
               
               
                 Li 2 O + Na 2 O + K 2 O 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 0.0 
                 0.0 
                 3.0 
                 0.0 
                 0.0 
               
               
                 Li 2 O + Na 2 O 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 0.0 
                 0.0 
                 3.0 
                 0.0 
                 0.0 
               
               
                 MgO + CaO + SrO 
                 17.5 
                 19.0 
                 17.5 
                 17.5 
                 17.2 
                 19.2 
                 3.0 
                 0.0 
                 0.0 
                 3.0 
               
               
                 Li 2 O + Na 2 O + K 2 O + MgO + CaO + 
                 27.5 
                 33.0 
                 27.5 
                 27.5 
                 27.1 
                 27.1 
                 3.0 
                 3.0 
                 0.0 
                 3.0 
               
               
                 SrO + BaO 
               
               
                 TiO 2 /B 2 O 3   
                 0.8 
                 1.0 
                 0.8 
                 0.9 
                 0.8 
                 0.8 
                 1.2 
                 1.2 
                 1.0 
                 1.2 
               
               
                 TiO 2  + WO 3  + Nb 2 O 5   
                 24.7 
                 24.2 
                 23.2 
                 23.7 
                 23.6 
                 23.6 
                 29.2 
                 29.2 
                 29.5 
                 29.5 
               
               
                 (ZrO 2  + Ta 2 O 5  + Nb 2 O 5 )/(SiO 2  + B 2 O 3 ) 
                 0.1 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
               
               
                 (Nb 2 O 5  + TiO 2  + WO 3  + Ta 2 O 5 )/ 
                 6.9 
                 6.7 
                 4.5 
                 3.6 
                 4.6 
                 4.6 
                 1.2 
                 1.2 
                 1.1 
                 1.1 
               
               
                 (La 2 O 3  + Gd 2 O 3  + Y 2 O 3  + Yb 2 O 3 ) 
               
               
                 MgO + CaO + SrO + BaO 
                 25.50 
                 31.00 
                 25.50 
                 25.50 
                 25.10 
                 27.10 
                 3.00 
                 0.00 
                 0.00 
                 3.00 
               
               
                 Refractive index (nd) 
                 1.85 
                 1.85 
                 1.85 
                 1.85 
                 1.85 
                 1.85 
                 1.97 
                 1.97 
                 1.98 
                 1.98 
               
               
                 Density (d) [g/cm 3 ] 
                 3.8 
                 4.0 
                 3.9 
                 3.9 
                 3.9 
                 4.0 
                 4.8 
                 4.8 
                 4.8 
                 4.9 
               
               
                 Devitrification temperature [° C.] 
                 1050 
                 1000 
                 988 
                 1025 
                 1025 
                 1012 
                 1150 
                 1150 
                 1125 
                 1137 
               
               
                 Temperature T1 [° C.] 
                 995 
                 999 
                 1022 
                 1010 
                 1009 
                 1015 
                 1102 
                 1082 
                 1099 
                 1101 
               
               
                 Devitrification viscosity 
                 0.6 
                 1.1 
                 1.3 
                 1.1 
                 1.1 
                 1.1 
                 0.7 
                 0.6 
                 0.8 
                 0.7 
               
               
                 Light transmittance (T360) [%] 
               
               
                 Glass transition point (Tg) [° C.] 
                 613 
                 607 
                 618 
                 624 
                 619 
                 633 
                 717 
                 696 
                 717 
                 717 
               
               
                 Young&#39;s modulus (E) [Gpa] 
               
               
                 Abbe number (νd) 
                 29.4 
                 29.8 
                 30.6 
                 30.5 
                 30.3 
                 30.2 
                 28 
                 28 
                 30 
                 29 
               
               
                 Coefficient of thermal expansion α 
                 82 
                 89 
                 85 
                 81 
                 85 
                 83 
                 89 
                 93 
                 91 
                 93 
               
               
                 (50-350° C.)[×10 −7 /K] 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
               
               
                   
                 ple 61 
                 ple 62 
                 ple 63 
                 ple 64 
                 ple 65 
                 ple 66 
                 ple 67 
                 ple 68 
                 ple 69 
                 ple 70 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 P 2 O 5   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 SiO 2   
                 15.8 
                 12.8 
                 14.8 
                 14.6 
                 14.6 
                 13.6 
                 14.6 
                 18.6 
                 10.6 
                 10.0 
               
               
                 B 2 O 3   
                 21.0 
                 24.7 
                 21.0 
                 24.5 
                 25.0 
                 23.8 
                 24.5 
                 20.5 
                 28.5 
                 23.1 
               
               
                 MgO 
               
               
                 CaO 
                 4.0 
                   
                 5.0 
                   
                   
                 3.0 
               
               
                 SrO 
               
               
                 BaO 
               
               
                 Li 2 O 
               
               
                 Na 2 O 
               
               
                 K 2 O 
               
               
                 Al 2 O 3   
               
               
                 Y 2 O 3   
                 4.0 
                 5.2 
                 4.0 
                 4.0 
                 3.5 
                 3.0 
                 4.0 
                 4.0 
                 4.0 
                 3.3 
               
               
                 TiO 2   
                 25.0 
                 24.0 
                 25.0 
                 24.0 
                 24.0 
                 24.0 
                 24.1 
                 24.0 
                 24.0 
                 32.4 
               
               
                 Ta 2 O 5   
               
               
                 WO 3   
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                   
                 0.2 
                 0.2 
                 0.3 
               
               
                 Nb 2 O 5   
                 4.0 
                 3.6 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 2.6 
               
               
                 La 2 O 3   
                 21.0 
                 23.0 
                 18.0 
                 22.7 
                 22.4 
                 22.3 
                 22.8 
                 22.7 
                 22.7 
                 18.2 
               
               
                 ZrO 2   
                 5.0 
                 6.5 
                 6.0 
                 6.0 
                 6.3 
                 6.1 
                 6.0 
                 6.0 
                 6.0 
                 6.9 
               
               
                 Bi 2 O 3   
               
               
                 ZnO 
               
               
                 Gd 2 O 3   
                   
                   
                 2.0 
                   
                   
                   
                   
                   
                   
                 3.3 
               
               
                 Sum 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 SiO 2  + Bi 2 O 3   
                 36.8 
                 37.5 
                 35.8 
                 39.1 
                 39.6 
                 37.4 
                 39.1 
                 39.1 
                 39.1 
                 33.1 
               
               
                 TiO 2  + Ta 2 O 5  + WO 3  + Nb 2 O 5  + 
                 59.2 
                 62.5 
                 59.2 
                 60.9 
                 60.4 
                 59.6 
                 60.9 
                 60.9 
                 60.9 
                 66.9 
               
               
                 ZrO 2  + Ln 2 O 3   
               
               
                 BaO/(MgO + CaO + SrO + BaO) 
                 0.0 
                 — 
                 0.0 
                 — 
                 — 
                 0.0 
                 — 
                 — 
                 — 
                 — 
               
               
                 SiO 2 /B 2 O 3   
                 0.8 
                 0.5 
                 0.7 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
                 0.9 
                 0.4 
                 0.4 
               
               
                 Li 2 O + Na 2 O + K 2 O 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Li 2 O + Na 2 O 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 MgO + CaO + SrO 
                 4.0 
                 0.0 
                 5.0 
                 0.0 
                 0.0 
                 3.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Li 2 O + Na 2 O + K 2 O + MgO + CaO + 
                 4.0 
                 0.0 
                 5.0 
                 0.0 
                 0.0 
                 3.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 SrO + BaO 
               
               
                 TiO 2 /B 2 O 3   
                 1.2 
                 1.0 
                 1.2 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.2 
                 0.8 
                 1.4 
               
               
                 TiO 2  + WO 3  + Nb 2 O 5   
                 29.2 
                 27.8 
                 29.2 
                 28.2 
                 28.2 
                 28.2 
                 28.1 
                 28.2 
                 28.2 
                 35.2 
               
               
                 (ZrO 2  + Ta 2 O 5  + Nb 2 O 5 )/(SiO 2  + B 2 O 3 ) 
                 0.2 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
               
               
                 (Nb 2 O 5  + TiO 2  + WO 3  + Ta 2 O 5 )/ 
                 1.2 
                 1.0 
                 1.2 
                 1.1 
                 1.1 
                 1.1 
                 1.0 
                 1.1 
                 1.1 
                 1.4 
               
               
                 (La 2 O 3  + Gd 2 O 3  + Y 2 O 3  + Yb 2 O 3 ) 
               
               
                 MgO + CaO + SrO + BaO 
                 4.00 
                 0.00 
                 5.00 
                 0.00 
                 0.00 
                 3.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 Refractive index (nd) 
                 1.98 
                 1.98 
                 1.97 
                 1.96 
                 1.96 
                 1.96 
                 1.96 
                 1.97 
                 1.96 
                 2.00 
               
               
                 Density (d) [g/cm 3 ] 
                 4.8 
                 5.0 
                 4.9 
                 4.8 
                 4.8 
                 4.8 
                 4.8 
                 4.8 
                 4.8 
                 4.9 
               
               
                 Devitrification temperature [° C.] 
                 1137 
                 1175 
                 1137 
                 1113 
                 1113 
                 1125 
                 1125 
                 1125 
                 1125 
                 1125 
               
               
                 Temperature T1 [° C.] 
                 1115 
                 1100 
                 1106 
                 1088 
                 1079 
                 1091 
                 1094 
                 1117 
                 1070 
                 1092 
               
               
                 Devitrification viscosity 
                 0.9 
                 0.5 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.9 
                 0.7 
                 0.8 
               
               
                 Light transmittance (T360) [%] 
               
               
                 Glass transition point (Tg) [° C.] 
                 714 
                 719 
                 718 
                 717 
                 715 
                 710 
                 717 
                 725 
                 708 
                 719 
               
               
                 Young&#39;s modulus (E) [Gpa] 
               
               
                 Abbe number (νd) 
                 34 
                 31 
                 29 
                 30 
                 30 
                 30 
                 30 
                 30 
                 30 
                 28 
               
               
                 Coefficient of thermal expansion α 
                 90 
                 91 
                 89 
                 90 
                 90 
                 92 
                 91 
                 90 
                 90 
                 92 
               
               
                 (50-350° C.)[×10 −7 /K] 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 8 
               
               
                   
                   
               
               
                   
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
                 Exam- 
               
               
                   
                 ple 71 
                 ple 72 
                 ple 73 
                 ple 74 
                 ple 75 
                 ple 76 
                 ple 77 
                 ple 78 
                 ple 79 
                 ple 80 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 P 2 O 5   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 SiO 2   
                 9.0 
                 10.0 
                 10.0 
                 9.0 
                 11.8 
                 11.8 
                 11.8 
                 11.8 
                 9.0 
                 11.8 
               
               
                 B 2 O 3   
                 23.0 
                 23.1 
                 23.1 
                 23.1 
                 19.8 
                 19.8 
                 19.8 
                 19.8 
                 22.6 
                 19.8 
               
               
                 MgO 
               
               
                 CaO 
               
               
                 SrO 
               
               
                 BaO 
               
               
                 Li 2 O 
               
               
                 Na 2 O 
               
               
                 K 2 O 
               
               
                 Al 2 O 3   
               
               
                 Y 2 O 3   
                 4.0 
                 3.3 
                 3.4 
                 4.6 
                 4.8 
                 4.5 
                 4.4 
                 4.8 
                 4.5 
                 4.8 
               
               
                 TiO 2   
                 30.9 
                 32.1 
                 31.5 
                 29.5 
                 24.0 
                 24.0 
                 24.0 
                 24.0 
                 24.0 
                 24.0 
               
               
                 Ta 2 O 5   
                   
                   
                   
                   
                   
                   
                   
                 1.5 
               
               
                 WO 3   
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.2 
                 3.0 
                 4.5 
                 1.7 
                 3.0 
                 0.2 
               
               
                 Nb 2 O 5   
                 2.6 
                 2.8 
                 3.1 
                 3.5 
                 4.8 
                 4.5 
                 3.5 
                 4.0 
                 4.5 
                 4.8 
               
               
                 La 2 O 3   
                 23.4 
                 18.9 
                 18.9 
                 19.1 
                 27.3 
                 25.6 
                 22.3 
                 25.1 
                 25.6 
                 27.3 
               
               
                 ZrO 2   
                 6.9 
                 6.3 
                 6.3 
                 6.6 
                 7.2 
                 6.8 
                 6.6 
                 7.2 
                 6.8 
                 7.2 
               
               
                 Bi 2 O 3   
               
               
                 ZnO 
               
               
                 Gd 2 O 3   
                   
                 3.3 
                 3.5 
                 4.4 
                   
                   
                 3.0 
               
               
                 Sum 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 SiO 2  + Bi 2 O 3   
                 32.0 
                 33.1 
                 33.1 
                 32.1 
                 31.6 
                 31.6 
                 31.6 
                 31.6 
                 31.6 
                 31.6 
               
               
                 TiO 2  + Ta 2 O 5  + WO 3  + Nb 2 O 5  + 
                 68.0 
                 66.9 
                 66.9 
                 67.9 
                 68.4 
                 68.4 
                 68.4 
                 68.4 
                 68.4 
                 68.4 
               
               
                 ZrO 2  + Ln 2 O 3   
               
               
                 BaO/(MgO + CaO + SrO + BaO) 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 SiO 2 /B 2 O 3   
                 0.4 
                 0.4 
                 0.4 
                 0.4 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
                 0.4 
                 0.6 
               
               
                 Li 2 O + Na 2 O + K 2 O 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Li 2 O + Na 2 O 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 MgO + CaO + SrO 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Li 2 O + Na 2 O + K 2 O + MgO + CaO + 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 SrO + BaO 
               
               
                 TiO 2 /B 2 O 3   
                 1.3 
                 1.4 
                 1.4 
                 1.3 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 1.1 
                 1.2 
               
               
                 TiO 2  + WO 3  + Nb 2 O 5   
                 33.7 
                 35.1 
                 34.8 
                 33.3 
                 29.0 
                 31.5 
                 32.0 
                 29.7 
                 31.5 
                 29.0 
               
               
                 (ZrO 2  + Ta 2 O 5  + Nb 2 O 5 )/(SiO 2  + B 2 O 3 ) 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.4 
                 0.4 
                 0.3 
                 0.4 
                 0.4 
                 0.4 
               
               
                 (Nb 2 O 5  + TiO 2  + WO 3  + Ta 2 O 5 )/ 
                 1.2 
                 1.4 
                 1.3 
                 1.2 
                 0.9 
                 1.0 
                 1.1 
                 1.0 
                 1.0 
                 0.9 
               
               
                 (La 2 O 3  + Gd 2 O 3  + Y 2 O 3  + Yb 2 O 3 ) 
               
               
                 MgO + CaO + SrO + BaO 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 Refractive index (nd) 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
                 2.00 
               
               
                 Density (d) [g/cm 3 ] 
                 5.0 
                 5.0 
                 5.0 
                 5.2 
                 5.1 
                 5.1 
                 5.2 
                 5.1 
                 5.1 
                 5.1 
               
               
                 Devitrification temperature [° C.] 
                 1200 
                 1100 
                 1113 
                 1175 
                 1238 
                 1225 
                 1225 
                 1225 
                 1225 
                 1225 
               
               
                 Temperature T1 [° C.] 
                 1097 
                 1103 
                 1098 
                 1104 
                 1152 
                 1131 
                 1138 
                 1133 
                 1117 
                 1143 
               
               
                 Devitrification viscosity 
                 0.5 
                 1.0 
                 0.9 
                 0.6 
                 0.5 
                 0.4 
                 0.4 
                 0.4 
                 0.4 
                 0.4 
               
               
                 Light transmittance (T360) [%] 
               
               
                 Glass transition point (Tg) [° C.] 
                 714 
                 719 
                 721 
                 729 
                 735 
                 727 
                 729 
                 730 
                 721 
                 735 
               
               
                 Young&#39;s modulus (E) [Gpa] 
               
               
                 Abbe number (νd) 
                 28 
                 28 
                 28 
                 29 
                 30 
                 29 
                 29 
                 30 
                 30 
                 30 
               
               
                 Coefficient of thermal expansion α 
                 96 
                 93 
                 93 
                 94 
                 100 
                 88 
                 83 
                 92 
                 88 
                 100 
               
               
                 (50-350° C.)[×10 −7 /K] 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 9 
               
               
                   
                   
               
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 81 
                 82 
                 83 
                 84 
                 85 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 P 2 O 5   
                   
                   
                   
                   
                 23.0 
               
               
                 SiO 2   
                 58.6  
                 60.6 
                 52.6 
                 34.6  
                 0.9 
               
               
                 B 2 O 3   
                 8.7 
                   
                   
                   
                 3.0 
               
               
                 MgO 
                   
                   
                 6.3 
               
               
                 CaO 
                 5.0 
                   
                 8.8 
                 1.8 
               
               
                 SrO 
                   
                   
                 4.0 
               
               
                 BaO 
                   
                   
                 14.8 
                   
                 1.8 
               
               
                 Li 2 O 
                 2.2 
                 20.2 
                   
                 26.2  
                 8.2 
               
               
                 Na 2 O 
                   
                 5.3 
                   
                 5.4 
                 6.8 
               
               
                 K 2 O 
                 9.9 
                 1.1 
                   
                 1.1 
                 2.1 
               
               
                 Al 2 O 3   
                   
                   
                 1.2 
               
               
                 Y 2 O 3   
                   
                   
                   
                 0.3 
               
               
                 TiO 2   
                   
                 6.4 
                 7.1 
                 5.0 
                 7.1 
               
               
                 Ta 2 O 5   
               
               
                 WO 3   
                   
                   
                   
                   
                 8.3 
               
               
                 Nb 2 O 5   
                 12.7  
                 6.4 
                 2.0 
                 20.9  
                 17.3 
               
               
                 La 2 O 3   
                   
                   
                 1.0 
               
               
                 ZrO 2   
                 0.8 
                   
                 2.2 
                 2.4 
               
               
                 Bi 2 O 3   
                   
                   
                   
                   
                 21.5 
               
               
                 ZnO 
                 1.0 
                   
                   
                 2.4 
               
               
                 Gd 2 O 3   
                 1.0 
                   
                   
                   
                   
               
               
                 Sum 
                 100    
                 100 
                 100 
                 100    
                 100 
               
               
                 SiO 2  + Bi 2 O 3   
                 67.3  
                 60.6 
                 52.6 
                 34.6  
                 3.9 
               
               
                 TiO 2  + Ta 2 O 5  + WO 3  + Nb 2 O 5  + 
                 14.5  
                 12.8 
                 12.3 
                 28.5  
                 32.6 
               
               
                 ZrO 2  + Ln 2 O 3   
               
               
                 BaO/(MgO + CaO + SrO + BaO) 
                 0.0 
                 — 
                 0.4 
                 0.0 
                 1.0 
               
               
                 SiO 2 /B 2 O 3   
                 6.7 
                 — 
                 — 
                 — 
                 0.3 
               
               
                 Li 2 O + Na 2 O + K 2 O 
                 12.1  
                 26.6 
                 0.0 
                 32.7  
                 17.1 
               
               
                 Li 2 O + Na 2 O 
                 2.2 
                 25.5 
                 0.0 
                 31.6  
                 15.0 
               
               
                 MgO + CaO + SrO 
                 5.0 
                 0.0 
                 19.1 
                 1.8 
                 0.0 
               
               
                 Li 2 O + Na 2 O + K 2 O + MgO + CaO + 
                 17.1  
                 26.6 
                 33.9 
                 34.4  
                 18.9 
               
               
                 SrO + BaO 
               
               
                 TiO 2 /B 2 O 3   
                 0.0 
                 — 
                 — 
                 — 
                 2.3 
               
               
                 TiO 2  + WO 3  + Nb 2 O 5   
                 12.7  
                 12.8 
                 9.1 
                 25.8  
                 32.6 
               
               
                 (ZrO 2  + Ta 2 O 5  + Nb 2 O 5 )/(SiO 2  + B 2 O 3 ) 
                 0.2 
                 0.1 
                 0.1 
                 0.7 
                 4.5 
               
               
                 (Nb 2 O 5  + TiO 2  + WO 3  + Ta 2 O 5 )/ 
                 12.3  
                 — 
                 9.1 
                 98.1  
                 — 
               
               
                 (La 2 O 3  + Gd 2 O 3  + Y 2 O 3  + Yb 2 O 3 ) 
               
               
                 MgO + CaO + SrO + BaO 
               
               
                 Refractive index (nd) 
                  1.74 
                 1.67 
                 1.70 
                  1.89 
                 2.00 
               
               
                 Density (d) [g/cm 3 ] 
                 3.2 
                 2.86 
                 3.66 
                  3.62 
                 5.10 
               
               
                 Devitrification temperature [° C.] 
                 1300&lt;    
                   
                 1070 
                 1400&lt;    
               
               
                 Temperature T1 [° C.] 
                   
                 1086 
                 1210 
               
               
                 Devitrification viscosity 
               
               
                 Light transmittance (T360) [%] 
                   
                   
                 64 
               
               
                 Glass transition point (Tg) [° C.] 
                 657    
                   
                   
                   
                 483 
               
               
                 Young&#39;s modulus (E) [Gpa] 
                   
                 96 
                   
                 115    
                 87 
               
               
                 Abbe number (νd) 
                   
                   
                   
                 24   
                 19 
               
               
                 Coefficient of thermal 
                 66   
                   
                   
                   
                 101 
               
               
                 expansion α 
               
               
                 (50-350° C.)[×10 −7 /K] 
               
               
                   
               
            
           
         
       
     
     The optical glasses of the examples (Examples 1 to 80) each have a refractive index (n d ) as high as 1.81 or more and a density as low as 6.0 g/cm 3  or less. These optical glasses each have a temperature T 1 , which is the temperature at which the glass viscosity is 10 1  dPa·s, of 900° C. to 1,200° C. and hence have satisfactory producibility. Furthermore, these optical glasses each have a devitrification temperature of 1,300° C. or lower and hence have satisfactory producibility. Consequently, these optical glasses are suitable for use in wearable appliances, in-vehicle cameras, and visual sensing for robots. 
     Meanwhile, the glasses of Examples 81 and 84, which are comparative examples, each have a devitrification temperature of higher than 1,300° C. and hence have poor producibility. The glasses of Examples 81, 82, and 83 each have a refractive index (nd) of lower than 1.81. The glass of Example 85 has an Si content of less than 5 mol %. 
     The optical glasses obtained from glasses obtained by melting the glass compositions of the examples (Examples 1 to 80) each have a temperature T 1  of 900° C. to 1,200° C. and hence have satisfactory producibility and contain a small number of residual bubbles, and the size of the bubbles is small. Glass plates free from defects such as bubbles, foreign matter, striae, and phase separation are hence obtained. Consequently, in the case where samples having the sizes shown above are formed, it is possible to obtain optical glasses having an LTV for an area of 25 cm 2  of 2 μm or less, a warpage (for disk-shaped glass plate with a diameter of 6 inches) of 30 μm or less, and an Ra of 2 nm or less. Furthermore, it is thought that since these optical glasses have devitrification temperatures of 1,300° C. or lower and are inhibited from being devitrified, it is possible to attain an LTV for an area of 25 cm 2  of 1.5 μm or less, a warpage (for disk-shaped glass plate with a diameter of 6 inches) of 18 μm or less, and an Ra of 1 nm or less. 
     Three defect-free glass plates of the examples were precisely polished and, as a result, the polished glass plates had LTV values for an area of 25 cm 2  of 1.0 μm, 1.2 μm, and 1.2 μm, warpage values of 45, 32, and 38, and Ra values of 0.198, 0.284, and 0.266. Consequently, an optical glass having an LTV for an area of 25 cm 2  of 2 μm or less, a warpage of 50 μm or less, and an Ra of 2 nm or less can be obtained by precisely polishing the defect-free glass plates of the examples of the present invention. 
     In the case where a glass of the present invention is chemically strengthened, a chemical strengthening treatment may be performed, for example, by immersing the glass for 30 minutes in a melt obtained by melting sodium nitrate by heating at 400° C. Thus, a strengthened glass can be obtained. 
     As described above, the optical glass of the present invention has a high refractive index, a low density, and satisfactory producibility and is suitable for use as an optical glass for wearable appliances and for mounting on vehicles or robots, etc. Optical components obtained by coating the optical glasses of the examples of the present invention with an antireflection film constituted of a multilayered dielectric film composed of 4 to 10 layers formed by alternately depositing a low-refractive index film of, for example, SiO 2 , and a high-refractive index film of, for example, TiO 2  are also suitable for use in wearable appliances and for mounting on vehicles and robots. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.