Optical element having compression stress layer

Abstract

To provide an optical element having excellent press-forming ability, a desired compression stress and compression layer depth, and an excellent thermal shock resistance.SOLUTION: The optical element has a compression stress layer on the surface thereof, the compression stress of the compression stress layer being 100 to 1000 MPa, the depth of the compression stress layer being 10 to 100 μm, and log η=8.5 (viscosity) being reached at a temperature lower than 700°C.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an optical element having a compressive stress layer.

Background Art

[0002] In recent years, the demand for projection optical system devices such as projectors and imaging optical system devices such as in-vehicle cameras and surveillance cameras has been increasing, and it has become necessary to take measures assuming various environments for the optical elements used inside the optical system.

[0003] In projection optical system devices, a light source using a high-power semiconductor light-emitting element is required to project an image onto a screen more clearly. Conventionally, high-pressure discharge or the like has been used, but due to the demand for compactness, LD and LED light sources have come to be used.

[0004] Since heat is also released from the LD and LED light sources of projection optical system devices together with light, the optical elements inside the projection optical system devices may be exposed to high temperatures. In particular, due to the use of high-power light sources, they are also exposed to large temperature changes. The temperature change is large, and the temperature of the optical element may reach about 200°C or higher. There has been a problem that the optical element cracks when cooled from that high temperature state to room temperature. Therefore, optical elements used in projection optical systems have come to be required to have high transmittance, high homogeneity, and heat shock resistance.

[0005] In addition, the optical elements inside the projection optical system devices need to produce aspherical lenses with complex shapes arranged in a matrix in the vertical and horizontal directions, and higher press formability than conventional optical elements is required.

[0006] In particular, since manufacturing an aspherical lens by grinding or polishing methods is costly and inefficient, as a method for manufacturing an aspherical lens, a gob or a glass block is heated and softened to form a preform material that has been cut and polished, and this is precision molded by pressure molding using a molding die having a highly accurate surface, thereby making it possible to omit the grinding and polishing processes, and realizing low-cost and mass production.

[0007] On the other hand, in imaging optical system devices, there has been a problem that the optical elements and the entire optical device are rapidly heated by direct sunlight, and after being heated, they are rapidly cooled by rain or water during car washing, etc., causing the optical elements to crack.

[0008] As a glass having good press formability, the invention described in Patent Document 1 is disclosed.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] However, in the invention disclosed in Patent Document 1, although a glass having good press formability is obtained by lowering the transition point, since the temperature at which logη = 8.5 is high, a complex aspherical lens cannot be manufactured, and furthermore, it is difficult to stabilize the thermal shock resistance.

[0011] The present invention has been made in view of the above problems, and an object thereof is to obtain an optical element having excellent press moldability, having a desired compressive stress and a depth of the compressive stress layer, and excellent in thermal shock resistance.

Means for Solving the Problems

[0012] As a result of intensive experimental research to solve the above problems, the present inventor has improved the press formability by setting the temperature at which logη = 8.5 (viscosity) to less than 700°C, and further has a compressive stress of 100 to 1000 MPa and a depth of the compressive stress layer of 10 to 100 μm, and by setting the ratio of the compressive stress to the thermal stress (compressive stress / thermal stress) to 1.00 or more, the present invention excellent in thermal shock resistance has been completed.

[0013] (1) An optical element having a compressive stress layer on its surface, wherein the compressive stress of the compressive stress layer is 100 to 1000 MPa, the depth of the compressive stress layer is 10 to 100 μm, and the temperature at which logη = 8.5 (viscosity) is less than 700°C.

[0014] (2) The optical element according to (1), wherein the refractive index (nd) is 1.45000 or more and 1.70000 or less, and the Abbe number (νd) is 45.00 or more and 70.00 or less.

[0015] (3) The optical element according to (1) or (2), wherein the ratio of the compressive stress to the thermal stress (compressive stress / thermal stress) is 1.00 or more.

[0016] (4) The optical element according to any one of (1) to (3) used in a projection optical system device.

[0017] (5) The optical element according to any one of (1) to (3) used in an imaging optical system device.

Mode for Carrying Out the Invention

[0018] The optical element of the present invention has a compressive stress layer on its surface. As methods for forming a compressive stress layer on the surface, there are a physical strengthening method and a chemical strengthening method, but the optical element of the present invention is formed by the chemical strengthening method. In addition, in the present invention, there are places where the mold press formability is described as the press formability.

[0019] The compressive stress of the compressive stress layer of the optical element of the present invention preferably has a lower limit of 100 MPa or more, more preferably 200 MPa or more, still more preferably 280 MPa or more, and most preferably 340 MPa or more. The greater the compressive stress, the higher the mechanical strength of the tempered glass. On the other hand, if an extremely large compressive stress is formed on the surface, the tensile stress CT inherent in the tempered glass will increase, and there is a risk of self-destruction. Therefore, the compressive stress of the compressive stress layer more preferably has an upper limit of 1000 MPa or less, more preferably 800 MPa or less, still more preferably 650 MPa or less, and most preferably 600 MPa or less.

[0020] The greater the depth of the compressive stress layer, the more difficult it is for the scratches formed on the surface to penetrate the compressive stress layer, and thus the crack resistance is improved. Therefore, it preferably has a lower limit of 10 μm or more, more preferably 15 μm or more, and most preferably 20 μm or more. On the other hand, the greater the depth of the compressive stress layer, the higher the tensile stress CT inherent in the tempered glass, and there is a risk of self-destruction. Further, it will affect the refractive index and Abbe number, making it difficult to use as an optical element. Therefore, the depth of the compressive stress layer preferably has an upper limit of 100 μm or less, more preferably 70 μm or less, more preferably 50 μm or less, still more preferably 40 μm or less, and most preferably 33 μm or less.

[0021] The ratio of the compressive stress to the thermal stress (compressive stress / thermal stress) is preferably 1.00 or more. Thereby, the thermal shock resistance is increased, and the optical element is less likely to crack even when a large temperature difference occurs. Therefore, it preferably has a lower limit value of 1.00 or more, more preferably 1.60 or more, more preferably 2.00 or more, still more preferably 2.20 or more, and most preferably 2.40 or more.

[0022] The thermal stress referred to here represents the thermal stress using the temperature difference when the temperature drops from 300 °C to 100 °C. Thermal stress (MPa) = Young's modulus (10 8 Pa) × average linear thermal expansion coefficient α (10 -7 / °C) × 200 (°C) × 10 -6

[0023] Due to the small thermal stress, the optical element of the present invention can suppress the occurrence of thermal cracking. Therefore, the thermal stress may preferably have an upper limit of 330 (MPa) or less, more preferably 230 (MPa) or less, and even more preferably 180 (MPa) or less.

[0024] The optical element of the present invention may have a small Young's modulus. In particular, the Young's modulus of the optical element of the present invention is preferably 1100 (10 8 Pa) or less, more preferably 1000 (10 8 Pa) or less, and even more preferably 950 (10 8 Pa) or less as the upper limit.

[0025] In the optical element of the present invention, the average linear thermal expansion coefficient α at 100 to 300 °C is preferably in the range of 40 to 150 (×10 -7 / °C). By having α in the above range, the optical glass of the present invention has good expansion matching with the mold during precision press molding without impairing the thermal shock resistance. This α is more preferably 40 to 150 (×10 -7 / °C), even more preferably 50 to 135 (×10 -7 / °C), even more preferably 60 to 115 (×10 -7 / °C), and particularly preferably 65 to 105 (×10 -7 / °C).

[0026] It is preferable that the temperature at which logη = 8.5 (viscosity) for the optical element of the present invention is less than 700 °C. As a result, the glass softens at a lower temperature, so it is easier to press-mold the glass at a lower temperature, and it is possible to press-mold an aspherical lens with a more complex shape. In addition, oxidation of the mold used for press molding can be reduced, and the life of the mold can be extended. Therefore, it is preferably less than 700 °C, more preferably 650 °C or less, even more preferably 620 °C or less, and even more preferably 610 °C or less.

[0027] The optical element of the present invention has a predetermined refractive index and an Abbe number within a predetermined range. The refractive index (nd) of the optical element of the present invention preferably has a lower limit of 1.45000 or more, more preferably 1.48000 or more. The upper limit of this refractive index is preferably 1.70000 or less, more preferably 1.65000 or less, more preferably 1.60000 or less, and still more preferably 1.55000 as the upper limit. The Abbe number (νd) of the optical element of the present invention preferably has a lower limit of 45.00 or more, more preferably 47.00 or more, and still more preferably 50.00 or more. The upper limit of this Abbe number is preferably 70.00 or less, more preferably 67.00 or less as the upper limit. The optical element of the present invention having such a refractive index and Abbe number is useful in optical design. In particular, while achieving high imaging characteristics and the like, it is possible to reduce the size of the optical system, so that the degree of freedom in optical design can be expanded.

[0028] Hereinafter, the glass constituting the optical element of the present invention will be described. [Glass components] In this specification, unless otherwise specified, the content of each component is expressed in cation % or anion % based on the molar ratio. First, the constituent components of the glass are divided into a cation component and an anion component. And "cation %" is a unit in which the content of each cation component is expressed as a percentage when the total content of all cation components contained in the glass is 100 mol%. "Anion %" is a unit in which the content of each anion component is expressed as a percentage when the total content of all anion components contained in the glass is 100 mol%.

[0029] [Regarding essential components and optional components] Si 4+ is a component that promotes stable glass formation, lowers the liquidus temperature, and can reduce devitrification (generation of crystals). In particular, by setting the content of Si 4+ to 37.0% or more, it becomes easier to vitrify, the coefficient of thermal expansion can be lowered, not only can a glass with excellent thermal shock resistance and devitrification resistance be obtained, but also the ion exchange performance is improved and the compressive stress tends to increase. Therefore, Si4+ The content of On the other hand, Si 4+ By setting the content of 4+ to 75.0% or less, it is possible to prevent the meltability and formability from being easily reduced and the thermal expansion coefficient from becoming too low. Therefore, the content of Si Si 4+ can use SiO2, K2SiF6, Na2SiF6, etc. as raw materials.

[0030] Li + is an ion-exchange component, and is also a component that reduces the high-temperature viscosity, enhances the meltability and formability, and is an optional component that increases the Young's modulus. Therefore, the content of Li + is preferably 0% or more, more preferably more than 0%, more preferably 5.0% or more, more preferably 10.0% or more, and most preferably 12.0% or more as the lower limit. On the other hand, when the content of Li + becomes extremely large, the compressive stress tends to decrease. The low-temperature viscosity decreases, stress relaxation is likely to occur, and the compressive stress may decrease. The thermal expansion coefficient becomes too high, the thermal shock resistance decreases, and it becomes difficult to match the thermal expansion coefficient of the surrounding materials. Therefore, the content of Li + is preferably 30.0% or less, more preferably 26.0% or less, and further preferably 24.0% or less as the upper limit. Li + can use Li2CO3, LiNO3, LiF, etc. as raw materials.

[0031] Na + is an ion-exchange component, and is an optional component that increases the compressive stress, reduces the high-temperature viscosity, enhances the meltability and formability, and increases the thermal expansion coefficient. Therefore, Na +The content is preferably 0% or more, more preferably 0.1% or more, and still more preferably 0.2% or more as the lower limit. On the other hand, by setting the content of Na + to 25.0% or less, devitrification due to excessive content can be reduced. Therefore, the content of Na + is preferably 25.0% or less, more preferably 12.5% or less, and still more preferably 10.0% or less as the upper limit. Na + can be used as raw materials such as Na2CO3, NaNO3, NaF, Na2SiF6, etc.

[0032] K + is a component that promotes ion exchange and is an optional component that easily increases the depth of the compressive stress layer among alkali metal oxides. It is also a component that reduces the high-temperature viscosity, enhances the meltability and formability, and improves the devitrification resistance. Therefore, the content of K + is preferably 0% or more, more preferably more than 0%, more preferably 1.0% or more, and most preferably 3.0% or more as the lower limit. On the other hand, if the content of K + is too much, the thermal expansion coefficient becomes too high and the thermal shock resistance decreases. Therefore, the content of K + is preferably 18.0% or less, more preferably 15.0% or less, and still more preferably 13.0% or less as the upper limit. K + can be used as raw materials such as K2CO3, KNO3, KF, KHF2, K2SiF6, etc.

[0033] B 3+ When the content contains more than 0%, it promotes stable glass formation and can lower the liquidus temperature. Therefore, it is an optional component that can enhance the devitrification resistance, improve the meltability of the glass raw materials, and improve the mechanical properties such as the strength and crack resistance of the glass. Therefore, the content of B 3+ is preferably 0% or more, more preferably more than 0%, more preferably 5.0% or more, still more preferably 10.0% or more, still more preferably 13.0% or more, and most preferably 15.0% or more as the lower limit. On the other hand, B 3+ will cause deterioration of chemical durability and phase separation when it increases. Therefore, the content of B 3+ is preferably 35.0% or less, more preferably 30.0% or less, still more preferably 27.0% or less, and most preferably 25.0% or less as the upper limit. B 3+ can be used as raw materials such as H3BO3, Na2B4O7, Na2B4O7·10H2O, BPO4, etc.

[0034] Zn 2+ is an optional component that can reduce the high-temperature viscosity. However, if the content of Zn 2+ is too high, the glass may phase-separate, the devitrification resistance may decrease, the depth of the compressive stress layer may become small, and stress relaxation may occur more easily, and conversely, the compressive stress may decrease. Therefore, the content of Zn 2+ is preferably 0% or more, more preferably more than 0%, still more preferably 0.1% or more, and further preferably 0.2% or more as the lower limit. On the other hand, by setting the content of Zn 2+ to 2.0% or less, phase separation of the glass and a decrease in devitrification resistance can be suppressed, and a decrease in compressive stress due to stress relaxation can be prevented. Therefore, the content of Zn 2+ is preferably 2.0% or less, more preferably 1.5% or less, still more preferably 1.0% or less, and most preferably 0.8% or less as the upper limit.

[0035] Nb 5+ and La 3+ , Gd 3+ , Y 3+ , Yb 3+ and other rare earth oxides are optional components that can increase rigidity. However, the cost of the raw materials themselves is high, and when added in a large amount, the devitrification resistance tends to decrease. Therefore, the content of the rare earth oxides is preferably 3% or less, preferably 2% or less, preferably 1% or less, preferably 0.5% or less, and particularly preferably 0.1% or less as the upper limit. Nb 5+ , La 3+ , Gd 3+ , Y 3+ and Yb3+ Nb2O5, La2O3, La(NO3)3·XH2O (X is an arbitrary integer), Y2O3, YF3, Gd2O3, GdF3, Yb2O3, etc. can be used as raw materials.

[0036] Ti 4+ is a component that enhances ion exchange performance and is an optional component that reduces high-temperature viscosity. Therefore, Ti 4+ content is preferably more than 0%, more preferably 0.02% or more, and most preferably 0.03% or more as the lower limit. On the other hand, by setting Ti 4+ to 10.0% or less, glass coloring can be reduced and internal transmittance can be increased. Therefore, the content of Ti 4+ is preferably 10.0% or less, more preferably 5.0% or less, and even more preferably 1.0% or less as the upper limit. Ti 4+ TiO2, etc. can be used as raw materials.

[0037] Al 3+ is an optional component that enhances ion exchange performance. If there is too little Al 3+ , the ion exchange performance cannot be fully exerted. Therefore, the content of Al 3+ is preferably more than 0%, more preferably 1.0% or more, even more preferably 1.5% or more, and most preferably 2.0% or more as the lower limit. On the other hand, by setting the content of Al 3+ to 10.0% or less, devitrification due to excessive content of Al 3+ can be reduced. Therefore, the content of Al3+ is preferably 10.0% or less, more preferably 5.0% or less, and even more preferably 3.0% or less as the upper limit. Al 3+ Al2O3, Al(OH)3, AlF3, etc. can be used as raw materials.

[0038] Mg 2+ is an optional component that can lower the melting temperature of the glass, reduce high-temperature viscosity, enhance meltability, and increase Young's modulus. On the other hand, Mg 2+By setting the content of [substance] to 10.0% or less, devitrification can be reduced while suppressing a decrease in the refractive index. Therefore, the content of Mg 2+ is preferably 10.0% or less, more preferably 5.0% or less, and even more preferably 1.0% or less as the upper limit. Mg 2+ can be used as raw materials such as MgO, MgCO3, MgF2, etc.

[0039] Ca 2+ is an optional component that can lower the high-temperature viscosity, enhance the meltability, and increase the Young's modulus without accompanying a decrease in devitrification resistance. However, if the content of Ca 2+ is too high, the density and thermal expansion coefficient will increase, and the glass composition will lack component balance, resulting in a tendency for the glass to be more prone to devitrification, a decrease in ion exchange performance, and an easier deterioration of the ion exchange solution. Therefore, the content of Ca 2+ is preferably 10.0% or less, more preferably 5.0% or less, even more preferably 3.0% or less, and most preferably 1.0 or less as the upper limit. Ca 2+ can be used as raw materials such as CaCO3, CaF2, etc.

[0040] Sr 2+ is an optional component that can lower the melting temperature of the glass, reduce the high-temperature viscosity, enhance the meltability, and increase the Young's modulus. On the other hand, by setting the content of Sr 2+ to 10.0% or less, the glass becomes stable and the devitrification of the glass can be suppressed. Therefore, the content of Sr 2+ is preferably 10.0% or less, more preferably 5.0% or less, and even more preferably 1.0% or less as the upper limit. Sr 2+ can be used as raw materials such as Sr(NO3)2, SrF2, etc.

[0041] Ba 2+ is an optional component that can lower the high-temperature viscosity and enhance the meltability and formability when the content is more than 0%. On the other hand, Ba 2+By setting the content of [substance] to 20.0% or less, devitrification during reheating pressing can be reduced, and the ion exchange reaction is less likely to be inhibited. Therefore, Ba 2+ The content of [substance] is preferably 20.0% or less, more preferably 15.0% or less, and even more preferably 12.0% or less as the upper limit. Ba 2+ As raw materials, BaCO3, Ba(NO3)2, etc. can be used.

[0042] Rn + (In the formula, Rn + is one or more selected from the group consisting of Li + Na + K + ) When the sum (mass sum) of the contents is 5.0% or more, it is an optional component that can enhance the meltability and formability. Therefore, the sum of Rn + is preferably 5.0% or more, more preferably 15.0% or more, and even more preferably 20.0% or more as the lower limit. On the other hand, by setting the sum (mass sum) of the content of Rn + to 35.0% or less, a decrease in the refractive index can be suppressed, and devitrification due to excessive content can be reduced. Therefore, it is preferably 35.0% or less, more preferably 32.0% or less, and even more preferably 30.0% or less as the upper limit.

[0043] Zr 4+ When the content is more than 0%, it is an optional component that can increase the refractive index and Abbe number of the glass. On the other hand, by setting the content of Zr 4+ to 10.0% or less, devitrification can be reduced, and a more homogeneous glass can be easily obtained. Therefore, the content of Zr 4+ is preferably 10.0% or less, more preferably 5.0% or less, and even more preferably 1.0% or less as the upper limit. Zr 4+ As raw materials, ZrO2, ZrF4, etc. can be used.

[0044] Ta 5+ When the content is more than 0%, it is an optional component that can increase the refractive index and increase the Abbe number. On the other hand, by making the content of Ta 5+ 10.0% or less, devitrification of the glass due to excessive content of Ta 5+ can be reduced. Therefore, the content of Ta 5+ is preferably 10.0% or less, more preferably 5.0% or less, still more preferably 3.0% or less, and even more preferably 1.0% or less as the upper limit. Ta 5+ can be Ta2O5 or the like as a raw material.

[0045] W 6+ is an optional component that can increase the refractive index, lower the Abbe number, and enhance the meltability of the glass raw material when its content exceeds 0%. On the other hand, by making the content of W 6+ 10.0% or less, coloring of the glass can be reduced and devitrification of the glass can be suppressed. Therefore, the content of W 6+ is preferably 10.0% or less, more preferably 5.0% or less, still more preferably 3.0% or less, and even more preferably 1.0% or less as the upper limit. W 6+ can be WO3 or the like as a raw material.

[0046] P 5+ is an optional component that can enhance the stability of the glass, improve the ion exchange performance, and increase the depth of the compressive stress layer when its content exceeds 0%. On the one hand, by making the content of P 5+ 10.0% or less, devitrification due to excessive content of P 5+ can be reduced, phase separation of the glass can be prevented, and a decrease in water resistance can be suppressed. Therefore, the content of P 5+ is preferably 10.0% or less, more preferably 5.0% or less, and still more preferably 3.0% or less as the upper limit. P 5+ can be Al(PO3)3, Ca(PO3)2, Ba(PO3)2, BPO4, H3PO4, etc. as a raw material.

[0047] Ge 4+is an optional component that can increase the refractive index when contained in an amount exceeding 0%. On the other hand, by setting the content of Ge 4+ to 10.0% or less, devitrification can be suppressed. Therefore, the content of Ge 4+ is preferably 10.0% or less, more preferably 5.0% or less, and even more preferably 3.0% or less as the upper limit. Ge 4+ can use GeO2 or the like as a raw material.

[0048] Sn 4+ is an optional component that can clarify (defoam) the melted glass and enhance the ion exchange performance when contained in an amount exceeding 0%. On the other hand, by setting the content of Sn 4+ to 1.0% or less, it is possible to make it difficult for the glass to be colored by the reduction of the molten glass and for the glass to devitrify. Also, since the alloying of Sn 4+ with the melting equipment (especially noble metals such as Pt) is reduced, the long life of the melting equipment can be achieved. Therefore, the content of Sn 4+ is preferably 1.0% or less, more preferably 0.5% or less, and even more preferably 0.1% or less as the upper limit. Sn 4+ can use SnO, SnO2, SnF2, SnF4, etc. as raw materials.

[0049] Sb 3+ is a component that promotes defoaming of the glass and clarifies the glass when contained in an amount exceeding 0%, and is an optional component in the optical glass of the present invention. Sb 3+ By setting the content relative to the total mass of the glass to 1.0% or less, it is possible to make it difficult for excessive foaming to occur during glass melting, and it is possible to make it difficult for Sb 3+ to alloy with the melting equipment (especially noble metals such as Pt). Therefore, the content rate of Sb 3+ is preferably 1.0% or less, more preferably 0.8% or less, and even more preferably 0.6% or less as the upper limit. Sb 3+ can use Sb2O3, Sb2O5, Na2H2Sb2O7·5H2O, etc. as raw materials.

[0050] Note that the components for fining and defoaming the glass are not limited to the above-mentioned Sb 3+ and known fining agents and defoaming agents in the field of glass manufacturing, or combinations thereof, can be used.

[0051] F - has the property of making the glass less likely to devitrify when contained in an amount exceeding 0%. On the other hand, when the content of F - is high, it excessively increases the Abbe number of the glass, decreases the refractive index, and decreases the liquidus temperature. Therefore, the content of F- is preferably 10.0% or less, more preferably 7.0% or less, and even more preferably 5.0% or less as the upper limit. F - can be contained in the glass by using fluorides of various cation components such as AlF3, MgF2, and BaF2 as raw materials.

[0052] <Components that should not be contained> Next, components that should not be contained in the optical glass of the present invention and components that are preferably not contained will be described.

[0053] Other components can be added as necessary within a range that does not impair the characteristics of the glass of the present invention. However, transition metal components such as V, Cr, Mn, Fe, Co, Ni, Cu, Ag, and Mo, excluding Ti, Zr, Nb, W, La, Gd, Y, Yb, and Lu, have the property that even when contained in a small amount alone or in combination, the glass is colored and absorbs at specific wavelengths in the visible region. Therefore, in optical glass that uses wavelengths in the visible region, it is preferably substantially free of them.

[0054] Also, lead compounds such as Pb 2+ and arsenic compounds such as As 3+ are components with a high environmental burden, so it is desirable to be substantially free of them, that is, not to contain them at all except for unavoidable contamination.

[0055] Furthermore, each of the components of Th, Cd, Tl, Os, Be, and Se has a tendency to refrain from being used as harmful chemicals in recent years, and environmental measures are required not only in the glass manufacturing process but also in the processing process and disposal after productization. Therefore, when emphasizing environmental impact, it is preferable not to substantially contain these components.

[0056] [Manufacturing Method] The optical glass used in the present invention is produced, for example, as follows. That is, the above raw materials are uniformly mixed so that each component is within a predetermined content range, and the prepared mixture is put into a platinum crucible, a quartz crucible, or an alumina crucible and roughly melted, and then put into a gold crucible, a platinum crucible, a platinum alloy crucible, or an iridium crucible and melted in a temperature range of 1000 to 1400 °C for 3 to 5 hours, stirred and homogenized to remove bubbles, etc., and then cooled to a temperature of 950 to 1250 °C and then finish-stirred to remove veins, and cast into a mold and slowly cooled.

[0057] [Optical Element] From the produced optical glass, a glass optical element can be produced, for example, by using means such as mold press forming. That is, a preform for mold press forming is produced from the optical glass, and the preform is subjected to mold press forming to produce a glass molded body, or for example, a preform produced by performing polishing is subjected to mold press forming to produce a glass optical element. Note that the means for producing the glass molded body is not limited to these means.

[0058] The optical element produced in this way is particularly preferably used for applications such as lenses and prisms. As a result, the blurring of colors due to chromatic aberration in the transmitted light of the optical system provided with the optical element is reduced. Therefore, when this optical element is used in a camera, the photographed object can be expressed more accurately, and when this optical element is used in a projector, a desired image can be projected with higher brilliance.

[0059] The chemical strengthening method of the optical element of the present invention is a method of introducing alkali ions with a large ionic radius onto the surface of glass by ion exchange treatment at a temperature below the strain point of the glass. If a compressive stress layer is formed by the chemical strengthening method, even when the plate thickness of the glass is small, the compressive stress layer can be properly formed. Also, after the compressive stress layer is formed, even if the strengthened glass is cut, unlike physical strengthening methods such as the air-cooling strengthening method, the strengthened glass does not easily break, which is a characteristic.

[0060] The chemical strengthening method can be carried out, for example, in the following steps. The crystallized glass base material is brought into contact with or immersed in a molten salt of a salt containing potassium or sodium, such as potassium nitrate (KNO3), sodium nitrate (NaNO3), or a mixed salt or double salt thereof. This treatment of bringing into contact with or immersing in the molten salt (chemical strengthening treatment) may be carried out in one step or in two steps.

[0061] For example, in the case of one-step chemical strengthening treatment, it is brought into contact with or immersed in a salt containing potassium or sodium, or a mixed salt thereof, heated at 350°C to 550°C for 30 minutes to 24 hours. In the case of two-step chemical strengthening treatment, first, it is brought into contact with or immersed in a sodium salt or a mixed salt of potassium and sodium heated at 350°C to 550°C for 30 minutes to 24 hours. Subsequently, second, it is brought into contact with or immersed in a potassium salt or a mixed salt of potassium and sodium heated at 350°C to 550°C for 30 minutes to 24 hours.

Example

[0062] Table 1 shows composition examples (a - c) of the glass used in this example and the comparative example. Note that the following examples are for illustrative purposes only and are not limited to these examples.

[0063] All the glasses in Table 1 were selected from high-purity raw materials such as the corresponding oxides, hydroxides, carbonates, nitrates, fluorides, metaphosphate compounds, etc. commonly used in optical glasses as raw materials for each component, weighed to the proportions shown in the composition of Table 1, uniformly mixed, and then put into a stone crucible (platinum crucible or alumina crucible may be used depending on the fusibility of the glass). After melting in an electric furnace at a temperature range of 1100 - 1400 °C for 0.5 - 5 hours according to the melting difficulty of the glass composition, it was transferred to a platinum crucible, stirred and homogenized to remove bubbles, etc., then the temperature was lowered to 1000 - 1200 °C, stirred and homogenized, and then cast into a mold and slowly cooled to produce the glass.

[0064]

Table 1

[0065] The refractive index (nd) and Abbe number (νd) of the glasses in Table 2 were measured according to the V-block method specified in JIS B 7071-2:2018.

[0066] The measurement method of the photoelastic constant of the glasses in Table 2 was as follows: the sample shape was ground by facing to a disc shape with a diameter of 25 mm and a thickness of 8 mm, a compressive load of 0 - about 100 kgf was applied in the side direction, the optical path difference at a wavelength of 546.1 nm generated at the center of the glass was measured, and it was obtained by the relational expression of δ = β·d·F. In the above formula, the optical path difference is expressed as δ (nm), the thickness of the glass is d (cm), and the stress is F (MPa).

[0067] The photoelastic constant is used to measure the compressive stress and the depth of the compressive stress layer on the surface.

[0068] The average linear thermal expansion coefficient α (100 - 300 °C) of the glasses in Table 2 was obtained from the thermal expansion curve obtained by measuring the relationship between temperature and the elongation of the sample according to the Japan Optical Glass Industry Association Standard JOGIS08-2003 "Measurement Method of Thermal Expansion of Optical Glass".

[0069] The Young's modulus of the glasses in Table 2 was measured according to the elastic modulus test method for fine ceramics specified in JIS R 1602:1995.

[0070] The temperature at which logη = 8.5 (viscosity) in Table 2 was measured by processing a glass sample into a shape of approximately φ7×7 mmt with parallel planes and using a glass parallel plate viscosity measuring device (model number: PPVM-1100).

[0071]

Table 2

[0072] Each sample (a-c) was cut and ground, and further polished on both sides to a thickness of approximately 1.0 mm to produce a glass plate.

[0073] Next, the glass plates produced were chemically strengthened under the conditions shown in Tables 3 to 5. The compressive stress and the depth of the compressive stress layer on the surface were calculated from the number and interval of interference fringes observed using a surface stress meter (FSM-6000LE manufactured by Oriehara Seisakusho). The measured values of the refractive index (nd) and the photoelastic constant (β) of each sample were used for the calculation.

[0074] Table 3 shows the examples and comparative examples of sample a subjected to chemical strengthening treatment.

[0075]

Table 3

[0076] Table 4 shows the examples and comparative examples of sample b subjected to chemical strengthening treatment.

[0077]

Table 4

[0078] Table 5 shows the examples of sample c subjected to chemical strengthening treatment.

[0079]

Table 5

[0080] As shown in Table 2, the temperature at which logη = 8.5 (viscosity) for the samples (a - c) used in this example and the comparative examples was less than 700°C. Therefore, it became clear that the samples (a - c) used in this example and the comparative examples were excellent in press formability.

[0081] Also, for all of the samples (a - c), the refractive index (nd) was 1.45000 or more and 1.70000 or less, and the Abbe number (νd) was 45.00 or more and 70.00 or less, which was within the desired range.

[0082] The compression stress of the examples shown in Tables 3 to 5 was 100 to 1000 Mpa, and the depth of the compression stress layer was 10 to 100 μm, which was within the desired range. Comparative example b - 1 shown in Table 4 had a compression stress layer depth of only 9.7 μm, which was outside the desired range.

[0083] As shown in Tables 3 to 5, the compression stress / thermal stress of the examples was 1.00 or more. Therefore, the examples shown in Tables 3 to 5 had good thermal shock resistance.

[0084] Although the present invention has been described in detail for illustrative purposes, it should be understood that this example is for illustrative purposes only, and those skilled in the art can make many modifications without departing from the spirit and scope of the present invention.

Claims

**Claim 1** An optical element having a compressive stress layer on its surface, wherein the compressive stress of the compressive stress layer is 100 to 1000 MPa, and the depth of the compressive stress layer is 10 to 100 μm, and the temperature at which logη = 8.5 (viscosity) is less than 700°C. **Claim 2** The optical element according to claim 1, wherein the refractive index (nd) is 1.45000 or more and 1.70000 or less, and the Abbe number (νd) is 45.00 or more and 70.00 or less. **Claim 3** The optical element according to claim 1 or 2, wherein the ratio of the compressive stress to the thermal stress (compressive stress / thermal stress) is 1.00 or more. **Claim 4** The optical element according to any one of claims 1 to 3, which is used in a projection optical system device. **Claim 5** The optical element according to any one of claims 1 to 3, which is used in an imaging optical system device.

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