WO2020158043A1 - Gradient index lens, optical product, optical device, glass composition for gradient index lens, and method for manufacturing gradient index lens - Google Patents

Abstract

A gradient index lens (1b) has a depth of field of 1.5 to 3.0 mm. The depth of field is determined by subtracting the minimum operating distance from the maximum operating distance. The value of a modulation transfer function (MTF) for a spatial frequency of six lines per millimeter is 30% or greater in an operating distance.

Description

Gradient index lens, optical product, optical device, glass composition for gradient index lens, and method for manufacturing gradient index lens

The present invention relates to a gradient index lens, an optical product, an optical device, a glass composition for a gradient index lens, and a method for manufacturing the gradient index lens.

Conventionally, a device for observing defects on the surface of a subject using a Charge-Coupled Device (CCD) is known. For example, Patent Document 1 describes a surface defect device including a light source, an irradiation unit, a condensing unit, and an observation unit. The observation means is composed of an imaging lens and a CCD.

Patent Document 2 describes an inspection device suitable for the appearance inspection of the photosensitive drum of an electrophotographic copying machine or printer. This inspection device is equipped with a camera device for photographing the photosensitive drum with a plurality of one-dimensional CCD cameras arranged in a line.

On the other hand, a contact image sensor (CIS) is also known as an image sensor. The CIS is equipped with a rod lens array. A gradient index lens is usually used for the rod lens array.

For example, Patent Documents 3 to 5 describe gradient index lenses. The gradient index lens or the gradient index rod lens is a rod-shaped (rod-shaped) lens having a gradient index distribution in which the refractive index continuously decreases from the center toward the outer periphery. In the gradient index lens described in Patent Document 3, the difference Δn between the refractive index on the peripheral surface of the lens and the refractive index on the central axis is 0.003 or more. According to Patent Document 3, when Δn is smaller than 0.003, the opening angle (2θ) becomes smaller than about 10°, which suggests that this is not desirable. In other words, it is considered that Patent Document 3 suggests that an opening angle (θ) of less than 5° is not desirable.

In Patent Document 4, the aperture angle of the gradient index lens according to the example is about 10.1 to 12.9°. In Patent Document 5, the aperture angle of the gradient index lens according to the example is 10.1 to 12.0°.

JP-A-7-27709 JP-A-2003-75906 Japanese Patent Publication No. 51-21594 JP 2005-289775 A Japanese Patent Laid-Open No. 2008-230956

Patent Documents 1 and 2 do not describe using a gradient index lens. The gradient index lenses described in Patent Documents 3 to 5 have a large aperture angle. This cannot be said to be advantageous from the viewpoint of realizing a large depth of field in the gradient index lens, and it is considered that the gradient index lenses described in Patent Documents 3 to 5 have a small depth of field.

In view of such circumstances, the present invention provides a gradient index lens having a large depth of field. The present invention also provides an optical product including such a gradient index lens and an optical device including the optical product. In addition, the present invention provides a glass composition for a gradient index lens, which is advantageous for increasing the depth of field of the gradient index lens. The present invention also provides an advantageous method for manufacturing a gradient index lens having a large depth of field.

The present invention is
With a depth of field of 1.5-3.0 mm,
The depth of field is determined by subtracting the minimum value from the maximum value of the working distance,
At the working distance, the value of the modulation transfer function (MTF) at a spatial frequency of 6 lines/mm is 30% or more,
A gradient index lens is provided.

The present invention is
An optical product provided with the above-mentioned gradient index lens is provided.

The present invention is
An optical device provided with the above optical product is provided.

The present invention is
Shown in mol%,
40%≦SiO ₂ ≦65%
0% ≤ TiO ₂ ≤ 10%
0.1%≦MgO≦22%
0.15%≦ZnO≦15%
0.5%≦Li ₂ O<4%
2% ≤ Na ₂ O ≤ 20%
0% ≤ B ₂ O ₃ ≤ 20%
0% ≤ Al ₂ O ₃ ≤ 10%
0%≦K ₂ O≦3%
0%≦Cs ₂ O≦3%
0% ≤ Y ₂ O ₃ ≤ 5%
0% ≤ ZrO ₂ ≤ 2%
0% ≤ Nb ₂ O ₅ ≤ 5%
0%≦In ₂ O ₃ ≦5%
0% ≤ La ₂ O ₃ ≤ 5%
0%≦Ta ₂ O ₅ ≦5%,
At least two selected from the group consisting of CaO, SrO, and BaO, each containing 0.1 mol% or more and 15 mol% or less,
Display in mol%,
2%≦MgO+ZnO,
0.07≦ZnO/(MgO+ZnO)≦0.93,
2.5%≦Li ₂ O+Na ₂ O<24%, and 0%≦Y ₂ O ₃ +ZrO ₂ +Nb ₂ O ₅ +In ₂ O ₃ +La ₂ O ₃ +Ta ₂ O ₅ ≦11%,
A glass composition for a gradient index lens is provided.

The present invention is
A method of manufacturing a gradient index lens, comprising:
Forming a glass wire made of a glass composition containing an oxide of a first alkali metal element,
The glass element wire is immersed in a molten salt containing a second alkali metal element different from the first alkali metal element, and the first alkali metal element in the glass element wire and the second alkali in the molten salt. By forming a refractive index distribution in the glass element wire by performing an ion exchange treatment with a metal element,
The glass composition, expressed in mol %,
40%≦SiO ₂ ≦65%
0% ≤ TiO ₂ ≤ 10%
0.1%≦MgO≦22%
0.15%≦ZnO≦15%
0.5%≦Li ₂ O<4%
2% ≤ Na ₂ O ≤ 20%
0% ≤ B ₂ O ₃ ≤ 20%
0% ≤ Al ₂ O ₃ ≤ 10%
0%≦K ₂ O≦3%
0%≦Cs ₂ O≦3%
0% ≤ Y ₂ O ₃ ≤ 5%
0% ≤ ZrO ₂ ≤ 2%
0% ≤ Nb ₂ O ₅ ≤ 5%
0%≦In ₂ O ₃ ≦5%
0% ≤ La ₂ O ₃ ≤ 5%
0%≦Ta ₂ O ₅ ≦5%,
The glass composition contains at least two selected from the group consisting of CaO, SrO, and BaO in an amount of 0.1 mol% or more and 15 mol% or less, respectively.
The glass composition is expressed in mol%,
2%≦MgO+ZnO,
0.07≦ZnO/(MgO+ZnO)≦0.93,
2.5%≦Li ₂ O+Na ₂ O<24%, and 0%≦Y ₂ O ₃ +ZrO ₂ +Nb ₂ O ₅ +In ₂ O ₃ +La ₂ O ₃ +Ta ₂ O ₅ ≦11%,
Provide a way.

The above gradient index lens has a large depth of field. Moreover, the glass composition for a gradient index lens described above is advantageous for increasing the depth of field of the gradient index lens.

FIG. 1 is a diagram illustrating a method of determining a depth of field of an example of a gradient index lens according to the present invention. FIG. 2 is a diagram showing an aperture angle of an example of the gradient index lens according to the present invention. FIG. 3A is a diagram showing an ion exchange process in an example of a method of manufacturing a gradient index lens according to the present invention. FIG. 3B is a graph conceptually showing the refractive index distribution in the gradient index lens. FIG. 4 is a perspective view showing an example of an optical product according to the present invention. FIG. 5 is a cross-sectional view showing an example of the optical device according to the present invention. FIG. 6 is a sectional view showing another example of the optical device according to the present invention. FIG. 7 is a diagram showing still another example of the optical device according to the present invention. FIG. 8 is a diagram showing still another example of the optical apparatus according to the present invention. FIG. 9 is a graph showing the relationship between the MTF value and the working distance of the gradient index lenses according to Example 2, Comparative Example 3, and Reference Example 1.

Image data collected in the visual inspection of the subject must have a resolution that can identify defects in the subject. When trying to obtain high-resolution image data using a camera equipped with a one-dimensional CCD sensor, the effective width that can be captured by one camera becomes small. Therefore, it may be difficult to image the entire subject with one camera. For example, when the pixel size corresponding to the required resolution in the image data is 90 μm, the width of the area that can be imaged by the camera equipped with the one-dimensional CCD sensor of 4096 pixels is about 370 mm. In this case, in order to thoroughly inspect an object having a width of 1200 mm, it is necessary to arrange four camera systems each including a one-dimensional CCD sensor and a camera lens in the width direction. Mounting a plurality of camera systems equipped with a one-dimensional CCD sensor increases the manufacturing cost of the device. In addition, it is necessary to adjust and maintain a plurality of camera systems every time the type of subject is changed, which increases the running cost of the examination.

Therefore, it is possible to use CIS to perform a visual inspection of the subject. The CIS includes a plurality of one-dimensional light receiving elements arranged on a substrate and a rod lens array. In CIS, the rod lens array forms an erecting equal-magnification image. If CIS is used, a one-dimensional image having a width of 1200 mm can be obtained by one unit. The rod lens array is an array of a plurality of rod-shaped gradient index lenses, for example. The gradient index lens has a refractive index distribution in the radial direction, and the refractive index changes from the central portion to the peripheral portion in the radial direction of the gradient index lens. The CIS including the rod lens array can reduce the distance between the image pickup element and the object to be photographed to about 1/10 of that of a conventional camera system including a CCD sensor and a lens, which makes the device compact. It is advantageous in terms of conversion. On the other hand, in CIS, the depth of field (DOF), which is a characteristic value indicating the allowable range of the distance between the object to be photographed and the lens, is small. This causes a problem that when the subject has a variation in thickness, a part of the subject that is in focus and a part that is not in focus occur. For this reason, the image of the out-of-focus portion is not clear, and oversight of the defect and erroneous recognition of the defect may occur.

As described above, the aperture angles of the gradient index lenses described in Patent Documents 3 to 5 are large, and this is hard to say from the viewpoint of increasing the DOF of the gradient index lens. Therefore, the present inventors drastically reviewed the conditions of the glass composition used for manufacturing the gradient index lens in order to realize the DOF in the desired range in the gradient index lens. As a result of many trials and errors, the present inventors finally found a gradient index lens that can realize a DOF in a desired range. The gradient index lens according to the present invention can be used not only in the field of visual inspection of a subject but also in the field of image formation of image scanners, copiers, facsimiles, printers and the like.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the following description relates to an example of the present invention, and the present invention is not limited to the following embodiments.

The gradient index lens 1b has a depth of field (DOF) of 1.5 to 3.0 mm. The DOF of the gradient index lens 1b is determined by subtracting the minimum value from the maximum value of the working distance. At the working distance of the gradient index lens 1b, the value of the modulation transfer function (MTF) at a spatial frequency of 6 lines/mm is 30% or more.

As shown in FIG. 1, in the DOF of the gradient index lens 1b, for example, the lens array 10a, the line pattern 3, and the light receiving element 2 are arranged at a predetermined interval in the optical axis direction, and the lens array 10a and the line pattern 3 are arranged. It can be determined by calculating the value of MTF while varying the distance between and. The lens array 10a is configured by arranging a plurality of gradient index lenses 1b in a direction perpendicular to the optical axis. The line pattern 3 has black and white line pairs corresponding to a spatial frequency of 6 lines/mm. The light receiving element 2 is, for example, a CCD sensor. For example, the light emitted from the halogen lamp is passed through the color filter and the light diffusion plate and then applied to the line pattern 3. The color filter may be, for example, one that transmits light in the wavelength range of 500 to 600 nm, or may be one that mainly transmits the wavelength of 530 nm. At this time, the value of MTF is formed on the light receiving element 2 by the lens array 10a with respect to the image (input image) of the line pattern 3 having a predetermined spatial frequency consisting of the bright portion and the dark portion before entering the lens array 10a. It can be determined as the reproducibility of the image obtained (output image).

A distance (distance between object point and image forming point) D _max between the line pattern 3 and the light receiving element 2 where the value of MTF is maximum is determined. After that, the line pattern 3 is moved in the positive direction (ΔL>0) and the negative direction (ΔL<0) of the Z axis parallel to the optical axis, and the MTF value is obtained at each position. Thereby, the maximum value and the minimum value of the working distance can be obtained by setting the allowable range of the predetermined MTF value. As a result, the DOF of the gradient index lens 1b can be determined. When ΔL>0, the distance between the line pattern 3 and the lens array 10a is larger than the working distance corresponding to the distance D _max . On the other hand, when ΔL<0, the distance between the line pattern 3 and the lens array 10a is smaller than the working distance corresponding to the distance D _max . At ΔL=0, the distance between the line pattern 3 and the lens array 10a is equal to the working distance corresponding to the distance D _max .

Since the DOF of the gradient index lens 1b is in the above range, it is advantageous for obtaining image data suitable for visual inspection of a subject having, for example, uneven thickness, steps, and irregularities. Therefore, the gradient index lens 1b can contribute to the improvement of the accuracy of the appearance inspection of the subject and the sophistication of the inspection standard.

DOF of the gradient index lens 1b is preferably 1.5 mm or more, more preferably 1.8 mm or more, and further preferably 2 mm or more. The DOF of the gradient index lens 1b is preferably 2.8 mm or less, more preferably 2.5 mm or less.

The working distance corresponding to the object point-imaging point distance D _max that maximizes the MTF value is, for example, 15 mm or more, and preferably 18 mm or more. Since the object point-imaging point distance D _max is in these ranges, the DOF becomes an appropriate range. By incorporating the rod lens array including the gradient index lens 1b into the inspection device, an object having a step can be inspected. In addition, the rod lens array can maintain an appropriate distance from the object, and the assembly of the optical system can be facilitated.

The gradient index lens 1b has an aperture angle θ of 3 to 6°, for example. Thereby, the DOF of the gradient index lens 1b is easily adjusted to a desired range. The aperture angle θ of the gradient index lens 1b is defined as shown in FIG. 2, for example. The opening angle θ is the maximum value of the angle formed by the light beam that can enter one end of the optical axis of the gradient index lens 1b and the optical axis. In FIG. 2, F1 is a subject surface, and F2 is a light receiving surface (image forming surface) of a light receiving element or the like. Z ₀ is the length of the gradient index lens 1b. L _o is the distance between the subject surface F1 and the gradient index lens 1b when the MTF value is maximum, and L _i is the refraction with the image formation plane F2 when the MTF value is maximum. It is a distance from the rate distribution type lens 1b. In FIG. 2, the lens array 10a constitutes a substantially erecting equal-magnification imaging system, and the distance L _i is substantially equal to the distance L _o . In FIG. 2, X ₀ is the field radius of the gradient index lens 1b. The aperture angle θ of the gradient index lens 1b can be determined, for example, according to the method described in the examples. In the method described in the examples, the aperture angle θ may be determined using the refractive index n ₀ at the center of the gradient index lens 1b instead of the refractive index Nc of the glass element wire before ion exchange. .. Nc or n ₀ is the Japanese Industrial Standard (JIS) B 7071-2: can be obtained by using the V-block method described in 2018.

The aperture angle θ of the gradient index lens 1b may be 3.5° or more, or 3.7° or more. The aperture angle θ of the gradient index lens 1b is preferably 5.5° or less, more preferably 5.2° or less.

The gradient index lens 1b may have, for example, a gradient index constant (√A) of 0.130 to 0.230 mm ⁻¹ . Note that √A means the square root of A. If the refractive index at the radius r of the gradient index lens is n(r), then n(r)=n ₀ ×{1-(A/2)×r ² } holds in the paraxial region of the lens. If the refractive index distribution constant √A is in such a range, the aperture angle θ of the gradient index lens 1b is likely to be within a desired range. As a result, the DOF of the gradient index lens 1b is easily adjusted to a desired range.

Refractive index distribution constant gradient index lens 1b may also be 0.140 mm ^-1 or more, may be 0.145 mm ^-1 or more. The refractive index distribution constant of the gradient index lens 1b is preferably 0.210 mm ^-1 or less, and more preferably 0.205 mm ^-1 or less.

The image forming distance of the erect image on the gradient index lens 1b is, for example, 45 to 80 mm. This is advantageous from the viewpoint of adjusting the DOF of the gradient index lens 1b within a desired range.

The image forming distance of the erect image on the gradient index lens 1b may be 47 mm or more, 50 mm or more, 53 mm or more, and 54 mm or more. The image forming distance of the erect image on the gradient index lens 1b may be 75 mm or less, 70 mm or less, or 67 mm or less.

The gradient index lens 1b is typically a rod-shaped or fiber-shaped lens. The gradient index lens 1b has, for example, a gradient index distribution as shown in FIG. 3B in the radial direction. Here, in FIG. 3B, the refractive index n _{0 at the} origin means the refractive index at the central axis of the gradient index lens 1b. r represents the position of the gradient index lens 1b in the radial direction.

The gradient index lens 1b prevents noise light (so-called white noise (stray light)) from being generated by reflecting incident light having an incident angle larger than the opening angle on the side surface of the lens, as necessary. It may have a structure. Such a structure may be, for example, a light absorbing layer or a light scattering layer provided on the side surface of the lens. For example, the gradient index lens 1b may have a core-clad structure in which a colored layer serving as a light absorbing layer is arranged on the side surface of the lens, or a fine uneven portion serving as a light scattering layer is located on the side surface. It may have a formed structure.

The gradient index lens 1b is typically a glass lens. The glass composition for a gradient index lens has a mol% of 40%≦SiO ₂ ≦65%, 0%≦TiO ₂ ≦10%, 0.1%≦MgO≦22%, 0.15%≦. ZnO≦15%, 0.5%≦Li ₂ O<4%, 2%≦Na ₂ O≦20%, 0%≦B ₂ O ₃ ≦20%, 0%≦Al ₂ O ₃ ≦10%, 0 %≦K ₂ O≦3%, 0%≦Cs ₂ O≦3%, 0%≦Y ₂ O ₃ ≦5%, 0%≦ZrO ₂ ≦2%, 0%≦Nb ₂ O ₅ ≦5%, Including 0%≦In ₂ O ₃ ≦5%, 0%≦La ₂ O ₃ ≦5%, and 0%≦Ta ₂ O ₅ ≦5%. In addition, the gradient index lens glass composition contains at least two selected from the group consisting of CaO, SrO, and BaO in an amount of 0.1 mol% or more and 15 mol% or less, respectively. Further, the glass composition for a gradient index lens is represented by mol %, 2%≦MgO+ZnO, 0.07≦ZnO/(MgO+ZnO)≦0.93, 2.5%≦Li ₂ O+Na ₂ O< The conditions of 24% and 0%≦Y ₂ O ₃ +ZrO ₂ +Nb ₂ O ₅ +In ₂ O ₃ +La ₂ O ₃ +Ta ₂ O ₅ ≦11% are satisfied. By using such a glass composition, it is possible to obtain a gradient index lens having a desired DOF.

The gradient index lens 1b can be manufactured, for example, by subjecting a glass element wire made of the above glass composition to an ion exchange treatment.

(SiO ₂ )
SiO ₂ is an essential component that forms a glass network structure. When the content of SiO ₂ is less than 40 mol %, the content of other components necessary for developing the optical characteristics as a gradient index lens after ion exchange becomes relatively large, and devitrification is likely to occur. Become. If the content is less than 40 mol%, the chemical durability of the glass composition will be significantly reduced. On the other hand, when the content exceeds 65 mol %, the content of other components such as an alkali component for forming a refractive index distribution, a refractive index increasing component, and a physical property value adjusting component is limited, which is practical. It becomes difficult to obtain a simple glass composition. Therefore, the content of SiO ₂ is 40 mol% or more and 65% mol or less.

(TiO ₂ )
TiO ₂ is an essential component that has the effect of increasing the refractive index of the glass composition. By increasing the refractive index of the base glass composition, the central refractive index of the gradient index lens obtained from the glass composition can be increased. Further, by increasing the content of TiO ₂ , the refractive index distribution in the gradient index lens can be brought closer to an ideal state, and it becomes possible to manufacture a gradient index lens having excellent resolution. When the content of TiO ₂ is 10 mol %, no decrease in the resolution of the image based on the obtained lens is observed, but when the content is less than 1 mol %, the resolution of the image is obviously decreased, which is not practical. I can't get a lens. On the other hand, when the content exceeds 10 mol %, the coloring becomes strong and the chromatic aberration increases, so that a practical lens cannot be obtained. Therefore, in order to obtain a lens with high image resolution and small chromatic aberration, the content of TiO ₂ is 1 mol% or more and 10 mol% or less. The content of TiO ₂ is desirably 2 mol% or more and 8 mol% or less.

(MgO)
MgO is an essential component that has a function of lowering the melting temperature of the glass composition and increasing the refractive index difference (Δn) between the lens central portion and the peripheral portion after ion exchange. If the MgO content exceeds 22 mol %, devitrification is likely to occur. On the other hand, if the content of MgO exceeds 22 mol %, the content of the other components is excessively reduced, and a practical glass composition cannot be obtained. Therefore, the MgO content is 0.1 mol% or more and 22 mol% or less. From the viewpoint of realizing a sufficient difference in refractive index, the content of MgO is preferably 2 mol% or more. When the content of MgO is 2 mol% or more, the content of the alkaline earth metal oxides (CaO, SrO, BaO) can be more appropriately controlled for the purpose of further reducing the mobility of alkali ions. .. That is, the content of MgO is preferably 2 mol% or more and 22 mol% or less, and more preferably 2 mol% or more and 16 mol% or less.

(ZnO, MgO+ZnO, ZnO/(MgO+ZnO))
ZnO has the function of improving the weather resistance of the glass composition and the gradient index lens. In the glass composition according to the present invention, ZnO may be added to replace a part of MgO. From the viewpoint of enhancing the weather resistance of the glass composition and the gradient index lens, the ZnO content is 0.15 mol% or more and 15 mol% or less. At this time, the content rates of MgO and ZnO are adjusted so that the total content rate of MgO and ZnO (MgO+ZnO) is 2 mol% or more. In addition, the content of MgO and ZnO is such that the ratio of the content of ZnO to the total content of MgO and ZnO (ZnO/(MgO+ZnO)) is 0.07≦ZnO/(MgO+ZnO)≦0.93. The rate is adjusted. From the viewpoint of further improving the weather resistance of the glass composition and the gradient index lens, the ZnO content is desirably 3% by mole or more and 15% by mole or less. In this case, MgO+ZnO may be 6 mol% or more, and the condition of 0.12≦ZnO/(MgO+ZnO)≦0.93 may be satisfied. From the viewpoint of devitrification resistance, the ZnO content is preferably 8 mol% or less. From the viewpoint of further improving the weather resistance of the glass composition and the gradient index lens, the ZnO content is more preferably 4 mol% or more and 15 mol% or less. In this case, MgO+ZnO may be 6 mol% or more, and MgO+ZnO may be 6 mol% or more and 22 mol% or less. MgO+ZnO may be 15 mol% or less. Further, ZnO/(MgO+ZnO) is preferably 0.07 or more and 0.9 or less, more preferably 0.25 or more and 0.85 or less, and further preferably 0.25 or more and 0.8 or less, It is particularly preferably 0.3 or more and 0.8 or less.

(Li ₂ O)
Li ₂ O is an essential component, and is one of the most important components in order to ion-exchange the glass composition of the present invention to obtain a gradient index lens. Conventionally, when the content of Li ₂ O in a glass composition is low, it is considered that a sufficient concentration distribution, that is, a sufficient refractive index distribution cannot be expressed by ion exchange, and an appropriate gradient index lens cannot be obtained. It was being done. However, the inventors of the present invention have a suitable refractive index distribution by performing ion exchange under predetermined conditions even for a glass composition having a Li ₂ O content of 4 mol% or less, and , Newly found that a gradient index lens having a large DOF can be manufactured. When the content of Li ₂ O exceeds 4 mol% or less, the aperture angle of the obtained gradient index lens tends to be large and the DOF tends to be small. The content of Li ₂ O is 0.5 mol% or more, preferably 0.7 mol% or more, and more preferably 1 mol% or more. The content of Li ₂ O is 4 mol% or less, preferably 3.5 mol% or less, more preferably 3 mol% or less, and further preferably 2 mol% or less.

One of the characteristics of the gradient index lens 1b is that the content of Li ₂ O is lower than that of various prior arts. In the past, there was a reason in the manufacturing process that the content of Li ₂ O could not be reduced. The inventors of the present invention have made a new device such as limiting the amount of glass strands to be processed per batch by the ion exchange method and reducing the initial content of Li in the molten salt, thereby making it possible to reduce the field curvature and the like. It was newly found that it is possible to obtain a gradient index lens having a smaller aperture angle and a practical resolution while suppressing the aberration of the lens.

(Na ₂ O)
At the time of ion exchange, Na ₂ O assists the ion exchange between Li and the ion of the ion exchange species (the ion contained in the molten salt) that replaces the Li ion by the so-called mixed alkali effect, so that the ion is easily exchanged. Keep the mobility moderate. By keeping the ion mobility moderate, the ion exchange rate can be adjusted appropriately, and the optical characteristics of the gradient index lens can be adjusted. If the content of Na ₂ O in the glass composition is less than 2 mol %, the glass becomes hard at the time of glass molding, which makes molding difficult. In addition, the melting temperature of the glass rises significantly, making it difficult to manufacture a lens. In addition, it is difficult to obtain a sufficient effect of keeping the mobility of ions moderate. On the other hand, when the content of Na ₂ O exceeds 20%, the chemical durability of the glass is lowered and the practicality is impaired. Therefore, the content of Na ₂ O is 2 mol% or more, preferably 5 mol% or more, and more preferably 10 mol% or more. The content of Na ₂ O is 20 mol% or less, preferably 17 mol% or less.

(Li ₂ O+Na ₂ O)
As described above, the total of the content of Li ₂ O and the content of Na ₂ O (Li ₂ O+Na ₂ O) in the glass composition is 2.5 mol% or more and less than 24 mol %. When Li ₂ O+Na ₂ O is in this range, an image with good resolution can be obtained by the gradient index lens manufactured using this glass composition. Li ₂ O+Na ₂ O is preferably 6 mol% or more, more preferably 10 mol% or more.

(Li ₂ O/Na ₂ O)
When Na ₂ O Li ₂ O ratio of content of the relative content of _{(Li 2 O / Na 2 O} ) is large, it is possible to improve resolution of refractive index distribution type lenses produced using the glass composition .. On the other hand, if Li ₂ O/Na ₂ O is excessively large (for example, 1.0 or more), the aperture angle of the gradient index lens manufactured using the glass composition tends to be large and its DOF tends to be small. is there. Therefore, Li ₂ O/Na ₂ O is, for example, 0.2 or less, preferably 0.15 or less, and more preferably 0.1 or less.

The above glass composition may further contain the following components.

(B ₂ O ₃ )
B ₂ O ₃ is an optional component that forms a network structure of glass, promotes vitrification of the glass composition and changes its viscosity without substantially changing the resolution and aperture angle θ of the obtained gradient index lens. Has the effect of adjusting. It also has a slight effect of slowing the ion exchange rate of the glass composition. B ₂ O ₃ has, for example, the content ratio of each of the above-mentioned essential components within the scope of the present invention, but when viewed as a composition, the content ratio of some components becomes relatively large, and B ₂ O ₃ is It may be added when the stability decreases (for example, devitrification tends to occur). By adding B ₂ O ₃ , it is possible to reduce the content ratios of the above-mentioned some components which have become relatively large, without changing the content ratios of the essential components. The content of B ₂ O _{3 that} can be added without changing the resolving power and the aperture angle of the obtained gradient index lens is, for example, 20% mol or less. Therefore, the content of B ₂ O ₃ is 0% or more and 20% or less. The content is preferably 0 mol% or more and 10 mol% or less, and when the glass composition contains B ₂ O ₃ , the content is preferably 1 mol% or more and 10 mol% or less.

(Al ₂ O ₃ )
The glass composition for a gradient index lens may contain Al ₂ O ₃ as an optional component, and the content thereof is 0 mol% or more and 10 mol% or less.

(SiO ₂ +TiO ₂ +B ₂ O ₃ )
In the glass composition for gradient index lenses, the total content of SiO ₂ , TiO ₂ , and B ₂ O ₃ (SiO ₂ +TiO ₂ +B ₂ O ₃ ) is, for example, 41 mol% or more and 70 mol% or less. It is preferably 50 mol% or more and 70 mol% or less.

_{(Y 2 O 3, ZrO 2} , Nb 2 O 5, In 2 O 3, La 2 O 3, Ta 2 O 5)
The glass composition for a gradient index lens includes Y ₂ O ₃ , ZrO ₂ , Nb ₂ O ₅ , and In for the purpose of adjusting the refractive index of the gradient index lens obtained after ion exchange or improving the weather resistance. _It may contain at least one component selected from the group consisting of ₂ O ₃ , La ₂ O ₃ , and Ta ₂ O ₅ . The total content of these components is 0 mol% or more and 11 mol% or less. When the glass composition for a gradient index lens contains these components, the total content of these components is preferably 0% or less. It is at least 2 mol% and at most 6 mol %. Further, it is desirable that the sum of the content rates of these components and the ZnO content is 15 mol% or less.

(Y ₂ O ₃ )
The content of Y ₂ O ₃ is desirably 0 mol% or more and 5 mol% or less.

(ZrO ₂ )
The content of ZrO ₂ is preferably 0 mol% or more and 2 mol% or less, and when the glass composition for a gradient index lens contains ZrO ₂ , the content is 0.2 mol% or more and 2 mol% or less. Is.

The respective contents of Nb ₂ O ₅ , In ₂ O ₃ , La ₂ O ₃ and Ta ₂ O ₅ are desirably 0 mol% or more and 5 mol% or less.

(K ₂ O, Cs ₂ O)
K ₂ O and Cs ₂ O are optional components having the action of reducing the mobility of alkali ions, like MgO, CaO, SrO, and BaO, due to the mixed alkali effect. Each of the content rates of K ₂ O and Cs ₂ O is, for example, 0 mol% or more and 3 mol% or less. From the viewpoint of increasing the water resistance of the glass composition for a gradient index lens, the content of Cs ₂ O is preferably less than 2 mol%, more preferably 0 mol% or more and 1 mol% or less, and further preferably Is 0.5 mol% or less. From the viewpoint of enhancing the water resistance of the glass composition for gradient index lenses, it is desirable that the glass composition for gradient index lenses does not substantially contain Cs ₂ O. The term "substantially free" as used herein means that the content of the component is less than 0.1 mol %.

(Other ingredients)
The glass composition for a gradient index lens may contain GeO ₂ as another component. The content of GeO ₂ may be 0 mol% or more and 10 mol% or less. Further, the glass composition for gradient index lens may contain at least one selected from the group consisting of SnO ₂ , As ₂ O ₃ , and Sb ₂ O ₃ as an additive. The content of each of SnO ₂ , As ₂ O ₃ , and Sb ₂ O ₃ may be 0 mol% or more and 1 mol% or less. The gradient index lens glass composition may consist essentially of the above components. In this case, the content rate of each component contained in the glass composition and the relationship between the content rates of the respective components (total and content ratio) satisfy each of the above-mentioned conditions. In the present specification, “consisting essentially of” means that the content of impurities is less than 0.1 mol %.

(PbO)
The glass composition for a gradient index lens does not substantially contain lead (a typical compound is PbO). Further, the gradient index lens 1b also contains substantially no lead.

In the glass composition for a gradient index lens, for example, the water resistance determined according to Japan Optical Glass Industry Association Standard (JOGIS) 06-2009 is first grade. In this case, the glass composition for a gradient index lens has high water resistance, and the gradient index lens manufactured using the glass composition for a gradient index lens also tends to have high water resistance. Even in the glass forming the gradient index lens, the water resistance determined according to JOGIS 06-2009 may be first grade.

The gradient index lens glass composition contains an oxide of a first alkali metal element. The gradient index lens 1b can be manufactured by, for example, a method including the following steps (I) and (II).
(I) A glass element wire 1a made of the above glass composition for a gradient index lens is formed.
(II) The glass element wire 1a is immersed in a molten salt S containing a second alkali metal element R different from the first alkali metal element Q contained in the glass composition for gradient index lens, The first alkali metal element Q and the second alkali metal element R in the molten salt are subjected to an ion exchange treatment to form a refractive index distribution in the glass element wire 1a.

In the step (II), for example, as shown in FIG. 3A, the glass wire 1a is put into the molten salt S inside the container V, and the glass wire 1a is immersed in the molten salt S for a predetermined time. In the molten salt S, for example, at least one of potassium nitrate and sodium nitrate is molten. When the glass element wire 1a is immersed in the molten salt S, for example, the cation of the first alkali metal element Q such as Li (lithium) contained in the glass element wire 1a is dissolved in the molten salt S. On the other hand, the cations of the second alkali metal element R such as K (potassium) in the molten salt S enter the glass element wire 1a. By adjusting the temperature of the molten salt S and the immersion time of the glass element wire 1a in the molten salt S, ion exchange between the cations of the first alkali metal element Q and the cations of the second alkali metal element R is appropriately performed. You can control. A specific monovalent cation concentration distribution occurs inside the glass element wire 1a, and a refractive index distribution as shown in FIG. 3B is formed on the glass element wire 1a according to this concentration distribution. Thereby, the gradient index lens 1b can be manufactured from the glass element wire 1a.

The optical product according to the present invention is not limited to a specific product as long as it has the gradient index lens 1b. For example, a predetermined lens array can be provided by using the gradient index lens 1b. In this case, the lens array may have a zero-dimensional array, a one-dimensional array, or a two-dimensional array with respect to the array of the gradient index lenses 1b. The 0-dimensional array is, for example, a configuration in which a single gradient index lens 1b is arranged, and is expected to have a desired action by an optical product including the single gradient index lens 1b. The one-dimensional arrangement is a configuration in which a plurality of gradient index lenses 1b are arranged in a line in a specific direction. The specific direction is called the main scanning direction, and the direction perpendicular to the main scanning direction and perpendicular to the optical axis is called the sub-scanning direction. The plurality of gradient index lenses 1b are arranged such that their optical axes are substantially parallel. The two-dimensional array is a configuration in which a plurality of lenses are arrayed in a direction different from that in addition to the one-dimensional array. For example, a configuration in which a plurality of gradient index lenses 1b are arranged in two or more rows along the main scanning direction may correspond to a two-dimensional arrangement. According to the lens array 10b, an erecting equal-magnification image in a wide range can be obtained even if the diameter of each gradient index lens is small.

For example, the lens array 10b shown in FIG. 4 can be provided by using the gradient index lens 1b. In the lens array 10b, a plurality of gradient index lenses 1b are arranged so that their optical axes are substantially parallel. In the lens array 10b, the plurality of gradient index lenses 1b are arranged in two rows so as to form a two-dimensional array. In the lens array 10b, the plurality of gradient index lenses 1b are arranged, for example, between a pair of fiber reinforced plastic (FRP) substrates 5. Between the pair of FRP substrates 5, a space between the plurality of gradient index lenses 1b and a space between the FRP substrate 5 and the gradient index lenses 1b are filled with the black resin 7. Thereby, the plurality of gradient index lenses 1b are integrated between the pair of FRP substrates 5. Such a lens array 10b can be manufactured as follows, for example. First, a plurality of gradient index lenses 1b are arranged substantially in parallel on the surface of one FRP substrate 5, and the other FRP substrate 5 holds the lenses. After that, the space between the pair of FRP substrates 5 is filled with the black resin 7, and the whole is integrated. Further, the end surface of the gradient index lens 1b is polished as needed.

The lens array 10b can be changed from various points of view, and the material of each part forming the lens array may be a known material in the production of the lens array. The array of the plurality of gradient index lenses 1b is not limited to two rows. The plurality of gradient index lenses 1b may be arranged in one line, or may be arranged in three or more lines. By arranging the gradient index lenses 1b in a large number of rows, it is possible to provide a lens array that can accommodate a large area.

The gradient index lens 1b may be a plastic rod lens having the above optical performance. The plastic rod lens can be produced by a method such as a copolymerization method, a sol-gel method, and an interdiffusion method. In particular, in the mutual diffusion method, resins are laminated concentrically in such a manner that the refractive index gradually decreases from the center to the outer periphery, and then the materials between layers are mutually diffused so that the refractive index becomes continuous. After such a treatment, the rod-shaped rod lens is obtained by further heating and stretching. Due to the characteristics of the material, the plastic rod lens is easy to handle, is generally inexpensive, and may be advantageous in some cases.

The lens array provided with the gradient index lens 1b has a large DOF and is excellent in weather resistance in some cases, and can be widely used for optical devices such as a scanner, a copying machine, a facsimile, a printer, a CIS, and a line camera. it can. Furthermore, since the lens array provided with the gradient index lens 1b is particularly excellent in water resistance (moisture resistance), it can be used not only for general air conditioning in offices, but also for factories, storage warehouses, or transportation units that are exposed to hot and humid conditions. It can be applied to the above-mentioned optical devices even in various environments including physical distribution such as trucks.

The lens array 10b can be used to provide the CIS scanner 100 shown in FIG. 5, for example. The CIS scanner 100 includes, for example, a lens array 10b, a housing 11, a linear light receiving element 12, a linear illumination device 13, and a document table 14. The line-shaped light receiving element 12 extends in the main scanning direction of the lens array 10b. In FIG. 5, the direction parallel to the X axis is the main scanning direction, and the direction parallel to the Y axis is the sub scanning direction. The line-shaped illumination device 13 extends in the main scanning direction of the lens array 10b. The document table 14 is formed of a glass plate. The glass plate forming the document table 14 is arranged so as to cover the opening of the housing 11. The lens array 10 b, the line-shaped light receiving element 12, and the line-shaped illumination device 13 are arranged inside the housing 11. Illumination light is linearly irradiated from the line-shaped illumination device 13 to the document P placed on the document table 14. The lens array 10b is arranged so that the light reflected on the surface of the document P is incident on the linear light receiving element 12. A two-dimensional image of the document P is obtained by scanning the scanner mechanism including the lens array 10b and the linear light receiving element 12 in the sub-scanning direction or by transporting the document P placed on the document table 14 in the sub-scanning direction. You can get the data.

Since the lens array 10b includes the gradient index lens 1b having a large DOF, the quality of the read image is high even in a portion where a part of the document P floats due to a wrinkle or a spread. Easy to be good.

Using the lens array 10b, for example, the scanner 300 shown in FIG. 6 can be provided. The scanner 300 includes a housing 31, a linear light receiving element 32, a linear illumination device 33, a first spacer 34a, a second spacer 34b, and a substrate 35. In the scanner 300, the linear lighting device 33 is arranged outside the housing 31. For example, in the scanner 300, in order to appropriately adjust the optical arrangement of the portion of the document P to be read and the line-shaped light receiving element 32, the lens array 10b includes a housing formed by the first spacer 34a and the second spacer 34b. It is positioned and fixed with respect to 31. The scanner 300 may be applied to an apparatus that inspects the appearance of a subject, and may be used to obtain an image from a subject (inspection object) instead of the document P. In this case, the light beam emitted from the line-shaped illumination device 33 is applied to the subject, and the light reflected on the surface of the subject is imaged on the line-shaped light receiving element 32 by the image forming action of the lens array 10b. The line-shaped light receiving element 32 can sequentially convert one-dimensional image information of the surface of the subject into an electric signal and output the electric signal.

By using the lens array 10b, for example, the printer 500 shown in FIG. 7 can be provided. The printer 500 includes a writing head 51, a photosensitive drum 52, a charging device 53, a developing device 54, a transfer device 55, a fixing device 56, an erasing lamp 57, a cleaning device 58, and a paper feeding cassette 59. I have it. The lens array 10b is arranged inside the write head 51. The printer 500 is an electrophotographic printer. The writing head 51 includes a lens array 10b and a light emitting element array (not shown). The lens array 10b constitutes an image forming optical system that exposes the light emitted from the light emitting element array onto the photosensitive drum 52. Specifically, the focal point of the lens array 10b is located on the surface of the photosensitive drum 52 and constitutes an erecting equal-magnification optical system. On the surface of the photosensitive drum 52, a photosensitive layer made of a photoconductive material (photoconductor) such as amorphous Si is formed. First, the surface of the rotating photosensitive drum 52 is uniformly charged by the charger 53. Next, the write head 51 irradiates the photosensitive layer of the photosensitive drum 52 with light of a dot image corresponding to the image to be formed, neutralizes the charge in the light-irradiated region of the photosensitive layer, and the latent image is formed on the photosensitive layer. An image is formed. Next, when toner is attached to the photosensitive layer by the developing device 54, the toner adheres to the portion of the photosensitive layer where the latent image is formed according to the charged state of the photosensitive layer. Next, the adhered toner is transferred to the paper sent from the cassette by the transfer device 55, and then the paper is heated by the fixing device 56, so that the toner is fixed on the paper and an image is formed. On the other hand, the charge on the photosensitive drum 52 after the transfer is neutralized by the erasing lamp 57 over the entire area, and then the cleaning device 58 removes the toner remaining on the photosensitive layer.

By using the lens array 10b, for example, the inspection device 700 shown in FIG. 8 can be provided. The inspection device 700 includes a CIS scanner 71, a linear illumination device 72, a controller 73, an output device 74, a transfer device 75, and a transfer control device 76. A lens array 10b is arranged inside the CIS scanner 71. The transport device 75 is, for example, a belt conveyor. The transport device 75 transports a subject T such as a printed circuit board, a textile, and paper. The transport control device 76 is a digital computer for controlling the transport device 75, and outputs a control signal for adjusting the transport speed of the transport device 75 to the transport device 75. The CIS scanner 71 and the line-shaped illumination device 72 are arranged, for example, above the transport device 75, and the subject T passes directly below the CIS scanner 71 by the transport device 75. The CIS scanner 71 and the line-shaped illumination device 72 are arranged so that clear image data of the subject T can be obtained. The controller 73 is a digital computer for forming image data of the subject T. When the subject T passes beneath the CIS scanner 71, the controller 73 continuously acquires one-dimensional image information from the CIS scanner 71. In addition, the controller 73 acquires the transport position information of the subject T from the transport control device 76. The controller 73 performs a calculation process based on the one-dimensional image information acquired from the CIS scanner 71 and the transportation position information acquired from the transportation control device 76 to form two-dimensional image information. The formed two-dimensional image information is compared with information stored in the controller 73 in advance and characterizing defects such as foreign matter, cracks, and pinholes. Thereby, the controller 73 identifies the presence/absence of a defect in the subject T, the number of defects, and the position of the defect. The controller 73 may judge pass/fail of the subject T based on the comparison result. The output device 74 is, for example, a monitor, and displays the two-dimensional image information formed by the controller 73.

Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited to the examples below.

(Preparation of glass composition and preparation of gradient index lens)
Glass raw materials were mixed so as to have the compositions shown in Table 1, and the mixtures were melted to obtain molten glass (glass compositions) according to Examples 1 to 4, Comparative Examples 1 to 3, and Reference Example 1. The numerical value in Table 1 shows mol%. Table 2 shows the relationship of the content percentage of the predetermined component in each glass composition based on mol %. Each molten glass was spun into a fiber shape, the obtained glass fiber was cut into a predetermined length, and the cut surface was polished. As a result, glass strands according to each example, each comparative example, and reference example 1 were obtained. The diameter (wire diameter) of each glass element wire was 560 μm. Next, each glass element wire was immersed in a molten salt of sodium nitrate heated near the glass transition temperature of the glass composition forming each glass element wire, and an ion exchange treatment was performed. Thereby, a refractive index distribution was formed in each glass element wire. Then, the glass element wire after the ion exchange treatment was cut into one cycle length, and the cut end face was polished to obtain a gradient index lens according to each of Examples, Comparative Examples and Reference Example 1.

(Characteristic evaluation)
The cut surface of the sample obtained by cutting the gradient index lens manufactured as described above into an appropriate length was mirror-polished. Next, a sheet on which a grid-like pattern is written is brought into contact with one end face of this sample, and an erect image of the pattern is observed from the other end face of the sample to determine the period length P of each gradient index lens. Decided. Next, the refractive index distribution coefficient √A of each gradient index lens was determined based on the relationship of √A=2π/P. Next, based on the relationship between the refractive index distribution coefficient √A, the radius r ₀ of the gradient index lens, the refractive index Nc of the glass element wire before the ion exchange treatment, and the following expression (1), The aperture angle θ of each gradient index lens was determined. The results are shown in Table 3. The refractive index Nc was 1.60 and could be regarded as the refractive index of the gradient index lens on the optical axis.
θ=sin ⁻¹ {√A·Nc·r ₀ } Equation (1)

The refractive index Nc was obtained by evaluating the refractive index of the glass composition according to each example, each comparative example, and reference example 1. A base material glass made of a glass composition was cut out to prepare a rectangular parallelepiped sample having a cross-sectional area of 15 mm square, and the refractive index Nc was evaluated according to the V block method described in JIS B7071-2:2018. In this method, the sample is placed on the V-block prism, and the deflection angle of the light beam bent by the sample is measured when the spectral light beam is passed. The present method is a method of relatively calculating the refractive index of the sample from the value of this deviation angle and the known refractive index of the V block prism. KPR-3000 manufactured by Shimadzu Corporation was used for evaluation.

(Water resistance evaluation)
The water resistance of each glass composition was evaluated according to JOGIS 06-2009. A sample prepared from each glass composition was placed in boiling water for 1 hour to measure the weight loss rate, and the water resistance of each glass composition was evaluated according to the weight loss rate. Water resistance according to JOGIS 06-2009 is classified into grades 1 to 6, and it can be said that a glass having a grade 1 water resistance has weather resistance, particularly excellent durability against moisture.

(DOF measurement)
A predetermined process (concavo-convex forming process) was performed on the side surface of each gradient index lens for the purpose of removing noise light. After that, a plurality of respective gradient index lenses were two-dimensionally arranged to prepare a lens array in which a plurality of gradient index lenses were arranged in two rows as shown in FIG. In this way, lens arrays according to Examples, Comparative Examples, and Reference Example 1 were obtained. A line pattern having 6 black-and-white line pairs at intervals of 1 mm was prepared. That is, this line pattern had a spatial frequency of 6 lines/mm. Light emitted from the halogen lamp was passed through a color filter (transmission center wavelength: 530 nm, full width at half maximum 15 nm) to irradiate a line pattern. As shown in FIG. 1, the line pattern, each lens array, and the light receiving element were arranged at the position where the value of MTF was maximum. The distance between the lens array and the light receiving element at this time was determined as the lens-imaging position distance L _o . The results are shown in Table 3. After that, the value of MTF was obtained at each position while moving the line pattern in the optical axis direction, and the range of working distance where the value of MTF was 30% or more was specified from the relationship between ΔL and the value of MTF. Then, the minimum value was subtracted from the maximum value of the working distance to determine the depth of field (DOF) of each gradient index lens. The results are shown in Table 3. Further, FIG. 9 shows the relationship between the value of MTF and ΔL in the lens arrays according to Example 2, Comparative Example 3, and Reference Example 1.

As shown in Table 1, the DOF in the lens array including the gradient index lens according to each example is in the range of 1.5 to 3.0 mm, and the gradient index lens according to each example is desired. It was suggested to have DOF. In addition, the water resistance of the glass composition according to each example was first grade. On the other hand, the DOF in the lens array including the gradient index lens according to each comparative example was small. The DOF of the lens array including the gradient index lens according to Reference Example 1 was 2.4 mm. However, the water resistance of the glass composition according to Reference Example 1 is grade 4, and it is suggested that the glass composition according to Reference Example 1 is inferior in water resistance as compared with the glass compositions according to Examples. Was done.

Claims (8)

1. With a depth of field of 1.5-3.0 mm,
   The depth of field is determined by subtracting the minimum value from the maximum value of the working distance,
   At the working distance, the value of the modulation transfer function (MTF) at a spatial frequency of 6 lines/mm is 30% or more,
   Gradient index lens.
2. The gradient index lens according to claim 1, having an aperture angle of 3 to 6°.
3. The gradient index lens according to claim 1 or 2, wherein an erect image has an image forming distance of 45 to 80 mm.
4. An optical product equipped with the gradient index lens according to any one of claims 1 to 3.
5. Optical equipment equipped with the optical product according to claim 4.
6. Shown in mol%,
   40%≦SiO ₂ ≦65%
   0% ≤ TiO ₂ ≤ 10%
   0.1%≦MgO≦22%
   0.15%≦ZnO≦15%
   0.5%≦Li ₂ O<4%
   2% ≤ Na ₂ O ≤ 20%
   0% ≤ B ₂ O ₃ ≤ 20%
   0% ≤ Al ₂ O ₃ ≤ 10%
   0%≦K ₂ O≦3%
   0%≦Cs ₂ O≦3%
   0% ≤ Y ₂ O ₃ ≤ 5%
   0% ≤ ZrO ₂ ≤ 2%
   0% ≤ Nb ₂ O ₅ ≤ 5%
   0%≦In ₂ O ₃ ≦5%
   0% ≤ La ₂ O ₃ ≤ 5%
   0%≦Ta ₂ O ₅ ≦5%,
   At least two selected from the group consisting of CaO, SrO, and BaO, each containing 0.1 mol% or more and 15 mol% or less,
   Display in mol%,
   2%≦MgO+ZnO,
   0.07≦ZnO/(MgO+ZnO)≦0.93,
   2.5%≦Li ₂ O+Na ₂ O<24%, and 0%≦Y ₂ O ₃ +ZrO ₂ +Nb ₂ O ₅ +In ₂ O ₃ +La ₂ O ₃ +Ta ₂ O ₅ ≦11%,
   A glass composition for a gradient index lens.
7. The glass composition for a gradient index lens according to claim 6, which has a first-class water resistance determined in accordance with the Japan Optical Glass Industry Association Standard (JOGIS) 06-2009.
8. A method of manufacturing a gradient index lens, comprising:
   Forming a glass strand made of a glass composition containing an oxide of a first alkali metal element,
   The glass element wire is immersed in a molten salt containing a second alkali metal element different from the first alkali metal element, and the first alkali metal element in the glass element wire and the second alkali in the molten salt. By forming a refractive index distribution in the glass element wire by performing an ion exchange treatment with a metal element,
   The glass composition, expressed in mol %,
   40%≦SiO ₂ ≦65%
   0% ≤ TiO ₂ ≤ 10%
   0.1%≦MgO≦22%
   0.15%≦ZnO≦15%
   0.5%≦Li ₂ O<4%
   2% ≤ Na ₂ O ≤ 20%
   0% ≤ B ₂ O ₃ ≤ 20%
   0% ≤ Al ₂ O ₃ ≤ 10%
   0%≦K ₂ O≦3%
   0%≦Cs ₂ O≦3%
   0% ≤ Y ₂ O ₃ ≤ 5%
   0% ≤ ZrO ₂ ≤ 2%
   0% ≤ Nb ₂ O ₅ ≤ 5%
   0%≦In ₂ O ₃ ≦5%
   0% ≤ La ₂ O ₃ ≤ 5%
   0%≦Ta ₂ O ₅ ≦5%,
   The glass composition contains at least two selected from the group consisting of CaO, SrO, and BaO in an amount of 0.1 mol% or more and 15 mol% or less, respectively.
   The glass composition is expressed in mol%,
   2%≦MgO+ZnO,
   0.07≦ZnO/(MgO+ZnO)≦0.93,
   2.5%≦Li ₂ O+Na ₂ O<24%, and 0%≦Y ₂ O ₃ +ZrO ₂ +Nb ₂ O ₅ +In ₂ O ₃ +La ₂ O ₃ +Ta ₂ O ₅ ≦11%,
   Method.

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