JP2002196251A - Lens system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compensate aberration very well by extremely small number of lenses. SOLUTION: This lens system for forming the enlarged image of an object includes a radial type refractive index distribution lens whose both sides are plane, at least one homogenous positive lens and at least one homogenous negative lens.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lens system used for a microscope objective lens, an imaging lens for a digital camera, or the like. The lens system has a large NA, is well corrected for various aberrations, and has about 3 to 4 components. And a lens system having a small number of lenses.

[0002]

2. Description of the Related Art A microscope objective lens is required to have a large NA, a large number of fields, and a long working distance.

As a conventional example of such a microscope objective lens, there is known a lens system described in JP-A-54-58038. The microscope objective lens described in this publication has a seven-element configuration, an NA of 0.3, and a field number of 30 m.
m and a working distance of about 8.4 mm.

A microscope objective lens of infinity design is designed so that an enlarged image of an object is formed at infinity, and an axial marginal ray is parallel to the optical axis on the image side of the objective lens.

In the microscope objective lens, the entrance pupil must be located at infinity in order to perform efficient illumination. Therefore, the principal ray is parallel to the optical axis on the object side of the objective lens. become.

FIG. 5 shows these states. In the drawing, 1 indicates an object, 2 indicates a lens system, and 3 indicates the direction of the enlarged image, and indicates that the enlarged image is formed in a pointless manner.
4 is an axial marginal ray emitted from the lens system 2;
Reference numeral 5 denotes an off-axis principal ray incident on the lens system 2.

In order to improve optical performance with a small number of lenses, it is effective to use a radial type gradient index lens (heterogeneous lens) as an optical element in the lens system. It is known that they have better Petzval sum and chromatic aberration correction ability than ordinary homogeneous lenses.

[0008] Is represented by the following equation (1). 2,...) Are coefficients representing the refractive index distribution at the wavelength λ.

The Abbe number V _i0 of the radial type gradient index lens is defined as follows. V _i0 = N _i0d / (N _i0F −N _i0C ) (i = 1, 2, 3,...) V ₀₀ = (N _00d −1) / (N _00F −N _00C ) (i = 0) N _i0d , N _i0F , N _i0C and N _00d , N _00F , N
_00C is the refractive index distribution coefficient N at d-line, F-line, and C-line, respectively.
_i0, it is N _00.

The partial dispersion ratio P _i0dC (i = 0, 1, 2, 3,...) _Of the radial type gradient index lens is defined as follows. P _i0dC = (N _i0d −N _i0C ) / (N _i0F −N _i0C )

When the radial type gradient index lens is a flat surface on both sides (in the case of a wood lens), its power φ
_m is approximately represented by the following equation. φ _m = −2N ₁₀ t _{m where} t _m is the thickness of the lens.

[0012]

SUMMARY OF THE INVENTION Japanese Patent Application Laid-Open No. 54-5803
The conventional microscope objective lens described in Japanese Patent Application Laid-Open No. 8-208 has the disadvantage that the number of lenses is large, the lens and the lens frame structure are complicated, and the cost is high.

An object of the present invention is to provide a lens system which can be used as a microscope objective lens or a photographing lens of a digital camera, in which the number of lens elements is as small as about three and various aberrations are well corrected. Another object of the present invention is to provide a lens system which has a small chromatic aberration over the entire visible light range including the g-line, has about four lenses, and can be used for a microscope objective lens, an imaging lens, and the like.

[0014]

A lens system according to the present invention forms an enlarged image of an object, and includes a two-sided flat radial type gradient index lens disposed on the most object side of the lens system.
It is characterized by including at least one homogeneous lens having a positive refractive power and at least one homogeneous lens having a negative refractive power.

In the lens system of the present invention, a radial type gradient index lens element having a high aberration correction capability is used so that various aberrations can be satisfactorily corrected with a small number of lenses. Further, by making both surfaces of this radial type gradient index lens flat, and making this lens a so-called wood lens, the lens surface is easily polished and easy to assemble, so that it has a practically useful value.

However, when a microscope objective lens, which is a lens system for forming, for example, an enlarged image of an object, with one wood lens, it is difficult to realize sufficiently satisfactory aberration correction. Included two homogeneous lenses, a wood lens, a negative lens and a positive lens.

Here, the radial type gradient index lens element 3
The following aberration coefficient will be described.

The third-order aberration coefficient of the radial type gradient index lens is given by the sum of the surface contribution a _si (i = 1, 2,...) And the medium contribution a _mi . In addition, the contribution of the surface is the contribution of the surface of the normal base glass (homogeneous lens of the same shape),
It is divided into additional contributions caused by the distribution of the refractive index. In the case of a wood lens, since both surfaces are flat, the additional contribution caused by the distribution of the refractive index is zero. Therefore, the third order aberration coefficient of the wood lens is the sum of the contribution of the surface by the normal base glass and the contribution of the medium.

J. J., published in 1970. O. S. A. 60 volumes,
As shown on page 1436, the contribution of spherical aberration a _s1 , the contribution of coma aberration a _s2 ,
The astigmatism contribution _as3 is _calculated by the following equations (6) and (7), respectively.
It is represented by (8).

Here, the suffix j (j = 1, 2) is
represents the j-th surface.

In the above equations (6), (7) and (8), a and q are obtained by the following equations. _{q = i b / i a (} 10)

The above formula (9), in (10), N ₀ is a refractive index on the optical axis, y _a, u ^_'a is the ray height and ray angle of each axial marginal ray, i _a, i _b are each The on-axis marginal optical axis and the angle of incidence of the chief ray, prime being:
It indicates that it is for a ray after refraction.

The contribution of the spherical aberration due to the medium a ^* ₁ ,
The contribution a ^* ₂ of coma and the contribution a ^* _{3 of} astigmatism are as shown in the following equations (11), (12) and (13).

In the above equations (11), (12) and (13), y _b and u _b ′ are the ray height and ray angle of the principal ray, respectively, and ▽ is the tangential plane on the surface of the radial type gradient index lens. It is the difference between the front and rear surfaces of the evaluated value.

The image position is almost infinite with one wood lens
When designing a designed microscope objective, the image of the lens system
On the side surface, that is, the image side surface of the wood lens,
The ray is vertical and therefore i_a= 0. Also
On the object-side surface of the wood lens, the off-axis chief ray is perpendicular to the surface.
And therefore i_b= 0. So the expression
Q in (7) and (8) a = 0. Because of this
The contribution of the aberration and the contribution of the astigmatism are zero in both planes.
You. On the image side surface of the wood lens, i_a= 0
Therefore, the contribution of spherical aberration is zero.

[0026] Finally, a microscope objective lens formed by only one wood lens, s contribution A ₁ of the spherical aberration, the contribution A ₂ of coma, astigmatism contribution A ₃ are each (14), (1
5) and (16). ^{_{A 1 = a s11 + a *}} 1 (14) A 2 = a * 2 (15) A 3 = a * 3 (16) where, a _s11 is a spherical aberration generated by the surface on the object side.

Further, considering that the axial marginal ray is perpendicular to the surface on the image side of the wood lens and the principal ray is perpendicular to the surface on the object side, the equations (15) and (1)
The first term of a ^* ₂ and a ^* ₃ given in 6) is 0.
Therefore, A ₁ , A ₂ , and A ₃ are calculated by the following equations (17) and (1).
8) and (19).

From the above equations (17), (18) and (19), it is possible to derive the characteristics of the lens system when the wood lens is applied to a microscope objective lens whose image position is designed to be almost infinite.

Next, the case where the microscope objective lens is traced backward will be described.

In the above equations (17), (18) and (19), attention is paid to the term including the integral among the terms.

In terms including these integrals, u ^′ _a , u
^' _b is 0 on one side of the integration interval and is a small value over the entire integration interval, and y _b is _a small value compared to ya. Therefore, the above equations (17), (18), (19)
U ^' _a · u ^' _b and u ^' _a
- second and subsequent terms containing y _a is assumed to be a small value. If N ₂₀ = 0, it is assumed that the first term in the integral becomes 0, and the whole integral term becomes a small value.
Therefore, the contribution A _{2 of the} coma aberration and the contribution A of the astigmatism
₃ is a small value.

The contribution A _{1 of the} spherical aberration has terms other than the terms including the integral (terms not including the integral), and all the terms not including the integral have a negative sign and have a size that cannot be ignored. And the sum of the spherical aberration contribution A ₁ is
It can be assumed that it has a large negative value.

Further, the amount of change of each aberration when N ₂₀ is changed has a positive slope proportional to the following values with respect to A ₁ , A ₂ , and A ₃ , respectively. The large and astigmatism are the smallest, and the coma is the middle slope.

FIG. 6A is composed of one wood lens.
Magnification × 10, the spherical aberration SA of the lens system of the numerical aperture NA is 0.3, coma COMA, a diagram showing a change in the astigmatism AS, shows a case where the taking N ₂₀ on the horizontal axis.

The aberration shown in these figures is a value obtained by dividing the contribution of each aberration by a marginal ray angle after the last surface of the lens system, and is a so-called aberration amount.

As shown in FIG. 6A, in this specification, N ₂₀
When = 0, the coma COMA and the astigmatism AS are almost zero, and a large negative aberration remains in the spherical aberration SA.

[0037] In N ₂₀ opposite to the spherical aberration SA becomes 0, coma COMA generates positive astigmatism AS is generated amount is very small.

FIG. 6 (b) shows the spherical aberration SA, coma COMA, and astigmatism AS of a lens system having a single wood lens, a magnification of × 20, and a numerical aperture NA of 0.5. shows the change when taking N ₂₀ to the shaft.

FIG. 6B shows the same thing as FIG. 6A. FIG. 6 also shows a lens system of another specification.
The same tendency as that shown in (a) and (b) is obtained.

From the above, when designing a microscope objective lens with realistic specifications using one wood lens, N
By setting ₂₀ = 0, coma and astigmatism can be reduced, but in this case, spherical aberration has a large negative value. Conversely, coma aberration occurs positively when to adjust the value of N ₂₀ to correct the spherical aberration.

One wood lens, magnification × 10, NA
An actual design example when N ₂₀ = 0 constitutes a lens system having specifications of 0.3 and is as shown in FIG. r ₁ = ∞ d ₁ = 21.22 n ₁ (refractive index lens) r ₂ = ∞ d ₂ = 11.2 r ₃ = ∞ d ₃ = 0.17 n ₂ = 1.52100 ν ₂ = 56.0 r ₄ = ∞ refractive index distribution lens N ₀₀ = 1.70000 N ₁₀ = −0.00150 N ₂₀ = 0 V ₀₀ = 40.0 V ₁₀ = 343 P _00dC = 0.295 P _10dC = 0.437 Magnification 10 × (f = 18 mm), NA / 0.3 Field of view 30 mm (object range 3.0 mm) ), WD = 11.2mm

CG (r _{3 to} r ₄ ) is a cover glass.

The aberration situation in this design example is as shown in FIG. As is clear from this aberration diagram, the lens system of the design example shown in FIG. 8 has small coma and astigmatism but large spherical aberration. This result is consistent with the description using the third-order aberration coefficient described above.

As is clear from the above description, it is difficult to configure the microscope objective lens with one wood lens.

As described above, if a lens system such as a microscope objective lens is constituted by a single radial type gradient index lens (wood lens) having both flat surfaces, a lens system having sufficiently good optical performance cannot be realized. .

However, if the lens system is made up of a wood lens and a homogeneous lens, it is possible to use a radial type gradient index lens which is easy to manufacture and to use a small lens system with a small number of lenses. A good lens system can be realized.

As described above, in the case of using only a wood lens, spherical aberration remains. Although it may correct the axial chromatic aberration by adjusting the value of V ₁₀ Wood lens chromatic aberration of magnification remains.

As described above, in order to correct spherical aberration and chromatic aberration of magnification, which cannot be corrected by the wood lens alone, it is necessary to use at least one homogeneous positive lens and one homogeneous negative lens in addition to the wood lens.

Therefore, the lens system according to the present invention forms an enlarged image of an object as described above. A radial type gradient index lens is disposed closest to the object, and at least one positive refractive index lens is disposed on the image side. It is characterized by including a homogeneous lens and at least one negative homogeneous lens. Further, a lens system having good optical performance can be formed with a small number of lenses by the above configuration.

In the above description, a lens system for forming an enlarged image of an object, such as a microscope objective lens, has been described. An image pickup lens can be formed by forming a reduced image of an object at a position (nearby) conjugate with the image sensor on the image side of the image sensor on the light receiving surface of the image sensor.

That is, another configuration of the present invention is a lens system for forming a reduced image of an object, which is a radial type gradient index lens having both flat surfaces disposed closest to the image side and a lens disposed on the object side. A lens system comprising at least one homogeneous positive lens and at least one homogeneous negative lens.

By adopting such a configuration, like a lens system for forming an enlarged image of an object such as the microscope objective lens, a reduced image of an object having good optical performance can be formed with a very small number of lenses. A lens system to be formed can be realized.

When N ₂₀ = 0, it is clear from the following description that the coefficient N ₁₀ has little effect on aberration.

That is, when N ₂₀ = 0, the coma and astigmatism have very small values, and the spherical aberration is given by the first and second terms of the equation (17). The spherical aberration is dependent only on the ray height and the ray angle of the axis on the refractive index N ₀₀ and the axial marginal light ray. But when varying the thickness changing the value of N ₁₀ to keep the power of the wood lens, changes in the ray height and the ray angle of the axial marginal light ray is small. Therefore, the value changes are allowed even if the spherical aberration of the N ₁₀ by changing the thickness of the wood lens, coma, the value of astigmatism is anticipated that does not change significantly.

[0055] The following table and FIG. 9 is a graph showing the changes in the aberrations when changing the value of N _10. Even by changing the value of N ₁₀ from this table, spherical aberration, coma aberration, the value of the astigmatism does not largely changed.

[0056] When changing the value of N ₁₀ in the wood lens, spherical aberration, coma aberration, the change in the value or the like of the astigmatism is as following table.

[0057] Table (A) (B) (C ) N 10 -0.0010 -0.0015 -0.0020 Δ n 0.029 0.044 0.058 d G (mm) 36.80 21.22 15 .24 WD (mm) 5.35 11.20 13.19 Spherical aberration -0.1384 -0.2100 -0.2357 Coma aberration 0.0026 -0.0060 -0.0080 Astigmatism 0.0019 0. 0021 0.0021

[0058] The above table (A), is N ₁₀ of (B), (C) -0.0010 , -0.0015, cross-sectional view of the lens when the -0.0020, respectively of FIG. 9 (A ), (B),
(C).

[0059] As described above, the aberration by changing the value of N ₁₀ Wood lens is not large change.

[0060] Therefore, small material clogging the value of N ₁₀ Δ
_n small material can be a lens of the optical performance close to big material delta _n by (Table (A)) of the thickness d _G of the lens as in any Figure 9 (A) to atmospheric. However, in this case, the working distance becomes shorter as shown in the above table.

Next, the lens system of the present invention is preferably made of a material which is easy to realize a radial type gradient index lens and which satisfies the following condition (A) in view of the size and balance of the lens system. Distance.

If the upper limit of 0.7 to condition (A) is exceeded, Δ
Since a material having a large n is required, it becomes extremely difficult to manufacture a gradient index lens. On the other hand, if the lower limit of 0.05 is exceeded, the refractive index distribution lens becomes too thick, and a necessary and sufficient working distance cannot be obtained. Further, it is conceivable to make the refractive index distribution lens thin by giving power to the surface, but it becomes difficult to correct aberrations generated on the surface. In particular, when the lens system of the present invention is used as a microscope objective lens, better aberration correction is required, and the refractive index distribution lens is preferably a two-sided flat surface.

Instead of the condition (A), the following condition (A-
It is more preferable to satisfy 1).

The lens system of the present invention desirably satisfies the following condition (B) in consideration of the balance between a material that can be easily realized and the size of the lens system. (B) 0.2 <d _G WD / f ² <2 where d _G is the thickness of the gradient index lens, and WD is the working distance of the lens system.

When the value exceeds the upper limit of 2 of the condition (B), the refractive index distribution lens becomes thick. If the lower limit of 0.2 is exceeded, it is necessary to increase Δn. However, it is difficult to manufacture a material for a gradient index lens having a large Δn.

It is more desirable that the condition (A) and the condition (B) be satisfied at the same time.

In the wood lens shown in FIG. 7, the spherical aberration remains. Axis aberration by appropriately selecting the value of V ₁₀ also is capable of correcting chromatic aberration of magnification is left. These remaining spherical aberration and chromatic aberration of magnification can be corrected by adding one homogeneous positive lens and one homogeneous negative lens. As described above, in a lens system composed of a wood lens, a homogeneous positive lens, and a homogeneous negative lens, the power of the wood lens is increased so that the wood lens has the necessary power in the entire lens system. The combined power of the lenses may be small, and it is desirable that the combined power be a negative power in order to increase the working distance.

The lens system of the present invention preferably satisfies the following condition (C) with the above configuration. _{(C) -0.8 <f G /} f 23 <-0.01 However, f _G is the focal length of the radial type refractive index distribution lens, f ₂₃ is the composite focal length of the lens other than the radial type refractive index distribution lens is there.

If the value of f _G / f ₂₃ in the condition (C) exceeds the lower limit of −0.8, the power of the radial type gradient index lens element becomes so small that it becomes difficult to satisfactorily correct various aberrations. Also, f _G / f ₂₃ is −0.01 which is the upper limit of the condition (C).
When the value exceeds, it is necessary to extremely increase the power of the radial type gradient index lens, which is not preferable in manufacturing a material.

In place of the condition (C), the following condition (C-
It is more desirable to satisfy 1). (C-1) −0.6 <f _G / f ₂₃ <−0.1

In a lens system such as a microscope objective lens in which it is important to properly correct chromatic aberration, there is a problem in handling a partial dispersion ratio.

As described in Applied Optics, Vol. 37, p. 622, published in 1998, a lens system using a radial type gradient index lens has P _10dC =
The approximation of 0.3 is often used. This is based on the assumption that the radial type gradient index lens has a partial dispersion ratio equivalent to that of ordinary glass. However, to have the same value as the partial dispersion ratio of the ordinary glass partial dispersion ratio of the radial type refractive index distribution lens, for large Abbe number V _10, it will not be susceptible making a material property, specifically V ₁₀ is less likely to make to be a more than 100 in.

Therefore, in the present invention, a radial type gradient index lens having a partial dispersion ratio which is easy to manufacture has been considered.

The radial type gradient index lens is considered to be a collection of minute glass. And it is a requirement for an easy-to-manufacture refractive index distribution lens to make each of these microscopic glasses into an ordinary dispersion glass that satisfies the Hertzberger relational expression. In other words, the condition of the partial dispersion ratio for making the material easy to produce can be obtained from the Hertzberger relational expression.

According to the Hertzberger relation, the refractive index of the homogeneous glass is given by the following equation (20). This is the Abbe number, and A (λ) and B (λ) are as shown in the following equations (21) and (22), respectively.

In the case of paraxial treatment, the refractive index distribution of the radial type gradient index lens is considered to be expressed by the following equation.

From equation (23), the refractive index at the center of the radial type gradient index lens element and at a position a minute distance δr from the center is represented by the following equations (24) and (25). Abbe number.

The following equation (26) is obtained from the equation (23).

By substituting equations (24) and (25) into equation (26), the following equation (27) is obtained.

Here, the Hertzberger equation has the following relationship. A (d) = B (d) = 0 (28) A (F) −A (C) = 1 (29) B (F) = B (C) (30)

From the equations (27) and (30), the following equation is obtained for the partial dispersion ratio P _10dC .

The equation (31) is a condition of the partial dispersion ratio for making the material easy to produce.

From this equation (31), the normal Abbe number is 5
Glass of about 0 has a partial dispersion ratio P _10dCBecomes 0.3
However, if the Abbe number is large, it may deviate from 0.3.
Understand. Therefore, to make the material easy to make,
It is desirable to satisfy the condition (D).

(D) −0.01 <(P _10dC −0.2756) / V ₁₀ <0.01

In place of the condition (D), the following condition (D-
It is more desirable to satisfy 1). (D-1) -0.0
_{04 <(P 10dC -0.2756) /} V 10 <0.006

Further, in the lens system of the present invention comprising a radial type gradient index lens, one homogeneous positive lens and one homogeneous negative lens, both axial chromatic aberration and chromatic aberration of magnification are excellent. It is desirable that the following condition (E) be satisfied in order to make correction. (E) 1 <v _n / v _p <3 where v _n and v _p are Abbe numbers of the homogeneous negative lens and the homogeneous positive lens, respectively.

In the above-mentioned lens system of the present invention, it is desirable that the power of the lens other than the radial type gradient index lens, that is, the combined power of the homogeneous positive lens and the homogeneous negative lens becomes negative. Therefore, this condition (E) is a condition for efficiently correcting the chromatic aberration generated by the homogeneous negative lens by the chromatic aberration generated by the homogeneous positive lens.

In condition (E), if the upper limit of 3 is exceeded, chromatic aberration generated by the homogeneous negative lens will be overcorrected. If the lower limit of 1 is exceeded, chromatic aberration will be insufficiently corrected, which is not preferable.

The lens system of the present invention uses a double-sided flat radial type gradient index lens element having a very small amount of chromatic aberration. Therefore, satisfying the condition (E) satisfactorily corrects the chromatic aberration of the lens system. It is important to do.
In particular, when the lens system of the present invention is applied to a lens system having a large magnification and difficult to correct chromatic aberration, such as a microscope objective lens, it is effective to satisfy the condition (E). .

In place of the condition (E), the following condition (E-
It is desirable to satisfy 1). (E-1) 1.5 <ν _n / ν _p <2.7

The above description of the conditions (A) to (E) has been mainly made with respect to a lens system for forming an enlarged image of an object such as a microscope objective lens. The present invention is also applicable to a lens system to be formed.

That is, this is a lens system for forming a reduced image of an object, which is a radial type gradient index lens having both flat surfaces disposed closest to the image side, at least one homogeneous positive lens and at least one homogeneous negative lens. In the lens system of the present invention including the lens, the conditions (A), (B), (C), (D),
A lens system that satisfies at least one of the conditions (E) has a desirable configuration for the above-described reason.

Next, a lens system having another configuration of the present invention is as follows.

That is, an optical system for forming an enlarged image of an object, which is a radial type gradient index lens having both flat surfaces disposed closest to the object, at least one homogeneous positive lens,
Including at least one homogeneous negative lens, the following condition (F)
It is a lens system that satisfies. Where φ _i is the power of the homogeneous lens, φ _T is the power of the entire lens system, and P _00gdi and v _i are the partial dispersion ratio between the g and d lines of the homogeneous lens and the Abbe number.

Here, _ng , n _d , n _F , and n _C are respectively
G line homogeneous lens, d line, F line, a refractive index of C-line, P _00Gdi is expressed as follows. P _00gdi = (n _gi −n _di ) / (n _Fi −n _Ci )

It is desirable that the refractive index distribution material which is easy to manufacture with the radial type gradient index lens (wood lens) used in the lens system of the present invention satisfies the expression (27) as described above. As an example to prove this,
SiO ₂ —BaO—TiO ₂ prepared by sol-gel method
It was measured regarding color dispersion characteristics of -K ₂ system samples.

This sample has chromatic aberration to some extent (V ₁₀ to 80), which is advantageous for examining the chromatic dispersion characteristics in detail.

The main characteristics of the above samples are as shown in the following table.

G, F, e, d,
FIG. 11 shows the result of measuring the chromatic aberration with respect to the wavelength corresponding to each line of C.

This measuring principle is based on OPTICAL REVI.
EW (Vol. 7, No. 4 (2000), P337)
-340) based on the method described in
Of the difference in the imaging position of the test target
The magnification at the time of measurement is -3.6. Also in this FIG.
The chromatic aberration shown is largely related to the chromatic dispersion characteristics of the medium.
And use the measured values for ray tracing and best fitting.
By law N_TenFIG. 12 shows the values converted to
You. In this figure, the symbol △ indicates the measurement point, and the solid line a indicates the equation (27).
In the theoretical curve given by, the theoretical value should be a fixed point
V to match _TenIs defined.

FIG. 13 shows that the measured value complies with the equation (27). Thus, equation (27) is considered to be an effective equation for expressing the chromatic dispersion characteristics of the material of the ordinary radial type gradient index lens element.

Here, the equation (27) has a form equivalent to the Hertzberger dispersion equation representing the wavelength dispersion of the homogeneous glass, and when this equation is followed, it is an ordinary dispersion.

Therefore, the chromatic dispersion of the measured sample is a normal dispersion.

FIG. 13 shows the measured value P _{10gd of the} sample.
Is plotted on a glass map showing the partial dispersion ratio.

As shown in FIG. 13, the measured value of the sample indicated by a triangle is almost on the straight line b given by the Hertzberger relational expression, and the material of the sample is expressed by the equation (27). This is shown in a different format from that of FIG. FIG. 1
In FIG. 3, the x indicates the partial dispersion ratio of a typical homogeneous glass.

Here, the partial dispersion ratio P _i0gd of the radial type refractive index distribution material is defined as follows. P _i0gd = (n _i0g −n _i0d ) / (n _i0F −n _i0C ) (i = 0, 1, 2,...) Where g represents the g line.

As described above, it is considered that the ordinary radial type refractive index distribution material has the ordinary dispersion characteristic given by the equation (27). Therefore, if the homogeneous lens in the lens system of the present invention has a normal dispersion, the entire lens system will also have a normal dispersion, and even if the chromatic aberration of the C line and the F line is corrected, the so-called secondary spectrum such as the g line remains. I do.

Therefore, by using an anomalous dispersion material for the homogeneous lens in the lens system, it
The next spectrum can be reduced. For that purpose, it is desirable to satisfy the following condition (F). This condition (F) is for keeping the secondary spectrum small. Indicates the deviation of the partial dispersion ratio from the normal dispersion.

In this condition (F), if the lower limit of 0 is exceeded, the secondary spectrum of the lens system will be undercorrected, and if it exceeds the upper limit of 0.3, the secondary spectrum of the lens system will be overcorrected.

Instead of the above condition (F), the following condition (F−
It is more desirable to satisfy 1).

That is, if the lower limit 0 of the condition (F) is changed to 0.02 and the upper limit 0.3 is changed to 0.2, the secondary spectrum can be further reduced.

In the lens system having the above-described configuration, it is more preferable that at least one of the above-described conditions (A), (B), and (D) is satisfied.

A lens system which satisfies the condition (F) or (F-1) so as to better correct chromatic aberration over the entire visible light range is also used as a microscope objective lens. An image sensor such as a CCD is arranged at the position of the object, and a reduced image of the object at a position (nearby) conjugate with the image sensor on the image side of the lens system is formed on the light receiving surface of the image sensor. And an imaging lens.

That is, a lens system for forming a reduced image of an object, which is a double-sided radial refractive index distribution lens disposed closest to the image side and at least one homogeneous positive lens disposed on the object side And at least one homogeneous negative lens, and can be used as an imaging lens in a configuration satisfying the condition (F) or the condition (F-1).

[0115]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the lens system according to the present invention will now be described based on Examples 1, 2, and 3 of a microscope objective shown in the drawings.

The embodiment of the present invention has the following data as shown in FIGS.

Example 1 r ₁ = 65.8780 d ₁ = 1.60 n ₁ = 1.84666 v ₁ = 23.8 r ₂ = 840.579 d ₂ = 7.40 r ₃ = -18.6432 d ₃ = 1.00 n ₂ = 1.88 300 v ₂ = 40.8 r ₄ = -37.5398 d ₄ = 0.40 r ₅ = ∞ d ₅ = 26.90 n ₃ (refractive index distribution lens 1) r ₆ = ∞ d ₆ = 9.07 r ₇ = ∞ d ₇ = 0.17 n ₄ = 1.52100 ν ₄ = 56.0 r ₈ =分布 Refractive index distribution lens 1 N ₀₀ = 1.70000 N ₁₀ = -0.00150 N ₂₀ = 5.83 × 10 ^-7 V ₀₀ = 40.0 V ₁₀ = 370.0 V ₂₀ = 370.0 P _00dC = 0.295 P _10dC = 0.450 P _20dC = 0.450 Magnification 10 × (F = 18 mm), NA / 0.3 Number of fields of view 30 mm (object range 3.0 mm), WD = 9.1 mm ^{_{d G · WD / f 2 =}} 0.756, f G / f 23 = -0.143 (P 10dC -0.2756) / V 10 = 0.000471, ν n / ν p = 1.71

Example 2 r ₁ = −25.6754 d ₁ = 1.515 n ₁ = 1.84666 ν ₁ = 23.8 r ₂ = −15.6076 d ₂ = 4.0000 r ₃ = −10.3209 d ₃ = 1.000 n ₂ = 1.64000 ν ₂ = 60.1 r ₄ = 49.6867 d ₄ = 4.447 r ₅ = ∞ d ₅ = 31.822 n ₃ (refractive index distribution lens 2) r ₆ = ∞ d ₆ = 3.000 r ₇ = ∞ d ₇ = 0.170 n ₄ = 1.52100 ν ₄ = 56.0 r ₈ = ∞ refractive index distribution lens 2 N ₀₀ = 1.65000 N ₁₀ = −0.002428 N ₂₀ = 2.05 × 10 ⁻⁶ N ₃₀ = 2.31 × 10 ⁻¹⁰ V ₀₀ = 40.0 V ₁₀ = 206.0 V ₂₀ = 206.0 V ₃₀ = 206.0 P _00dC = 0.295 P _10dC = 0.373 P _20dC = 0.373 P _30dC = 0.373 Magnification 20 × (f = 9.0 mm), NA / 0.4 Field of view 26.5 mm (object range 1.325 mm), WD = 3.0 mm d _G · WD / f ² = 1.179, f _G / f ₂₃ = −0.429 (P _10dC −0.2756) / V ₁₀ = 0.000473, v _n / v _p = 2.53

Example 3 r ₁ = -63.9185 d ₁ = 1.57 n ₁ = 1.84666 ν ₁ = 23.8 r ₂ = -21.6931 d ₂ = 2.57 r ₃ = -14.3229 d ₃ = 1.00 n ₂ = 1.88300 ν ₂ = 40.8 r _{4 = ∞ d 4 = 0.15 r} 5 = 25.8888 d 5 = 3.74 n 3 = 1.43875 ν 3 = 95.0 r 6 = -28.9168 d 6 = 0.15 r 7 = ∞ d 7 = 27.92 n 4 ( refractive index distribution lens 3) r _{8 = ∞ d 8 = 9.00 r} 9 = ∞ d 9 = 0.17 n 5 = 1.52100 ν 5 = 56.0 r 10 = ∞ refractive index distribution lens _{3 n 00 = 1.70000 n 10 =} -0.00120 n 20 = 8.51 × 10 -7 V ₀₀ = 40.0 V ₁₀ = 370 V ₂₀ = 370 P _00dC = 0.295 P _10dC = 0.452 P _20dC = 0.452 P _00gd = 1.284 P _10gd = 0.538 P _20gd = 0.538 Magnification 10 × (f = 18 mm), NA / 0.3 field of view Several 30 mm (object range 3.0 mm), WD = 9.0 mm d _G · WD / f ² = 0.776 (P _10dC −0.2756) / V ₁₀ = 0.000477 Where r ₁ , r ₂ ... Are the radii of curvature of the respective surfaces of the lens,
.., d ₁ , d _2, ...
n _1, n ₂ ··· is a refractive index of each lens, ν _1, ν ₂ ··· is the Abbe number of each lens. Note that r ₁ , r ₂ .
The unit of length such as d ₁ , d ₂ ... is mm.

The first embodiment has a configuration as shown in FIG. In this figure, the right side is the object side, and the light ray is incident from the image side, and the reverse tracing is performed as described.

In the first embodiment, a radial type gradient index lens LG (r
₆ ~r ₅₎ a homogeneous negative lens LN (r ₄ ~r ₃₎ a homogeneous positive lens LP (r ₂ ~r ₁₎ become more. In addition, parallel plane plate CG
(R ₈ ~r ₇₎ is a cover glass.

Example 1 is a microscope objective lens having a magnification of 10 × and an NA of 0.3. The aberration curve chart when traceback of Example 1 (by the incidence of the parallel light flux from the image side is imaged on the object surface r ₈₎ are as shown in FIG. The g line in the aberration curve diagram is based on the value calculated by the equation (27).

Embodiment 2 is as shown in FIG. 2 and is based on reverse tracking as in FIG. 1, and the right side is the object side.

In the second embodiment, in order from the object side, a radial type gradient index lens LG (r _{6 to} r ₅ ), a homogeneous negative lens LN (r _{4 to} r ₃ ), and a homogeneous positive lens LP (r _Two
To r ₁ ).

The second embodiment is a microscope objective lens having a magnification of 20 × and an NA of 0.4, and a parallel flat plate CG on the object side.
(R ₈ ~r ₇₎ is a cover glass.

The aberration situation in this embodiment is as shown in FIG. 4 (by reverse tracking), and the g line in the figure is based on the value calculated by the equation (27).

The third embodiment is a lens system having the structure shown in FIG. 10 and includes, in order from the object side (in order from the right side in the drawing), a wood lens, that is, a refractive index distribution lens LG (r _{8 to} r ₇ ).
A homogeneous positive lens LP1 (r ₆ ~r _5), homogeneous negative lens LN
This is a microscope objective lens composed of four lenses (r _{4 to} r ₃ ) and a homogeneous positive lens LP2 (r _{2 to} r ₁ ) with a magnification of 10 × and an NA of 0.3. Further, flat glass CG which is the object side (r ₁₀ ~r ₉₎ is a cover glass.

The longitudinal chromatic aberration of the third embodiment is as shown in FIG. It can be seen that the secondary spectrum of the chromatic aberration (marked by ○) of the present embodiment is better corrected than the chromatic aberration of normal achromat (marked by □). Further, the aberration situation (due to reverse tracking) of the third embodiment is as shown in FIG. As is clear from the aberration curve diagrams shown in FIGS. 14 and 15, in Example 3 satisfying the condition (F), the chromatic aberration is corrected extremely well.

The data of the third embodiment is also based on reverse tracking.

The third embodiment differs from the first and second embodiments in that it has a four-group configuration using three homogeneous lenses and that the radial type gradient index lens is made of a material satisfying the condition (F). Different. That is, in the first and second embodiments, the conditions (A), (A-1), condition (B), condition (C), and (C
-1), the conditions (D), (D-1) and the condition (E) are satisfied, and in Example 3, the conditions (A), (A-1), the condition (B), the condition (D), (D-1) and condition (F),
Satisfies (F-1).

[0131] The above embodiments both shows the case of using the lens system of the present invention as a microscope objective lens, r ₈ of FIG. In the vicinity (Examples 1 and 2 of the object position of these examples, performed In Example 3, an image sensor such as a CCD is arranged at r ₁₀ ), and an image of a distant object (an object conjugate with the image sensor) on the left side of the figure is formed on the light receiving surface of the image sensor. If used, it can be used as an imaging lens. In this case, the imaging lens can be a very bright telephoto lens.

In addition to the lens systems described in the claims, the lens systems described in the following items can also achieve the object of the present invention.

(1) The lens system according to claim 1, 2 or 3, wherein the following condition (B) is satisfied. (B) 0.2 <d _G · WD / f ² <2

(2) An optical system for forming a magnified image of an object, which is a radial type gradient index lens having both flat surfaces disposed closest to the object, at least one homogeneous positive lens, and at least one A lens system that satisfies the following condition (F) including a homogeneous negative lens.

(3) An optical system for forming a reduced image of an object, comprising: a radial type gradient index lens having both flat surfaces disposed closest to the image side; at least one homogeneous positive lens;
A lens system that includes at least one homogeneous negative lens and satisfies the following condition (F).

(4) The lens system according to claim 1, 2 or 3, or the lens system according to (1),
A lens system characterized by satisfying the following condition (C). (C) −0.8 <f _G / f ₂₃ <−0.01

(5) Claims 1, 2 or 3 of the claims or the above (1), (2), (3) or (4)
Wherein the partial dispersion ratio of the radial type gradient index lens satisfies the following condition (D): _{(D) -0.01 <(P 10dC} -0.2756) / V 10 <0.01

(6) The lens system according to claim 1, 2 or 3, or the lens system according to (1), (4) or (5), wherein the lens system is a radial type refractive index distribution. A lens system comprising a lens, one homogeneous positive lens and one homogeneous negative lens, and satisfying the following condition (E). (E) 1 <ν _n / ν _p <3

(7) The lens system described in the above item (2) or (3), wherein the following condition (A) is satisfied.

(8) The lens system described in the above item (2) or (3), wherein the following condition (B) is satisfied. (B) 0.2 <d _G · WD / f ² <2

[0141]

According to the present invention, it is possible to realize a lens system suitable for a microscope objective lens and an imaging lens system in which the number of lenses constituting the lens is as small as about three and the aberration is well corrected.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a first embodiment of a lens system according to the present invention.

FIG. 2 is a sectional view of Embodiment 2 of the lens system of the present invention.

FIG. 3 is an aberration curve diagram of the first embodiment.

FIG. 4 is an aberration curve diagram of the second embodiment.

FIG. 5 is a diagram showing a situation of an on-axis marginal ray and an off-axis chief ray in a microscope objective lens designed at infinity.

FIG. 6 is a diagram showing changes in spherical aberration, coma aberration, and astigmatism in a lens system including one radial type gradient index lens having both flat surfaces.

FIG. 7 is a diagram showing a configuration of a lens system composed of only a radial type gradient index lens having both flat surfaces.

8 is an aberration curve diagram of the lens system shown in FIG.

FIG. 9 shows N of a radial type gradient index lens element having both flat surfaces.
Diagram showing lens changes when ₁₀ is changed

FIG. 10 is a sectional view of a lens system according to a third embodiment of the present invention.

FIG. 11 is a diagram showing measured values of axial chromatic aberration of the radial type gradient index material of Example 3 of the present invention.

Figure 12 shows the chromatic dispersion characteristic in terms of coefficient N ₁₀ of the radial type refractive index distribution material of Example 3 of the present invention

FIG. 13 is a diagram showing a partial dispersion ratio of a radial type gradient index material according to Example 3 of the present invention.

FIG. 14 is a diagram showing axial chromatic aberration according to a third embodiment of the present invention.

FIG. 15 is an aberration curve diagram according to the third embodiment of the present invention.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H087 KA03 KA09 LA01 NA14 PA03 PA04 PA17 PB03 PB04 QA02 QA03 QA05 QA12 QA22 QA25 QA31 QA41 QA42 QA45 QA46 RA22 RA42

Claims (3)

[Claims] 1. An optical system for forming an enlarged image of an object, comprising: a radial-type gradient index lens having two flat surfaces disposed closest to the object, at least one homogeneous positive lens, and at least one homogeneous lens. Lens system including negative lens. 2. An optical system for forming a reduced image of an object, comprising: a radial type gradient index lens having both flat surfaces disposed closest to the image side; at least one homogeneous positive lens; Lens system including negative lens. 3. The distribution formula of the radial type gradient index lens element is represented by the following equation (1). The lens system according to claim 1 or 2, wherein 2,...) Are coefficients representing the refractive index distribution at the wavelength λ, and f is the focal length of the lens system.