Double convex collimator lens, illumination device, and microscope

1. A collimator lens consisting of a double convex single lens formed with a resinous material, for collimating a light flux emitted from a light source, wherein:

at least one of two opposing surfaces of the collimator lens is formed as an aspherical surface, and the collimator lens satisfies the following conditions:

2<| R 1/ R 2|<10;

NA>0.6; and

nF<f/WD , where:

R 1 is a curvature radius of a first surface, which is a lens surface on a side of the light source of the collimator lens,

R 2 is a curvature radius of a second surface, which is a lens surface on a side opposite to the first surface of the collimator lens,

NA is a numerical aperture on the side of the light source of the collimator lens,

nF is a refractive index with respect to an F line of the collimator lens,

f is a focal length of the collimator lens, and

WD is a distance on an optical axis between the light source and the first surface.

2. The collimator lens according to claim 1 , wherein the second surface is the aspherical surface.

3. The collimator lens according to claim 2 , wherein the aspherical surface is defined by an expression:

Z

=

Y

2

R

0

+

R

0

⁢

1

-

(

K

+

1

)

⁢

(

Y

R

0

)

2

+

∑

i

=

1

⁢

(

A

2

⁢

⁢

i

⁢

Y

2

⁢

i

)

and satisfies a condition:

| A 2i |<0.1

where:

Z is a coordinate in a direction of the optical axis,

Y is a coordinate in a direction orthogonal to the optical axis,

K is a conic constant,

R 0 is a paraxial curvature radius of the aspherical surface, and

A 2i is an aspherical coefficient of an even-numbered order.

4. A collimator lens consisting of a double convex single lens formed with a resinous material, for collimating a light flux emitted from a light source,

wherein at least one of two opposing surfaces of the collimator lens is formed as an aspherical surface, and the collimator lens satisfies the following conditions:

2<| R 1/ R 2|<10; and

NA>0.6, where:

R 1 is a curvature radius of a first surface, which is a lens surface on a side of the light source of the collimator lens,

R 2 is a curvature radius of a second surface, which is a lens surface on a side opposite to the first surface of the collimator lens, and

NA is a numerical aperture on the side of the light source of the collimator lens,

wherein the second surface is the aspherical surface,

wherein the aspherical surface is defined by an expression:

Z

=

Y

2

R

0

+

R

0

⁢

1

-

(

K

+

1

)

⁢

(

Y

R

0

)

2

+

∑

i

=

1

⁢

(

A

2

⁢

⁢

i

⁢

Y

2

⁢

i

)

and satisfies a condition:

| A 2i |<0.1, where:

Z is a coordinate in a direction of an optical axis,

Y is a coordinate in a direction orthogonal to the optical axis,

K is a conic constant,

R 0 is a paraxial curvature radius of the aspherical surface, and

A 2i is an aspherical coefficient of an even-numbered order,

wherein the aspherical surface satisfies a condition between aspherical coefficients of an even-numbered order equal to or higher than a second order:

| A 2i |<|A 2(i-1) |

where A 2(i-1) is an aspherical coefficient of an even-numbered order lower than the A 2i .

5. An illumination device, comprising:

a low heat generation light source; and

a collimator lens consisting of a double convex single lens formed with a resinous material, for collimating a light flux emitted from a light source,

wherein at least one of two opposing surfaces of the collimator lens is formed as an aspherical surface, and the collimator lens satisfies the following conditions:

2<| R 1/ R 2|<10; and

NA>0.6, where:

R 1 is a curvature radius of a first surface, which is a lens surface on a side of the light source of the collimator lens,

R 2 is a curvature radius of a second surface, which is a lens surface on a side opposite to the first surface of the collimator lens, and

NA is a numerical aperture on the side of the light source of the collimator lens.

6. The illumination device according to claim 5 , further comprising a second lens on a side of the second surface of the collimator lens,

wherein the illumination device satisfies a condition:

| f/fs|< 0.1

where f is a focal length of the collimator lens and fs is a focal length of the second lens.

7. A microscope, comprising:

the illumination device according to claim 5 ; and

an image forming optical system for forming an image of a specimen with light from the specimen via an objective lens.