Lens array, lens unit, exposure device, LED head, and image forming apparatus

1. A lens array comprising a plurality of lenses arranged so as to form an array, wherein

each of the plurality of lenses has an optical axis,

each of the plurality of lenses is configured so that a contour focal length which is a focal length of a ray incident on a first position separate from the optical axis is longer than a paraxial focal length which is a focal length of a ray incident on a second position in a vicinity of the optical axis, and

in each of the plurality of lenses, the second position is closer to the optical axis than is the first position.

2. The lens array according to claim 1 , wherein each of the plurality of lenses is an aspherical lens.

3. A lens unit comprising:

a first lens array including a plurality of first lenses arranged so as to form an array; and

a second lens array including a plurality of second lenses arranged so as to form an array, wherein

optical axes of the plurality of second lenses respectively coincide with optical axes of the plurality of first lenses,

each of the plurality of first lenses is configured so that a first contour focal length which is a focal length of a ray incident on a first position separate from the optical axis is longer than a first paraxial focal length which is a focal length of a ray incident on a second position in the vicinity of the optical axis,

each of the plurality of second lenses is configured so that a second contour focal length which is a focal length of a ray incident on a third position separate from the optical axis is longer than a second paraxial focal length which is a focal length of a ray incident on a fourth position in the vicinity of the optical axis,

in each of the plurality of first lenses, the second position is closer to the optical axis than is the first position, and

in each of the plurality of second lenses, the fourth position is closer to the optical axis than is the third position.

4. The lens unit according to claim 3 , wherein

the first lens array is arranged on a side of an object while the second lens array is arranged on a side of an image formation surface,

a first distance from the object to the first lens array and a second distance from the second lens array to the image formation surface are substantially equal to each other, and

an upright isometric image of the object is formed on the image formation surface.

5. The lens unit according to claim 4 , further comprising a light blocking member that includes a plurality of openings each having an opening radius RA and is disposed between the first lens array and the second lens array, wherein

the plurality of openings of the light blocking member face the plurality of first lenses and the plurality of second lenses,

the first lens array and the second lens array are formed of material that allows light to pass through, and

the first lens array and the second lens array satisfies

0.45×Δ FB≤ΔF≤ 1.79×Δ FB,

where ΔF represents each of a first difference between the first paraxial focal length and the first contour focal length and a second difference between the second paraxial focal length and the second contour focal length,

ΔFB is determined by

Δ

⁢

⁢

FB

=

n

⁢

⁢

FE

-

n

2

⁢

FE

2

-

(

K

-

1

)

2

⁢

RA

2

K

,

n represents a refractive index of the first lens array and the second lens array,

FE represents each of the first contour focal length and the second contour focal length,

PI represents a distance from each of principal planes of the second lenses to the image formation surface,

FA represents each of the first paraxial focal length and the second paraxial focal length, and

K represents a ratio PI/FA.

6. The lens unit according to claim 3 , wherein

a first difference between the first paraxial focal length and the first contour focal length is within a range of 0.04 to 0.159, and

a second difference between the second paraxial focal length and the second contour focal length is within a range of 0.04 to 0.159.

7. The lens unit according to claim 3 , wherein

each of the plurality of first lenses is an aspherical lens, and

each of the plurality of second lenses is an aspherical lens.

8. An exposure device comprising:

a light emission section; and

the lens unit according to claim 3 , wherein the lens unit according to claim 3 condenses each of light beams emitted from the light emission section.

9. An LED head comprising:

an LED array; and

the lens unit according to claim 3 , wherein the lens unit according to claim 4 that condenses each of light beams emitted from the LED array.

10. An image forming apparatus comprising:

an electrostatic latent image bearing body that bears an electrostatic latent image;

a light emission section; and

the lens unit according to claim 3 , wherein the lens unit according to claim 4 condenses each of light beams emitted from the light emission section on a surface of the electrostatic latent image bearing body.

11. The lens array according to claim 1 , wherein a difference between the paraxial length and the contour focal length is within a range of 0.04 to 0.159.