Image reading lens, image reading device, and image forming device

1. An image reading lens, comprising:

a front lens group disposed on an object side, comprising first, second, third, fourth, and fifth lenses including a positive lens and a negative lens; and

a rear lens group disposed on an image side, comprising a negative plastic lens, wherein

the image reading lens has an angle of view of 56 degrees or more, and

the image reading lens is formed to satisfy the following four conditions:

0<Σ dn N /dt ( N )−Σ dn p /dt ( P )<25,   (1)

0.10< fa/f×d 11 /D< 0.90,   (2)

−1.5< fb/f<− 0.5, and   (3)

0.35< d 11 /D< 0.70,   (4)

where:

Σdn N /dt(N) is a sum of temperature coefficients of refractive indexes, multiplied by 10 6 , of the negative lenses of the front lens group along a He—Ne line at a temperature of 40° C. to 60° C.,

Σdn p /dt(P) is a sum of temperature coefficients of refractive indexes, multiplied by 10 6 , of the positive lenses of the front lens group along a He—Ne line at a temperature of 40° C. to 60° C., respectively,

fa is a focal length of the front lens group,

fb is a focal length of the rear lens group,

f are focal length of the entire image reading lens,

d 11 is an on-axis distance between the front lens group and the rear lens group, and

D is a total length of the image reading lens.

2. The image reading lens according to claim 1 , wherein the front lens group is formed to satisfy the following condition:

5<ν P −ν N <30,   (5)

where:

ν P is a mean value of abbe numbers of the positive lens relative to an e-line, and

ν N is a mean value of abbe numbers of the negative lens relative to the e-line.

3. The image reading lens according to claim 1 , wherein the front lens group is formed to satisfy the following condition:

−1.2< R 3 /R 11 <−0.4,   (6)

where:

R 3 is a curvature radius of an object-side surface of the first lens of the front lens group, and

R 11 is a curvature radius of an image-side surface of the fifth lens of the front lens group.

4. The image reading lens according to claim 1 , wherein the image reading lens is formed to satisfy the following condition:

0.25<(φ b−φa )/ d 11 <0.65,   (7)

where:

φa is an effective diameter of an image-side surface of the fifth lens of the front lens group,

φb is an effective diameter of an object-side surface of the negative lens of the rear lens group, and

d 11 is the on-axis distance between the front lens group and the rear lens group.

5. The image reading lens according to claim 1 , wherein

at least one surface of the rear lens group is aspheric.

6. The image reading lens according to claim 1 , wherein

both surfaces of the negative plastic lens of the rear lens group are aspheric.

7. The image reading lens according to claim 1 , wherein

the front lens group comprises a biconvex lens, a biconcave lens, a negative meniscus lens, a biconvex lens, and a negative meniscus lens arranged in order from the object side.

8. The image reading lens according to claim 1 , wherein

the first lens and the second lens of the front lens group are a cemented lens and a fourth lens and a fifth lens are a cemented lens.

9. The image reading lens according to claim 1 , wherein

at least one surface of the front lens group is aspheric.

10. The image reading lens according to claim 1 , wherein

an outer shape of the negative plastic lens of the rear lens group is not rotationally symmetric relative to an optical axis.

11. The image reading lens according to claim 10 , wherein

the outer shape of the negative plastic lens of the rear lens group is a short strip long in a main scanning direction.

12. An image reading device comprising the image reading lens according to claim 1 .

13. The image forming device comprising the image reading device according to claim 12 .