Large numerical aperture objective lens and night vision optical device using the same
View Patent ↗A large numerical aperture objective lens is disclosed having three lens groups. In order from the object side, these lens groups include: a first lens group of positive refractive power that is formed of two positive lens elements and an achromatic set of paired lens elements formed of a lens element of negative refractive power and a lens element of positive refractive power, respectively; a second lens group of negative refractive power; and a third lens group of positive refractive power, with the surface of the third lens group nearest the object side being convex. Specified conditions are satisfied in order to provide a compact, large numerical aperture objective lens having its aberrations favorably corrected as well as to correct for shifts of focus caused by expansion/contraction of the lens barrel with changes in temperature. The objective lens of the invention is particularly suitable for use in a night vision optical device having a photoelectron amplifier tube and an eyepiece, wherein aberrations in the eyepiece are balanced by aberrations of the objective lens.
1. An objective lens comprising, in order from the object side:
a first lens group having positive refractive power and formed of, in order from the object side, two positive lens elements and an achromatic set of paired lens elements formed of a negative lens element and a positive lens element;
a second lens group having negative refractive power; and
a third lens group having positive refractive power, the surface of the third lens group nearest the object side being convex;
wherein the following Conditions (1)-(4) are satisfied
0.45<|f G1 |/f<0.9 Condition (1)
0.3<|f G2 |/f<0.7 Condition (2)
0.15<|R G31 |/f<0.6 Condition (3)
0.4<D G2G3 /f<0.8 Condition (4)
where
f is the focal length of the objective lens,
f G1 is the focal length of the first lens group,
f G2 is the focal length of the second lens group,
R G31 is the radius of curvature of the surface of the third lens group nearest the object side, and
D G2G3 is the on-axis distance between the second lens group and the third lens group.
2. The objective lens of claim 1 , said third lens group being formed of an achromatic set of paired lenses having overall positive refractive power that is formed of a positive lens element and a negative lens element.
3. The objective lens of claim 2 , wherein at least one lens element, of among the lens elements having negative refractive power in the first and second lens groups, is made of a material satisfying the following Condition (5):
dn/dT>7×10 −6 Condition (5)
where
dn/dT is the temperature coefficient of the index of refraction of the material at room temperature.
4. The objective lens of claim 2 , wherein at least one lens element in said first lens group is made of a material satisfying the following Conditions (6) and (7):
dn/dT<0 Condition (6)
α>9×10 −6 Condition (7)
where
dn/dT is the temperature coefficient of the index of refraction of the material at room temperature, and
α is the linear expansion coefficient of the material at room temperature.
5. A night vision optical device, comprising the objective lens of claim 2 in combination with a photoelectron amplifier tube and an eyepiece.
6. The objective lens of claim 1 , wherein at least one lens element, of among the lens elements having negative refractive power in the first and second lens groups, is made of a material satisfying the following Condition (5):
dn/dT>7×10 −6 Condition (5)
where
dn/dT is the temperature coefficient of the index of refraction of the material at room temperature.
7. The objective lens of claim 3 , wherein at least one lens element in said first lens group is made of a material satisfying the following Conditions (6) and (7):
dn/dT<0 Condition (6)
α>9×10 −6 Condition (7)
where
dn/dT is the temperature coefficient of the index of refraction of the material at room temperature, and
α is the linear expansion coefficient of the material at room temperature.
8. A night vision optical device, comprising the objective lens of claim 6 in combination with a photoelectron amplifier tube and an eyepiece.
9. The objective lens of claim 1 , wherein at least one lens element in said first lens group is made of a material satisfying the following Conditions (6) and (7):
dn/dT<0 Condition (6)
α>9×10 −6 Condition (7)
where
dn/dT is the temperature coefficient of the index of refraction of the material at room temperature, and
α is the linear expansion coefficient of the material at room temperature.
10. A night vision optical device, comprising the objective lens of claim 9 in combination with a photoelectron amplifier tube and an eyepiece.
11. A night vision optical device, comprising the objective lens of claim 1 in combination with a photoelectron amplifier tube and an eyepiece.