OPTICAL SYSTEM AND IMAGE PICKUP APPARATUS HAVING THE SAME

Abstract

An optical system images an object with light with a wavelength of 8 μm or higher, and includes a diaphragm and an optical element having an aspherical surface and disposed at a position different from that of the diaphragm. In a section including an optical axis, a thickness of an optical element monotonously increases from an on-axis to an outermost off-axis or the optical element is the thinnest at a position other than an on-axis and an outermost off-axis. A predetermined condition is satisfied.

Claims

1. An optical system configured to image an object with light with a wavelength of 8 μm or longer, the optical system comprising: a diaphragm; and an optical element having an aspherical surface and disposed at a position different from that of the diaphragm, wherein in a section including an optical axis, a thickness of an optical element monotonously increases from an on-axis to an outermost off-axis, and wherein the following conditional expression is satisfied:
0.0<|f/Pf1|<0.3 where f is a focal length of the optical system, and Pf1 is a focal length of the optical element.

2. The optical system according to claim 1, further comprising a first lens closest to the object, wherein the following conditional expression is satisfied:
0.1<|f1/f|<6.0 where f1 is a focal length of the first lens.

3. The optical system according to claim 1, further comprising a first lens closest to the object, and a second lens adjacent to the first lens, wherein the following conditional expression is satisfied:
0.1<|f2/f|<20.0 where f2 is a focal length of the second lens.

4. The optical system according to claim 1, further comprising a first lens closest to the object, a second lens adjacent to the first lens, and a third lens disposed on an image side of the second lens, wherein the following conditional expression is satisfied:
0.1<|f3/f|<5.0 where f3 is a focal length of the third lens.

5. An optical system configured to image an object with light with a wavelength of 8 μm or longer, the optical system comprising: a diaphragm; and an optical element having an aspherical surface and disposed at a position different from that of the diaphragm, wherein in a section including an optical axis, the optical element is the thinnest at a position other than an on-axis and an outermost off-axis, and wherein the following conditional expression is satisfied:
0.0<|f/Pf1|<0.3 where f is a focal length of the optical system, and Pf1 is a focal length of the optical element.

6. The optical system according to claim 5, wherein the aspherical surface has a shape represented by a function of a sixth order or longer.

7. The optical system according to claim 5, further comprising a first lens closest to the object, wherein the optical element is disposed on an object side of the diaphragm, and wherein the following conditional expression is satisfied:
PZ/LP<0.8 where PZ is a distance from the diaphragm to the optical element, and LP is a distance from the diaphragm to the first lens.

8. The optical system according to claim 5, wherein the optical element is disposed on an image side of the diaphragm, and wherein the following conditional expression is satisfied:
PZ/LS<0.8 where PZ is a distance from the diaphragm to the optical element, and LS is a distance from the diaphragm to an image plane.

9. The optical system according to claim 5, further comprising a first lens closest to the object, wherein the following conditional expression is satisfied:
0.1<|f1/f|<5.0 where f1 is a focal length of the first lens.

10. The optical system according to claim 5, further comprising a first lens closest to the object, and a second lens adjacent to the first lens, wherein the following conditional expression is satisfied:
0.1<f2/f<10.0 where f2 is a focal length of the second lens.

11. The optical system according to claim 5, further comprising a first lens closest to the object, a second lens adjacent to the first lens, and a third lens disposed on an image side of the second lens, wherein the following conditional expression is satisfied:
0.1<f3/f<10.0 where f3 is a focal length of the third lens.

12. The optical system according to claim 1, wherein the following conditional expression is satisfied:
0.05≤T≤1.00 where T [mm] is a thickness of the optical element on an optical axis.

13. An optical system configured to image an object with light with a wavelength of 8 μm or longer, the optical system comprising: a diaphragm; and an optical element having an aspherical surface and disposed at a position different from that of the diaphragm, wherein the following conditional expressions are satisfied:
20≤(N10−1)/(N8−−N12)≤800
0.0<|f/Pf2|<0.3 where N8 is a refractive index of a material of an optical element at a wavelength of 8 μm, N10 is a refractive index of the material at a wavelength of 10 μm, N12 is a refractive index of the material at a wavelength of 12 μm, f is a focal length of the optical system, and Pf2 is a focal length of the optical element.

14. The optical system according to claim 13, wherein the optical element is disposed on an image side of the diaphragm, and wherein the following conditional expression is satisfied:
0.3<PZ/LS<1.0 where PZ is a distance from the diaphragm to the optical element, and LS is a distance from the diaphragm to an image plane.

15. The optical system according to claim 13, further comprising a first lens closest to the object, wherein the following conditional expression is satisfied:
0.01<|f1/Pf2|<0.80 where f1 is a focal length of the first lens.

16. The optical system according to claim 13, further comprising a first lens closest to the object, and a second lens adjacent to the first lens, wherein the following conditional expression is satisfied:
0.01<f2/Pf2<0.80 where f2 is a focal length of the second lens.

17. The optical system according to claim 13, further comprising a first lens closest to the object, a second lens adjacent to the first lens, and a third lens disposed on an image side of the second lens, wherein the following conditional expression is satisfied:
0.1<f3/Pf2<0.80 where f3 is a focal length of the third lens.

18. The optical system according to claim 1, wherein the optical element is made of a silicon material.

19. The optical system according to claim 1, further comprising another optical element made of a silicon material or a germanium material.

20. An image pickup apparatus comprising: the optical system according to claim 1; and an image sensor configured to receive light from the optical system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 is a sectional view of an optical system according to Example 1.

[0009] FIG. 2 is an MTF diagram of the optical system according to Example 1.

[0010] FIG. 3 is a sectional view of an optical system according to Example 2.

[0011] FIG. 4 is an MTF diagram of the optical system according to Example 2.

[0012] FIG. 5 is a sectional view of an optical system according to Example 3.

[0013] FIG. 6 is an MTF diagram of the optical system according to Example 3.

[0014] FIG. 7 is a sectional view of an optical system according to Example 4.

[0015] FIG. 8 is an MTF diagram of the optical system according to Example 4.

[0016] FIG. 9 is a sectional view of an optical system according to Example 5.

[0017] FIG. 10 is an MTF diagram of the optical system according to Example 5.

[0018] FIG. 11 is a sectional view of an optical system according to Example 6.

[0019] FIG. 12 is an MTF diagram of the optical system according to Example 6.

[0020] FIG. 13 is a sectional view of an optical system according to Example 7.

[0021] FIG. 14 is an MTF diagram of the optical system according to Example 7.

[0022] FIG. 15 is a sectional view of an optical system according to Example 8.

[0023] FIG. 16 is an MTF diagram of the optical system according to Example 8.

[0024] FIG. 17 is a sectional view of an optical system according to Example 9.

[0025] FIG. 18 is an MTF diagram of the optical system according to Example 9.

[0026] FIG. 19 is a sectional view of an optical system according to Example 10.

[0027] FIG. 20 is an MTF diagram of the optical system according to Example 10.

[0028] FIG. 21 is a sectional view of an optical system according to Example 11.

[0029] FIG. 22 is an MTF diagram of the optical system according to Example 11.

[0030] FIG. 23 is a sectional view of an optical system according to Example 12.

[0031] FIG. 24 is an MTF diagram of the optical system according to Example 12.

[0032] FIG. 25 is a schematic view of principal part of a camcorder as an illustrative image pickup apparatus according to Example 13.

DESCRIPTION OF THE EMBODIMENTS

[0033] Referring now to the accompanying drawings, a detailed description will be given of embodiments according to the present invention. Corresponding elements in respective figures will be designated by the same reference numerals, and a duplicate description thereof will be omitted.

[0034] A silicon material or a germanium material in the description of each example means a material containing silicon or germanium as a main component, and is not limited to a material exclusively consisting of silicon (Si) or germanium (Ge) but may contain a small amount of impurities.

Example 1

[0035] FIG. 1 is a sectional view of an optical system 100 according to this example. The optical system 100 is an infrared optical system having a focal length of 18 mm and an F-number of 0.8. The infrared optical system, as used herein, is an optical system that images an object with light having a wavelength of 8 μm or longer. The optical system 100 includes, in order from the object side to the image side, a first lens L11 having a positive refractive power and made of a silicon material, a thin aspherical plate (optical element) P11 made of a silicon material and having an aspherical portion with a thickness that gradually increases from the center to the periphery (or with a thickness that monotonously increases from the on-axis to the outermost off-axis in the section including the optical axis), a diaphragm (aperture stop) S1, a second lens L12 having a positive refractive power and made of a silicon material, a third lens L13 having a positive refractive power and made of a silicon material, and a thin aspherical plate (optical element) P12 made of a silicon material and having an aspherical portion with a thickness that gradually increases from the center to the periphery. The thin aspherical plates P11 and P12 are assumed to have a very thin thickness of several hundred μm. Therefore, it can be manufactured by easy processing such as processing used to manufacture a Schmidt correction plate. Light in the infrared range (with a wavelength 8 to 14 μm) guided by the optical system 100 passes through a cover glass CG1 and forms an image on an infrared sensor IM1. A window material may be provided between the first lens L11 and the object, or the cover glass CG1 may include an infrared material other than the germanium material. Table 1 shows numerical data of the optical system 100. The unit of radius of curvature and spacing (distance or interval) is mm.

TABLE-US-00001 TABLE 1 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT PLANE — INFINITY L11 SPHERICAL SURFACE 16.30 1.50 SILICON SPHERICAL SURFACE 17.91 5.76 P1 FLAT SURFACE FLAT SURFACE 0.20 SILICON ASPHERICAL SURFACE 11 1.00E+16 4.16 S2  FLAT SURFACE — 4.10 L12 SPHERICAL SURFACE 10.19 1.60 SILICON SPHERICAL SURFACE 9.44 3.52 L13 SPHERICAL SURFACE 17.60 1.40 SILICON SPHERICAL SURFACE 27.55 1.82 P2  ASPHERICAL SURFACE 12 1.00E+16 0.20 SILICON FLAT SURFACE FLAT SURFACE 2.00 CG1 FLAT SURFACE FLAT SURFACE 1.00 GERMANIUM FLAT SURFACE FLAT SURFACE 3.60 IMAGE PLANE

[0036] Table 2 shows aspherical shape data.

TABLE-US-00002 TABLE 2 ASPHERICAL ASPHERICAL SURFACE 11 SURFACE 12 PARAXIAL RADIUS OF  1.00E+16  1.00E+16 CURVATURE R CONICAL COEFFIENT k 0.00 0.00  4TH-ORDER COEFFICIENT A  1.15E−05 −3.67E−05  6TH-ORDER COEFFICIENT B −7.80E−08  1.24E−07  8TH-ORDER COEFFICIENT C  1.20E−09  7.32E−09 10TH-ORDER COEFFICIENT D −1.07E−11 −6.89E−10 12TH-ORDER COEFFICIENT E  4.02E−14  1.36E−11 14TH-ORDER COEFFICIENT F −3.76E−17 −8.20E−14

[0037] The aspherical shape is expressed as follows:

[00001]$\begin{array}{cc}Z=\frac{\left(1/R\right)h^2}{1+\sqrt{1-\left(1+k\right){\left(1/R\right)}^2{h}^2}}+A{h}^4+B{h}^6+C{h}^8+D{h}^{10}+E{h}^{12}+F{h}^{14}& \left(1\right)\end{array}$

where Z is a displacement amount from a surface apex in an optical axis direction, h is a height from an optical axis in a direction orthogonal to the optical axis, R is a paraxial radius of curvature, k is a conical coefficient, and A to F are fourth to fourteenth order aspherical coefficients.

[0038] FIG. 2 is an MTF diagram of the optical system 100. A pixel pitch of a general infrared sensor is several tens of μm. This example uses the infrared sensor IM1 having a pixel pitch of 17 μm by an implementation. In this case, the Nyquist frequency is about 30 lp/mm. In order to resolve the object at this Nyquist frequency, the MTF value of about 30% is empirically sufficient. The MTF value at the Nyquist frequency in this example represented by reference numeral 11 in FIG. 2 is 28%. Since the MTF value is about 30% at the Nyquist frequency of the infrared sensor IM1, the optical system 100 exhibits excellent optical performance.

[0039] The silicon material has a high refractive index and a low dispersion in the infrared range, and can provide high optical performance with an aspherical surface as part of the lens made of the silicon material. In order to process an optical element having an aspherical surface of a silicon material, a highly difficult process such as grinding or polishing is required. Accordingly, a thin aspherical plate is useful in which it is easy to process an aspherical portion without any highly difficult processes such as grinding or polishing. The thin aspherical plate can be manufactured by a photolithography process and a process for the Schmidt correction plate, which is manufactured by adsorbing an aspherical prototype on a thin lens substrate, by transferring the shape of the aspherical prototype, and by polishing it.

[0040] Table 3 shows a refractive index N10 and an Abbe number ν10 of a material that transmits infrared light. The refractive index N10 is a refractive index at a wavelength of 10 μm. The Abbe number ν10 is expressed by the following expression (2) where N8 is a refractive index of the material at a wavelength of 8 μm and N12 is a refractive index of the material at a wavelength of 12 μm. In general, the larger the Abbe number is, the smaller the refractive index change (dispersion) becomes due to the wavelength. Since numerical values are slightly different depending on each glass material manufacturer, Table 3 shows approximate numerical values. In particular, the refractive index and Abbe number of chalcogenides vary greatly depending on the type of compound using the oxygen group element of Group 16 in the periodic table. Depending on the type of compound, the refractive index N10 varies from 2.0 to 4.0 and the Abbe number ν10 varies from 100 to 800.

TABLE-US-00003 TABLE 3 GERMA- CALCO- ZINC ZINC NIUM SILICON RESIN GENIDE SULFIDE SELENIDE REFRACTIVE 4.0 3.4 1.5 2.5 2.2 2.4 INDEX N10 DISPERSION 861 1860 45 109 23 57 VALUE ν 10

[00002]$\begin{array}{cc}v10=\frac{N10-1}{N8-N12}& \left(2\right)\end{array}$

[0041] Germanium and silicon each have a higher refractive index and a smaller dispersion than other materials. A general N-unit optical system has a configuration that satisfies the following expression (3) in order to correct the chromatic aberration. In the expression (3), f1, f2, f3, . . . , and fn are focal lengths of a first lens, a second lens, a third lens, . . . , and an n-th lens, respectively, and ν1, ν2, ν3, . . . , and νn are Abbe numbers of the first lens, the second lens, the third lens, . . . , and the n-th lens, respectively.

[00003]$\begin{array}{cc}\frac{1}{f1v1}+\frac{1}{f2v2}+\frac{1}{f3v3}+\mathrm{.Math.}+\frac{1}{fnvn}=0& \left(3\right)\end{array}$

[0042] A lens usually has a positive Abbe number, and thus at least one lens may have a negative focal length in order to reduce the chromatic aberration. Therefore, the optical system for correcting the chromatic aberration has a combination of a positive lens and a negative lens. Since the silicon lens has a very small dispersion, the chromatic aberration is small even with the silicon lens having a positive refractive power alone, but there may be an element that plays a role of a negative lens. In this example, the thin aspherical plate has an aspherical portion with a thickness that gradually increases from the center to the periphery, so that the chromatic aberration can be corrected with high accuracy. An optical system having a short focal length needs to converge light beams with a plurality of angles of view, so that the first lens may be a negative lens.

[0043] In order to correct the curvature of field, the optical system needs to satisfy the following expression (4) so as to reduce the Petzval sum. Since the Petzval sum correlates with the curvature of field, the curvature of field can be reduced by reducing the Petzval sum. In the expression (4), f1, f2, f3, . . . , and fn are focal lengths of the first lens, the second lens, the third lens, . . . , and the n-th lens, respectively, and N1, N2, N3, . . . , and Nn are refractive indexes of the first lens, the second lens, the third lens, . . . , and the n-th lens, respectively.

[00004]$\begin{array}{cc}\frac{1}{f1N1}+\frac{1}{f2N2}+\frac{1}{f3N3}+\mathrm{.Math.}+\frac{1}{fnNn}=0& \left(4\right)\end{array}$

[0044] A lens usually has a positive refractive index, and thus at least one lens may have a negative focal length in order to reduce the Petzval sum. Therefore, the optical system for correcting a curvature of field has a combination of a positive lens and a negative lens. Since a silicon lens has a very large refractive index, the Petzval sum can be reduced only by the silicon lens having a positive refractive power, but there may be an element that plays a role of a negative lens. In this example, the Petzval sum can be corrected with high accuracy by the thin aspherical plate having an aspherical portion with a thickness that gradually increases from the center to the periphery. An optical system having a short focal length needs to converge light beams with a plurality of angles of view, so that the first lens may be a negative lens.

[0045] For high optical performance, it is important to correct a spherical aberration with high accuracy that is proportional to the pupil diameter of the lens. It is useful to share the spherical aberration with each lens and to correct it. In each example, the spherical lens having the main refractive power may be a positive lens. Thereby, the light beams can be gently converged, and the spherical aberration can be suppressed. An optical system having a short focal length needs to converge light beams with a plurality of angles of view, so that the first lens may be a negative lens.

[0046] A bright F-number lens such as an infrared lens needs to correct a high-order curvature of field with high accuracy in order to obtain excellent imaging performance. Accordingly, the present invention places the thin aspherical plate P11 at a position different from that of the diaphragm S1, and causes the aspherical portion of the thin aspherical plate P11 to correct the higher-order curvature of field. Each example places the thin aspherical plate P12 having a small refractive power at a position close to the diaphragm S1 on the object side or the image side of the diaphragm S1 and corrects a higher-order curvature of field and spherical aberration. That is, the conditions for Petzval sum and chromatic aberration correction are shared with spherical lenses having main refractive powers, and a thin aspherical plate having a small refractive power corrects a higher-order spherical aberration and a higher-order curvature of field. It is particularly difficult to correct the high-order curvature of field only with a spherical lens, and thus a thin aspherical plate may be disposed at a position where the on-axis ray and the off-axis ray are separated. Due to this configuration, the optical system 100 can exhibit high optical performance.

[0047] Compared to other spherical lenses, the focal length of the thin aspherical plate gives only the action of the aspherical portion as an effect, so it is unnecessary to have a high refractive power. Therefore, the optical systems according to Examples 1 to 8 satisfy the following conditional expression (5):

0.0<|f/Pf1|<0.3  (5)

where f is a focal length of the optical system and Pf1 is a focal length of the thin aspherical plate.

[0048] The numerical range of the conditional expression (5) may be set to that of the following conditional expression (5a):

0.00<|f/Pf1|<0.25  (5a)

[0049] As described above, the configuration according to this example can realize an optical system that is easy to manufacture and has high optical performance in the infrared range.

[0050] In the optical systems according to Examples 1 to 8, the thin aspherical plate may have a thin shape from the viewpoint of the transmittance of the silicon material. Hence, the following conditional expression (6) may be satisfied:

0.05≤T≤1.00  (6)

where T [mm] is a central thickness (thickness on the optical axis).

[0051] If the center thickness T is located out of the range of the conditional expression (6), the transmittance may be significantly reduced and the optical performance may be deteriorated.

[0052] The numerical range of the conditional expression (6) may be set to that of the following conditional expression (6a):

0.05≤T≤0.60  (6a)

[0053] The optical systems according to Examples 1 to 4 may satisfy at least of the following conditional expressions (7) to (9):

0.1<|f1/f|<6.0  (7)

0.1<f2/f<20.0  (8)

0.1<f3/f<5.0  (9)

where f1, f2, and f3 are focal lengths of the first to third lenses L11, L12, and L13, respectively.

[0054] The conditional expressions (7) to (9) are set to the optical systems according to Examples 1 to 4 so as to satisfactorily correct various aberrations such as the chromatic aberration, the spherical aberration, and the curvature of field and to exhibit high optical performance. If the value is higher than the upper limit or lower than the lower limit in each of the conditional expressions (7) to (9), a correction balance between the curvature of field and the spherical aberration destroys, and the optical performance deteriorates.

[0055] The numerical ranges of the conditional expressions (7) to (9) may be set to those of the following conditional expressions (7a) to (9a):

0.1<|f1/f|<5.0  (7a)

0.5<f2/f<15.0  (8a)

0.1<f3/f<4.0  (9a)

[0056] Table 4 shows numerical values corresponding to the conditional expressions according to Examples 1 to 4.

TABLE-US-00004 TABLE 4 f f1 f2 f3 Pf1 Pf2 f1/f f2/f f3/f f/Pf1 f/Pf2 EX. 1 18 45.3 104.7 16.3 0 0 2.52 5.82 1.05 — — EX. 2 14 57.5 147.6 13.2 0 0 4.11 10.54 0.95 — — EX. 3 50 70.6 59.7 — 0 0 1.41 1.19 — — — EX. 4 6 −14.16 11.3 15.08 51.7 49.6 −2.36 1.88 2.51 0.011605 0.012097

Example 2

[0057] FIG. 3 is a sectional view of an optical system 200 according to this example. The optical system 200 is an infrared optical system having a focal length of 14 mm and an F-number of 0.8. The optical system 200 includes, in order from the object side to the image side, a first lens L21 having a positive refractive power and made of a silicon material, a thin aspherical plate P21 made of a silicon material and having an aspherical portion with a thickness that gradually increases from the center to the periphery, a thin aspherical plate P22 made of a silicon material and having an aspherical portion with a thickness that gradually decreases from the center to the periphery, a diaphragm S2, a second lens L22 having a positive refractive power and made of a silicon material, a thin lens L23 having a positive refractive power and made of a silicon material, and a thin aspherical plate P23 made of a silicon material and having an aspherical portion with a thickness that gradually increases from the center to the periphery. The thin aspherical plates P21, P22, and P23 are assumed to have a very thin thickness of several hundred μm. Therefore, each of them can be manufactured by easy processing such as processing used to manufacture the Schmidt correction plate. This example disposes a thin aspherical plate P22 in order to correct a high-order spherical aberration. The light in the infrared range guided by the optical system 200 passes through a cover glass CG2 and forms an image on an infrared sensor IM2. Table 5 shows numerical data of the optical system 200. The unit of radius of curvature and spacing is mm.

TABLE-US-00005 TABLE 5 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT PLANE — INFINITY L21 SPHERICAL SURFACE 16.30 1.20 SILICON SPHERICAL SURFACE 17.91 3.74 P21 FLAT SURFACE FLAT SURFACE 0.20 SILICON ASPHERICAL SURFACE 21 1.00E+16 3.42 P22 FLAT SURFACE FLAT SURFACE 0.70 SILICON ASPHERICAL SURFACE 22 1.00E+16 0.83 S2  FLAT SURFACE — 1.43 L22 SPHERICAL SURFACE 10.19 1.60 SILICON SPHERICAL SURFACE 9.44 2.87 L23 SPHERICAL SURFACE 17.60 1.40 SILICON SPHERICAL SURFACE 27.55 2.23 P23 ASPHERICAL SURFACE 23 1.00E+16 0.20 SILICON FLAT SURFACE FLAT SURFACE 2.00 CG2 FLAT SURFACE FLAT SURFACE 1.00 GERMANIUM FLAT SURFACE FLAT SURFACE 3.00 IM2

[0058] Table 6 shows aspherical shape data. The aspherical shape is represented by the expression (1).

TABLE-US-00006 TABLE 6 ASPHERICAL ASPHERICAL ASPHERICAL SURFACE 21 SURFACE 22 SURFACE 23 PARAXIAL RADIUS OF  1.00E+16  1.00+16  1.00E+18 CURVATURE R CONICAL COEFFIENT k 0.00 0.00 0  4TH-ORDER COEFFICIENT A  4.19E−05 −4.65E−05 −0.00012  6TH-ORDER COEFFICIENT B  2.50E−08  1.72E−07 −1.04E−06  8TH-ORDER COEFFICIENT C −1.63E−10 −5.57E−09  1.02E−07 10TH-ORDER COEFFICIENT D  6.34E−11  7.43E−10 −3.42E−09 12TH-ORDER COEFFICIENT E −9.48E−13 −6.76E−11  5.33E−11 14TH-ORDER COEFFICIENT F  6.30E−15  2.63E−14 −3.01E−13

[0059] FIG. 4 is an MTF diagram of the optical system 200. This example uses the infrared sensor IM2 having a pixel pitch of 17 μm as an implementation. In this case, the Nyquist frequency is about 30 lp/mm. The MTF value at the Nyquist frequency in this example represented by reference numeral 21 in FIG. 4 is 43%. Since the MTF value is 30% or higher at the Nyquist frequency of the infrared sensor IM2, the optical system 200 exhibits excellent optical performance.

Example 3

[0060] FIG. 5 is a sectional view of an optical system 300 according to this example. The optical system 300 is an infrared optical system having a focal length of 50 mm and an F-number of 0.8. The optical system 300 includes, in order from the object side to the image side, a first lens L31 made of a silicon material and having a positive refractive power, a thin aspherical plate P31 made of a silicon material and having an aspherical portion with a thickness that gradually increases from the center to the periphery, a diaphragm S3, a second lens L32 having a positive refractive power and made of a silicon material, and a thin aspherical plate P32 made of a silicon material and having an aspherical portion with a thickness that gradually increases from the center to the periphery. Therefore, it can be manufactured by easy processing such as a bending process used to manufacture the Schmidt correction plate. The thin aspherical plates P31 and P32 are assumed to have a very thin thickness of several hundred μm. Light in the infrared range guided by the optical system 300 passes through a cover glass CG3 and forms an image on an infrared sensor IM3. Table 7 shows numerical data of the optical system 300. The unit of radius of curvature and spacing is mm.

TABLE-US-00007 TABLE 7 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT PLANE — INFINITY L31 SPHERICAL SURFACE 16.30 3.5 SILICON SPHERICAL SURFACE 17.91 9.0 P31 FLAT SURFACE FLAT SURFACE 0.3 SILICON ASPHERICAL SURFACE 31 1.00E+16 0.7 S3  FLAT SURFACE FLAT SURFACE 31.6 L32 SPHERICAL SURFACE 52.95227 1.8 SILICON SPHERICAL SURFACE 82.412 14.0 P32 ASPHERICAL SURFACE 32 1.00E+16 0.2 SILICON FLAT SURFACE FLAT SURFACE 3.7 CG3 FLAT SURFACE FLAT SURFACE 1.0 GERMANIUM FLAT SURFACE FLAT SURFACE 3.0 IM3

[0061] Table 8 shows aspherical shape data. The aspherical shape is represented by the expression (1).

TABLE-US-00008 TABLE 8 ASPHERICAL ASPHERICAL SURFACE 31 SURFACE 32 PARAXIAL RADIUS OF  1.00E+18  1.00E+18 CURVATURE R CONICAL COEFFIENT k 0.00 0.00  4TH-ORDER COEFFICIENT A  3.27E−07 −7.33E−06  6TH-ORDER COEFFICIENT B −1.17E−10  3.85E−08  8TH-ORDER COEFFICIENT C  4.20E−14  9.48E−11 10TH-ORDER COEFFICIENT D −2.11E−17 −1.46E−11 12TH-ORDER COEFFICIENT E  6.93E−21  1.69E−13 14TH-ORDER COEFFICIENT F  0.00E+00 −5.85E−16

[0062] FIG. 6 is an MTF diagram of the optical system 300. This example uses the infrared sensor IM3 having a pixel pitch of 17 μm as an implementation. In this case, the Nyquist frequency is about 30 lp/mm. The MTF value at the Nyquist frequency of this example represented by reference numeral 31 in FIG. 6 is 32%. Since the MTF value is 30% or higher at the Nyquist frequency of the infrared sensor IM3, the optical system 300 exhibits excellent optical performance.

Example 4

[0063] FIG. 7 is a sectional view of an optical system 400 according to this example. The optical system 400 is an infrared optical system having a focal length of 6 mm and an F-number of 0.9. The optical system 400 includes, in order from the object side to the image side, a first lens L41 having a negative refractive power and made of a silicon material, a thin aspherical plate P41 made of a silicon material and having an aspherical portion with a thickness that gradually increases from the center to the periphery, a second lens L42 having a positive refractive power and made of a silicon material, a diaphragm S4, a third lens L43 having a positive refractive power and made of a silicon material, and a thin aspherical plate P42 made of a silicon material and having an aspherical portion with a thickness that gradually increases from the center to the periphery. Therefore, it can be manufactured by easy processing such as a bending process used to manufacture the Schmidt correction plate. The thin aspherical plates P41 and P42 are assumed to have a very thin thickness of several hundred μm. Light in the infrared range guided by the optical system 400 passes through a cover glass CG4 and forms an image on an infrared sensor IM4. Table 9 shows numerical data of the optical system 400. The unit of radius of curvature and spacing is mm.

TABLE-US-00009 TABLE 9 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT PLANE — INFINITY L41 SPHERICAL SURFACE 9.745272 1.0 SILICON SPHERICAL SURFACE 7.033832 6.3 P41 ASPHERICAL SURFACE 41 1249.747 0.2 SILICON FLAT SURFACE FLAT SURFACE 5.4 L42 SPHERICAL SURFACE −155.564 1.5 SILICON SPHERICAL SURFACE −2.34E+01 0.6 S4  FLAT SURFACE FLAT SURFACE 6.5 L43 SPHERICAL SURFACE 51.53527 1.0 SILICON FLAT SURFACE −122.591 1.3 P42 ASPHERICAL SURFACE 42 1200.627 0.2 SILICON FLAT SURFACE FLAT SURFACE 2.0 CG4 FLAT SURFACE FLAT SURFACE 1.0 GERMANIUM FLAT SURFACE FLAT SURFACE 3.0 IM4

[0064] Table 10 shows aspherical shape data. The aspherical shape is represented by the expression (1).

TABLE-US-00010 TABLE 10 ASPHERICAL ASPHERICAL SURFACE 41 SURFACE 42 PARAXIAL RADIUS OF  1.25E+03  1.20E+03 CURVATURE R CONICAL COEFFIENT k 0.00 0.00  4TH-ORDER COEFFICIENT A −7.89E−05 −7.80E−05  6TH-ORDER COEFFICIENT B −2.17E−07 −1.43E−06  8TH-ORDER COEFFICIENT C −1.46E−08  1.44E−07 10TH-ORDER COEFFICIENT D  5.68E−10 −6.54E−09 12TH-ORDER COEFFICIENT E −8.71E−12  1.36E−10 14TH-ORDER COEFFICIENT F  8.49E−14 −1.10E−12

[0065] FIG. 8 is an MTF diagram of the optical system 400. This example uses the infrared sensor IM4 having a pixel pitch of 17 μm as an implementation. In this case, the Nyquist frequency is about 30 lp/mm. The MTF value at the Nyquist frequency in this example represented by reference numeral 41 in FIG. 8 is 36%. Since the MTF value is 30% or higher at the Nyquist frequency of the infrared sensor IM4, the optical system 400 exhibits excellent optical performance.

Example 5

[0066] FIG. 9 is a sectional view of an optical system 500 according to this example. The optical system 500 is an infrared optical system having a focal length of 4.5 mm and an F-number of 0.8. The optical system 500 includes, in order from the object side to the image side, a first lens L51 having a negative refractive power and made of a silicon material, a thin aspherical plate P51 made of a silicon material and having an aspherical portion that is the thinnest between the center and the outermost periphery not inclusive (with a thickness that becomes the thinnest at a position other than an on-axis and an outermost off-axis (between the on-axis and the outermost off-axis not inclusive) in a section including the optical axis), a second lens L52 having a positive refractive power and made of a silicon material, a diaphragm S5, and a third lens L53 having a positive refractive power and made of a silicon material. The thin aspherical plates P51 and P52 are assumed to have a very thin thickness of several hundred μm. Therefore, it can be manufactured by easy processing such as a bending process used to manufacture the Schmidt correction plate. Light in the infrared range guided by the optical system 500 passes through a cover glass CG5 and forms an image on an infrared sensor IM5. Table 11 shows numerical data of the optical system 500. The unit of radius of curvature and spacing is mm.

TABLE-US-00011 TABLE 11 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT PLANE — INFINITY L51 SPHERICAL SURFACE 8.53 0.45 SILICON SPHERICAL SURFACE 6.31 10.48 P51 ASPHERICAL SURFACE 51 196.43 0.20 SILICON FLAT SURFACE FLAT SURFACE 7.22 L52 SPHERICAL SURFACE −4.39E+01 1.50 SILICON SPHERICAL SURFACE −22.31 0.10 S5  FLAT SURFACE FLAT SURFACE 4.04 L53 SPHERICAL SURFACE 14.84 1.65 SILICON SPHERICAL SURFACE 24.16 2.26 SPHERICAL SURFACE FLAT SURFACE 2.00 SILICON FLAT SURFACE FLAT SURFACE 1.00 CG5 FLAT SURFACE FLAT SURFACE 2.00 GERMANIUM FLAT SURFACE FLAT SURFACE 2.10 IM5

[0067] Table 12 shows aspherical shape data. The aspherical shape is represented by the expression (1).

TABLE-US-00012 TABLE 12 ASPHERICAL SURFACE 51 PARAXIAL RADIUS OF CURVATURE R  1.96E+02 CONICAL COEFFIENT k 0.00  4TH-ORDER COEFFICIENT A −4.11E−05  6TH-ORDER COEFFICIENT B  2.12E−07  8TH-ORDER COEFFICIENT C  0.00E+00 10TH-ORDER COEFFICIENT D  0.00E+00 12TH-ORDER COEFFICIENT E  0.00E+00 14TH-ORDER COEFFICIENT F  0.00E+00

[0068] FIG. 10 is an MTF diagram of the optical system 500. This example uses the infrared sensor IM5 having a pixel pitch of 17 μm as an implementation. In this case, the Nyquist frequency is about 30 lp/mm. The MTF value at the Nyquist frequency in this example represented by reference numeral 51 in FIG. 10 is 48%. Since the MTF value is 30% or higher at the Nyquist frequency of the infrared sensor IM5, the optical system 500 exhibits excellent optical performance.

[0069] One method of correcting a curvature of field with high accuracy is a method of placing a thin aspherical plate having an aspherical effect at a position where light rays at respective angles of view are separated. The position where the light rays at respective angles of view are separated is different from the position of the diaphragm. Scattering of rays at respective angles of view is corrected by the aspherical surface portion. Since the curvature of field correction up to the fourth-order aspherical term is not enough, it is necessary to have an aspherical shape represented by a function having a sixth-order or higher aspherical term. It is necessary for the aberration correction that the shape has the thinnest thickness between the center and the outermost periphery (not inclusive).

[0070] The thin aspherical plate may be disposed at a position different from the position where the diaphragm is disposed from the viewpoint of curvature of field correction. The following expression (10) may be satisfied when the thin aspherical plate is disposed on the object side of the diaphragm:

PZ/LP<0.8  (10)

where LP is a distance from the first lens to the diaphragm, and PZ is a distance from the diaphragm to the thin aspherical plate.

[0071] When the thin aspherical plate is disposed on the image side of the diaphragm, the following conditional expression (11) may be satisfied:

PZ/LS<0.8  (11)

where LS is a distance from the diaphragm to the image plane, and PZ is a distance from the diaphragm to the thin aspherical plate.

[0072] The numerical ranges of the conditional expressions (10) and (11) may be set to those of the following conditional expressions (10a) and (11a).

0.1<PZ/LP<0.7  (10a)

0.1<PZ/LS<0.7  (11a)

[0073] The optical systems according to Examples 5 to 8 may satisfy at least one of the following conditional expressions (12) to (14):

0.1<|f1/f|<5.0  (12)

0.1<f2/f<10.0  (13)

0.1<f3/f<10.0  (14)

[0074] The conditional expressions (12) to (14) are set to the optical systems according to Examples 5 to 8 so as to satisfactorily correct various aberrations such as the chromatic aberration, the spherical aberration, and the curvature of field and to exhibit high optical performance. If the value is higher than the upper limit or lower than the lower limit in each of the conditional expressions (12) to (14), the correction balance between the curvature of field and the spherical aberration destroys, and the optical performance is deteriorated.

[0075] The numerical ranges of the conditional expressions (12) to (14) may be set to those of the following conditional expressions (12a) to (14a).

0.1<|f1/f|<3.5  (12a)

0.5<f2/f<7.0  (13a)

0.1<f3/f<4.0  (14a)

[0076] Tables 13 and 14 show numerical values corresponding to the conditional expressions according to Examples 5 to 8.

TABLE-US-00013 TABLE 13 f f1 f2 f3 Pf f1/f f2/f f3/f f/Pf EX. 5 4.5 −11.68 17.91 15.14 81.34 −2.60 3.98 3.14 0.055 EX. 6 4.5 −10.34 16.23 13.47 72.62 −2.30 3.61 2.99 0.062 EX. 7 3 −5.795 18.257 9.4242 31.166 −1.93 6.09 3.14 0.096 EX. 8 6 −13.09 22.268 12.316 28.474 −2.18 3.71 2.05 0.211

TABLE-US-00014 TABLE 14 LP LS PZ PZ/LP PZ/LS T1 EX. 5 15.05 19.95 8.81 059 — 0.20 EX. 6 16.69 18.3 10.39 — 0.57 0.22 EX. 7 7.23 19.73 12.01 — 0.61 0.33 EX. 8 8.54 23.74 15.79 — 0.67 0.45

Example 6

[0077] FIG. 11 is a sectional view of an optical system 600 according to this example. The optical system 600 is an infrared optical system having a focal length of 4.5 mm and an F-number of 0.8. The optical system 600 includes, in order from the object side to the image side, a first lens L61 made of a silicon material and having a negative refractive power, a second lens L62 made of a silicon material and having a positive refractive power, a diaphragm S6, a third lens L63 having a positive refractive power and made of a silicon material, and a thin aspherical plate P61 made of a silicon material and having an aspherical portion that is the thinnest between the center and the outermost periphery (not inclusive). Light in the infrared range guided by the optical system 600 passes through a cover glass CG6 and forms an image on an infrared sensor IM6. Table 15 shows numerical data of the optical system 600. The unit of radius of curvature and spacing is mm.

TABLE-US-00015 TABLE 15 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT — INFINITY PLANE L61 SPHERICAL SURFACE 9.46 0.45 SILICON SPHERICAL SURFACE 6.63 14.65 L62 SPHERICAL SURFACE 49.33 1.50 SILICON SPHERICAL SURFACE −186.71 0.10 S6 FLAT SURFACE FLAT SURFACE 7.51 L63 SPHERICAL SURFACE 14.94 1.70 SILICON SPHERICAL SURFACE 25.41 1.18 P61 ASPHERICAL SURFACE 61 175.36 0.22 SILICON SPHERICAL SURFACE FLAT SURFACE 2.00 CG6 FLAT SURFACE FLAT SURFACE 1.00 GERMANIUM FLAT SURFACE FLAT SURFACE 4.68 IM6 — —

[0078] Table 16 shows aspherical shape data. The aspherical shape is represented by the expression (1).

TABLE-US-00016 TABLE 16 ASPHERICAL SURFACE 61 PARAXIAL RADIUS OF CURVATURE R 175.359 CONICAL COEFFICIENT k 0.00 4TH-ORDER COEFFICIENT A −6.07E−05  6TH-ORDER COEFFICIENT B 3.43E−07 8TH-ORDER COEFFICIENT C 0.00E+00 10TH-ORDER COEFFICIENT D 0.00E+00 12TH-ORDER COEFFICIENT E 0.00E+00 14TH-ORDER COEFFICIENT F 0.00E+00

[0079] FIG. 12 is an MTF diagram of an optical system 600. In this example, an infrared sensor IM6 having a pixel pitch of 17 μm is used as an example. In this case, the Nyquist frequency is about 30 lp/mm. The MTF value at the Nyquist frequency of this example represented by reference numeral 61 in FIG. 12 is 50%. Since the MTF value is 30% or higher at the Nyquist frequency of the infrared sensor IM6, the optical system 600 exhibits excellent optical performance.

Example 7

[0080] FIG. 13 is a sectional view of an optical system 700 according to this example. The optical system 700 is an infrared optical system having a focal length of 3 mm and an F-number of 0.8. The optical system 700 includes, in order from the object side to the image side, a first lens L71 having a negative refractive power and made of a silicon material, a diaphragm S7, a second lens L72 having a positive refractive power and made of a silicon material, a third lens L73 having a positive refractive power and made of a silicon material, and a thin aspherical plate P71 made of a silicon material and having an aspherical portion that is the thinnest between the center and the outermost periphery (not inclusive). Light in the infrared range guided by the optical system 700 passes through a cover glass CG7 and forms an image on an infrared sensor IM7. Table 17 shows numerical data of the optical system 700. The unit of radius of curvature and spacing is mm.

TABLE-US-00017 TABLE 17 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT — INFINITY PLANE L71 SPHERICAL SURFACE 9.72 0.45 SILICON SPHERICAL SURFACE 7.43 10.05 S7 SPHERICAL SURFACE FLAT SURFACE 0.10 L72 SPHERICAL SURFACE 19.87 1.00 SILICON FLAT SURFACE 28.08 8.70 L73 SPHERICAL SURFACE 15.16 2.00 SILICON SPHERICAL SURFACE 29.26 2.62 F71 ASPHERICAL SURFACE 71 83.69 0.22 SILICON SPHERICAL SURFACE FLAT SURFACE 2.00 CG7 FLAT SURFACE FLAT SURFACE 1.00 GERMANIUM FLAT SURFACE FLAT SURFACE 4.50 IM7 — —

[0081] Table 18 shows aspherical shape data. The aspherical shape is represented by the expression (1).

TABLE-US-00018 TABLE 18 ASPHERICAL SURFACE 71 PARAXIAL RADIUS OF CURVATURE R 75.259 CONICAL COEFFICIENT k 0.00 4TH-ORDER COEFFICIENT A −1.35E−04  6TH-ORDER COEFFICIENT B 6.62E−07 8TH-ORDER COEFFICIENT C 0.00E+00 10TH-ORDER COEFFICIENT D 0.00E+00 12TH-ORDER COEFFICIENT E 0.00E+00 14TH-ORDER COEFFICIENT F 0.00E+00

[0082] FIG. 14 is an MTF diagram of the optical system 700. This example uses the infrared sensor IM7 having a pixel pitch of 17 μm as an implementation. In this case, the Nyquist frequency is about 30 lp/mm. The MTF value at the Nyquist frequency in this example represented by reference numeral 71 in FIG. 14 is 49%. Since the MTF value is 30% or higher at the Nyquist frequency of the infrared sensor IM7, the optical system 700 exhibits excellent optical performance.

Example 8

[0083] FIG. 15 is a sectional view of an optical system 800 according to this example. The optical system 800 is an infrared optical system having a focal length of 6 mm and an F-number of 0.8. The optical system 800 includes, in order from the object side to the image side, a first lens L81 having a negative refractive power and made of a silicon material, a diaphragm S8, a second lens L82 having a positive refractive power and made of a silicon material, a third lens L83 having a positive refractive power and made of a silicon material, and a thin aspherical plate P81 made of a silicon material and having an aspherical portion that is the thinnest between the center and the outermost periphery (not inclusive). Light in the infrared range guided by the optical system 800 passes through a cover glass CG8 and forms an image on an infrared sensor IM8. Table 19 shows numerical data of the optical system 800. The unit of radius of curvature and spacing is mm.

TABLE-US-00019 TABLE 19 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT — INFINITY PLANE L81 SPHERICAL SURFACE 10.79 0.45 SILICON SPHERICAL SURFACE 7.81 8.09 S8 SPHERICAL SURFACE FLAT SURFACE 0.10 L82 SPHERICAL SURFACE 24.50 1.00 SILICON FLAT SURFACE 43.71 9.98 L83 SPHERICAL SURFACE 16.13 2.00 SILICON SPHERICAL SURFACE 32.18 2.71 P81 ASPHERICAL SURFACE 81 68.76 0.45 SILICON SPHERICAL SURFACE FLAT SURFACE 2.00 CG8 FLAT SURFACE FLAT SURFACE 1.00 GERMANIUM FLAT SURFACE FLAT SURFACE 4.50 IM8 — —

[0084] Table 20 shows aspherical shape data. The aspherical shape is represented by the expression (1).

TABLE-US-00020 TABLE 20 ASPHERICAL SURFACE 81 PARAXIAL RADIUS OF CURVATURE R 68.759 CONICAL COEFFICIENT k 0.00 4TH-ORDER COEFFICIENT A −1.02E−04  6TH-ORDER COEFFICIENT B 3.50E−07 8TH-ORDER COEFFICIENT C 0.00E+00 10TH-ORDER COEFFICIENT D 0.00E+00 12TH-ORDER COEFFICIENT E 0.00E+00 14TH-ORDER COEFFICIENT F 0.00E+00

[0085] FIG. 16 is an MTF diagram of an optical system 800. This example uses the infrared sensor IM8 having a pixel pitch of 17 μm as an implementation. In this case, the Nyquist frequency is about 30 lp/mm. The MTF value at the Nyquist frequency in this example represented by reference numeral 81 in FIG. 16 is 54%. Since the MTF value is 30% or higher at the Nyquist frequency of the infrared sensor IM8, the optical system 800 exhibits excellent optical performance.

Example 9

[0086] FIG. 17 is a sectional view of an optical system 900 according to this example. The optical system 900 is an infrared optical system having a focal length of 4.5 mm and an F-number of 0.8. The optical system 900 includes, in order from the object side to the image side, a first lens L91 having a negative refractive power and made of a germanium material, a second lens L92 having a positive refractive power and made of a silicon material, a diaphragm S9, a third lens L93 having a positive refractive power and made of a silicon material, and an optical element (aspherical lens) ASP91 having an aspherical surface and made of a chalcogenide material. Chalcogenide contains a compound using an oxygen group element of Group 16 in the periodic table, and the refractive index and Abbe number change depending on the type of the compound. In this example, IRG206 of NHG (Hubei New Huagung Information Materials Co., Ltd.) is used as chalcogenide, but the compound is not limited to this example as long as it is a compound using an oxygen group element of Group 16 in the periodic table. Light in the infrared range guided by the optical system 800 passes through a cover glass CG9 and forms an image on an infrared sensor IM9. Table 21 shows numerical data of the optical system 900. The unit of radius of curvature and spacing is mm.

TABLE-US-00021 TABLE 21 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT — INFINITY PLANE L91 11.4429 0.45 GERMANIUM 8.208534 28.08 L92 33.64908 1.50 SILICON 103.3151 0.96 S9 FLAT SURFACE 10.01 L93 12.39436 1.70 SILICON 17.8448 2.07 ASP91 358.0504 1.00 CALOGENIDE (ASPHERICAL SURFACE 91) FLAT SURFACE 2.00 CG9 FLAT SURFACE 1.00 GERMANIUM FLAT SURFACE 4.56 IM9 —

[0087] Table 22 shows aspherical shape data. The aspherical shape is represented by the expression (1).

TABLE-US-00022 TABLE 22 ASPHERICAL SURFACE 91 PARAXIAL RADIUS OF CURVATURE R 358.050 CONICAL COEFFICIENT k 0.00 4TH-ORDER COEFFICIENT A −1.05E−04  6TH-ORDER COEFFICIENT B 2.95E−06 8TH-ORDER COEFFICIENT C −1.13E−07  10TH-ORDER COEFFICIENT D 3.21E−09 12TH-ORDER COEFFICIENT E −4.79E−11  14TH-ORDER COEFFICIENT F 2.81E−13

[0088] FIG. 18 is an MTF diagram of the optical system 900. This example uses the infrared sensor IM9 having a pixel pitch of 17 μm as an implementation. In this case, the Nyquist frequency is about 30 lp/mm. The MTF value at the Nyquist frequency in this example represented by reference numeral 91 in FIG. 18 is 57%. Since the MTF value is 30% or higher at the Nyquist frequency of the infrared sensor IM9, the optical system 900 exhibits excellent optical performance.

[0089] Silicon materials and germanium materials each have a high refractive index and a low dispersion in the infrared range and can exhibit high optical performance using an aspherical surface used for part of the lens made of silicon material or germanium material. However, in order to process an optical element having an aspherical surface of a silicon material or a germanium material, a highly difficult process such as grinding or polishing is required.

[0090] On the other hand, chalcogenide, zinc selenium (ZnSe), zinc sulfide (ZnS), resin (high density polyethylene) and the like are materials that can be molded by heat or the like, but exhibits a large chromatic aberration amount due to a large dispersion. If a diffraction structure is provided on the lens surface in order to correct the chromatic aberration, unnecessary light may be incident on the infrared sensor due to scattering in the diffraction structure, which may deteriorate the optical performance.

[0091] A lens usually has a positive Abbe number, and thus at least one lens may have a negative focal distance in order to reduce the chromatic aberration. Using materials with a large dispersion such as chalcogenide, zinc selenium, zinc sulfide, and resin (high density polyethylene) can suppress the chromatic aberration by increasing the focal distance of the lens (or by reducing the refractive power).

[0092] One method of correcting the curvature of field with high accuracy is a method of placing an optical element having an aspherical surface at a position where light rays of respective angles of view are separated. In an attempt to mainly correct the curvature of field, it is effective to dispose an aspherical lens on the image side. When such a moldable material as chalcogenide, zinc selenide, zinc sulfide, and resin (high density polyethylene) is used for the aspherical lens, the chromatic aberration becomes a problem. In order to solve this problem, an aspherical lens having a long focal length is used in this example.

[0093] Thus, this example can improve optical performance by disposing the optical element having an aspherical surface with a small refractive power at a position different from that of the diaphragm. The optical element having an aspherical surface may satisfy the conditional expression of 20≤ν10≤2000. In order to improve the ease of manufacturing (molding and processing) an optical element having an aspherical surface, the optical element may be made of a material having a large dispersion. More specifically, the optical element having an aspherical surface may satisfy the conditional expression of 20≤ν10≤800.

[0094] Compared to aspherical lenses, aspherical lenses using materials with large dispersion need to suppress the chromatic aberration while giving only the action of the aspherical part as an effect. Therefore, the following expression (15) may be satisfied:

0.0<|f/Pf2|<0.3  (15)

where Pf2 is a focal length of the aspherical lens.

[0095] The numerical range of the conditional expression (15) may be set to that of the following conditional expression (15a).

0.0<|f/Pf2|<0.1  (15a)

[0096] The numerical range of the conditional expression (15) may be set to that of the following conditional expression (15b).

0.00<|f/Pf2|<0.07  (15b)

[0097] The aspherical lens using a material having a large dispersion may be disposed on the image side of the diaphragm from the viewpoint of the curvature of field correction. The following expression (16) may be satisfied:

0.3<PZ/LS<1.0  (16)

where PZ is a distance from the diaphragm to the aspherical lens.

[0098] The numerical range of the conditional expression (16) may be set to that of the following conditional expression (16a).

0.4<PZ/LS<0.9  (16a)

[0099] The optical systems according to Examples 9 to 12 may satisfy at least one of the following conditional expressions (17) to (19).

0.01<|f1/Pf2|<0.80  (17)

0.01<|f2/Pf2|<0.80  (18)

0.01<|f3/Pf2|<0.80  (19)

[0100] The conditional expressions (17) to (19) are set to the optical systems according to Examples 9 to 12 so as to satisfactorily correct various aberrations such as the chromatic aberration, the spherical aberration, and the curvature of field and to exhibit high optical performance. If the value is higher than the upper limit or lower than the lower limit in each of the conditional expressions (17) to (19), the correction balance between the curvature of field and the spherical aberration destroys, and the optical performance is deteriorated.

[0101] The numerical ranges of the conditional expressions (17) to (19) may be set to those of the following conditional expressions (17a) to (19a).

0.01<|f1/Pf2|<0.50  (17a)

0.01<|f2/Pf2|<0.50  (18a)

0.01<|f3/Pf2|<0.50  (19a)

[0102] Tables 23 and 24 show numerical values corresponding to the conditional expressions according to Examples 9 to 12.

TABLE-US-00023 TABLE 23 f f1 f2 f3 Pf f1/f f2/f f3/f f/Pf EX. 9 4.5 −9.95 18.15 13.60 90.16 −2.21 4.03 3.02 0.050 EX. 10 4.5 −10.24 17.92 13.54 130.83 −2.28 3.98 3.01 0.034 EX. 11 4.5 −9.834 17.78 13.538 140.82 −2.19 3.95 3.01 0.032 EX. 12 4.5 −10.66 17.884 13.547 203.53 −2.37 3.97 3.01 0.022

TABLE-US-00024 TABLE 24 LS PZ PZ/LS EX.9 22.34 13.78 0.62 EX.10 21.05 12.54 0.60 EX.11 21.45 12.74 0.59 EX.12 20.4 12.24 0.60

Example 10

[0103] FIG. 19 is a sectional view of an optical system 1000 according to this example. The optical system 1000 is an infrared optical system having a focal length of 4.5 mm and an F-number of 0.8. The optical system 1000 includes, in order from the object side to the image side, a first lens L101 having a negative refractive power and made of a silicon material, a second lens L102 having a positive refractive power and made of a silicon material, a diaphragm S10, a third lens L103 having a positive refractive power and made of a silicon material, and an optical element (aspherical lens) ASP101 having an aspherical surface and made of a zinc sulfide material. Light in the infrared range guided by the optical system 1000 passes through a cover glass CG10 and forms an image on an infrared sensor IM10. Table 25 shows numerical data of the optical system 1000. The unit of radius of curvature and spacing is mm.

TABLE-US-00025 TABLE 25 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT — INFINITY PLANE L101 10.8801 0.45 SILICON 7.33552 16.85 L102 52.42293 1.50 SILICON −242.625 0.10 S10 FLAT SURFACE 9.32 L103 14.17181 1.70 SILICON 22.89453 1.53 ASP101 151.7479 1.00 ZINC (ASPHERICAL SULFIDE SURFACE 101) 1.73E+04 2.00 CG10 FLAT SURFACE 1.00 GERMANIUM FLAT SURFACE 4.56 IM10 —

[0104] Table 26 shows aspherical shape data. The aspherical shape is represented by the expression (1).

TABLE-US-00026 TABLE 26 ASPHERICAL SURFACE 101 PARAXIAL RADIUS OF CURVATURE R 151.748 CONICAL COEFFICIENT k 0.00 4TH-ORDER COEFFICIENT A −1.34E−04  6TH-ORDER COEFFICIENT B 4.16E−06 8TH-ORDER COEFFICIENT C 1.68E−07 10TH-ORDER COEFFICIENT D 4.26E−09 12TH-ORDER COEFFICIENT E −5.50E−11  14TH-ORDER COEFFICIENT F 2.81E−13

[0105] FIG. 20 is an MTF diagram of the optical system 1000. This example uses the infrared sensor IM10 having a pixel pitch of 17 μm as an implementation. In this case, the Nyquist frequency is about 30 lp/mm. The MTF value at the Nyquist frequency in this example represented by reference numeral 101 in FIG. 20 is 52%. Since the MTF value is 30% or higher at the Nyquist frequency of the infrared sensor IM10, the optical system 1000 exhibits excellent optical performance.

Example 11

[0106] FIG. 21 is a sectional view of an optical system 1100 according to this example. The optical system 1100 is an infrared optical system having a focal length of 4.5 mm and an F-number of 0.8. The optical system 1100 includes, in order from the object side to the image side, a first lens L111 having a negative refractive power and made of a silicon material, a second lens L112 having a positive refractive power and made of a silicon material, a diaphragm S11, a third lens L113 having a positive refractive power and made of a silicon material, and an optical element (aspherical lens) lens ASP111 having an aspherical surface and made of a zinc selenium material. Light in the infrared range guided by the optical system 1100 passes through a cover glass CG11 and forms an image on an infrared sensor IM11. Table 27 shows numerical data of the optical system 1100. The unit of radius of curvature and spacing is mm.

TABLE-US-00027 TABLE 27 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT — INFINITY PLANE L111 11.24434 0.45 SILICON 7.41501 16.50 L112 42.69496 1.50 SILICON 7444.902 0.10 S11 FLAT SURFACE 9.50 L113 14.16753 1.70 SILICON 22.88135 1.54 ASP111 154.7086 1.00 ZINC (ASPHERICAL SELENIDE SURFACE 111) 7.29E+02 2.00 CG11 FLAT SURFACE 1.00 GERMANIUM FLAT SURFACE 4.70 IM11 —

[0107] Table 28 shows aspherical shape data. The aspherical shape is represented by the expression (1).

TABLE-US-00028 TABLE 28 ASPHERICAL SURFACE 111 PARAXIAL RADIUS OF CURVATURE R 154.709 CONICAL COEFFICIENT k 0.00 4TH-ORDER COEFFICIENT A −1.23E−04 6TH-ORDER COEFFICIENT B  3.58E−06 8TH-ORDER COEFFICIENT C  1.48E−07 10TH-ORDER COEFFICIENT D  3.92E−09 12TH-ORDER COEFFICIENT E −5.29E−11 14TH-ORDER COEFFICIENT F  2.81E−13

[0108] FIG. 22 is an MTF diagram of the optical system 1100. This example uses the infrared sensor IM10 having a pixel pitch of 17 μm as an implementation. In this case, the Nyquist frequency is about 30 lp/mm. The MTF value at the Nyquist frequency in this example represented by reference numeral 111 in FIG. 22 is 57%. Since the MTF value is 30% or higher at the Nyquist frequency of the infrared sensor IM10, the optical system 1100 exhibits excellent optical performance.

Example 12

[0109] FIG. 23 is a sectional view of an optical system 1200 according to this example. The optical system 1200 is an infrared optical system having a focal length of 4.5 mm and an F-number of 0.8. The optical system 1200 includes, in order from the object side to the image side, a first lens L121 having a negative refractive power and made of a silicon material, a second lens L122 having a positive refractive power and made of a silicon material, a diaphragm S12, a third lens L123 having a positive refractive power and made of a silicon material, and an optical element (aspherical lens) ASP121 having an aspherical surface made of a resin material. A resin material amount includes high-density polyethylene (HDPE), which is polyethylene having a density of 0.942 [kg/m.sup.3] or higher. Light in the infrared range guided by the optical system 1200 passes through a cover glass CG12 and forms an image on an infrared sensor IM12. Table 29 shows numerical data of the optical system 1200. The unit of radius of curvature and spacing is mm.

TABLE-US-00029 TABLE 29 RADIUS OF GLASS CURVATURE SPACING MATERIAL OBJECT — INFINITY PLANE L121 10.34174 0.45 SILICON 7.150503 17.55 L122 83.9322 1.50 SILICON −87.8344 0.10 S12 FLAT SURFACE 8.88 L123 14.08102 1.70 SILICON 22.61232 1.66 ASP121 120.1702 1.00 RESIN (ASPHERICAL SURFACE 121) −7.64E+02 2.00 CG12 FLAT SURFACE 1.00 GERMANIUM FLAT SURFACE 4.16 IM12 —

[0110] Table 30 shows aspherical shape data. The aspherical shape is represented by the expression (1).

TABLE-US-00030 TABLE 30 ASPHERICAL SURFACE 121 PARAXIAL RADIUS OF CURVATURE R 120.170 CONICAL COEFFICIENT k 0.00 4TH-ORDER COEFFICIENT A −2.77E−04 6TH-ORDER COEFFICIENT B  1.14E−05 8TH-ORDER COEFFICIENT C −4.22E−07 10TH-ORDER COEFFICIENT D  8.76E−09 12TH-ORDER COEFFICIENT E −8.50E−11 14TH-ORDER COEFFICIENT F  2.81E−13

[0111] FIG. 24 is an MTF diagram of the optical system 1200. This example uses the infrared sensor IM7 having a pixel pitch of 17 μm as an implementation. In this case, the Nyquist frequency is about 30 lp/mm. The MTF value at the Nyquist frequency in this example represented by reference numeral 121 in FIG. 24 is 43%. Since the MTF value is 30% or higher at the Nyquist frequency of the infrared sensor IM12, the optical system 1200 exhibits excellent optical performance.

Example 13

[0112] In this example, an infrared camcorder (video camera) is an illustrative image pickup apparatus using the optical system according to each example. FIG. 25 is a schematic view of principal part of the camcorder according to this example. The camcorder has an imaging optical system 11 including a camera body 13 and an optical system according to any one of Examples 1 to 12. The camera body 13 includes an image sensor (infrared sensor) 12 such as a microbolometer that receives (photoelectrically converts) an object image formed by the imaging optical system 11. The infrared sensor can use, for example, one formed by using vanadium oxide or amorphous silicon. An acquired image can be confirmed on a display device 14. The display device 14 may be incorporated into the camera body 13 or may be separated from but communicable with the camera body 13 via wireless communications. The optical system according to each example is applicable to an image pickup apparatus such as an in-vehicle camera or a surveillance camera.

[0113] Each example can provide an optical system having high optical performance in the infrared range and easy to manufacture, and an image pickup apparatus having the same.

[0114] While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

[0115] This application claims the benefit of Japanese Patent Application No. 2020-108208, filed on Jun. 23, 2020, which is hereby incorporated by reference herein in its entirety.