JPH04230717A - Optical system for infrared image pickup - Google Patents

Abstract

PURPOSE:To increase the latitude of material selection with the IR image pickup optical system which is simultaneously corrected in chromatic aberration and thermal aberration. CONSTITUTION:This optical system consists of lens materials which are respectively different IR transparent materials of high dispersion. The chromatic aberration and thermal aberration of the IR image pickup optical system are simultaneously corrected by the 1st lens 11 consisting of the material having the smallest ratio of the Abbe number of the lens material and the thermal Abbe number expressed by formula: thermal Abbe number =1/{(alphan/alphaT)/(n-1)-alpha}, where n: refractive index, T: temp., alpha: coefft. of thermal expansion, and having a negative refracting power the 2nd lens 12 having the above-mentioned ratio smallest next to the above ratio of the 1st lens 11 and having a positive refracting power the 3rd lens 13 having the largest ratio and a negative refracting power.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an infrared imaging optical system used in an infrared camera.

0002

2. Description of the Related Art Generally, changes in ambient temperature affect the resolution of an infrared imaging optical system. When the temperature changes, the imaging position of the infrared imaging optical system moves due to changes in the refractive index of the lens material, changes in the shape of the lens due to thermal expansion and contraction, changes in the distance between lenses due to thermal expansion and contraction of the lens holder, etc., and the resolution deteriorates ( (hereinafter referred to as thermal aberration). In order to suppress this thermal aberration, it is necessary to correct the movement of the imaging position by some method.

For example, two lenses consisting of lens A and lens B
Correction of chromatic aberration and thermal aberration will be explained using a set of thin close-up lenses as an example. As is well known, the axial chromatic aberration Δf of a thin proximity lens is given by the following equation. Δf=f2 {1/(fA VA )+1/(fB
VB )} (1) However, fA: Focal length of lens A fB: Focal length of lens B f: Combined focal length of lenses A and B VA: Abbe number of the material of lens A VB: Abbe number of the material of lens B number

Therefore, the condition for correcting chromatic aberration is Δf=
From 0, fA = {(VA - VB )/VA }・f

(2) fB = {(VB - VA)/VB }
・f
(3) becomes.

Next, thermal aberration will be explained. The composite focal length is given by the following equation. 1/f=1/fA +1/fB = (nA -1) (1/RA1-1/
RA2)+ (nB-1)(1/
RB1-1/RB2)
(4) However, nA: refractive index of the material of lens A nB: refractive index of the material of lens B RA1: radius of curvature of the first surface of lens A RA2: radius of curvature of the second surface of lens A RB1: radius of curvature of the second surface of lens B Radius of curvature of the first surface RB2: Radius of curvature of the second surface of lens B

The change in focal length due to temperature change is expressed by equation (4)
) with respect to temperature T. ∂f/∂T=(f/fA)2 (∂fA/∂T
)+(f/fB)2 ・
(∂fB /∂T) = (f
/fA )2 {-fA 2 (1/RA1-1/RA
2) (∂nA /∂T)
+fA αA}+ (f/
fB )2 {-fB2(1/RB1-1/RB2)
(∂nB /∂T)+fB
αB } =-f2 [1/
fA {(∂nA /∂T)/(nA −1)−αA
}+1
/fB {(∂nB /∂T)/(nB −1)−αB
}]

(5) However, αA
: Coefficient of linear expansion αB of material of lens A : Coefficient of linear expansion of material of lens B

Here, similar to the Abbe number in chromatic aberration, the thermal Abbe number is expressed as VT = 1/{(∂n/∂T)/(n-1)-α
By defining (6), we obtain an equation similar to equation (1). ∂f/∂T=-f2 {1/(fA VTA)+1
/(fB VTB)} (7) However, VTA
: Thermal Abbe number of the material of lens A VTB : Thermal Abbe number of the material of lens B

Therefore, the conditions for correcting thermal aberration, like chromatic aberration, are fA = {(VTA-VTB)/VTA}·f

(8) fB = {(VTB-VTA)/VTB}
・f
(9) becomes.

From the above, formula (2), formula (3), and formula (8
) and equation (9), the conditions for simultaneously correcting chromatic aberration and thermal aberration VA /VTA=VB /VTB

(10) is found.

FIG. 15 is a diagram illustrating combinations of materials that satisfy formula (10). The horizontal axis is the Abbe number, and the vertical axis is the thermal Abbe number. The condition for simultaneously correcting chromatic aberration and thermal aberration means a combination of materials that lie on a straight line passing through the origin in FIG. In this case, lens A made of a material with a larger Abbe number becomes a lens with positive refractive power,
Lens B becomes a lens with negative refractive power.

FIG. 16 is a diagram showing the Abbe number and thermal Abbe number of typical infrared transmitting materials at wavelengths around 8 to 12 μm. As can be seen from the figure, there is no ideal combination of materials that satisfies equation (10). Therefore, it is difficult to sufficiently correct chromatic aberration and thermal aberration with a lens made of two types of materials, and it becomes necessary to correct this with a lens made of three types of materials.

FIG. 17 shows a conventional infrared imaging optical system of this type composed of lenses made of three types of materials.
NO. No. 4,679,891. In the figure, 1 is a first lens made of zinc selenide (ZnSe), 2 is a second lens made of zinc sulfide (ZnS), and 3 is a first lens made of zinc selenide (ZnSe).
is the third lens made of germanium (Ge), and 4 is the first lens.
This is a lens holder that holds a lens 1, a second lens 2, and a third lens 3.

This infrared imaging optical system consisting of the first lens 1, second lens 2, third lens 3, and lens holder 4 collects infrared rays with a wavelength in the vicinity of 8 to 12 μm emitted from an object to be imaged. light to form an infrared image of the object to be imaged. Thereafter, although not shown here, the signal is converted into an electrical signal by a photoelectric converter placed at the imaging position of the infrared imaging optical system, which is then passed through a signal processing circuit and displayed on a display device. There is.

The first lens 1 made of zinc selenide, which has a larger Abbe number than zinc sulfide, has a positive refractive power, and the second lens 2 has a positive refractive power.
has a negative refractive power, and each refractive power is a combination determined to correct the chromatic aberration of the pair of lenses, and the pair of lenses has a positive refractive power. However, when the temperature changes, the refractive power of this pair of lenses changes. The direction is the direction in which the refractive power increases as the temperature increases.

On the other hand, germanium, which is the material of the third lens 3, has a small thermal Abbe number and is suitable for the first lens and the second lens.
The Abbe number is extremely large compared to the material of lens 2. Therefore, since the chromatic aberration is negligibly small, the chromatic aberration in the entire infrared imaging optical system can be sufficiently corrected only by correcting the chromatic aberration of the pair of lenses. Further, the refractive power of the third lens 3 is set to a negative refractive power that cancels out temperature changes in the refractive powers of the pair of lenses, thereby correcting thermal aberrations in the entire infrared imaging optical system. As mentioned above,
This infrared imaging optical system has chromatic aberration and thermal aberration corrected, and the imaging position hardly moves even when the temperature changes, so that resolution does not deteriorate.

[0016]

[Problems to be Solved by the Invention] Since the conventional infrared imaging optical system is constructed as described above, the material for the third lens 3 should have a large Abbe number so that chromatic aberration can be ignored, and a small thermal Abbe number. Special materials had to be used. However, such materials have a problem in that there are only limited materials such as germanium that have wavelengths around 8 to 12 μm in the infrared wavelength range, and the range of material selection is limited.

[0017] Furthermore, the lenses made of the two materials have positive refractive power,
Since the lens made of the remaining material must have a negative refractive power, there is a problem in that there are design constraints.

The first invention was made in order to solve the above-mentioned problems, and instead of using the above-mentioned special extremely low-dispersion materials, different general relatively high-dispersion materials can be used. The objective is to increase the degree of freedom in design by constructing an infrared imaging optical system from materials such as

[0019] The second invention has been made to solve the above-mentioned problems, and has a configuration in which the first lens and the third lens have positive refractive power, and the second lens has negative refractive power. The purpose of this is to increase the degree of freedom in designing the infrared imaging optical system.

[0020]

[Means for Solving the Problems] An infrared imaging optical system according to a first aspect of the invention is an infrared imaging optical system that corrects chromatic aberration and thermal aberration, and is comprised of lenses made of different infrared transmitting materials each having a relatively high dispersion. a first lens with negative refractive power made of a material with the smallest ratio of the Abbe number to the thermal Abbe number; a second lens with positive refractive power made of a material with the second smallest ratio after the first lens; and a third lens having a negative refractive power made of a material having the largest ratio.

Further, the infrared imaging optical system according to the second invention is an infrared imaging optical system that corrects chromatic aberration and thermal aberration, and is comprised of lenses made of different infrared transmitting materials, and the Abbe number and thermal Abbe number of the materials are different from each other. A first lens with a positive refractive power made of a material with the smallest ratio, a second lens with a negative refractive power made of a material with the second smallest ratio after the first lens, and a positive refractive power made of a material with the largest ratio. It is equipped with a third lens that has power.

[0022]

In the first and second inventions, chromatic aberration and thermal aberration are corrected by a combination of the first to third lenses each made of a different infrared transmitting material.

[0023]

[Embodiment] FIG. 1 is a sectional view showing an embodiment of the first invention. In the figure, 4 is a lens holder similar to the conventional infrared imaging optical system shown in FIG. A first lens made of zinc sulfide (Zns), a second lens 12 made of arsenic selenide (AsSe), a third lens 13 made of cadmium telluride (CdTe), and the first lens 11 and the second lens 12 are made of arsenic selenide (AsSe). 12, the third lens 13, and the lens holder 4. This infrared imaging optical system condenses infrared light with a wavelength of, for example, 8 to 12 μm.

Table 1 shows the refractive index, Abbe number, temperature change in refractive index, linear expansion coefficient, and thermal Abbe number of each material for each wavelength light.

[Table 1]

[0025] The ratio of Abbe number and thermal Abbe number is
Arsenic selenide and cadmium telluride are the smallest. Both materials have small Abbe numbers and high dispersion so that chromatic aberration cannot be ignored. For example, when an infrared imaging optical system is constructed using only one of these materials, chromatic aberration is large and good imaging performance cannot be obtained. On the other hand, when an infrared imaging optical system is constructed using only a material with a large Abbe number and low dispersion, such as germanium, which is used in conventional infrared imaging optical systems, chromatic aberration is small, so good imaging close to the diffraction limit is achieved. performance can be obtained.

FIG. 2 is a diagram showing the characteristics of each lens material. The horizontal axis is the reciprocal of the Abbe number, and the vertical axis is the reciprocal of the thermal Abbe number. The first lens 11 with the smallest ratio of Abbe number and thermal Abbe number has a negative refractive power, and the second lens 1 with the second smallest ratio has a negative refractive power.
2 has a positive refractive power, and the largest third lens 13 has a negative refractive power, and the entire infrared imaging optical system has a positive refractive power. The composite Abbe number V13 and composite thermal Abbe number VT13 of the first lens 11 and the third lens 13 are defined as follows. V13=f13/Δf13

(11) VT13 = −f13/(∂
f13/∂T)
(12) However, f13: first
Combined focal length Δf13 of lens 11 and third lens 13: Axial chromatic aberration of first lens 11 and third lens 13

For simplicity, ignoring the lens thickness and spacing, equations (1) and (11), equations (7) and equations (12)
From, 1/V13=(f13/f1)(1/V1)+(f
13/f3 ) (1/V3 ) (13) 1/VT13
=(f13/f1)(1/VT1)+(f13/f3
) (1/VT3)

(1
4) However, f1: Focal length of the first lens 11 f3:
Focal length V1, VT1 of the third lens 13: Abbe number and thermal Abbe number of the material of the first lens 11 V3, VT3: Abbe number and thermal Abbe number of the material of the third lens 13. From the composition of focal lengths, f13/f1 + f13/f3 = 1

(15).

[0028] Also, f13/f1 >0, f13/f3 >0

(16), so the point (1/V13, 1/VT13
) changes on the line segment shown by the solid line in FIG. 2 depending on the combination of f1 and f3. Point (1/V13,
1/VT13) is the above line segment, the origin and the point (1/V2
, 1/VT2) so that f1 becomes the intersection point P of the straight lines connecting
If you combine it with f3. V13/VT13 =V2/VT2

(17), which satisfies the condition for simultaneously correcting chromatic aberration and thermal aberration as shown in equation (10).

As described above, the second lens 12 having a positive refractive power and the first lens 11 and the third lens 13 having a negative refractive power allow infrared rays of different wavelengths emitted from an object to be imaged to be almost completely focused. Since the images are formed at the same imaging position, deterioration due to chromatic aberration is suppressed, and since the imaging position hardly moves even if the temperature changes, deterioration of resolution due to temperature changes is suppressed.

The above description has been made while ignoring the lens thickness and spacing for the sake of simplicity. However, in an actual optical system, since both must be taken into account, the above equation is not necessarily satisfied, but the above explanation is generally followed. Table 2 shows numerical examples of the infrared imaging optical system according to the first invention shown in FIG. 1, standardized with a focal length of 20. And (a) in Figure 3
(c) shows various aberrations, and (d) shows the amount of movement of the imaging position with respect to temperature.

[0031]

[Table 2]

FIG. 4 shows another embodiment of the first invention, in which a first lens made of 11 zinc sulfide (Zns),
12 is a second lens made of zinc selenide (ZnSe);
13 is a third lens made of cadmium telluride (CdTe). Refractive index of zinc selenide (ZnSe) for each wavelength of light, Abbe number, temperature change in refractive index, coefficient of linear expansion,
Thermal Abbe numbers are shown in Table 3.

[0033]

[Table 3]

The second lens 12 having the second smallest ratio of the Abbe number to the thermal Abbe number has a positive refractive power, and the smallest first lens 11 and the largest third lens 13 have a negative refractive power. Similarly, chromatic aberration and thermal aberration are corrected by the infrared imaging optical system. In this way, the same effect can be obtained even if the material and arrangement of the lenses are different. Table 4 shows
A numerical example of the infrared imaging optical system shown in FIG. 4 is shown standardized with a focal length of 20. And (a) in Figure 5
(c) shows various aberrations, and (d) shows the amount of movement of the imaging position with respect to temperature.

[0035]

[Table 4]

Furthermore, FIG. 6 shows still another embodiment of the first invention, in which 11 is a first lens made of zinc sulfide (ZnS), 12 is a second lens made of zinc selenide (ZnSe), and 13a and 13b is cadmium telluride (C
3a and 3b lenses made of dTe), which are made of a material with the largest ratio of Abbe number to thermal Abbe number.
The combination of these two pieces provides negative refractive power. As in the above embodiments, chromatic aberration and thermal aberration are corrected in the entire infrared imaging optical system. In this way, the same effect can be obtained even if any lens is divided into two or more lenses. Table 5 shows numerical examples of the infrared imaging optical system shown in FIG. 6, standardized with a focal length of 20. Then, various aberrations are shown in (a) to (b) of Fig. 7, and (d)
shows the amount of movement of the imaging position with respect to temperature.

[0037]

[Table 5]

[0038] In each of the embodiments of the first invention, 8 to 1
Although the infrared imaging optical system that focuses infrared rays on a wavelength around 2 μm has been described, it goes without saying that similar effects can be obtained with imaging optical systems used for other wavelength bands.

Further, in the first invention, an infrared imaging optical system of the following aspect can be implemented. - An infrared imaging optical system in which at least one of the first lens, second lens, or third lens is divided into two or more lenses. - Infrared imaging optics in which the ratio of the change in refractive power due to temperature and the change in refractive power due to wavelength of the lens group consisting of the second lens and the third lens is approximately equal to the ratio of the first lens. system. - An infrared imaging optical system in which the ratio of the lens group consisting of the first lens and the second lens is approximately equal to the ratio of the third lens. - An infrared imaging optical system in which the ratio of the lens group consisting of the first lens and the third lens is approximately equal to the ratio of the second lens.

Next, FIG. 8 is a sectional view showing an embodiment of the second invention. In the figure, 21 is arsenic selenide (AsS).
e), the first lens 22 is made of gallium arsenide (GaA
s), 23 is germanium (Ge)
The third lens consists of the first lens 21, the second lens 21, and
This infrared imaging optical system consisting of the lens 22, the third lens 23, and the lens holder 4 condenses infrared light with a wavelength of 8 to 12 μm, for example. The refractive index for each wavelength of each material,
The Abbe number, temperature change in refractive index, linear expansion coefficient, and thermal Abbe number are shown in Table 6. The ratio of the Abbe number to the thermal Abbe number is the smallest for arsenic selenide, gallium arsenide, and germanium, in that order.

[0041]

[Table 6]

FIG. 9 is a diagram showing the characteristics of each material. The horizontal axis is the reciprocal of the Abbe number, and the vertical axis is the reciprocal of the thermal Abbe number. The first lens 21 with the smallest ratio of Abbe number to thermal Abbe number has positive refractive power, the second lens 22 with the next smallest ratio has negative refractive power, and the third lens 23 with the largest ratio has positive refractive power. The entire system has positive refractive power.

Here, for the sake of simplicity, the combined Abbe number V13 and the combined thermal Abbe number VT13 of the first lens 21 and the third lens 23 will be considered, ignoring the lens thickness and lens spacing. The point (1/V13, 1/VT13) is the first
Similar to the invention, the combination of f1 and f3 results in Fig. 9
It changes on the line segment shown as a solid line. Point (1/V13,1
/VT13) is the above line segment, the origin and the point (1/V2,
If f1 and f3 are combined so as to form an intersection point P of straight lines connecting and thermal aberrations are corrected at the same time. As described above, in this infrared imaging optical system, infrared rays of different wavelengths emitted from the object to be imaged are focused on almost the same imaging position, so deterioration due to chromatic aberration is suppressed, and even if the temperature changes, the above Since the imaging position hardly moves, deterioration in resolution due to temperature changes is suppressed.

Although the explanation above has been made while ignoring the lens thickness and spacing for simplicity, the above explanation generally follows even when these are taken into consideration. Table 7 shows a numerical example of the infrared imaging optical system according to the second invention shown in FIG. 8, standardized with the focal length set to 100. Various aberrations are shown in (a) to (b) of FIG.
(d) shows the amount of movement of the imaging position with respect to temperature.

[0045]

[Table 7]

FIG. 11 shows another embodiment of the second invention, in which 21 is a first lens made of zinc selenide (ZnSe), 22 is a second lens made of gallium arsenide (GaAs), and 23 is a first lens made of zinc selenide (ZnSe). is a third lens made of cadmium telluride (CdTe). Table 8 shows the refractive index, Abbe number, temperature change in refractive index, linear expansion coefficient, and thermal Abbe number of zinc selenide and cadmium telluride for each wavelength light.

[0047]

[Table 8]

The first lens 21 has the smallest ratio of Abbe number and thermal Abbe number, and the second lens 2 has the second smallest positive refractive power.
2 has negative refractive power, and the largest third lens has positive refractive power, and chromatic aberration and thermal aberration are corrected in the entire infrared imaging optical system as in the above embodiment. In this way, the same effect can be obtained even if the material and arrangement of the lenses are different. Table 9 shows numerical examples of the infrared imaging optical system shown in FIG. 11, standardized to a focal length of 20. Figure 12(a)
(c) shows various aberrations, and (d) shows the amount of movement of the imaging position with respect to temperature.

[0049]

[Table 9]

Furthermore, FIG. 13 shows still another embodiment of the second invention, in which 21 is a first lens made of zinc selenide (ZnSe), and 22a and 22b are gallium arsenide (ZnSe).
a second a lens and a second b lens made of GaAs), 2
3 is a third lens made of cadmium telluride (CdTe). The second lens 22a and the second lens 22b are split into two lenses made of a material with the second smallest ratio of Abbe number to thermal Abbe number, and the combination of these two lenses has a negative refractive power. . As in the above embodiments, chromatic aberration and thermal aberration are corrected in the entire infrared imaging optical system. In this way, the same effect can be obtained even if any lens is divided into two or more lenses. Table 10 shows numerical examples of the infrared imaging optical system shown in FIG. 13, standardized with a focal length of 20. Various aberrations are shown in (a) to (c) of Fig. 14, and (d
) shows the amount of movement of the imaging position relative to temperature.

[0051]

[Table 10]

[0052] In each of the embodiments of the second invention, 8 to 1
Although the infrared imaging optical system that focuses infrared rays on a wavelength around 2 μm has been described, it goes without saying that similar effects can be obtained with imaging optical systems used for other wavelength bands.

Furthermore, in the second invention, an infrared imaging optical system of the following aspect can be implemented. - An infrared imaging optical system in which at least one of the first lens, second lens, or third lens is divided into two or more lenses. - Infrared imaging optics in which the ratio of the change in refractive power due to temperature and the change in refractive power due to wavelength of the lens group consisting of the second lens and the third lens is approximately equal to the ratio of the first lens. system. - An infrared imaging optical system in which the ratio of the lens group consisting of the first lens and the second lens is approximately equal to the ratio of the third lens. - An infrared imaging optical system in which the ratio of the lens group consisting of the first lens and the third lens is approximately equal to the ratio of the second lens.

[0054]

As described above, according to the infrared imaging optical system of the first invention, each lens is made of a different highly dispersive infrared transmitting material, and the ratio of the Abbe number to the thermal Abbe number of the materials is the smallest. a first lens having a negative refractive power made of a material; a second lens having a positive refractive power made of a material having the second smallest ratio after the first lens; and a third lens having a negative refractive power made of a material having the largest ratio. Since the lens simultaneously corrects chromatic aberration and thermal aberration, it has the effect of increasing the degree of freedom in selecting materials for the infrared imaging optical system.

Further, according to the infrared imaging optical system of the second invention, each lens is made of a different infrared transmitting material,
A first lens having a positive refractive power made of a material having the smallest ratio of the Abbe number to the thermal Abbe number of the material, and a second lens having a negative refractive power made of a material having the second smallest ratio after the first lens; Since chromatic aberration and thermal aberration are corrected simultaneously by the third lens having positive refractive power made of the material with the highest ratio, the first and third lenses are called positive refractive power, and the second lens is called negative refractive power. This configuration has the effect of increasing the degree of freedom in designing the infrared imaging optical system.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an infrared imaging optical system showing a first embodiment of the first invention.

FIG. 2 is a characteristic diagram illustrating Example 1 of the first invention.

[Figure 3] Various aberrations (a) to (c) of the infrared imaging optical system in Figure 1.
) and the amount of movement (d) of the imaging position with respect to temperature.

FIG. 4 is a sectional view of an infrared imaging optical system showing a second embodiment of the first invention.

[Figure 5] Various aberrations (a) to (c) of the infrared imaging optical system in Figure 4.
) and the amount of movement (d) of the imaging position with respect to temperature.

FIG. 6 is a sectional view of an infrared imaging optical system showing Example 3 of the first invention.

[Figure 7] Various aberrations (a) to (c) of the infrared imaging optical system in Figure 6.
) and the amount of movement (d) of the imaging position with respect to temperature.

FIG. 8 is a sectional view of an infrared imaging optical system showing Example 4 of the second invention.

FIG. 9 is a characteristic diagram illustrating a fourth embodiment of the second invention.

FIG. 10: Various aberrations (a) to () of the infrared imaging optical system in FIG.
It is a characteristic diagram showing the amount of movement (d) of the imaging position with respect to c) and temperature.

FIG. 11 is a cross-sectional view of an infrared imaging optical system showing Example 5 of the second invention.

[Figure 12] Various aberrations (a) of the infrared imaging optical system in Figure 11 ~
It is a characteristic diagram showing the amount of movement (d) of the imaging position with respect to (c) and temperature.

FIG. 13 is a sectional view of an infrared imaging optical system showing Example 6 of the second invention.

[Figure 14] Various aberrations (a) of the infrared imaging optical system in Figure 13 ~
It is a characteristic diagram which shows the amount of movement (d) of the imaging position with respect to (c) and temperature.

FIG. 15 is an explanatory diagram illustrating a conventional infrared imaging optical system.

FIG. 16 is a characteristic diagram of a typical infrared transmitting material.

FIG. 17 is a cross-sectional view of a conventional infrared imaging optical system.

[Explanation of symbols]

11, 21 1st lens 12, 22 Second lens 13, 23 3rd lens

Claims (2)

[Claims] Claim 1: An infrared imaging optical system for correcting chromatic aberration and thermal aberration, comprising lenses made of different infrared transmitting materials with relatively high dispersion, the materials having the smallest ratio of the Abbe number to the thermal Abbe number. a first lens with a negative refractive power of , a second lens with a positive refractive power made of a material with the second smallest ratio after the first lens, and a third lens with a negative refractive power of a material with the largest ratio. An infrared imaging optical system characterized by comprising a lens. 2. An infrared imaging optical system for correcting chromatic aberrations and thermal aberrations, each consisting of lenses made of different infrared transmitting materials, with the positive refractive power of the material having the smallest ratio between the Abbe number and the thermal Abbe number of said materials. a second lens having a negative refractive power made of a material having the second smallest ratio after the first lens; and a third lens having a positive refractive power made of a material having the largest ratio. An infrared imaging optical system featuring: