GB2434878A

GB2434878A - Wide-angle optical system for infrared evaluation unit

Info

Publication number: GB2434878A
Application number: GB0701395A
Authority: GB
Inventors: Joerg Baumgart
Original assignee: Diehl BGT Defence GmbH and Co KG
Current assignee: Diehl BGT Defence GmbH and Co KG
Priority date: 2006-02-06
Filing date: 2007-01-25
Publication date: 2007-08-08
Anticipated expiration: 2027-01-25
Also published as: FR2897165B1; FR2897165A1; GB0701395D0; DE102006005171B4; DE102006005171A1; GB2434878B

Abstract

A wide-angle optical system (10, 60) for the infrared spectral region with an evaluation unit, particularly a detector (16), with a stop (22) disposed in front of the evaluation unit, has, in direction from an object side to an image side, a lens system with a primary lens system (12) and a secondary lens system (14). An intermediate image plane is located between the primary lens system (12) and the secondary lens system (14). An entrance pupil (26) is located on the object side of the primary lens system (12), which is the real image of the stop (22). An exit pupil (24), located on the image side of the secondary lens system (14), which coincides with the stop (22).

Description

I

Wide-angle Optical System The invention relates to a wide-angle optical system for the infrared spectral region with an evaluation unit, particularly a detector, and with a stop disposed in front of said evaluation unit.

Optical systems for the infrared spectral region are, for example, used in surveillance equipment -such as night viewers, aiming devices and the detection units of missiles.

With equipment of this type, it is often necessary to be able to capture a large field of view.

This is possible thanks to an appropriate design of the optical system.

US 6,292,293 BI discloses a wide-angle imaging system for the infrared spectral region which is capable of detecting very hot or burning objects. For this purpose, the imaging system has an entrance pupil located between its lenses, a physical aperture stop being located at the position of said entrance pupil. By way of this aperture stop, the infrared radiation incident on the imaging system can be controlled in such a way that saturation of the infrared detector of the imaging system is prevented and detection of individual burning objects is facilitated. It is a disadvantage with such an imaging system that it is hard, if not impossible, to detect objects emitting infrared radiation of low intensity. In fact, in such a case, the internal aperture stop itself constitutes a source of radiation which outshines the radiation of the weakly emitting objects in the field of view and thus makes their detection impossible.

US 5,479,292 discloses a wide-angle optical system in the form of a single lens for a temperature-measuring system. Located in front of the wide-angle optical system is an aperture stop which at the same time forms the entrance pupil of this wide-angie optical system. Systems of this type, having the aperture stop located in front of the wide-angle optical system, involve the disadvantage that an aperture stop of large diameter is necessary in order to be able to cover a large field of view, and thus lenses are needed which are also large in diameter and consequently expensive.

It is therefore the objective of the present invention to propose a wide-angle optical system for the infrared spectral region, which is compact and small in structure and yet nevertheless provides a good quality of image, and which has an evaluation unit, particularly a detector, and a stop disposed in front of said evaluation unit.

This objective is achieved by a wide-angle optical system for the infrared spectral region with an evaluation unit, particularly a detector, and with a stop disposed in front of the evaluation unit, the wide-angle optical system according to the invention comprising, in the direction from an object side to an image side, a lens system consisting of a primary lens system and a secondary lens system and the lens combination being configured in such a way that a. an intermediate image plane is located between the primary lens system and the secondary lens system, b. it has an entrance pupil located on the object side of the primary lens system which is the real image of the stop, and c. it has an exit pupil, located on the image side of the secondary lens system, which coincides with the stop.

The invention proceeds in a first step from the fact that there are applications for wide-angle optical systems for the infrared spectral region which require operation under extreme environmental conditions. Such extreme environmental conditions are present, for instance, when the wide-angle optical system is used in aviation. When the wide-angle optical system is used in an aeroplane or a missile for example, then it must be protected against external influences, such as fluctuations in temperature and mechanical damage caused by erosion or stones, so as to ensure a good quality of image. For this reason, wide-angle optical systems are generally located inside the aeroplane or missile and "look out" through a window located in the outer shell of the aeroplane or missile.

The invention further proceeds from the knowledge that windows having large geometric dimensions may adversely affect the aerodynamic structure of an aeroplane or missile. So as to be able to minimize this adverse effect on the aerodynamic structure, it is to be recommended that such windows are made as small as possible in terms of their geometric dimensions.

In a further step, the invention proceeds from the observation that, with a given intensity of radiation of a wide-angle optical system, the window, through which the wide-angle optical system is meant to "look9, can be produced with a minimum size in respect of its diameter if the position of the window coincides with the entrance pupil of the wide-angle optical system. In this way, such a window, having a diameter corresponding to the entrance pupil, does not have a vignetting effect on infrared radiation passing through said window.

In another step, the invention proceeds from the knowledge that, for the detection of infrared radiation, evaluation units, particularly detectors, are used with a stop being disposed in front of them. The detectors are then mostly placed in a heat-insulating housing, i.e. in a so-called Dewar vessel. The stop located in front of the detector or evaluation unit is mostly cooled -like the detector itself -in order, as effectively as possible, to prevent thermally generated stray-light fractions which can lead to distortion of images on the detector or evaluation unit.

In addition, the invention proceeds from the observation that, from the radiometric point of view, it is desirable if such a cooled stop -as mentioned above and hereinafter referred to as a cold stop -coincides with the exit pupil of a wide-angle optical system. In this way, it is possible to achieve a geometric cold-stop efficiency of one, which ensures defined radiometric conditions and thus a clearly improved stray-light behaviour and higher-quality images on the evaluation unit or detector.

In a final step, the invention proceeds from the knowledge that a wide-angle optical system having an entrance pupil located on the object side and an exit pupil located on the image side which coincides with a stop disposed in front of the evaluation unit, particularly the detector, can only be realized by way of a wide-angle optical system comprising a primary lens system and a secondary lens system with an intermediate image plane located between them. After all, only in this way is it possible to realize the entrance pupil as a real image of the stop in front of the wide-angle optical system. When viewed from the direction of the evaluation unit, the stop is then imaged by way of the secondary lens system as a virtual image in the intermediate image plane and imaged by way of the primary lens system as a real image on the object side.

According to the invention, a wide-angle optical system for the infrared spectral region, having an evaluation unit, particularly a detector, and a stop disposed in front of said evaluation unit, is therefore produced, which can be placed behind a window of small diameter and yet is capable of capturing a large field of view with a high quality of image.

This results in a reduction in the costs associated with a large diameter of window, and also reduces the concomitant obstructions caused by aerodynamic structures, such as a missile for example, inside which a wide-angle optical system of this type is to be located.

The primary lens system of the wide-angle optical system usefully has an f-number of less than one. The f-number determines the light-transmitting capacity or speed of a lens. In this context, we speak of a high-speed lens, since with a lens of great light-transmitting capacity, relatively short exposure times can be selected for the production of high-quality images by means of a detector. Particularly when using wide-angle optical systems for a detector or evaluation unit in the infrared spectral region in missiles and for the purpose of detecting rapidly moving targets, wide-angle optical systems or primary lens systems with a low f-number are needed to facilitate the speedy acquisition of said targets by means of the detector. Since short exposure times are therefore required for a qualitatively high-value recording of a field of view by means of the detector, the wide-angle optical system with the "high-speed" primary lens system together with the detector or evaluation unit can, for example, be lined up particularly quickly with another field of view.

It is advantageous if the primary lens system, in direction from object side to image side, comprises a doublet consisting of a positive lens and a negative lens. In this context, a negative lens is understood to be a diverging lens and a positive lens is understood to be a converging lens. Owing to the fct that the primary lens system comprises a doublet consisting of two lenses, with a suitable embodiment as regards the geometry and material of the negative and positive lenses a mutual compensation of their image defects (in particular of chromatic aberration) can be achieved without any additional lenses being necessary for the purpose. This results in a saving in costs and also provides for a compact structure of the primary lens system due to a reduced space requirement.

It is advantageous if the negative lens and the positive lens of the primary lens system are meniscus lenses. With meniscus lenses, one of the two outer faces of a lens is convex whilst the other is concave. A meniscus lens is thus sickle shaped. Thanks to a suitable geometric configuration of the positive meniscus lens and of the following negative meniscus lens, it is possible to place these a small distance apart. If the opposed outer faces of the negative meniscus lens and of the positive meniscus lens have the same radius of curvature, it is even possible to place the two lenses so that they rest directly against one another. As a result, the primary lens system, and thus also the wide-angle optical system which includes this, can be kept particularly compact and can even be used where there is only a small amount of space available for said system.

In practice, at least one outer face of the positive lens of the primary lens system has an aspherical shape and the other outer faces of the primary lens system have a spherical shape. It is possible to correct the aperture defect or so-called spherical aberration of the lenses to some extent just by combining a negative lens with a positive lens in the primary lens system, since these in each case produce aberrations having opposite signs which consequently, at least in part, compensate one another. As a result of the negative lens of the primary lens system having outer faces with an aspherical shape, it is possible to compensate the aperture defect produced by the spherically shaped outer faces of the positive lens. A good image quality can thus be obtained with the primary lens system of the wide-angle optical system. In addition, costs can be reduced by using at least one lens having outer faces which are spherical in shape, since outer faces of this type can be produced more cost-effectively than outer faces having an aspherical shape.

In practice, the positive lens of the primary lens system is made of germanium or silicon and the negative lens of the primary lens system is made of calcium fluoride or magnesium fluoride. By combining a lens made of germanium, that is to say of a highly refractive material having low dispersion, with a lens made of calcium fluoride, that is to say of a material having low refractive power and high relative dispersion, it is possible to compensate effectively for Longitudinal colour aberration of the primary lens system.

Longitudinal colour aberration, also known as chromatic aberration, occurs because the effective focal length of an individual lens differs for radiation of different wavelengths (=colour). A poor correction of same leads to colour fringing in images, which makes it difficult or even impossible to detect or identify objects. By combining a lens made of germanium with a lens made of calcium fluoride, it is possible to correct this longitudinal chromatic aberration because it is possible with these materials to have different spectral portions of infrared radiation coincide. Compensation of the longitudinal chromatic aberration can also be achieved if the negative lens is made of silicon or another highly refractive material with low dispersion which is transparent in respect of the infrared spectral region. Likewise, the longitudinal chromatic aberration can be corrected if the positive lens is made of magnesium fluoride or another material with low refractive power and with high relative dispersion which is transparent in respect of the infrared spectral region.

In an alternative advantageous embodiment of the invention, the primary lens system comprises a positive meniscus lens made of germanium, its first outer face having an aspherical shape and also a diffractive surface. Owing to the fact that the primary lens system here only comprises a single lens, the wide-angle optical system can be produced as a whole in such a way as to be very compact and thus can be used for applications where there is only little available space. A positive meniscus lens made of germanium has the advantage that its alteration of the imaging properties brought about as a result of material dispersion is so slight that such a primary lens system can be used both in a spectral region of 3-5 tm (mid IR) and in a spectral region of 8-12 jm (long-wave ER). The design of the positive meniscus lens with an outer face having an aspherical shape has the advantage that an image defect, based on spherical aberration and leading to degradation of the wide-angle optical system, is not introduced into the wide-angle optical system via the primary lens system. With a primary lens system of this type, a longitudinal chromatic aberration can thus be avoided in that the outer face on the object side of the positive meniscus lens has a diffractive surface. The material dispersion produced by the material from which the positive meniscus lens of the primary lens system is made, can be compensated by suitable shaping of the ditfractive surface. it is possible to produce the diffractive surface in a single operation together with production of the aspherically shaped outer face on the object side.

The secondary lens system advantageously comprises, in direction from object side to image side, a negative lens, a first positive lens and a second positive lens. By a suitable configuration of the secondary lens system comprising a total of three lenses, a mutual compensation of the image defects of the three lenses can be achieved without this involving the need for additional lenses, which would lead to a greater space requirement and increased costs for the wide-angle optical system.

The negative lens, the first positive lens and the second positive lens of the secondary lens system are preferably formed as meniscus lenses. This makes it possible to minimize the distances between the lenses and thus produce an arrangement which is as compact as possible. It is particularly inventive if the negative lens is made so as to be convex on the image side and the first positive lens is made so as to be concave on the object side.

Appmpriate radii of curvature of the opposed outer faces of the negative lens and of the first positive lens make it possible for the lenses to be placed relatively closely one behind the other.

It is advantageous if the negative lens of the secondary lens system is made of calcium fluoride, the first positive lens of the secondary lens system is made of silicon and the second positive lens of the secondary lens system is made of germanium. Thanks to the negative lens made of calcium fluoride and the second positive lens made of germanium, it is possible to correct the longitudinal chromatic aberration of the secondary lens system and thus to optimize the image quality of the wide-angle optical system as a whole. Costs can be reduced owing to the fact that the first positive lens of the secondary lens system is made of silicon, since silicon lenses are substantially less expensive than, for example, germanium lenses.

It is advantageous if one of the outer faces of the second positive lens of the secondary lens system has an aspherical shape. The aperture defect caused by spherically formed outer faces of the negative lens and of the first positive lens can be compensated by the aspherical form of the one outer face of the second positive lens.

With a suitable configuration of the lenses of the primary lens system and of the secondary lens system as specified above, a wide-angle optical system can be achieved with a diffraction-limited image quality right through to an aperture number of one with a field of view of approximately 50 .

Working examples of the invention are described in greater detail below with reference to the attached drawings, which are as follows: Fig. 1 shows a wide-angle optical system with a primary lens system comprising two lenses and with a secondary lens system comprising three lenses and Fig. 2 shows a wide-angle optical system with a primary lens system comprising a single lens and with a secondary lens system which corresponds to the one shown in Fig. 1.

Functionally identical parts are provided with the same reference numbers in the drawings.

Table 1 shows the design data of the wide-angle optical system according to Fig. 2.

Fig. 1 represents a wide-angle optical system 10, comprising a primary lens system 12 and a secondary lens system 14. On the image side of the secondary lens system 14, a cadmium-telluride-based detector 16 is located. The detector is placed in a Dewar vessel which is not shown. The Dewar vessel has a window 18 which is made of silicon and thus is transparent in respect of the infrared spectral region. Disposed inside the Dewar vessel in front of the detector 16 is a cold filter 20. The cold filter 20 is also made of silicon and thus meets the requirements as regards transparency in the infrared spectral region. The cold filter 20 made of silicon is an optical filter used for the purpose of blocking off background radiation outside of the desired spectral range and thus for reducing noise.

Located between the window 18 and the cold filter 20, there is a stop which serves to mechanically limit the bundle of rays of the optical image and to block off thermal radiation. The window 18, the cold filter 20 and the position of the stop 22 have an optical effect and therefore constitute part of the design of the wide-angle optical system 10.

The Dewar vessel with its window 18 and the elements disposed inside it, such as stop 22, cold filter 20 and detector 16, is disposed relative to the primary lens system 12 and secondary lens system 14 in such a way that the position of the stop 22 coincides with the position of the exit pupil 24 of the wide-angle optical system. The stop 22 and exit pupil 24 are represented in broken lines in Fig. 1. By way of the secondary lens system 14, the stop 22 is imaged as a virtual image in the intermediate image plane between the secondary lens system 14 and the primary lens system 12. For its part, the primary lens system 12 now reproduces the virtual image of the stop 22 as area! image of the stop 22. This real image of the stop 22 constitutes the entrance pupil 26 of the wide-angle optical system 10.

The primary lens system 12 comprises two lenses 28 and 30. Both the lens 28 on the object side and the lens 30 on the image side are meniscus lenses. The lens 28 made of germanium is convexo-concave and has a convergent, thus positive, effect. The lens 30 made of calcium fluoride is also convexo-concave and has a divergent, thus negative, effect The lens 28 has a first outer face 32 with an aspherical shape and a second outer face 34 with a spherical shape. The lens 30, on the other hand, has two outer faces 36,40 with a spherical shape. The radii of curvature of the opposed outer faces 34 and 36 of the lenses 28 and 30 are the same and this makes it possible to arrange the two lenses 28 and so that they rest directly one against the other. The primary lens system represented in Fig. 1 has an f-number of less than one. The longitudinal chromatic aberration of the primary lens system 12 is compensated by the doublet consisting of lens 28 and lens 30.

The secondary lens system 14 results in an improvement in the image quality of the wide-angle optical system 10. The secondary lens system 14 comprises three lenses 42,44 and 46. The lenses 42,44 and 46 are in the form of meniscus lenses, the distance between them being minimized in order to achieve as compact an arrangement as possible. The lens 42 is a concavo-convex negative lens made of calcium fluoride. This also serves in the secondary lens system 14 to compensate longitudinal chromatic aberration. The lens 44 following the lens 42 is a concavo-convex convergent lens and is made of silicon. The lens 46 is a convexo-concave positive lens made of germanium. The lenses 42 and 44 have outer faces 48 and 50 and 52 and 54 respectively, each having a spherical shape. The first outer face 56 of the lens 46, on the other hand, has an aspherical shape in order to correct the aperture defect of the secondary lens system 14. However, the second outer face 58 of the lens 46 has a spherical shape.

The wide-angle optical system 60 shown in Fig. 2 represents a variant of the wide-angle optical system 10 shown in Fig. 1. The primary lens system 61 here comprises only a single lens 62. The lens 62 is made of germanium. Its first outer face 64 has an aspherical shape, its second outer face 66 a spherical shape. The lens 62 is a convexo-concave lens and has a positive effect. Its outer face 64 on the object side is a diffractive surfüce 68, which serves to compensate the longitudinal chromatic aberration of the primary lens system 61. The secondary lens system 14 of the wide-angle optical system 60 is constructed so as to be functionally identical to the secondary lens system 14 of the wide-angle optical system 10 represented in Fig. 1.

The precise design data of the wide-angle optical system 60 can be seen in detail in Table 1. The data relating to the lenses' outer faces having an aspherical shape are then defined in accordance with the following formula for aspherical surfaces: z = cv + adr4 + ae + aft8 + agr' 1 + [1 -cv(cc + in which r designates the radius, cv the curvature and cc the conic constant. With ad, ae, af, ag, it involves the aspherical coefficients. Aspherical coefficients which are not specified are zero in the present example.

In the same way as with the formula for the aspherical surface, with the diffractive surface the so-called phase 0 (r) is expressed by an equation in the following fonn: 0(r)2n(dfo+dfi r2+df?'r4+ p..)

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Table 1: Design data of the wide-angle optical system according to Fig. 2 No. of the Radius Thickness Aperture Material Comments optical (mm) (mm) radius (mm) plane or Distance ___ (mm) ____ _______ ____ ____ ____ ____ Air ________ __________ __________ Air Entrance pupil 26 1 __________ 29 11.188509 Air Distance to the lens 2 31.835 9 26 Germanium Lens 62 3 57.078 14.608484 24 Air Distance to the next 4 _________ 9.162888 9 Air lens -10.181 15 9 Calcium fluoride Lens 42 6 -51.724 4 23 Air Distance to the next __________ ___________ __________ __________ ________________ lens 7 -43.234 9 26 Silicon Lens 44 8 -37.417 0.1 29 Air Distance to the next ____ ____ ____ ______ lens 9 64.812227 9 32 Germanium Lens 46 179.71 17.789538 31 Air Distance to the window 11 _________ _________ 12.99873 Air of the Dewar vessel 12 _________ 3 18 Silicon Window 18 13 2 18 Air Distance to the stop 14 __________ __________ 11.066843 Air __________________ Aperture 30.00033 11.900616 Air Stop 22, stop __________ __________ __________ _______________ exit pupil 24 16 -10.442 9.0823 13 Air Distance to the cold _________ _________ _________ _____________ filter 17 __________ 1 12 Silicon Cold filter 20 18 __________ 10 12 Air Distance to the detector 19 _________ 0.4 9.091998 Cadmium telluride Detector 16 __________ 0.000306 9.082124 Air ___________________ Image 9.082 124 plane __________ __________ __________ _______________ __________________ Aspherical data (conic and po ynomial) _______ _______ _________ _______ _______ No. of the cc ad ae af ag Comments optical plane __________ ___________ ___________ _____________ ___________ ___________ 2 -0.108959 -2.4034E-06 -8.4633E-l0 2.359E-13 -l.699E-15 Lens 62 9 __________ -5.6319E-07 -8.4106E-12 -2.8028E-14 8.4263E-18 Lens 46 Data of the diffractive surface of lens 62 __________ ____________ __________ __________ No. of the Comments optical plane __________ ___________ ___________ _____________ ___________ ___________ 2 __________ __________ __________ DFI -0.000298 __________ Lens 62 List of Reference Numerals Wide-angle optical system 12 Primary lens system 14 Secondary lens system 16 Detector 18 Window Cold filter 22 Stop 24 Exit pupil 26 Entrance pupil 28 Lens Lens 32 Outer face 34 Outer face 36 Outer face Outer face 42 Lens 44 Lens 46 Lens 48 Outer face Outer face 52 Outer face 54 Outer face 56 Outerface 58 Outer face Wide-angle optical system 61 Primary lens system 62 Lens 64 Outer face 66 Outer face 68 Diffractive surface

Claims

Patent Claims A wide-angle optical system (10, 60) for the infrared

spectral region with an evaluation unit, particularly a detector (16), and with a stop (22) disposed in front of the evaluation unit, the wide-angle optical system (10, 60), in direction from an object side to an image side, comprising a lens system having a primary lens system (12, 61) and a secondary lens system (14) and the lens combination being configured in such a way that a. an intermediate image plane is located between the primary lens system (12, 61) and the secondary lens system (14), b. it has an entrance pupil (26), located on the object side of the primary lens system (12,61), which is the real image of the stop (22), and c. it has an exit pupil (24), located on the image side of the secondary lens system (14), which coincides with the stop (22).

2. A wide-angle optical system (10,60) according to Claim 2, characterized in that the primary lens system (12,61) has an f-number of less than one.

3. A wide-angle optical system (10) according to one of the preceding claims, characterized in that the primary lens system (12), in direction from the object side to the image side, comprises a doublet consisting of a positive lens (28) and a negative lens (30).

4. A wide-angle optical system (10) according to Claim 3, characterized In that the positive lens (28) and the negative lens (30) of the primary lens system (12) are meniscus lenses.

5. A wide-angle optical system (10) according to Claim 3 or 4, characterized in that the first outer face (32) of the positive lens (28) of the primary lens system (12) has an aspherical shape and the second outer face (34) of the positive lens (28) of the primary lens system as well as the outer faces (36, 40) of the negative lens (30) of the primary lens system (12) have a spherical shape.

6. A wide-angle optical system (10) according to one of Claims 3 to 5, characterized in that the positive lens (28) of the primary lens system (12) is made of germanium or silicon and the negative lens (30) of the primary lens system (12) is made of calcium fluoride or magnesium fluoride.

7. A wide-angle optical system (60) according to Claim 1 or 2, characterized in that the primary lens system (61) comprises a positive lens (62) which a. is in the form of a meniscus lens, and b. is made of germanium, c. its first outer face (64) (on the object side) having an aspherical shape and a diffractive surface.

8. A wide-angle optical system (10,60) according to one of the preceding claims, characterized in that the secondary lens system (14), in direction from the object side to the image side, comprises a negative lens (42), a first positive lens (44) and a second positive lens (46), the lenses (42, 44, 46) preferably taking the form of meniscus lenses.

9. A wide-angle optical system (10,60) according to Claim 8, characterized in that a. the negative lens (42) of the secondary [ens system (14) is made of calcium fluoride, b. the first positive lens (44) of the secondary lens system (14) is made of silicon and c. the second positive lens (46) of the secondary lens system (14) is made of germanium.

10. A wide-angle optical system (10, 60) according to one of Claims 8 or 9, characterized in that the first outer face (56) of the second positive lens (46) of the secondaiy lens system (14) has an aspherical shape.

11. A wide-angJe optical system (10, 60) as substantially described herein with reference to figures 1 and 2 of the drawings