GB2420632A

GB2420632A - Wide angle infrared optical system with five lenses

Info

Publication number: GB2420632A
Application number: GB0522717A
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: 2004-11-26
Filing date: 2005-11-08
Publication date: 2006-05-31
Anticipated expiration: 2025-11-08
Also published as: FR2878625A1; DE102004057096B4; GB2420632B; DE102004057096A1; FR2878625B1; GB0522717D0

Abstract

A wide-angle optical system (10, 11), for a detector (32) for the infrared spectral region, comprise a lens system which, in the direction from the object side to the image side, consists of: <SL> <LI>a. an inverse Galileo telescope (20) with a negative lens (22) and a positive lens (24), and <LI>b. an arrangement (12) of three additional lenses (14, 16, 18), one lens (18) being negative and two lenses (14, 16) being positive. </SL> Mirror 34 may be present between lenses 22 and 24. The lenses may be made of germanium or silicon. Window 26, cold filter 28 and cold stop 30 are shown.

Description

Wide-angle Optical System The invention relates to a wide-angle optical

system for a detector for the infrared spectral region.

Optical systems for the infrared spectral region are used in surveillance equipment, for example, - such as night security devices, aiming devices and the detection units of missiles. With equipment of this type, it is often necessary to be able to cover a large field of view. This is possible thanks to an appropriate design of the optical system. Such optical systems, which allow a total field angle on the object side of more than 450, are known as wide-angle optical systems.

In order to obtain high-quality recordings of a field of view on a detector with preceding wide-angle optical system, a homogeneous and strong illumination of the image field is necessary, i.e. the f-number must be less than one. A high- speed wide- angle objective lens for the near-infrared spectral region (1.5 - 2.5 tm) is known from EP 0 195 747 A2. With a wide-angle objective lens as described there, a total field angle of up to 60 can be achieved in this spectral region with an f-number of 0.7.

It is a disadvantage that a total field angle of more than 60 cannot be achieved with a high-aperture wide-angle optical system according to the prior art. This total field angle, achievable up till now in the nearinfrared spectral region, is however often not adequate for particular applications, such as, for example, the detection of flying targets on the basis of their emissions in the mid-infrared spectral region, or else is unsuitable owing to the spectral limitation to the near-infrared. When only a small total field angle is viewable via the optical system for the detector, flying targets and their emissions, due to their high velocity, frequently vanish too quickly from the field of view of a detector with an optical system located in front of it for any useful acquisition of the target to be achieved.

It is therefore the objective of the present invention to propose a wideangle optical system for a detector for the infrared spectral region, in particular for the mid-infrared spectral region, by means of which a larger total field angle than that of the prior art can be obtained.

This objective is achieved according to the invention in respect of a wide-angle optical system for a detector for the.infrared spectral region in that the wide-angle optical system comprises a lens system which, in the direction from the object side to the image side, consists of: a. an inverse Galileo telescope with a negative and a positive lens and b. an arrangement of three additional lenses, one lens being negative and two lenses being positive.

In a first step, the invention proceeds from the knowledge that the magnification of a Galileo telescope depends on the ratio of the focal length of the objective lens to the focal length of the ocular or eyepiece, that is to say on the ratio of the focal length of the positive lens to the focal length of the negative lens. With a Galileo telescope, the viewing angle, i.e. the angle which the outermost rays coming from a viewed object form together with one another, is thus enlarged. By simple modification of the focal lengths of the negative and positive lenses of the Galileo telescope, variability in the

field of view from 0 to 180 is possible.

In a second step, the invention proceeds from the knowledge that by using an arrangement of three lenses, in which case two lenses are positive and one lens is negative, images of good quality can be produced, since image defects of the two positive lenses, particularly due to chromatic aberration, can be corrected by means of the negative lens.

In a subsequent step, the invention proceeds from the knowledge that, by appropriate selection of focal lengths and diameters of the three additional lenses of the arrangement, a high-aperture optical system, in other words, an optical system having an f-number of less than 1.1, can be produced.

In a further step, the invention proceeds from the consideration that the addition of an inverse Galileo telescope in front of an arrangement as previously described of three additional lenses, with appropriate selection of the diameters and focal lengths of the lenses of the Galileo telescope, leads to a high-aperture wide-angle optical system, with a total field angle which can be enlarged up to 180 by way of the focal lengths of the negative and positive lenses of the inverse Galileo telescope whilst at the same time having a low f-number. Specifically, if a ray enters the inverse Galileo telescope at a steep angle, then it leaves it at a flat angle.

According to the invention, a high-speed wide-angle optical system is thus produced, of which the total field angle can be adjusted in the range from 0 to 1800 for the purposes of its specific application by simple modification of the focal lengths of the positive and negative lenses of the inverse Galileo telescope. The f-number can also be influenced by appropriate selection of the lens diameters and their focal lengths.

A mutual compensation of the image defects can be achieved without involving the need for additional lenses - possibly having aspherical outer faces which are difficult to manufacture from the technical point of view - by selecting the appropriate configuration of the positive and negative lenses of the inverse Galileo telescope, as well as the appropriate configuration of the arrangement of the three additional lenses. This reduces costs and guarantees a good, defect-free image quality of the wide-angle optical system.

The wide-angle optical system advantageously has an f-number of less than 1.1. The f-number establishes the power and speed of an optical system. The lower the f- number, the more powerful the optical system. In this connection, we are talking of a fast optical system since shorter exposure times can be selected with a powerful optical system for the purpose of obtaining high- quality recordings by means of a detector. Particularly in respect of the use in missiles of wide-angle optical systems for a detector in the infrared spectral region for detecting fast-moving targets, wide- angle optical systems with a low f-number are necessary so as to be able to acquire these quickly by means of the detector. Since only short exposure times are necessary for a high-quality recording of a field of view by means of the detector, the wide-angle optical system together with the detector can thus, for example, be oriented particularly rapidly to a new field of view.

The lenses of the wide-angle optical system advantageously have spherically formed outer faces. On the one hand, due to dispensing with the use of aspherical lenses, costs can be reduced since the manufacture of aspherical lenses is considerably more complicated than that of spherical lenses and thus is more expensive. On the other hand, this can further reduce the level of flare since spherical lenses can be produced with considerably greater precision and particularly with a lower surface roughness than aspherical lenses.

In a further advantageous embodiment of the invention, a number of bending elements are provided between the negative and positive lenses of the inverse Galileo telescope. Since the distance between the negative and the positive lenses of the inverse Galileo telescope can be varied based on the focal lengths of the two lenses, bending elements can be inserted in this intermediate space. The bending elements may, for example, involve deflecting mirrors, gratings, prisms or the like. Depending on the intended purpose, an alteration of the direction of the beam path as well as, if necessary or desired, the screening out of a specific spectral region can be facilitated by the bending elements.

It is of advantage if, in the case of the bending element disposed between the negative and positive lenses of the inverse Galileo telescope, it involves a deflecting mirror.

The insertion of a deflecting mirror makes it possible for the wide-angle optical system to be made more compact in structure. In missiles, where for example there is only limited space available for a wide-angle optical system, the longitudinal dimension of the wide-angle optical system can be reduced by the insertion of a deflecting mirror between the negative and the positive lenses of the inverse Galileo telescope, said deflecting mirror producing a deflection of the beam path by for example 90 . Thus a space available for the wide-angle optical system can optimally be made full use of as regards its length and width, without wasting valuable space which can now be used for the installation of other components.

It is conceivable for the negative lens of the inverse Galileo telescope on the object side and the deflecting mirror to be designed so that they are rotatable. This presents the possibility of surveying the entire horizon by means of the wide-angle optical system or, with a total field angle of the wide-angle optical system of 1800, of surveying the entire sphere.

In another advantageous embodiment of the invention, a number of bending elements are provided between the last lens of the inverse Galileo telescope facing the image side and the arrangement of the three additional lenses. In this way - as already mentioned above - it is possible to adapt the longitudinal and transverse dimensions of the wideangle optical system to the space available. The spectral transmission as regards the spectral regions concerned can also be influenced in this way.

In practice, in the case of one of the bending elements disposed between the last lens of the inverse Galileo telescope facing the image side and the arrangement of the additional lenses, it involves a deflecting mirror or a prism. A deflecting mirror presents the possibility of aligning the arrangement of the three additional lenses in such a way relative to the optical axis of the inverse Galileo telescope as the actual spatial conditions allow. If, in the case of the deflecting mirror, it involves a mirror which deflects one portion of the beam path at a certain angle and allows the other portion of the beam path to pass without hindrance, then two arrangements of three additional lenses, variously designed as regards their focal lengths, can be provided after the inverse Galileo telescope. In this way, two different fields of view can then be covered, for example one which covers a large region with lower resolution, and another which covers a smaller region thereof, but with a higher resolution. A prism can also be used instead of the deflecting mirror. Moreover, the use of a deflecting mirror with appropriate coating or of a prism offers the possibility of a division into two beam paths with different spectral regions. Again, in this case, different arrangements of three additional lenses - depending on the required information to be ascertained via a detector - can also be provided.

The wide-angle optical system advantageously has an exit pupil located after the optical system's last lens facing the image side. A freely accessible exit pupil makes it possible to have this coincide with an actual aperture stop and thus produce defined radiometric conditions. As a result, substantially improved stray light behaviour can be achieved, which ensures higher-quality images on a detector located after it.

In practice, the freely accessible exit pupil coincides with a cold stop located before the detector. Such cold stops - as the name implies - are kept at a low temperature so as to prevent thermally produced scattered light components which lead to falsification of the images on a detector located after them. Thus it is possible to produce a geometric cold stop efficiency of one, which ensures defmed radiometric conditions and thus a clearly improved scattered light behaviour and consequently high-quality images on the detector.

In a novel way, the negative lens and the positive lens of the inverse Galileo telescope and the two positive lenses of the arrangement of three additional lenses are made of silicon and the negative lens of the arrangement of three additional lenses is made of germanium. The arrangement enables all the additional lenses to be made of silicon with the exception of the germanium lens necessary for colour correction. The costs in respect of the wide-angle optical system can be reduced due to largely abandoning the use of expensive germanium lenses and using clearly less expensive silicon lenses instead. It is particularly ingenious that, in respect of its geometric configuration, the negative lens, made of germanium which is necessary for colour correction is formed with as small a volume as possible so as further to reduce costs.

Working examples of the invention are described in greater detail below with the aid of drawings, which are as follows: Fig. 1 shows a wide-angle optical system having an arrangement of three lenses in front of a detector, and a preceding inverse Galileo telescope, Fig. 2 shows a wideangle optical system according to Fig. 1, but with a deflecting mirror between two lenses of the inverse Galileo telescope.

In the drawings, functionally identical components are provided with the same reference numerals.

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

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

Fig. I shows a wide-angle optical system 10 comprising an arrangement 12 of three lenses 14, 16 and 18 and a preceding inverse Galileo telescope 20 comprising two lenses 22 and 24. All five lenses 14, 16, 18, 22, 24 have spherical outer faces. The two positive lenses 14 and 18 of the arrangement 12 are convex as regards their outer faces on the object side and concave in respect of their outer faces on the image side and are made of silicon. The negative lens 16 disposed between the lenses 14 and 18 is made of germanium and is bi-concave. The lens 16 serves to correct the colour defects of the positive lenses 14 and 18. The lenses 22, 24 of the inverse Galileo telescope 20 are made of silicon. With the lens 22, it involves a negative lens with a concave outer face on the object side and a convex outer face on the image side. The lens 24, on the other hand, is a positive lens with a concave outer face on the object side and a convex outer face on the image side. The radii of the outer faces of the lenses 14, 16, 18, 22 and 24, their thickness, their distance from one another, their aperture radius and the material from which they are made, can be seen in detail in Table 1. Located after the last lens 18 of the arrangement 12, after a window 26, a cold filter 28 and a cold stop 30, is a detector 32 which is sensitive in the infrared spectral region. The window 26 in this case is the window in a Dewar vessel in which the detector 32 is located. The window 26 and the cold stop 30 are both made of germanium. The cold filter 28 is made of silicon. Thus - in precisely the same way as lenses I 4, 16, 18, 22, 24 - these components also meet the requirements as regards transmission in the infrared spectral region. With the detector 32, it can involve a cadmium telluride based detector. The geometric dimensions of window 26, cold filter 28, cold stop 30 and detector 32 and their distances from one another can again be seen in Table 1. The design of the wide-angle optical system 10 is such that its exit pupil coincides with the position of the cold stop 30.

Table 1: Design Data of Wide-angle Optical System according to Fig. 1 Radius (mm) Thickness (mm) or Aperture radius (mm) Material Comments distance (mm) Object plane Air 1 ____ Air 2 -86.849163 7.990233 40 Silicon Lens 22 3 -160.17772 146.550497 45 Air Distance to the next lens 4 -551.959594 10 50 Silicon Lens 24 -286.98 1429 0.1 50 Air Distance to the next lens 6 99.320892 12 48 Silicon Lens 14 7 286.548419 7 48 Air Distance to the next lens 8 -507.299039 8 48 Germanium Lens 16 9 357.988211 28.781476 48 Air Distance to the next lens 106.225498 16 30 Silicon Lens 18 11 377.539283 5 30 Air Distance to the window 12 3 20 Germanium Window 26 13 1 20 Air Distance to the cold filter 14 3 20 Silicon Cold filter 28 0.5 20 Air Distance A erture stop 14.03003 1 16.92 Air to the cold stop 17 1.1 13.082117 Germanium Cold stop 30 18 10 13.066494 Air Distance to the detector 19 0.4 7.68 Cadmium tellurjde Detector 32 Air Image plane The illustrated wide-angle optical system 10 has a focal length f of 21.63 mm and a numerical aperture NA of 0.5682. The high-aperture and high-speed wide- angle optical system 10 according to the present design has an f-number of 0.86 and a total field angle of 60 . By altering the focal lengths of the lenses 22 and 24 of the inverse Galileo telescope 20, it is possible for a person skilled in the art to reduce or enlarge the

total field angle.

Fig. 2 shows a wide-angle optical system 11, comprising elements which are functionally identical to those of the wide-angle optical system 10 illustrated in Fig. 1, but which includes a deflecting mirror 34 disposed in the space available between the negative lens 22 and the positive lens 24 of the inverse Galileo telescope 20. After the lens 22, the deflecting mirror 34 deflects the incident beam path on the object side via the deflecting mirror 34 through 90 to the following lens 24 and the arrangement 12 of the three additional lenses 14, 16 and 18. According to the design which can be seen in Table 2, the high-aperture and high-speed wide-angle optical system 11 has a total field angle of 90 and an fnumber of 1. The focal length f of the wide-angle optical system 11 is 9.2 mm. The detector 32 is disposed in a Dewar vessel 36. It cannot be seen in Fig. 2 that the lens 22 and the deflecting mirror 34 are designed to be rotatable about a common axis. As a result, in surveillance equipment for example, monitoring of the entire horizon is possible with a total field angle of 90 .

Table 2: Design Data of Wide-angle Optical System according to Fig. 2 Radius (mm) Thickness (mm) or Aperture radius (mm) Material Comments distance (mm) Obect lane Air I Air 2 -37.049853 3.408633 18 Silicon Lens 22 3 -68.331815 32.656442 19.197 Air Distance to the deflectin minor Reflection coatjn Deflectin mirror 34 4 -29. 862 28 Air Distance to the next lens 235.465963 -4.266 21.33 Silicon Lens 24 6 122.426277 -0.04266 21.33 Air Distance to the next lens 7 -42.370293 -5.1192 20.4768 Silicon Lens 14 8 -122.24 1555 -2.9862 20.4768 Air Distance to the next lens 9 216.41377 -3.4128 20.4768 Germanium Lens 16 -152.717771 -12.278178 20.4768 Air Distance to the next lens 11 -45.3 15798 -6.8256 12.798 Silicon Lens 18 12 -161.058258 -2.133 12.798 Air Distancetothewindow 13 -1.2798 8.532 Germanium Window 26 14 -0.4266 8.532 Air Distance to the cold filter -1.2798 8.532 Silicon Cold filter 28 16 -0.2133 8.532 Air Distance A erture sto -5.985211 5.2 13395 Air to the cold sto 18 -0.46926 8.532 Germanium Cold sto 30 19 -4.266 8.532 Air Distance to the detector -0.17064 3.276288 Cadmium tellurjde Detector 32 21 Air Ima e lane Reference Numerals Wide-angle optical system 11 Wide-angle optical system 12 Arrangement 14 Positive lens 16 Negative lens 18 Positive lens Inverse Galileo telescope 22 Negative lens 24 Positive lens 26 Window 28 Cold filter Cold stop 32 Detector 34 Deflecting mirror 36 Dewar vessel

Claims

Patent Claims 1. A wide-angle optical system (10, 11) for a detector (32)

for the infrared spectral region, the wide-angle optical system (10, 11) comprising a lens system which, in the direction from the object side to the image side, consists of: a. an inverse Galileo telescope (20) with a negative lens (22) and a positive lens (24), and b. an arrangement (12) of three additional lenses (14, 16, 18), one lens (16) being negative and two lenses (14, 18) being positive.
2. A wide-angle optical system (10, 11) according to Claim 1 having an fnumber oflessthan 1.1.
3. A wide-angle optical system (10, 11) according to one of the preceding claims, in which the lenses (14, 16, 18, 22, 24) have spherically formed outer faces.
4. A wide-angle optical system (10, 11) according to one of the preceding claims, in which a number of bending elements are disposed between the negative lens (22) and the positive lens (24) of the inverse Galileo telescope (20).
5. A wide-angle optical system (10, 11) according to Claim 4, in which one bending element is a deflecting mirror (34).
6. A wide-angle optical system (10, 11) according to one of the preceding claims, in which a number of bending elements are disposed between the last lens (24), facing the image side, of the inverse Galileo telescope (20) and the arrangement (12) of the three additional lenses (14, 16, 18).
7. A wide-angle optical system (10, 11) according to Claim 6, in which one bending element is a deflecting mirror or a prism.
8. A wide-angle optical system (10, 11) according to one of the preceding claims with an exit pupil located after the last lens (18), facing the image side, of the wide-angle optical system (10, Il).
9. A wide-angle optical system (10, 11) according to Claim 8, in which the exit pupil coincides with a cold stop (30) located before the detector (32).
10. A wide-angle optical system (10, 11) according to one of the preceding claims, in which the negative lens (22) and the positive lens (24) of the inverse Galileo telescope (20) and the two positive lenses (14, 18) of the arrangement (12) of the three additional lenses (14, 16, 18) are made of silicon and the negative lens (16) of the arrangement (12) of the three additional lenses (14, 16, 18) is made of germanium.
11. A wide-angle optical system (10,11) as substantially described herein with reference to figures 1 and 2 of the drawings.