WO2009109979A2

WO2009109979A2 - Compact infra-red lens assembly

Info

Publication number: WO2009109979A2
Application number: PCT/IL2009/000256
Authority: WO
Inventors: Eliyahu Bender; Nissim Asida
Original assignee: Ophir Optronics Ltd.
Priority date: 2008-03-06
Filing date: 2009-03-08
Publication date: 2009-09-11
Also published as: WO2009109979A3

Abstract

An infra red camera assembly, with elements efficient in raw material use, simple to produce and which provides a high level of performance. The assembly uses a three element design, with a main lens, and with both the entrance window and the detector array protection window having some optical power, such that the refractive power of the lens assembly comprises up to four surfaces spread over three elements. The lens assembly comprises (i) a front element which has one surface flat, and which can thus also serve as the input window, (ii) a rear element which has one surface flat, and which can thus also serve as the detector window, and (iii) a single central lens element which, because of the combined use of the two other optically active window elements, does not require any steeply formed surfaces, such that it is also material efficient and economical to produce.

Description

COMPACT INFRA-RED LENS ASSEMBLY

FIELD OF THE INVENTION

The present invention relates to the field of lens assemblies for infra-red thermal imaging, especially for automotive use with uncooled detector arrays.

BACKGROUND OF THE INVENTION

With the advent of uncooled detectors for the thermal infra-red (IR) region, a large number of uses have developed for applications of thermal imaging for use in consumer-type of applications. One such use is for the provision of forward looking infra-red ((FLIR) imaging systems for automotive use, such that the driver can be provided with much better visibility in conditions of poor weather or darkness.

One of the main requirements of such consumer-use systems is low cost, and a secondary requirement is for compact dimensions. Whatever design is used must be very durable, such that, for instance, it can be mounted on the front of a vehicle where it is exposed to dust, stones and moisture for a use period which should extend to a number of years before replacement is required. The high cost of the raw materials and of the forming of the optical surfaces, which is usually performed by diamond turning in order to provide sufficiently good image quality, limit the extent to which these targets can be achieved. Typical optical requirements for such an automotive lens are for a focal length of 19mm, combined with an f-number of 1.1. In order to achieve the optical performance required, prior art designs generally use multiple lens elements, typically a 2- element assembly and the lenses of such a 2-element solution are generally material intensive, and thus costly. In addition, the lens assembly needs a protective front or input window, and the detector array also needs to be supplied with an IR protective window The overall cost of the lens assembly may thus be significant. There is therefore need for new lens designs aimed at providing such lens assemblies with optimal performance and reduced cost.

The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety. SUMMARY OF THE INVENTION

The present disclosure describes new compact lens assemblies for use in the infra-red, which are efficient in raw material use, whose elements are comparatively simple to produce, and which nevertheless provide the high level of performance required. These objectives are achieved by using a three element lens assembly, rather than the generally used two element, prior art designs. The cost of the lens assembly is kept reasonable by the use of elements having parameters designed for low material costs and low production costs. In order to achieve these goals, such an exemplary lens may thus comprise: (i) a front element which is almost flat, and which can thus also serve as the input window,

(ii) a rear element which has one flat surface, and which can thus also serve as the detector window, and

(iii) a central lens element which, because of the combined use of the two other optically active elements, does not require any steeply formed surfaces, such that' it is also a material-efficient lens and an economical lens to produce.

The use of a front element as the window of the lens assembly saves the i cost of a separate window, and provides an optical surface generally unavailable in prior art 2-element lens designs, to use in designing the complete lens assembly to achieve the desired performance. It is thus generally sufficient for the front element to have a diamond turned back surface, which is part of the optical design, and a flat front surface provided with a hard coating, such as a hard diamond coating to withstand the environmental requirements of the input window. The only additional cost over a conventional input flat window is therefore the need to diamond turn one surface, and if the lens assembly is designed such that this turned surface is shallow, there is little material wastage, and comparatively low production costs in turning the surface.

Likewise, the use of a rear element as the window of the detector array saves the cost of a separate detector window, and provides an additional optical surface, unavailable in prior art 2-element lens designs, to use in designing the complete lens assembly to achieve the desired performance. The rear element may generally have a spherically polished front surface, which is part of the optical design, and a flat rear surface which is used as the window to seal the detector array. The only additional cost over a conventional flat detector window is therefore the need to produce one surface spherical rather than flat. Since this element is generally a thin element, and the departure from flatness is generally slight, the production of the non-flat surfaces should involve very little waste of material. The detector array may advantageously be supplied by the detector manufacturer with the specified flat/curved optical window already fitted.

The central element of the assembly, which is the lens which does most of the refractive work, is designed to be as simple and as material cost-effective as possible. If this lens is germanium, it is possible to use one flat surface, with the other surface aspheric. If this lens is of zinc selenide, then it cannot generally provide an acceptable performance with a flat surface, and it is generally necessary to use a spherical/aspherical design, and with a diffractive element turned on the aspherical side.

The central element provides most of the refractive power of the lens, while the front and rear elements, because of the constraints laid upon them to be quasi-flat, are designed to behave like corrector plates. The front element is designed to operate as a corrector plate to maintain or improve the performance of the lens over a wider field of view. The typical requirement for a lens for automotive use is for the above mentioned optical parameters to be achieved over a total field angle of up to about 30 degrees. The rear element operates as a field curvature corrector plate, adapting the curved output field of the main lens to the flat geometry of the detector array plane. If a curved detector element were provided, with a curvature matching the curvature of the output wavefront of the main lens, then the rear element could be a simple flat-flat window for the detector array.

Thus, a feature of the various examples of lens assemblies described in this application is that the input window may be only slightly deviant on one surface from being a flat/flat element, while the detector array window may be a thin spherical/flat element.

The optical design may be optimized by using, in addition to the conventional conditions for the design of the central lens, the rather unconventional constraints of defining one flat surface for the input element, a flat surface for the rear element, and that both of those elements should have their non-flat surfaces designed such that the material loss in preparing those elements is minimal, commensurate with providing the required optical performance of the entire lens assembly. The optimization may generally be performed to provide the required focal length and MTF over the entire required field of view. A typical requirement for such an automotive application is for an MTF of 20cy/mm of 50% on axis, falling to no less than 30% at the field edge.

In order to maintain performance over a wide temperature range, such as is encountered in an automotive application, thermal compensation for the change in the refractive index of the central element may be required, this being the element where most of the refractive effect of the lens is accomplished. This is accomplished by provision of a thermal compensation element, such as a plastic ring having a predetermined thermal expansion characteristic, which pushes the central element towards the detector array as the temperature rises, thus compensating for the rise in the refractive index of the lens material with rise in temperature, as is known in the art.

Although the lens assemblies have been described in terms of their use foπ. automotive applications, it is to be understood that this is just one exemplary use of such assemblies. The lens assemblies described in the present disclosure are not intended to be limited to this use or application, but are understood to be generally applicable wherever a cost sensitive infra-red camera using standard n detector arrays is to be provided, with requirement for good protection from environmental damage to its front viewing port.

According to various examples of the infra red camera assemblies described in this disclosure, one exemplary camera may comprise: (i) an input window having a flat surface and a non-flat surface, (ii) a detector array having a protective window with a concave surface and a flat surface, and

(iii) a single lens disposed between the input window and the protective window, the lens having at least one aspheric surface.

In examples of the above described camera, the non-flat surface of the input window, the lens surfaces, and the concave surface of the protective window may be optimized to provide a predetermined optical performance for the camera. In so doing, the non-flat surface of the input window may be selected to correct residual aberration in the combination of the lens and the protective window, and it may be designed to increase the field of view over which the desired MTF of the lens assembly is maintained, in comparison with a similar camera with a flat/flat input window. Furthermore, this non-flat surface of the input window may be constrained to have a curvature such that wastage of material in production of the input window is minimized. In particular, the departure from flatness of the non-flat surface of the input window may be such as to not exceed 1/200 of the diameter of the input window. The non-flat surface of the input window may be aspheric.

With regard to the concave surface of the protective window, according to one exemplary design of this surface, it may be spherical. Whether spherical or not, its departure from flatness may be such as not to exceed 1/15 of the diameter of the protective window. Furthermore, the protective window may be adapted to operate as a field curvature corrector plate, adapting the curved output field of the single lens to the flat detector array plane.

With regard to the single lens of the above described camera, the lens surfaces may advantageously be constrained to have curvatures such that _; wastage of material in production of the lens is minimized. According to this constraint, the single lens may be such that it does not have a meniscus lens form. The single lens may preferably be constructed of germanium or of zinc selenide.

Another exemplary implementation of the infra red imaging camera described in this disclosure may comprise: (i) an input window having at least one flat surface,

(ii) a detector array having a protective window with at least one flat surface, and (iii) a single lens disposed between the input window and the protective window, wherein the refractive power of the camera is vested in no more than four surfaces spread over the input window, the lens and the protective window. The refractive power of such a camera may also be vested in just three surfaces spread over the input window, the lens and the protective window.

Yet other implementations of the claimed invention described in this application perform a method of reducing the number of elements in an infra red camera, the method comprising: (i) providing an input window,

(ii) providing a detector array having a protective window, and (iii) disposing a single lens between the input window and the protective window, the lens having at least one aspheric surface, wherein the input window has a flat surface and a non-flat surface and the protective window has a concave surface and a flat surface.

The above mentioned method may further comprise the step of optimizing the non-flat surface of the input window, the lens surfaces, and the concave surface of the protective window to provide a predetermined optical performance for the camera. This exemplary method may involve the step of selecting the non- flat surface of the input window to correct residual aberration in the combination of the lens and the protective window. The step of selecting the non-flat surface of the input window may also be performed in order to increase the field of view over which the desired MTF of the lens assembly is maintained, in comparison with a similar camera with a flat/flat input window. Furthermore, this non-flat surface of the input window may be constrained to have a curvature such that wastage of material in production of the input window is minimized. In particular, the departure .. from flatness of the non-flat surface of the input window may be designed so as to . not exceed 1/200 of the diameter of the input window. The non-flat surface of the . input window may be made aspheric.

In the above described method, the concave surface of the protective window, according to one exemplary design of this surface, may be constructed to be spherical. Whether spherical or not, Its departure from flatness may be such as . not to exceed 1/15 of the diameter of the protective window. Furthermore, the protective window may be operated as a field curvature corrector plate, adapting the curved output field of the single lens to the flat detector array plane.

In some exemplary implementations of this method, the lens surfaces may advantageously be constrained to have curvatures such that wastage of material in production of the lens is minimized. According to this constraint, the single lens may be constructed such that it does not have a meniscus lens form. The single lens may preferably be made of germanium or of zinc selenide.

Additionally, in other implementations of any of the above-described methods, the camera may be designed such that its refractive power is vested in no more than four surfaces spread over the input window, the lens and the protective window. The camera may be designed such that its refractive power may also be vested in just three surfaces spread over the input window, the lens and the protective window. Still other example implementations involve a method of reducing the number of elements in an infra red camera, comprising: (i) providing an input window having a at least one flat surface, (ii) providing a detector array having a protective window with at least one flat surface, and

(iii) disposing a single lens between the input window and the protective window, wherein the refractive power of the camera may be vested in no more than four surfaces spread over the input window, the lens and the protective window. The camera may be designed such that its refractive power may also be vested in just three surfaces spread over the input window, the lens and the protective window.

BRIEF DESCRIPTION OF THE DRAWINGS

The present claimed invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

Fig. 1 illustrates schematically a prior art automotive IR imaging camera;

Fig. 2 illustrates schematically an example of an IR imaging camera lens assembly, constructed and operative according to one implementation of the present claimed invention, using a germanium main lens;

Fig. 3 illustrates schematically an alternative design in which the central lens is constructed of zinc selenide; and

Fig. 4 illustrates an IR imaging camera lens assembly such as is shown in Fig. 2 or Fig. 3, and including a thermal expansion collar adapted to compensate for the changes in refractive index of the main lens with change in temperature of the assembly.

DETAILED DESCRIPTION

Reference is now made to Fig. 1 , which illustrates schematically a prior art automotive IR imaging camera 10, showing a 2-element 12,14, optical design with a flat input entrance window 16 and an IR detector array 17 with its own built-in protection window 18. As is observed, this design requires a total of four IR elements, of which two are active optical elements providing the refractive power of the lens, and two function as refractively inactive window elements. Reference is now made to Fig. 2, which illustrates schematically an exemplary IR imaging camera lens assembly 20, in which a single central refractive element 21 is used, but in which the input window 22 and the detector protection cover 25 each have one surface formed to provide some refractive effect in the lens assembly. The range of operation of the camera may be the infra-red thermal range, typically covering a spectral band of 7.5 to 15.5 microns.

The outer surface 23 of the input window 22 may be flat, while its inner surface 24 is almost flat, but is shaped to add a small correction to the entering wavefront so that the desired MTF of the lens assembly is maintained over a larger field of view than would be obtained from an identical lens with a flat/flat entrance window. This window may have a hard coating on the outer surface 23, in order to provide good protection from the rigors of the environment in which it is to operate. This window thus has the double function of acting as the input protective window of the camera, and yet also contributing some refractive functionality to the lens assembly, providing an additional refractive surface for use in optimizing the complete lens assembly. It is logical to mount the input window with the flat surface outermost, such that if the outer surface is damaged, as is likely in the course of time, it is more economical to refurbish the window by plane repolishing the flat surface, rather than reforming the aspheric surface, which would be the case if this surface were outermost. However, it is to be understood that the claimed lens assemblies are not meant to be thus limited, and would be operable also with the flat surface facing inwards.

The detector array protection window 25 may have a flat surface 26 facing the detector array 27, and its outer surface 28 spherically concave, to provide field curvature correction to the wavefront exiting the central lens. This window 25 thus has the double function of acting as the front protective window of the detector array 27, and yet also contributing some refractive functionality to the lens assembly, providing an additional refractive surface for use in optimizing the complete lens assembly. Though it is most logical to mount the protective window with its flat surface on the detector array housing, to provide a good seal, it is to be understood that the claimed lens assemblies are not meant to be thus limited, and would be operable also with the flat surface facing outwards from the detector array. The complete lens array thus has four surfaces to adjust in order to optimize the lens performance, like that of the prior art lens assembly shown in Fig. 1 , but since these four surfaces are now spread over three elements instead of two, greater flexibility is available to the lens designer to provide an optimal solution. Some designs of the imaging camera optics may be such that the central lens 21 also has one surface flat, such that the camera contains only three surfaces contributing to the refractive power, these three surfaces being spread over three elements.

All four of the elements are constructed of IR transparent material, with the central refractive element being preferably made of germanium (Fig. 2) or zinc selenide (Fig. 3) to provide good refractive power. The input window may also be of germanium to provide physical strength and a suitable substrate for the hard coating, and the detector window may be of silicon, a comparatively low cost ^• material as often used for IR detector windows. Since the detector window is thin, - the slight absorbance of silicon within the thermal IR range is not too detrimental to performance. It is to be understood however, that these materials are only exemplary materials commonly used for these functions, and that other suitable materials may also be used without departing from the present claimed invention.

The lens assembly may conveniently be constructed according to the * results of ray tracing optimizations to minimize a merit function (this generally being an error function), which is preferably built of a number of desired performance parameters of the camera. The merit function may contain three components, according to the method used to design the exemplary implementation shown in Fig. 2 (and in Fig. 3 below). These three components may be the effective focal length of the lens assembly, the RMS spot radius, and the RMS wavefront, which ensures that the optical path length of every ray traced is essentially the same, providing phase uniformity across the image. The result of optimizing these three parameters is an optimized modulation transfer function (MTF) of the lens assembly as a function of field position. It is to be understood though, that these parameters are only typically-used parameters for optimizing lens performance, and that other parameters and combinations may equally well be used in the merit function. The assemblies described in this disclosure are not thus deemed to be limited to the specific parameters mentioned hereinabove. The optimization procedure results, inter alia, in defined values of the radii of curvature of the element surfaces, and their thicknesses, for defined values of the index of refraction of the lens material. For embodiments using one or more aspheric surfaces, the surfaces are defined also by means of conic constant and aspheric coefficients. Some lens designs for some materials may also require a radial phase function applied to the aspheric surface by the addition of a diffractive optical element turned on the surface. This is particularly so for a zinc selenide lens. The lens assemblies thus defined then have optimal optical performance with minimal aberrations within the design criteria chosen for the merit function.

In order to illustrate the design methods and the resulting lens assemblies which may be obtained, an optimization program was run on this lens assembly, to provide the optimum performance for an f/1.1 lens of 19mm effective focal length, with a 30 degree field of view. The required MTF at 20cy/mm is 50% on- axis, and 30% at the field edge. The optimization was first performed, using the above-described merit function, on the combination of the central lens 21 and the detector window 25. The central lens 21 was constrained to have one surface flat, and the input window 22 was constrained to be flat/flat. The optimization could not generate the desired performance using only one central lens 21 and its field curvature correction plate 25, though the level of residual aberration was small. - One non-flat surface 24 of the input window was then added to the optimization procedure, in order to improve the MTF. Since the level of residual aberration was small, the desired MTF over the required field of view was achieved with only a small departure from flatness of this surface. A surface figure of 35 microns on one surface of the input window was found to be sufficient to optimize the complete assembly to the above-mentioned required performance. A 35 micron sag in an element of 18 mm. diameter represents a departure from flatness of 1/500, i.e. 0.2%. This small departure fulfils the design requirements of minimal material wastage, and minimal production costs.

This surface should be prevented from developing significant curvature during the optimization process, and hence from showing significant departure from its original flat form, by constraining the positions of the other elements in the assembly to remain essentially unchanged. It is apparent that there is a trade-off between the allowed extent of departure from flatness of the input window surface, and the refractive power demanded from the surfaces of the central lens 21. The greater the degree of departure from flatness allowed in the input window, the more relaxed are the curvatures required for the surfaces of the central lens 21. However, since a larger departure from flatness of the input window generally involves a higher production cost, while small changes in the surface form of the main lens do not appreciably affect the production cost of this lens, it is advantageous to apply the minimum curvature possible to the input window for fulfilling its refractive function. A useful criterion for the advisable extent of departure from flatness of the input window is that its sag should not exceed 1/200 of the element diameter, e.g. 100 microns for a 20 mm diameter window, though it is to be understood that in specific applications where performance may overrule cost considerations, a larger sag may be permissible.

One of the features of the elements of the exemplary lens assembly of Fig. 2, and of any other lens assembly according to other implementations shown in this disclosure, is that all of the elements are effectively constrained to avoid ^s having deep curvatures, in order to provide optimum material usage for each element. For this reason, the designs avoid the use of deeply meniscus elements, or even of any meniscus elements, which are wasteful of raw material, whose starting point is generally a cylindrical disc of the raw material. The less the final shape deviates from such a cylindrical disc, the less material wastage there is in manufacture. Furthermore, curvatures in a meniscus lens are generally more sensitive to the form of the surfaces than a lens having a conventional convex or plano-convex shape. Therefore, in order to achieve a lens having a predetermined performance, a lens design using a deep meniscus form generally requires closer manufacturing tolerances, and is thus more expensive to manufacture, than a lens having conventional convex or plano-convex shape with shallow curvatures.

In the example shown in the sample lens of Fig. 2, the central lens is constructed of germanium, and to save material costs and manufacturing costs, one surface is made flat. The main results of one optimization performed resulted in the following parameters:

Input window: Ge, 3mm thick, with surfaces flat/slightly aspheric (within

(18mm. dia) 35 microns of being flat).

Central element: Ge, 2.5mm. thick, with surfaces flat/53 mm convex with (19mm. dia) optimized aspheric perturbations. Rear window: Si, 0.7mm. thick, with surfaces 58mm concave spherical

(1 Omm. dia) (within 250 microns of being flat) /flat.

The thicknesses cited are the element central thicknesses. A reasonable criterion for the limit of the allowed extent of departure from flatness of the rear window is that the sag should not exceed 1/20 of the element diameter, i.e. 500 microns for a 10 mm diameter window, though it is to be understood that in specific applications where performance may overrule cost considerations, a larger sag may be permissible.

Reference is now made to Fig. 3, which shows an alternative example of an IR imaging camera lens assembly 30, similar to that shown in Fig. 2, but in which the central lens 31 is constructed of zinc selenide. Since such a lens cannot generally provide an acceptable performance with a flat surface, and it is generally necessary to use a spherical/aspherical design, and with a diffractive element having 12 rings turned on the aspherical side. The dimensions of the diffractive rings are too small to be seen on the drawing of Fig. 3. The input window 32 and the detector protection cover 35 each have one surface formed to provide some refractive effect in the lens assembly. This lens assembly was designed to provide the optimum performance for an f/1.1 lens of 18.4 mm effective focal length, with a 30 degree field of view.

The outer surface 33 of the input window 32 may be flat, while its inner surface 34 may be almost flat, but is shaped to add a small correction to the entering wavefront so that the desired MTF of the lens assembly is maintained over a larger field of view than would be obtained from an identical lens with a flat/flat entrance window. This window may have a hard coating on the outer surface 33, such as a diamond-like, hard carbon coating in order to provide good protection from the rigors of the environment in which it is to operate.

The detector array protection window 35 preferably has a flat surface 36 facing the detector array 37, and its outer surface 38 spherically concave, to provide field curvature correction to the wavefront exiting the central lens. This window 35 thus has the double function of acting as the front protective window of the detector array 37, and yet also contributing some refractive functionality to the lens assembly, providing an additional refractive surface for use in optimizing the complete lens assembly. The main results of one optimization of the embodiment of Fig. 3 are shown as follows:

Input window: Ge, 3mm thick, with surfaces flat/slightly aspheric

(18mm. dia) (within 35 microns of being flat).

Central element: ZnSe, 3.8mm. thick, with aspheric surfaces convex 49mm/ (19mm. dia) 45 mm and with a binary diffractive element machined on the second surface.

Rear window: Si, 0.7mm. thick, with surfaces 37mm concave spherical/flat.

(10mm. dia) (within 350 microns of being flat)

The thicknesses cited are the element central thicknesses. A useful criterion for the allowed extent of departure from flatness of the rear window is that the sag should not exceed approximately 1/15 of the element diameter, i.e. approximately 700 microns for a 10 mm diameter window, though it is to be understood that in . specific applications where performance may overrule cost considerations, a larger sag may be permissible.

Reference is now made to Fig. 4 which illustrates an IR imaging camera lens assembly 40 constructed according to one of the exemplary designs described hereinabove, and including a thermal expansion collar 42 adapted to compensate for the changes in refractive index of the main lens 44 with change in <„ temperature of the assembly. The lens is kept in contact with the collar by means of spring loading 46, as is known in the art. The use in the exemplary lens assemblies described herewithin, of a single element 44 to perform the majority of the refractive effort simplifies the design of the thermal compensation collar, as compared with prior art lens assemblies which have two lenses to perform the refractive work.

It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.

Claims

CLAIMS We claim:

1. An infra red imaging camera, comprising: an input window having a flat surface and a non-flat surface; a detector array having a protective window with a concave surface and a flat surface; and a single lens disposed between said input window and said protective window, said lens having at least one aspheric surface.

2. An infra-red camera according to claim 1 wherein said non-flat surface of said input window, said lens surfaces, and said concave surface of said protective window are optimized to provide a predetermined optical performance for said camera.

3. An infra-red camera according to either of claims 1 and 2 wherein said non- flat surface of said input window is selected to correct residual aberration in the combination of said lens and said protective window.

4. An infra-red camera according to claim 3 wherein said non-flat surface of said input window is designed to increase the field of view over which the desired MTF of the lens assembly is maintained, in comparison with a similar camera with a flat/flat input window.

5. An infra-red camera according to any of the previous claims, wherein said non-flat surface of said input window is constrained to have a curvature such that wastage of material in production of said input window is minimized.

6. An infra-red camera according to any of the previous claims, wherein the departure from flatness of said non-flat surface of said input window does not exceed 1/200 of the diameter of said input window.

7. An infra-red camera according to any of the previous claims, wherein the departure from flatness of said concave surface of said protective window does not exceed 1/15 of the diameter of said protective window.

8. An infra-red camera according to any of the previous claims, wherein said non-flat surface of said input window is aspheric.

9. An infra-red camera according to any of the previous claims, wherein said concave surface of said protective window is spherical.

10. An infra-red camera according to claim 9, wherein said protective window is adapted to operate as a field curvature corrector plate, adapting the curved output field of said single lens to the flat detector array plane.

11. An infra-red camera according to any of the previous claims, wherein said lens surfaces are constrained to have curvatures such that wastage of material in production of said lens is minimized.

12. An infra-red camera according to claim 11 wherein said single lens does not have a meniscus lens form.

13. An infra red imaging camera according to any of the previous claims, wherein said single lens is constructed of germanium.

14. An infra red imaging camera according to any of previous claims 1 to 12, wherein said single lens is constructed of zinc selenide.

15. An infra red imaging camera comprising: an input window having at least one flat surface; a detector array having a protective window with at least one flat surface; and a single lens disposed between said input window and said protective window, wherein the refractive power of said camera is vested in no more than four surfaces spread over said input window, said lens and said protective window.

16. An infra red imaging camera according to claim 15, wherein the refractive power of said camera is vested in three surfaces spread over said input window, said lens and said protective window.

17. A method of reducing the number of elements in an infra red camera, comprising: providing an input window; providing a detector array having a protective window; and disposing a single lens between said input window and said protective window, said lens having at least one aspheric surface, wherein said input window has a flat surface and a non-flat surface and said protective window has a concave surface and a flat surface.

18. A method according to claim 17, further comprising the step of optimizing said non-flat surface of said input window, said lens surfaces, and said concave surface of said protective window to provide a predetermined optical performance £ for said camera.

19. A method according to claim 17, further comprising the step of selecting said non-flat surface of said input window to correct residual aberration in the combination of said lens and said protective window.

20. A method according to claim 17, further comprising the step of selecting said non-flat surface of said input window to increase the field of view over which the desired MTF of the lens assembly is maintained, in comparison with a similar camera with a flat/flat input window.

21. A method according to any of claims 17 to 20, wherein said non-flat surface of said input window is constrained to have a curvature such that wastage of material in production of said input window is minimized.

22. A method according to any of claims 17 to 21 , wherein the departure from flatness of said non-flat surface of said input window does not exceed 1/200 of the diameter of said input window.

23. A method according to any of claims 17 to 22, wherein the departure from flatness of said concave surface of said protective window does not exceed 1/15 of the diameter of said protective window.

24. A method according to any of claims 17 to 23, wherein said non-flat surface of said input window is aspheric.

25. A method according to any of claims 17 to 24, wherein said concave surface of said protective window is spherical.

26. A method according to claim 25, wherein said protective window is adapted to operate as a field curvature corrector plate, adapting the curved output field of said single lens to the flat detector array plane.

27. A method according to any of claims 17 to 26, wherein said lens surfaces are constrained to have curvatures such that wastage of material in production of said lens is minimized.

28. A method according to claim 27 wherein said single lens does not have a meniscus lens form.

29. A method according to any of claims 17 to 28, wherein said single lens is constructed of germanium.

30. A method according to any of claims 17 to 28, wherein said single lens is constructed of zinc selenide.

31. A method according to any of claims 17 to 30, wherein the refractive power of said camera is vested in no more than four surfaces spread over said input window, said lens and said protective window.

32. A method according to any of claims 17 to 30, wherein the refractive power of said camera is vested in three surfaces spread over said input window, said lens and said protective window.

33. A method of reducing the number of elements in an infra red camera, comprising: providing an input window having a at least one flat surface; providing a detector array having a protective window with at least one flat surface; and disposing a single lens between said input window and said protective window, wherein the refractive power of said camera is vested in no more than four- surfaces spread over said input window, said lens and said protective window.

34. A method according to claim 33 wherein the refractive power of said camera is vested in three surfaces spread over said input window, said lens and said protective window.