DESC:TECHNICAL FIELD
[0001] The present invention relates to a field of lens module for thermal imagers, more particularly, the present invention relates to a three field of view (FOV) discrete zoom lens for thermal imagers.
BACKGROUND
[0002] A thermal imager is a non-contact type detection device. Conventionally, single field of view (FOV) optics module is used in the thermal imager for capturing an image. Various lenses are known in the art.
[0003] US4469396 discloses about the afocal dual magnification refractor telescope. Eight lens elements of which five lens elements are used in the high magnification mode and eight lens elements are used in the low magnification mode. The high and low magnification lens systems are readily coupled and made interchangeable by utilizing a single rotary mechanism.
[0004] US5229880 discloses the infrared refractive reimaging afocal telescopic system includes an objective lens group and eyepiece lens group. A switching lens group, which is selectively positioned into and out of the radiation beam with respect to the objective lens group, receives energy to change the field of view. Due to switching mechanism, the alignment is critical in both field of views.
[0005] US8294988 discloses an optical system consisting of two lens groups. The optical system further includes a detector disposed behind the second lens group and a mechanism for Switching a configuration of the optical system between a narrow field of view (NFOV) configuration and a wide field of view (WFOV) configuration.
[0006] US8369008 discloses a dual-field imaging system. The optical combination has refractive groups of optics on the optical axis. The imaging system has a cooled detector. Since the refractive groups have lenses, at least three different materials including CaF2 are used for the lenses of the front group.
[0007] However, prior dual/triple FOV systems as described above have been utilized in whole or part. These dual/triple FOV systems generally have one or more of the following disadvantages:
a) The systems are composed of a prohibitively large number of optical lens elements such that the overall transmission is significantly lowered.
b) The lens material is composed of Znse/Zns, CaF2, and AMTIR. The manufacturing of lenses with these materials is critical and these materials are not abundantly available.
c) The systems often do not allow a sufficiently high magnification in the narrow field of view.
d) The systems may allow only two of the three magnification field of view modes.
e) The systems are often of undesirably large track length and size.
f) There is an incomplete correction of optical aberrations, most notably chromatic and spherical Aberrations.
g) The multiple fields of views may be achieved using the either flip in out mechanism or rotating drum mechanisms.
[0008] Hence, there is a need for a three FOV Thermal Imaging system that alleviates the aforementioned drawbacks of the conventional systems and which is compact, lightweight, with abundant available materials, fixed front lens and with high zoom ratio.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
[0009] The detailed description is described with reference to the accompanying figures. The same numbers are used throughout the drawings to reference like features and modules.
[0010] Figure 1 illustrates an isometric view of a Three Field of View (FOV) discrete zoom lens module of the present invention.
[0011] Figure 2 illustrates a schematic view of the Three Field of View (FOV) discrete zoom lens of the present invention.
[0012] Figures 3(a), 3(b) and 3(c) illustrate schematic views of the FOV lens of the present invention in the Narrow FOV, Middle FOV and Wide FOV with the MTF performance respectively.
[0013] Figures 4(a), 4(b) and 4(c) illustrate spot diagrams at Narrow FOV, Middle FOV and Wide FOV respectively.
[0014] Figures 5(a), 5(b) and 5(c) illustrate graphical representations of the energy concentration at detector pixel element at Narrow FOV, Middle FOV, and Wide FOV respectively.
[0015] Figure 6 illustrates a schematic view of FOV lens of the present invention.
[0016] Figure 7 illustrates a schematic view of a graphical user interface (GUI) of the FOV lens of the present invention.
[0017] It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative methods embodying the principles of the present invention. Similarly, it will be appreciated that any flow charts, flow diagrams, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.

OBJECTIVE OF THE INVENTION
[0018] The main objective of the present invention is to provide a three field of view (FOV) discrete zoom lens for thermal imagers which can be used for surveillance applications.
SUMMARY OF THE INVENTION

[0019] An aspect of the present invention is to address the above-mentioned problems and/or disadvantages and to provide at least the advantages described below.
[0020] Accordingly, the present invention relates to a three Field of View (FOV) discrete zoom lens for thermal imagers, the three field of view discrete zoom lens comprising: a front fixed lens configured to correct spherical and chromatic aberration in an image of a target, wherein the front fixed lens is an aspherical lens comprising diffractive optical surface towards a detector, a variator group configured to change field of view by axially shifting the variator group, wherein the variator group comprises a concave aspherical lens, a compensator group configured to restore an image shift into a fixed position in all three field of view by moving the compensator group with respect to the variator group, wherein the compensator group comprises a convex aspherical lens, a focusing lens group configured to maintain the image in focus with respect to a target range by axially moving the focusing lens group, wherein the focusing lens group comprises a convex DOE lens, two cam barrels include: a FOV barrel and a focus barrel and are configured to move the variator group, the compensator group and the focusing lens group to obtain a specified field of view and sharp focus in the image and a lens control card configured to control a FOV motor and a focus motor in order to rotate the two cam barrels simultaneously to move the variator group, the compensator group and the focusing lens group into the specified field of view and to adjust the focus with respect to the target range on receiving inputs from a user.
[0021] Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.
DETAILED DESCRIPTION
[0022] In the following description, for purpose of explanation, specific details are set forth in order to provide an understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these details. One skilled in the art will recognize that embodiments of the present invention, some of which are described below, may be incorporated into a number of systems.
[0023] However, the systems and methods are not limited to the specific embodiments described herein. Further, structures and devices shown in the figures are illustrative of exemplary embodiments of the present invention and are meant to avoid obscuring of the present invention.
[0024] It should be noted that the description merely illustrates the principles of the present invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present invention. Furthermore, all examples recited herein are principally intended expressly to be only for explanatory purposes to help the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.
[0025] The present invention relates to a three field of view (FOV) discrete zoom lens for thermal imagers. The FOV lens of the present invention is now described with reference to accompanying figures 1 to 7.
[0026] Three field of view objective assembly for thermal imager is a complex, lightweight and compact. This is the lightest lens in its category. The system features a novel, optimum geometry carousel, which provides three FOVs with a dual CAM mechanism. The design is modular and flexible, offering zoom ratio up to 7.5X between the widest and narrowest field. Hybrid optical elements assist in achieving performance with minimum element count, and a variant of the design featuring Germanium and Silicon materials extended temperatures gives good performance and active athermalisation for specific applications.
[0027] In these three FOVs, the wide field of view (WFOV) provides wider searching area for possible targets of interest, the narrow field of view (NFOV) is used for giving the specified target ranges and third additional FOV (Super NFOV) is used to enhance target ranges.
[0028] The three-field system solution evolved from a series of key design aims. Constraints on weight, space volume and balance are the most critical in stabilized configurations, demanding a lightweight, compact system, realization of which is facilitated by a novel lens carousel geometry. Configuration of the design for different sets of fields must be possible with minimum change to structure, and especially, to mechanisms. Flexibility and modularity are also key features of the design. Hybrid optical elements are featured to simplify the design, reduce weight, and facilitate assembly.
[0029] The design of lens includes two major aspects, viz., optical design and mechanical design.
[0030] Broadly, the optical design is divided into three phases, viz., initial phase, aberration balancing phase and final phase.
[0031] In the initial phase, it is to be decided which zooming form (like optically compensated zoom or mechanically compensated zoom) should be adapted and calculated the moving distance of variable group and compensation group in the discrete zooming process. In this phase the focal length range, aperture size and track length and number of lenses are fixed.
[0032] The Aberration balancing Phase is now elaborated. Unlike lenses with a fixed focal length where only aberrations caused by focusing have to be stabilized, for zoom lenses the variation of aberrations due to zooming has also to be stabilized. Since during zooming every group encounters an aperture change or a conjugate change or both changes at the same time, it is obvious to expect that aberrations are variable with zooming.
[0033] There are three important conditions to be fulfilled in order to stabilize aberrations successfully. The first condition implies that the aberration contributions from all groups have to be small. The second condition is that the increasing aberrations from different groups have to have opposite signs and compensate each other, at least in part. The third condition is to add the means of correction to selected groups in order to adapt their aberration contribution to reduce aberration residuals. The most effective means which can be added to a lens group is aspheric and diffractive optical elements.
[0034] There are a limited number of lens materials available in the MWIR spectral bands, so to balance chromatic aberration, the silicon material is adapted as positive focal lens because it has high index and low dispersion. The germanium, which has a larger dispersive factor than other lenses in the MWIR band, it is adapted as negative lens because it benefits to correct for axial colour in the narrow field of view (NFOV).
[0035] Advanced Diffractive Optical Element surfaces are used to correct the lateral colour in the NFOV and aspherics is adapted to balancing coma and off-axis aberrations in all FOVs. These DOE and aspherical lenses are used to reduce the number of lenses which will result the system to get the more transmission, less alignment errors, compact in size and weight. All above mentioned parameters are taken into account to ensure the system has good imaging quality.
[0036] The final phase is now elaborated. According to the relative motion of variable group and compensation group, using the interpolation method to select three focal length positions in the system, and optimizing until each focal length has good image quality. On the basis of the data obtained by the software MATLAB, the discrete CAM curves are generated.
[0037] Mechanical design: In discrete zoom lenses, the mechanical design comprises very complicated internal construction. There are quite a number of both large and small parts. Magnification is achieved by movement of the variator optics group and compensator optics group relatively as per the optical design paths. Since the zooming and focusing are to be achieved by pressing button within very little time and image has to stay focused at any of the discrete FOV positions, these stringent requirements need optical elements to be located within a tolerance of 30µm.
[0038] Improperly machined retaining ring or spacer can cause tilt and decentre of optical components which results in performance degradation.
[0039] The three field of view (FOV) discrete zoom mechanism of the present invention not only moves the various optical groups with the correct rates of movement, but also precisely maintains optical alignment. These features are incorporated while design stage by considering the tolerance stack-up analysis.
[0040] One critical challenge in designing of a discrete zoom lens is to cater for strain free holding of lens elements across wide temperature zone i.e. -30° C to +55° C. The lenses do not get expanded proportion to mechanical elements. This will be accommodated with using RTV epoxy, which has an appropriately large CTE as well as a low elastic modulus.
[0041] Locating lens elements as per the designed optical spacing is achieved by helical grooves machined on cam barrels. Movement of the cam barrels is created by means of two different DC motors, for getting desired FOV position and sharp Focus. These motors coupled with precision gear head and incremental encoder for catering torque requirements of the optics load and electronically calibrating cam position with respect to encoders. Various magnifications are achieved by placing lens elements with encoder readout from motors. These encoder values and cam profiles are calibrated at factory for getting designed magnifications and sharp image at position across the temperature range.
[0042] Three Field of View (FOV) discrete zoom lens for thermal Imagers has been configured using Military grade components to meet the technical requirements of the surveillance applications.
[0043] Three Field of View (FOV) discrete zoom lens for thermal imagers has been designed by considering tolerances in component, assembly and system level for optical and mechanical components so that the Three Field of View (FOV) discrete zoom lens is not only producible but also easily repairable and maintainable.
[0044] A three field of view (FOV) discrete zoom lens 100 of the present invention comprises a front fixed lens 110, a variator group 120, a compensator group 130, a focusing lens group 140, cam barrels 150, limit sensors, a lens control card, and a graphical user interface 160.
[0045] The front fixed lens 110 is used as a singlet of comprising diffractive optical surface towards the detector. The diffractive optical surface is used to reduce spherical aberration and lateral colour. The front objective is an aspherical singlet bent in an optimum shape to reduce the spherical aberration. This surface has DOE properties which are used for colour-correction. By placing the DOE surface here, the axial-colour correction in both the narrow and the wide field-of-view will be corrected in all FOV modes simultaneously. The front fixed lens 110 is configured to correct spherical and chromatic aberration in the image of the target. Further, the front fixed lens 110 is aspherical lens.
[0046] The variator group 120 comprises one concave aspherical lens. The magnification of the zoom system is changed by axially shifting a component, called the variator, but this change in magnification is always accompanied by a blurring of image due to shift of the image plane. The main optical power of this group comes from the Germanium element and the aspherical surface is used here to correct the coma and off-axis aberrations in all FOV’s simultaneously. The variator group 120 is configured to change field of view (magnification) by axially shifting the variator group 120.
[0047] The compensator group 130 is used to restore the image shift into a fixed position in all zoom conditions (all three field of view). It comprises one convex aspherical lens. More specifically, the final image is restored to the original image plane by moving the compensator. This restoration of the final image usually demands a nonlinear movement of the compensator 130 with respect to the variator 120.
[0048] The focusing lens group 140 comprises one convex DOE lens. The image always is in focus with respect to a target range by axially moving the focusing lens group 140. The focusing lens group 140 can also be termed as re-imaging group. A re-imaging design is chosen in such a way to minimize the objective lens diameter while maintaining 100% cold shield efficiency. The re-imager is also useful for focus adjustment. The focus lens also gives the allowance for thermal expansion and misalignment. The focusing lens group 140 has the X-Y movement across the optical axis for the inter field deviation (IFD) adjustment. With this feature, the inter field deviation can be easily achieved and caters tolerances in manufacturing of mechanical components.
[0049] In an embodiment, the variator group 120, the compensator group 130 and the focus group 140 are chosen to get the High MTF performance which is equal to the diffraction limited MTF value. Further, the variator group 120, the compensator group 130 and the focus group 140 are chosen to get the Encircled energy maximum value i.e., up to 80%, which is equal to the diffraction limited value.
[0050] The cam barrels 150 (FOV barrel and Focus barrel) are used to move the variator, compensator and focusing groups into specified zoom positions. Zoom (Magnification) is achieved by movement of the variator optics group 120 and compensator optics group 130 relatively as per the optical design paths. Since the zooming and focusing are to be achieved by press of a button with in very little time and image has to stay focused at any of the positions, these stringent requirements need optical elements to be located within a tolerance of 30µm at any magnification level. These placements of the optical elements are catered by rotating two cam barrels 150 simultaneously, thus making the design of barrels very critical. The two cam barrels 150 are configured to move the variator group 120, the compensator group 130 and the focusing lens group 140 to obtain specified field of view and sharp focus in the image. The specified field of view is a desired field of view including the wide field of view or the narrow field of view or the super narrow field of view. Further, the FOV barrel comprises only two cams, a first cam belongs to the variator group 120, and a second cam belongs to the compensator group 130.
[0051] The limit sensors are mounted at the extreme Cam positions. In an embodiment, optical limit sensors are used. For this zoom lens, four set of limit sensors are used. These limit sensors are used to easily identify the issues in the zoom assembly during BIT test. The limit sensors are used to easily diagnose the problem of the objective assembly, monitoring the cam position.
[0052] The lens control card comprises four major blocks. i.e., Power supplies block, Memory block, Communication interface Block and control processor block. The power supply block is used to supply required power to the motors. The Memory block memory is an on-board EPROM which is used for storing auto focus values for complete zoom range, communication block and control processor block receives external commands or inputs from a user depending on the user input for focus or FOV change. With the predefined commands the lens control card is used to move the variator, compensator and focusing groups into the specified zoom positions and also used to adjust the focus with respect to range by pressing respective buttons in the Thermal Imager. The Lens control card is developed for control of Field Of View (FOV) motor and focus motors.
[0053] In an embodiment, the lens control card is configured to control the FOV motor and the focus motor in order to rotate the two cam barrels 150 simultaneously to move the variator group 120, the compensator group 130 and the focusing lens group 140 into the specified field of view and to adjust the focus with respect to the target range on receiving inputs from the user.
[0054] In an embodiment, the three Field of View (FOV) discrete zoom lens 100 comprises limit sensors configured to identify issues in the three field of view discrete zoom lens 100 by monitoring position of the two cams.
[0055] In the present invention, the field of view of the wide field of view is 13.6°, the field of view of the narrow field of view is 4.0° and the field of view of the super narrow field of view is 1.83°.
[0056] In an embodiment, the three field of view discrete zoom lens 100 is configured to provide all the three field of view and to maintain the sharp focus across a wide temperature range of -30° C to +55° C.
[0057] The GUI 160 is developed to evaluate the basic lens parameters such as FOV and focus. A screenshot of GUI 160 is shown in figure 7. The GUI 160 is able to read the FOV and Focus motors encoder values in any stage. It is easy to synchronize the concern thermal imager with the other electro optical equipment like day camera and SWIR camera. The main features of the GUI 160 are as following:
a) The BIT test is used to ensure all the FOV and FOC motors are properly working or not.
b) WFOV, NFOV and SNFOV buttons are used to fix the encoder values as per the designed FOVs.
c) Focus+ and Focus- buttons are used to fix the focus encoder values at the three FOVs.
[0058] In an embodiment, the Graphical User Interface, GUI 160 is configured to: read encoder values of the FOV motor and the focus motor, fix the encoder values of the FOV motor as per the wide field of view or the narrow field of view or the super narrow field of view on receiving inputs from the user and fix the encoder values of the focus motor in all the three field of view on receiving inputs from the user.
[0059] In an embodiment, the three FOV zoom lens comprises only four lenses including the front fixed lens (110), the variator group (120), the compensator group (130) and the focusing lens group (140).
[0060] In the lens 100, Germanium and silicon lenses are used. Since these materials are abundantly available, manufacturing of lenses using Germanium and Silicon lenses is simple when compared with materials like ZnSe, Zns, AMTIR and CaF2 used in conventional lenses.
[0061] In an embodiment, the apsherical lens of the front fixed lens 110 is manufactured using silicon, the concave aspherical lens of the variator group 120 is manufactured using germanium, the convex apsherical lens of the compensator group 130 is manufactured using silicon and the convex DOE lens of the focusing lens group 140 is manufactured using germanium.
[0062] The design parameters of the FOV lens system are now elaborated. The boundaries on the design are defined by a 1.8° horizontal field, limited to be the smallest field required to satisfy intended missions. At the other extreme, a field of 13.6° is selected, with flexibility of choice of the three fields within these limits subject to a 7.5:1 ratio overall. A maximum aperture of 50 mm is maintained for satisfying the sensitivity requirements.
S. No Parameter Specification
01 Wavelength range 3.7 µm - 4.8 µm
02 Detector size 320 x256 pixels
03 Pixel pitch 20 microns
04 WFOV 13.6° x 11.0°(Typical)
05 NFOV 4.0° x 3.21° (Typical)
06 SNFOV 1.8° x1.5° (Typical)
07 F–number 4.0
09 Cold stop distance 16mm
10 Cold stop efficiency 100%
12 Number of lenses 4

OTable 1 Optical design parameters
[0063] Four-component mechanically compensated zoom lenses are adapted to achieve the design parameters. Figure 1 shows Three Field of View (FOV) discrete zoom lens module 100 of the present invention. A mechanically compensated zoom lens maintains a sharp image over a fixed image plane during the discreetly variation of magnification in image over the zoom range. Figure 2 shows the schematic diagram of the discrete zoom imaging system in MWIR spectral region.
Modulation Transfer Function (MTF)
The maximum spatial frequency of an image, which can effectively be resolved by a pixelated sensor without aliasing is Nyquist frequency. The Nyquist frequency (Clp) can be calculated by the following equation (1).
Clp = ½ a pix Where a pix is the size of pixel………….Equation (1)
The optical MTF is an important image quality evaluation means for optical system. As the pixel size is 20µm×20µm, so the system Nyquist frequency is 33 lp/mm. Figure 3(a), Figure 3(b) and Figure 3(c) respectively show the schematic diagram of the lens 100 in the Narrow FOV, Middle FOV, and Wide FOV with the MTF performance. From the MTF curves it can be seen that the optical system has high image quality at the Nyquist frequency and it is closely equal to the diffraction limited value.
Spot diagram
Spot diagrams are the geometrical image blur formed by the lens when imaging a point object. Figure 4(a), Figure 4(b) and Figure 4(c) respectively show the diameters of the diffraction-blur spot matching the detector pixels dimensions i.e. 20 microns in the Narrow FOV, Middle FOV and Wide FOV which indicate that the lens 100 has well corrected aberrations and fine imaging quality.
Encircled energy
Geometric encircled energy can calculate the minimum radius contain energy ratio and also calculate the encircled energy in each field of the radial energy distribution. Figure 5(a), 5(b) and 5(c) respectively show the energy concentration ratio greater than 80% within the sensing element of detector.
[0064] The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the invention.

,CLAIMS:
1. A three Field of View (FOV) discrete zoom lens (100) for thermal imagers, the three field of view discrete zoom lens (100) comprising:
a front fixed lens (110) configured to correct spherical and chromatic aberration in an image of a target, wherein the front fixed lens (110) is an aspherical lens comprising diffractive optical surface towards a detector;
a variator group (120) configured to change field of view by axially shifting the variator group (120), wherein the variator group (120) comprises a concave aspherical lens;
a compensator group (130) configured to restore an image shift into a fixed position in all three field of view by moving the compensator group (130) with respect to the variator group (120), wherein the compensator group (130) comprises a convex aspherical lens;
a focusing lens group (140) configured to maintain the image in focus with respect to a target range by axially moving the focusing lens group (140), wherein the focusing lens group (140) comprises a convex DOE lens;
two cam barrels (150) include: a FOV barrel and a focus barrel and are configured to move the variator group (120), the compensator group (130) and the focusing lens group (140) to obtain a specified field of view and sharp focus in the image; and
a lens control card configured to control a FOV motor and a focus motor in order to rotate the two cam barrels (150) simultaneously to move the variator group (120), the compensator group (130) and the focusing lens group (140) into the specified field of view and to adjust the focus with respect to the target range on receiving inputs from a user.
2. The three field of view discrete zoom lens (100) as claimed in claim 1, wherein the three field of view includes a wide field of view, a narrow field of view and a super narrow field of view and the three field of view discrete zoom lens (100) comprises only four lenses including the front fixed lens (110), the variator group (120), the compensator group (130) and the focusing lens group (140).
3. The three field of view discrete zoom lens (100) as claimed in claim 1, wherein the apsherical lens of the front fixed lens (110) is manufactured using silicon, the concave aspherical lens of the variator group (120) is manufactured using germanium, the convex apsherical lens of the compensator group (130) is manufactured using silicon and the convex DOE lens of the focusing lens group (140) is manufactured using germanium.
4. The three field of view discrete zoom lens (100) as claimed in claim 1, wherein the FOV barrel comprises only two cams, a first cam belongs to the variator group (120), and a second cam belongs to the compensator group (130).
5. The three field of view discrete zoom lens (100) as claimed in claim 1, wherein the three Field of View (FOV) discrete zoom lens (100) comprises:
limit sensors configured to identify issues in the three field of view discrete zoom lens (100) by monitoring position of the two cams.
6. The three field of view discrete zoom lens (100) as claimed in claim 1, wherein the three field of view discrete zoom lens (100) comprises:
a Graphical User Interface, GUI (160) configured to: read encoder values of the FOV motor and the focus motor, fix the encoder values of the FOV motor as per the wide field of view or the narrow field of view or the super narrow field of view on receiving inputs from the user and fix the encoder values of the focus motor in all the three field of view on receiving inputs from the user.
7. The three field of view discrete zoom lens (100) as claimed in claim 1, wherein the focusing lens group (140) is given X-Y movement across an optical axis for an inter field deviation (IFD) adjustment to achieve an inter field deviation and to cater tolerances in manufacturing of mechanical components.
8. The three field of view discrete zoom lens (100) as claimed in claim 1, wherein a field of view of the wide field of view is 13.6°, the field of view of the narrow field of view is 4.0° and the field of view of the super narrow field of view is 1.83°.
9. The three field of view discrete zoom lens (100) as claimed in claim 1, wherein the variator group (120), the compensator group (130) and the focusing lens group (140) are configured to provide a high modulation transfer function performance which is equal to a diffraction limited modulation transfer function value.
10. The three field of view discrete zoom lens (100) as claimed in claim 1, wherein the variator group (120), the compensator group (130) and the focusing lens group (140) are configured to provide an encircled energy maximum value which is equal to a diffraction limited value.
11. The three field of view discrete zoom lens (100) as claimed in claim 1, wherein the three field of view discrete zoom lens (100) is configured to provide all the three field of view and to maintain the sharp focus across a wide temperature range of -30° C to +55° C.