Claims:
We claim:
1. A motorized medium wavelength infrared continuous zoom system for capturingthermal images and maintaining focus during continuous changes in field of views, said system comprises:

an image acquisition block, a cam barrel block, and a control block;

saidimage acquisition block acquires thermal images within a field of view, said image acquisition block comprises:

a front fixed lens for reducing spherical abberations;

a variator lens for varying magnification of said system upon zooming by axial movements;

atleast onecompensator for maintaining focus of said front fixed lens upon zooming; and

a focusing lens for maintaining focus on image with respect to a range of zoom positions by axial movements across an optical axis.

said cam barrel block comprises atleast two cam barrels connected to said image acquisition block and configured for moving lensesin said image acquisition block for obtaining desired zoom position;

said control block is coupled to said cam barrel block for controlling said one or more lenses of said image acquisition block through said atleast two cam barrels of said cam barrel block, said control block comprises:

a communication interface for receiving a choice of focus and field of view;

a control processor coupled to said communication interface and configured for producing speed control signals based on said received choice of focus and field of view; and

atleast two motor elements connected to said cam barrel block and configured for moving said atleast two cam barrels based on said speed control signals.

2. The system of claim 1,wherein said first cam barrel block comprises said variator and said atleast one compensator for changing said field of view.

3. The system of claim 1, wherein said second cam barrel block comprises said focusing lens with a diffractive optical element surface which for maintaining focus at multiple ranges of field of view.

4. The system of claim 1, wherein said focusing lens group in the acquisition block moves across said optical axis in X-Y direction for achieving a through zoom bore sight adjustmentand for catering tolerances in manufacturing of mechanical components.

5. The system of claim 1, wherein said image acquisition block comprises six lenses.that are made of Germanium and/or Silicon lenses..

6. The system of claim 1,wherein said image acquisition blockcomprises a track length of about 74mmby the useof said diffractive optical element surface out of 12 optical surfaces (six lenses).

7. The system of claim 1,wherein said diffractive optical element surface of said lens controls reduction in transmittance and aids to make said system compact..
8. The system of claim 1,wherein said cam barrel block is configured for providing smooth cam profiles for reducing burden on said atleast two motor elements.

9. The system of claim 1,wherein said variator, said atleast one compensator, and said focus lens of said image acquisition block provide high modulation transfer function performance thatis equal to diffraction limited modulation transfer function value.

10. The system of claim 1, wherein field of view of said system varies from about 1.96 degrees of field of view to about 27 degrees of field of view.

11. The system of claim 1, wherein said cam barrel block allows continuous changes in field of view.

12. The system of claim 1, wherein said cam barrel block comprises a first cam barrel for moving said variator and said atleast one compensator and a second cam barrel for moving said focusing lens.

13. The system of claim 12, wherein said control block comprises a field of view motor and a focus motor, said field of view motor is coupled to said first cam barrel and said focus motor is coupled to said second cam barrel.

14. The system of claim 1, wherein movement of said focusing lens compensates for image plane shift with respect to temperature variations and provides active athermalization.
Dated this 22nd day of March, 2019
FOR BHARAT ELECTRONICS LIMITED
(By their Agent)

(D. Manoj Kumar)
Patent Agent No. IN/PA-2110
KRISHNA & SAURASTRI ASSOCIATES LLP

, Description:FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(SEE SECTION 10, RULE 13)

MOTORIZED MEDIUM WAVELENGTH INFRARED CONTINUOUS ZOOM SYSTEM

BHARAT ELECTRONICS LIMITED,
OUTER RING ROAD, NAGAVARA, BANGALORE – 560045,
KARNATAKA, INDIA

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

TECHNICAL FIELD

[0001] The present disclosure relates generally to zoom lens that is an assembly of lenses for which the focal length can be varied and particularly relates to an automated zoom lens that maintains focus at infinity during changes in field of views.

BACKGROUND

[0002] The prior art Japanese Patent Laid-Open Nos. 4-14006 ( U.S. Pat.No. 5134524) discloses a zoom lens comprising a first lens unit having positive refractive power, an aperture stop, a second lens unit having negative refractive power , a third lens unit having positive refractive power, and fourth lens unit having positive refractive power, which are arranged in this order from the object side. For zooming, from the wide-angle end to the telephoto end, the first lens unit is moved toward the object side and the second lens unit is moved toward the image side. The fourth lens unit is moved for focusing, while the aperture stop is always kept fixed during the zooming.

[0003] The prior art Japanese patent Laid-Open No. 11-242160 disclosed a zoom lens comprising a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power and a fourth lens unit having positive refractive power, which are arranged in this order from the object side. For zooming from the wide-angle end to the telephoto end, the first and fourth lens units are moved toward the object side and the second lens unit is moved toward the image side. The third lens unit and an aperture stop are always kept fixed during the zooming.

[0004] The prior art U.S. Patent No.4632498 discloses about the IR Zoom lens device which has two independently moving parts to keep the lens device in focus. The zoom lens comprises the ZnSe lenses without employing silicon, which is less desirable than the use of germanium and silicon because of their toxic in nature and difficult to manufacture.

[0005] The prior art U.S.Pat.Nos.6191896 and 6233099 discloses a four-group zoom lens. During zooming, all the lens groups move along the optical axis in such a manner that the distance between every two lens groups is changed.

[0006] Therefore there is a need for a thermal imaging system that is compact, lightweight, with abundant available materials, fixed front lens andprovides continuous changes in field of view from a narrow field of view to the wide field of view and visa-s-versa with high zoom ratio and active thermal compensation.

SUMMARY

[0007] An aspect of the present invention is to address at least one of the above-mentioned problems and/or disadvantages and to provide the advantages described below.

[0008] A motorized medium wavelength infrared continuous zoom system for capturing thermal images and maintaining focus during continuous changes in field of views. The system comprises an image acquisition block, a cam barrel block, and a control block. The image acquisition block acquires thermal images within a field of view. The image acquisition block comprises a front fixed lens for reducing spherical aberrations, a variator lens for varying magnification of the system upon zooming by axial movements, atleast one compensator for retaining focus which was changed due to the variator movement for magnification upon zooming, a focusing lens for maintaining focus on image with respect to a range of zoom positions by axial movements along an optical axis; and a detector for receiving and capturing infrared radiation from the focusing lens produced by objects within the field of view. The cam barrel block comprises atleast two cam barrels connected to the image acquisition block and comprises the variator lens, the atleast one compensator configured for moving lenses in the image acquisition block for obtaining desired zoom position. The control block is coupled to the cam barrel block for controlling the one or more lenses of the image acquisition block through the cam barrels of the cam barrel block. The control block comprises a communication interface for receiving a choice of focus and field of view, a control processor coupled to the communication interface and configured for producing speed control signals based on the received choice of focus and field of view, and atleast two motor elements connected to the cam barrel block and configured for moving the cam barrels based on the speed control signals.

[0009] 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

BRIEF DESCRIPTION OF DRAWINGS

[0010] The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:

[0011] FIG. 1 exemplarily illustrates a block diagram of a medium wavelength infrared continuous zoom system.

[0012] FIG. 2exemplarily illustrates a system diagram of the medium wavelength infrared continuous zoom system

[0013] FIG. 3exemplarily illustrate zoom lens optical layout.

[0014] FIGS. 4A - 4B exemplarily illustrate modulation transfer function curve at narrow field of view and wide field of view respectively.

[0015] FIGS. 5A - 5B exemplarily illustrate spot diagrams at narrow field of view and wide field of view respectively.

[0016] FIG. 6 exemplarily illustrates cam profile of the zoom lens.

[0017] Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION OF DRAWINGS

[0018] The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

[0019] The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

[0020] It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces.

[0021] By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic is intended to provide.

[0022] FIGS. 1 through 6, discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way that would limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged communications system. The terms used to describe various embodiments are exemplary. It should be understood that these are provided to merely aid the understanding of the description, and that their use and definitions, in no way limit the scope of the invention. Terms first, second, and the like are used to differentiate between objects having the same terminology and are in no way intended to represent a chronological order, unless where explicitly stated otherwise. A set is defined as a non-empty set including at least one element.

[0023] Those skilled in this technology can make various alterations and modifications without departing from the scope and spirit of the invention. Therefore, the scope of the invention shall be defined and protected by the following claims and their equivalents.

[0024] FIGS. 1-6 are merely representational and are not drawn to scale. Certain portions thereof may be exaggerated, while others may be minimized. FIGS. 1-6 illustrate various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art.

[0025] In the foregoing detailed description of embodiments of the invention, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the invention require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the detailed description of embodiments of the invention, with each claim standing on its own as a separate embodiment.

[0026] FIG. 1 exemplarily illustrates a block diagram of a medium wavelength infrared (MWIR) continuous zoom system 100 for providing continuous change in field of view. FIG. 2 exemplarily illustrates a system diagram of the medium wavelength infrared continuous zoom system 100. The motorized MWIR continuous zoom system 100 henceforth shall be referred to as continuous zoom system 100. The continuous zoom system 100 comprises an image acquisition block 101, a cam barrel block 108, and a control block 111. The image acquisition block 101 comprises six lenses, that is twelve surfaces, as shown in FIG. 3. The image acquisition block 101 acquires thermal images within a field of view (FOV). The image acquisition block 101 comprises a front fixed lens 102 for reducing spherical aberrations, a variator lens 103 for varying magnification of the continuous zoom system 100 upon zooming by axial movements. The image acquisition block 101 further comprises atleast one compensator that is a first compensator 104 and a second compensator 105 for retaining focus which was changed due to the variator lens 103 movement upon zooming. The image acquisition block 101 further comprises a focusing lens 106 for maintaining focus on image with respect to the range by axial movements along an optical axis. A detector 107, present outside the system 100 receives and captures infrared radiation from the focusing lens produced by objects within the field of view. The detector 107 is f/5.5 cooled detector. The continuous zoom system 100 comprises some of the lenses are made of silicon that may translate to a reduction in cost.

[0027] The cam barrel block 108 comprises atleast two cam barrels that is a first cam barrel 109 and a second cam barrel 110. The first cam barrel 109 and the second cam barrel 110are connected to the image acquisition block101 and configured for moving lenses in the image acquisition block 101 for obtaining a desired zoom position. The control block 111 is coupled to the cam barrel block 108 for controlling the lenses of the image acquisition block 101 through the cam barrels 109 and 110 of the cam barrel block 108. The control block 111 comprises a communication interface 115 for receiving a choice of focus and field of view. The control block 111 further comprises a control processor 116 coupled to the communication interface115 and configured for producing speed control signals based on the received choice of focus and field of view. The control block 111 further comprises atleast two motors 112 and 113 connected to the cam barrel block 108 and configured for moving the two cam barrels 109 and 110 based on the speed control signals.

[0028] The first cam barrel 109 comprises the variator lens103, the first compensator 104, and the second compensator 105. The first cam barrel 109is employed for moving the variator lens 103, the first compensator 104, the second compensator 105into a desired FOV position, and the second cam barrel 110 is employed for moving the focussing lens106 into a desired focus position. The first cam barrel 109, also called as field of view (FOV) barrel, is employed for movingthe variator lens 103, the first compensator 104, and the second compensator 105.The movements of the variator lens 103 and compensators104 and 105are obtained by an outer revolution of the first cam barrel 109 by turning around body of the cam barrel block108along bearings of surface tracks. The second cam barrel 110 also called as the focus barrel moves the focusing lens 106. The first cam barrel 109 and the second cam barrel 110 have two helicoidally machined ramps. The ramps receive two drive-rollers respectively. The mechanical play of the drive rollers in the ramps is accomplished by using springs. Manufacturing of the cam barrels 109 and 110 affect in achieving the critical design parameters like field of view (FOV) and through zoom bore sight. The cam barrels 109 and 110 require critical dimensional stability and higher surface finish for achieving smooth discrete zooming movement.

[0029] The first cam barrel 109 moves the variator lens 103and the compensator 104 and 105relatively as per the optical design paths for achieving magnification, that is, zoom. The placement of optical elements such as the variator lens 103, the first compensator 104, the second compensator 105, and the focusing lens 106 are brought about by rotating the two cam barrels 109and 110 simultaneously. The function of zooming and focusing are to be achieved in a short duration and the image has to be focused at any of the positions and these functions are stringent requirements that are to be taken care of. The stringent requirements need the optical elements to be located within a tolerance of 30µm at any magnification level. The placement of the optical elements, that are the lenses, within the tolerance are catered by rotating both the cam barrels 109 and 110 simultaneously. For zooming from the wide-angle end to the telephoto end, the variator lens 103 is moved toward the image side, the first compensator 104 is moved toward the object side and the second compensator 105 is moved towards to the image side. The focusing lens 106 is moved axially for focusing.

[0030] The control block 111controlsthe variator lens 103, the compensators 104 and 105, the focusing lens 106, and the two cam barrels 109 and 110 based on a choice of focus and field of view received by the control block 111. The control block 111 comprises power supply unit 114, communication interface unit115, control processor unit 116 and memory unit 117. The power supply unit 114 supplies required voltages to the focus motor 113 and field of view (FOV) motor 112. The control block 111 further comprises a RS422 serial communication in communication interface unit 115. The RS422 serial communication interface 115 receives external commands or inputs from a user depending on the user input for focus or FOV change, for example, a FPGA board sends respective commands to a control PCB via a power supply board assembly 114. The field of view (FOV) motor 112 and focus motor 113 are connected to the control block 111. Based on the received commands the control block 111 moves the FOV motor 112and the focus motor113to appropriate positions. The control assembly further comprises a memory unit117on board EPROM, for storing auto focus values for complete zoom range. Controlling the speed of cam barrels 109 and 110 for moving the cam barrels 109 and 110 at different positions for focusing and zooming may be performed by a set of program codes stored in the memory.

[0031] The zoom ratio of the continuous zoom system 100is about 14X. The continuous zoom system 100 provides continuous changes in the field of view from the narrow field of view, that is about 1.96°, to the wide field of view, that is about 27°. The continuous zoom system 100 provides zoom ratio of about 14X for the f/5.5 cooled detector 107, image quality closer to an ideal image, active thermal compensation, short zoom path and smooth zoom focus. The continuous zoom system 100 is compact and lightweight. The continuous zoom system 100 maintains focus at infinity in all field of view (FOV) positions and changes the FOV with retention of focus at infinity. For any surveillance application, area of surveillance should be as large as possible and simultaneously the continuous zoom system 100 obtains details of the interested object. For such requirements, optical modules with variable focal length with large area coverage and to get the details of the interested object at longer ranges are essential. A discrete FOV system may be used for the above mentioned requirement; however a disadvantage of the discrete FOV system is that the surveillance may be performed only at specified values of FOV. The continuous zoom system 100 overcomes the above disadvantage by providing flexibility to a user to stop at any FOV to get the target or the object details.

[0032] Size, weight and power are critical requirements in all defence applications. The continuous zoom system 100 comprises telephoto configuration for obtaining maximum performance within minimal space. In the telephoto configuration, the length of the continuous zoom system 100 is reduced by employing the second lens, that is the variator lens 103, as negative lens.

[0033] Most of the infrared optical materials, such as germanium, are sensitive to temperature variations. The sensitivity is reflected in a temperature dependent refractive index, which in turn affects the focal length and results in deterioration of the image quality with temperature variations. To minimize the deterioration in image quality, the focusing lens 106 is designed with a positive power. Also, the residual degradation of the image quality due to temperature variation is corrected automatically by adjusting the position of the focusing lens 106 as per a look up table. The continuous zoom system 100 may be used in space applications, military applications, weapon systems, etc. The continuous zoom system 100 has a high degree producibility with image performance closer to ideal even at corners. The design of control block 111 ensures that smooth change of field of view and focus at infinity at all zoom positions. The continuous zoom system 100 works on an entire temperature range and seamless user interface. The continuous zoom system 100 provides high zoom ratio (14X) for detectors 107 such as f/5.5 cooled detectors.

[0034] FIG. 3 exemplarily illustrates zoom lens optical layout, that is, the arrangement of lenses in the image acquisition block 101. The lenses in the image acquisition block 101 are made of germanium(Ge) and silicon (Si) and manufacturing of lenses using Ge and Si material is simple when compared with materials like ZnSe and Zns. The front fixed lens 102 is a single lens, for example is a IP 67 sealed front lens, the variator lens 103 is single lens. The first compensator 104 comprises a single lens, the second compensator 105 comprises two lenses, and the focusing lens 106 is a single convex lens. A surface of the focusing lens 106 comprises a diffractive optical element (DOE) to reduce the lateral color and maintaining an allowable track length and the usage of one DOE surface also aids in achieving compactness. The track length of the continuous zoom system 100 is about 74 millimetres that is achieved by using one DOE surface out of twelve surfaces. In an embodiment, the track length of the continuous zoom system 100 may be less than 74 millimetres. The limits of distance between the variator lens 103 and the compensators 104 and 105 is determined to ensure compactness of the continuous zoom system 100. The limits of the distance were determined to ensure an acceptable image quality. The first compensator 104 is a convex aspherical lens and the second compensator 105 comprises two meniscus aspherical lenses. The image acquisition block 101comprises a track length of about 74mm by the use of said diffractive optical element surface out of 12 optical surfaces of six lenses.

[0035] The movement of the focusing lens 106, that is axial movement, compensates for image plane shift with respect to temperature variations and provides active athermalization. A zoom bore-sight achieved by minor adjustment of lens for ease of production. The focusing lens 106 can be moved in a X-Y direction across the optical axis for through zoom bore sight adjustment with respect to the detector. The movement of the focusing lens 106 aids in achieving the through zoom bore sight adjustment and also caters tolerances in manufacturing of mechanical components.

[0036] The front fixed lens 102, also called as front objective lens is an aspherical singlet lens that is bent in an optimum shape for reducing spherical aberration. An aspherical lens aids in compacting size of the continuous zoom system 100. A single lens is used as a front fixed lens 102. A problem of smooth sag variation is considered and solved for reducing complexity in production. The variator lens 103 is axially shifted for providing magnification. However, the magnification results in blurring of an image due to shift of image plane. The main power to the variator lens 103 is provided by using germanium as a lens material. An aspherical surface of the continuous zoom system 100 corrects the coma and off-axis aberrations in all fields of views simultaneously. The first compensator 104 and the second compensator 105, are employed for retaining the focus of the which was due to the variator 103 movement. The compensators 104 and 105 are moved for restoring a final image to the original image plane where the movement is non-linear with respect to the variator lens 103 and the compensator has a linear movement. The focusing lens 106 is employed for maintaining focus or focus adjustment on the image with respect to the range by axial movements along an optical axis. The focusing lens 106 is also called as re-imaging lens that minimizes the front fixed lens 102 diameter and maintains cold shield efficiency of about 100 percent. The focusing lens 106 provides allowance for thermal expansion and misalignment. The modulation transfer function of the image acquisition block is about diffraction modulation transfer function value. The image acquisition block uses only one DOE surface to achieve compactness. Normally more than one DOE lens is to be used to achieve compactness as achieved in this case. By using only one DOE lens, reduction in transmittance is controlled without compromising the performance and maintaining the required allowable track length.

[0037] FIGS. 3A-3B exemplarily illustrate modulation transfer function (MTF) curve at narrow field of view (NFOV) and wide field of view (WFOV) respectively. The MTF is a benchmarking parameter that defines the quality of an image obtained by using the respective optics module. The best MTF that can be achieved for the current pixel pitch requirement at 17lp/mm considering the diffraction limitation comes out to a maximum value of about 0.5. The MTF curves for the NFOV and the WFOV positions are shown in FIGS. 3A - 3B. The MTF values achieved with current design is about 0.5 as shown in. As the achieved MTF value is equal to a best achievable value, the image quality obtained may be considered as good. The variator103, the first compensator 104, second compensator 105, and the focusinglens106 of the image acquisition block 101 was chosen to get the high modulation transfer function performance that is equal to diffraction limited modulation transfer function value

[0038] FIGS. 4A - 4B exemplarily illustrate spot diagrams at narrow field of view and wide field of view respectively. The spot diagram, also called as diffraction blur, may also be used for evaluating quality of an image. An ideal image has a diffraction blur that does not extend beyond one pixel and as shown in the FIG. 4A and 4B the diffraction blur is around 15 µ in this case that is closer to the ideal image. From the spot diagram shown in FIG. 4A and 4B for both NFOV and WFOV positions, it can be observed that the diffraction blur is only 13 µ. This shows that the designed system conforms provides an image with qualities closer to the ideal image.

[0039] FIG. 5 exemplarily illustrates cam profile of the zoom lens with the zoom curves that are fitted. The cam profile of the zoom lens is obtained by using an interpolation software tool. Based on the zoom curves shown in the FIG. 6, the variator lens has a nonlinear movement, the first compensator 104 has a linear movement and the second compensator 105 has a nonlinear movement, thereby reducing burden on the field of view (FOV) motor 112 and the focus motor 113. These optical elements have relative curvilinear movement to compensate deviation of image plane, that makes the image plane stable and the curves smooth.

[0040] It is understood that the above description is intended to be illustrative, and not restrictive. It is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein,” respectively.