DESC:FIELD OF INVENTION
[0001] The present invention relates generally to imaging devices and specifically to a zoom lens.

BACKGROUND
[0002] Infrared Zoom Lenses are used in commercial, medical and surveillance applications. They are extensively used in aerial platforms, both for targeting systems and infrared search and track
[0003] Chinese patent application no. CN102866485b discloses a long-wavelength infrared continuous zoom lens for 640x512 elements, 25-micron pixel size focal plane array detectors. Zoom lens comprises of total 10 optical elements (8 lenses and 2 mirrors) with 3 diffractive surfaces. This zoom lens has lower transmission efficiency because of 10 optical elements and 3 diffractive surfaces.
[0004] Russian patent application no. RU2569424 discloses a zoom lens consist of five lens groups with total 7 lenses. The second and third groups moves along the optical axis to achieve the required zoom and to keep the focus on detector focal plane array for infinity target. This zoom lens has the total track length i.e. From first lens to image plane is 485 mm for maximum focal length 500 mm in telephoto end. The ratio between the total length to telephoto focal length is 485/500=0.97, which make the system bulk with respect to physical dimensions and weight. However, such zoom lens cannot be used, where weight and space are critical.
[0005] English patent application no. GB2474762 discloses a zoom lens for spectral range 8-12 µm with six lens groups with total of 9 lenses. The second group consists of negative lens moves along the optic axis to act as variator. The third group moves along the optical axis to act as compensator. The sixth and seventh groups re-images the intermediate image on to the detector FPA. This zoom lens is it comprises of ZnSe lenses. But ZnSe is not desirable as it releases toxic gasses during manufacturing.
[0006] US patent application no. US5022724 discloses zoom lens used in mirror scanning type of infrared optical system. This zoom lens consists of 5 lens groups with total 7 lenses made of germanium and ZnSe materials. The first and fourth group are fixed groups, whereas second, third groups moves axially on rails with help of electric motors to achieve the required zooming. The fifth group is collimator group moves axially for active athermalization i.e. for focus compensation during change in temperature of the system. This zoom lens useful in scanning type cooled thermal imaging systems and not suitable for FPA array detectors.
[0007] In view of the above, there is a need for an effective zoom lens that addresses at least one of the above mentioned problems or disadvantages.

SUMMARY
[0008] This summary is provided to introduce concepts related to long wave infrared continuous zoom lens. This summary is neither intended to identify essential features of the present invention nor is it intended for use in determining or limiting the scope of the present invention.
[0009] In an embodiment of the present invention, a Long Wave Infra-Red (LWIR) zoom lens is provided. The LWIR zoom lens includes a front objective lens, a variator lens group, first and second compensator lens groups, a relay lens group, a mechanical housing assembly, and a lens control module. The mechanical housing assembly includes a main housing, a zoom sleeve, a focus sleeve, a zoom motor, a focus motor, a Non-Uniform Correction (NUC) shutter, and an NUC motor. The front objective lens collects IR radiation from a target. The front objective lens has positive power and aspheric diffractive profile on concave surface. The front objective lens is made of Germanium. The variator lens group is configured to move along an optical axis. The variator lens group has a single lens having negative refractive power with one aspheric surface. The aforesaid single lens is made of Germanium. The first and second compensator lens groups are configured to move along the optical axis relative to each other. The first compensator lens group has one positive power aspheric lens made of Germanium and the second compensator lens group has one negative power aspheric lens made of Germanium. The relay lens group is configured to move along the optical axis. The relay lens group is configured to adjust focus and to provide thermal compensation. The relay lens group has two aspheric lenses made of Germanium. The zoom sleeve and the focus sleeve are both rotatable to move the variator lens group, the first and second compensator lens groups, and the relay lens group along the optical axis. The zoom motor and the focus motor are configured to rotate the zoom sleeve and the focus sleeve. The NUC motor is configured to move the NUC shutter. The lens control module is configured to control the zoom and the focus and to provide power to the zoom motor, the focus motor, and the NUC motor.
[0010] In an exemplary embodiment, the aspheric diffractive profile on the concave surface of the front objective lens corrects spherical and chromatic aberrations.
[0011] In another exemplary embodiment, the first and second compensator lens groups restore the focus on to an image plane of the target in all zoom levels.
[0012] In yet another exemplary embodiment, the relay lens group is configured to match exit pupil of the LWIR zoom lens with cold shield of detector while limiting diameter of the front objective lens to value calculated from F-number and focal length in narrow field of view.
[0013] In yet another exemplary embodiment, the LWIR zoom lens comprises six lenses made of Germanium without employing ZnSe.
[0014] In yet another exemplary embodiment, the LWIR zoom lens has a telephoto ratio of 0.5 (120 mm/239 mm), thereby providing compactness.
[0015] In yet another exemplary embodiment, the variator lens group moves away from object side, the first compensator lens group moves away from object side, and the second compensator lens group moves towards object side while zooming from wide field of view to narrow field of view.
[0016] In yet another exemplary embodiment, the relay lens group moves lateral to the optical axis to correct a zoom bore-sight lower than 0.1 mrad (milli radians).
[0017] In yet another exemplary embodiment, the LWIR zoom lens has a diffraction limited MTF performance and optical distortion less than 2% across zoom positions.
[0018] In yet another exemplary embodiment, the LWIR zoom lens is compatible with 640X512, 15-micron pixel size LWIR cooled detectors.
[0019] In yet another exemplary embodiment, the zoom sleeve allows continuous change of field of view.
[0020] In yet another exemplary embodiment, the LWIR zoom lens includes a focus barrel to compensate for focus shift in different temperatures.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
[0021] The detailed description is described with reference to the accompanying figures.
[0022] Fig. 1 illustrates optical layout of Long Wave Infra-Red (LWIR) zoom lens in accordance with an embodiment of the present invention.
[0023] Fig. 2 illustrates Modulation Transfer Function (MTF) curves for Wide Field of View (WFOV) and Narrow Field of View (NFOV) positions of LWIR zoom lens in accordance with an embodiment of the present invention.
[0024] Fig. 3 illustrates distortion graphs for NFOV and WFOV positions of LWIR zoom lens in accordance with an embodiment of the present invention.
[0025] Fig. 4 illustrates mechanical slot profiles of LWIR zoom lens in accordance with an embodiment of the present invention.
[0026] Figs. 5 illustrates sub-modules of LWIR zoom lens in accordance with an embodiment of the present invention.
[0027] Fig. 6 illustrates mechanical housing of LWIR zoom lens in accordance with an embodiment of the present invention.
[0028] It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative systems 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.

DETAILED DESCRIPTION
[0029] The various embodiments of the present invention provide a compact motorized long wave infrared continuous zoom lens for cooled detectors.
[0030] 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.
[0031] 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.
[0032] Furthermore, connections between components and/or modules within the figures are not intended to be limited to direct connections. Rather, these components and modules may be modified, re-formatted or otherwise changed by intermediary components and modules.
[0033] References in the present invention to “an embodiment” or “another embodiment” mean that a particular feature, structure, characteristic, or function described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in an embodiment” in various places in the specification are not necessarily all referring to the same embodiment. The phrase “embodiment of the present invention” used in the present invention may refer to various embodiments of the present invention.
[0034] Referring now to Fig. 1, an optical layout of the LWIR zoom lens (100) is shown in accordance with an embodiment of the present invention. The LWIR zoom lens (100) includes a front objective lens (102), a variator lens group (104), a first compensator lens group (106), a second compensator lens group (108), a relay lens group (110), and a detector Focal Plane Array (FPA) (112).
[0035] The LWIR zoom lens (100) of the present invention operates in the Long Wave infrared (LWIR) spectrum for 640X512 elements, pixel size 15-micron LWIR cooled detector. The LWIR zoom lens (100) provides continuous change in focal length from 59 mm to 236 mm with constant F number of 2.7 to provide: (1) Continuous optical zoom; (2) Best image quality; (3) Compact and lightweight; (4) Smooth mechanical slots profiles; and (5) Thermal focus compensation.
[0036] For any thermal imager, area of surveillance should be as large as possible. At the same time, it is also required to resolve more details of the target. For such requirements, optics assembly with variable focal length are very much essential, which are accomplished by discrete Field of View (FOV) systems. However, in such systems, a user is not able to monitor the targets continuously while switching from Wide FOV (WFOV) to Narrow FOV (NFOV) as the target is in focus at only two discrete positions or FOVs. Whereas, continuous optical zoom systems overcome this limitation and give flexibility to the user to monitor the target continuously without losing the sight during zooming.
[0037] Referring now to Fig. 2, Modulation Transfer Function (MTF) curves for the WFOV and NFOV positions of the LWIR zoom lens (100) are shown in accordance with an embodiment of the present invention. MTF and distortion are the important performance parameters for any optical systems used in surveillance cameras and these define the quality of the image obtained using the respective optics module. The LWIR zoom lens (100) is used with 15-micron pixel pitch detector where the system performance is limited by the detector, as the cut off spatial frequency of the detector is less than the optics cut off frequency. At each frequency, the MTF of values are theoretically limited by diffraction effects in optic, called diffraction limited MTF performance. The zoom lens is designed with near diffraction limited MTF performance throughout the zoom range.
[0038] Referring now to Fig. 3, distortion graphs for NFOV and WFOV positions of the LWIR zoom lens (100) are shown in accordance with an embodiment of the present invention. One more parameter for judging quality of thermal imager is optical distortion. The zoom lens is designed with distortion values are less than 2% through the zoom range.
[0039] Compact physical dimension and light weight are the critical requirements in any surveillance camera. The LWIR zoom lens (100) is designed with one variator lens group (104), two compensator lens groups (106 and 108) and advanced optical elements like diffractive and aspheric lenses such that the total length of system 120 mm while NFOV focal length is 239 mm, which makes the LWIR zoom lens (100) compact and light weight.
[0040] Mechanical Slots profiles of the LWIR zoom lens (100) depends on the how the variator lens group (104) and the first and second compensator lens groups (106 and 108) are moved along the optical axis for zooming. In the optical design, it is ensured that the variator lens group (104) and the first and second compensator lens groups (106 and 108) move in a smooth path while zooming.
[0041] Referring now to Fig. 4, the mechanical slot profiles of the LWIR zoom lens (100) are shown in accordance with an embodiment of the present invention. While rotating the zoom sleeve at angle theta, the Movement of Variator group is linear (104). The movement the first compensator lens group (106) and the second compensator lens group (108) are Non Linear.
[0042] The infrared radiation transmitting materials are highly sensitive to temperature variations due to large variation in refractive index with temperature. This results in variation of the focus with temperature for same distant target. The relay lens group (110) is moved axially along the optical axis to compensate this focus variation due to temperature. The temperature sensor in lens control module senses the system temperature and send commands to the relay lens group motor to move the relay lens group (110) to a position calibrated for that temperature.
[0043] Referring now to Fig. 5, sub-modules of the LWIR zoom lens (100) are shown in accordance with an embodiment of the present inventions.
[0044] The front objective lens (102) is a fixed lens with positive power. The front objective lens (102) is made of Germanium. The front objective lens (102) consists of a convex spherical surface facing towards object and a concave aspheric and diffractive surface facing the variator lens group (104). The aspheric and diffractive surface is used to minimize spherical and chromatic aberrations.
[0045] The zooming i.e. variation of the focal length or the FOV is achieved by moving the variator lens group (104) axially along the optical axis. The variator lens group (104) consists of a single lens with negative refractive power with a concave aspheric surface facing the front objective lens (102) and a concave spherical surface facing the second compensator lens group (108). The variator lens group (104) is also made of Germanium.
[0046] The first and second compensator lens groups (106 and 108) move along the optical axis relatively to each other and relatively to the variator lens group (104) to keep the image always on the FPA (112) of the detector. The first compensator lens group (106) consists of one positive refractive power aspheric lens made of Germanium. The second compensator lens group (108) consists of one negative refractive power aspheric lens made of Germanium.
[0047] The LWIR zoom lens (100) is designed with the relay lens group (110) to match exit pupil of the LWIR zoom lens (100) with cold shield of the detector while limiting a diameter of the front objective lens (102) to a value calculated from the F-Number and the focal length in NFOV. The relay lens group (110) moves axially for focus adjustment and thermal focus compensation. The relay lens group (110) consists of two aspheric lenses made of Germanium.
[0048] Referring now to Fig. 6, a mechanical housing of the LWIR zoom lens (100) is shown in accordance with an embodiment of the present invention. It comprises of a main housing (602), a zoom sleeve (608), and a focus sleeve (610). The main housing (602) comprises of four linear slots. The zoom sleeve (608) consists of one linear and two non-linear slots. The focus sleeve (610) comprises of one linear slot. The placements of the variator lens group (104), the first compensator lens group (106) and the second compensator lens group (108) are maintained by rotating the zoom sleeve (608) and the focus sleeve (610). The required rotation of the zoom sleeve (608) and the focus sleeve (610) is attained by a mechanism consisting of two gear train sets (one spur gear and one spur pinion) that are driven by two DC motors independently.
[0049] In LWIR cooled FPA detectors, the detector elements or pixels response varies with time, so it essential to correct non-uniformities when degradation in thermal imager performance is found using image processing. This is known as Non-Uniform Correction (NUC). The NUC requires that all the detector elements of FPA see a uniform source or target. This accomplished by the incorporating an anodized aluminium shutter, an NUC shutter (614) into the LWIR zoom lens (100). The NUC shutter (614) can be placed in or out of the optical path by sending commands to an NUC motor (612) from the lens control module.
[0050] The lens control module takes approximately +12V DC power input from a power supply board assembly. The power supply board assembly supplies required voltages to the focus motor (604), the zoom motor (606) and the NUC motor (612). The power supply board assembly incorporates a RS422 serial communication interface. Depending on the user input for focus or FOV change or NUC, an FPGA board sends respective commands to the lens control module via the power supply board assembly. Based on these commands, the lens control module moves the focus motor (604), the zoom motor (606) and the NUC motor (612) to appropriate positions. In an example, provision is provided to store auto focus values for complete zoom range and for different temperatures in a permanent memory in the lens control module.
[0051] In an embodiment of the present invention, the LWIR zoom lens (100), for capturing LWIR radiation and maintaining the focus during the continuous change in the FOV is provided. The LWIR zoom lens (100) includes the front objective lens (102) for collecting the IR radiation from the target. The front objective lens (102) is fixed one with positive power and is made of Germanium material having aspheric diffractive profile on concave surface for correction of spherical and chromatic aberrations. The variator lens group (104) moves along the optical axis for varying the focal length of the LWIR zoom lens (100). The variator lens group (104) consists of a single lens with negative refractive power with one aspheric surface, which is made of Germanium. The first compensator lens group (106) and the second compensator lens group (108) moves along the optical axis in a predefined path relative to each other as per the optical design to restore the focus on to the image plane in all zoom positions.
[0052] Further, the first compensator lens group (106) includes one positive power aspheric lens made of Germanium. The second compensator lens group (108) includes one negative refractive power aspheric lens made of Germanium. The relay lens group (110) to match the exit pupil of the LWIR zoom lens (100) with the cold shield of the detector while limiting the diameter of the front objective lens (102) to the value calculated from the f-number and the focal length in NFOV. The relay lens group (110) moves along the optical axis for focus adjustment and thermal focus compensation. The relay lens group (110) consists of two aspheric lenses made of Germanium.
[0053] Further, a mechanical housing of the LWIR zoom lens (100) consists of the main housing (602), the zoom sleeve (608), and the focus sleeve (610). The zoom sleeve (608) and the focus sleeve (610) are rotated to move the variator lens group (104), the first compensator lens group (106), the second compensator lens group (108) and the relay lens group (110) along the optical axis in the designed paths while zooming to get the good image quality. The zoom motor (606) and the focus motor (604) rotate the zoom sleeve (608) and the focus sleeve (610). The NUC shutter (614) for non-uniform correction of the detector is provided. The NUC motor (612) moves the NUC shutter (614). The lens control module gives required power to the zoom motor (606), the focus motor (604) and the NUC motor (612) and to control the zoom and focus. The LWIR zoom lens (100) includes only six lenses made of Germanium without employing Znse material. The LWIR zoom lens (100) has telephoto ratio of only 0.5 (120 mm/239 mm) which makes the LWIR zoom lens (100) compact. The LWIR zoom lens (100) includes only one diffractive lens and total six lenses, which results in good transmission efficiency. While zooming from WFOV to NFOV, the variator lens group (104) moves away from the object side, the first compensator lens group (106) moves away from the object side and the second compensator lens group (108) moves towards the object side. The relay lens group (110) moves lateral to the optical axis to correct the through zoom bore sight lower than 0.1 mrad (milliradians). The LWIR zoom lens (100) has the diffraction limited MTF performance and optical distortion less than 2% across zoom positions. The LWIR zoom lens (100) is compatible 640X512, 15-micron pixel size LWIR cooled detectors. The zoom sleeve (608) allows continuous change of FOV. The LWIR zoom lens (100) has focus barrel allows for compensation of focus shift for different temperatures.
[0054] In an embodiment of the present invention, a compact LWIR zoom lens for large FPA cooled detectors is provided. The LWIR zoom lens comprises complex opto-mechanical and control electronics module involving innovative optical design backed up by equally innovative mechanical and lens control module design. The optical design is based on aspherical and diffractive optical elements. The mechanical design has taken care of complex optical alignment issues and provision has been made for adjustment of relay lens to achieve through zoom bore-sighting. The electronics module (PCB) has been developed for control of FOV motor, focus motors and NUC motor. The electronics also calibrates the differential encoders and performs BITE of zoom lens on Power ON. The electronics module caters for automatic focus at infinity at various zoom positions and temperature compensation of focus. The electronics interfaces with motor encoders and also with the signal processing electronics. The electronics accepts commands from signal processing electronics for controlling the zoom lens and also provides accurate FOV, focus and NUC shutter position feedback.
[0055] Advantageously, the LWIR zoom lens that meets requirements of compact size, diffraction limited performance and compactness is provided. The optical design of the LWIR zoom lens has a smaller number of lenses made of Germanium without compromising the optical performance. The mechanical design positions the lenses precisely at desired locations during zooming to get a good image quality at all zoom positions. The lens control module ensures that the images are always in focus while zooming in or zooming out within an operating temperature range. The LWIR zoom lens provides continuous change in FOV from wide FOV to narrow FOV. The LWIR zoom lens has: (i) Continuous optical zoom for f/2.7, 640X512, 15 micron pixel size cooled detectors; (ii) Compactness in total length with telephoto ratio of 120/239 = 0.5 - achieved by using one variator and two compensators; (iii) High transmission efficiency due to use of single diffractive surface out of twelve optical surfaces and only six optical lenses; (iv) Diffraction limited performance throughout the zoom, which is achieved with less number of lenses by using aspheric and diffractive surfaces; (v) Thermal focus compensation; (vi) High MTF and low distortion performance; (vii) Maintains focus for infinity targets throughout the zoom; (viii) Minimized through zoom bore-sight; (ix) Motorized shutter is incorporated for field non-uniform correction of detector FPA, which is essential for cooled LWIR detectors; (x) Cost effective design as only six lenses are used; and. The LWIR zoom lens is very suitable for applications where weight, size, and accuracy are critical.
[0056] 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 spirit and 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 Long Wave Infra-Red (LWIR) zoom lens (100) comprising:
a front objective lens (102) for collecting IR radiation from a target, said front objective lens (102) having positive power and aspheric diffractive profile on concave surface, and said front objective lens (102) made of Germanium;
a variator lens group (104) configured to move along an optical axis, said variator lens group (104) having a single lens having negative refractive power with one aspheric surface, said single lens made of Germanium;
first and second compensator lens groups (106 and 108) configured to move along the optical axis relative to each other, said first compensator lens group (106) having one positive power aspheric lens made of Germanium and said second compensator lens group (108) having one negative power aspheric lens made of Germanium;
a relay lens group (110) configured to move along the optical axis, said relay lens group (110) configured to adjust focus and to provide thermal compensation, and said relay lens group (110) having two aspheric lenses made of Germanium;
a mechanical housing assembly, comprising;
a main housing (602);
a zoom sleeve (608) and a focus sleeve (610), both rotatable to move the variator lens group (104), the first and second compensator lens groups (106 and 108), and the relay lens group (110) along the optical axis;
a zoom motor (606) and a focus motor (604) configured to rotate the zoom sleeve (608) and the focus sleeve (610);
a Non-Uniformity Correction (NUC) shutter (614); and
an NUC motor (612) configured to move the NUC shutter (614); and
a lens control module configured to control the zoom and the focus and to provide power to the zoom motor (606), the focus motor (604), and the NUC motor (612).

2. The LWIR zoom lens (100) as claimed in claim 1, wherein the aspheric diffractive profile on the concave surface of the front objective lens (102) corrects spherical and chromatic aberrations.

3. The LWIR zoom lens (100) as claimed in claim 1, wherein the first and second compensator lens groups (106 and 108) restore the focus on to an image plane of the target in all zoom levels.

4. The LWIR zoom lens (100) as claimed in claim 1, the relay lens group (110) is configured to match an exit pupil of the LWIR zoom lens (100) with a cold shield of a detector while limiting a diameter of the front objective lens (102) to a value calculated from an F-number and a focal length in a narrow field of view.

5. The LWIR zoom lens (100) as claimed in claim 1, comprising six lenses made of Germanium without employing ZnSe.

6. The LWIR zoom lens (100) as claimed in claim 1, having a telephoto ratio of 0.5 (120 mm/239 mm), thereby providing compactness.

7. The LWIR zoom lens (100) as claimed in claim 1, wherein the variator lens group (104) moves away from object side, the first compensator lens group (106) moves away from object side and the second compensator lens group (108) moves towards object side while zooming from wide field of view to narrow field of view.

8. The LWIR zoom lens (100) as claimed in claim 1, wherein the relay lens group (110) moves lateral to the optical axis to correct a zoom bore-sight lower than 0.1 mrad (milli radians).

9. The LWIR zoom lens (100) as claimed in claim 1, having a diffraction limited MTF performance and optical distortion less than 2% across zoom positions.

10. The LWIR zoom lens (100) as claimed in claim 1, compatible with 640X512, 15-micron pixel size LWIR cooled detectors.

11. The LWIR zoom lens (100) as claimed in claim 1, wherein the zoom sleeve (608) allows continuous change of field of view.

12. The LWIR zoom lens (100) as claimed in claim 1, comprising a focus barrel to compensate for focus shift in different temperatures.