DESC:TECHNICAL FIELD
[0001] The present invention relates generally to the field of thermal imaging cameras. More particularly, the present invention concerns a motorized Long Wave Infrared (LWIR) dual Field of View lens for cooled detectors used in Thermal Imaging Cameras.
BACKGROUND
[0002] Thermal imaging cameras are used for detection, recognition, and identification of objects during day and night. The requirement of thermal imaging cameras with dual FOV lens module is to provide an image of a scene having a wide field of view (WFOV) and a low magnification and another image of the scene having a narrow field of view (NFOV) and relatively higher magnification.
[0003] One of the conventional techniques discloses compact second generation Forward-looking Infrared (FLIR) kit which comprises of Long Wavelength Infrared (LWIR) dual field of view optics designed for LWIR cooled 2nd generation detectors which require scanning mirror. Such dual FOV Lens requires Scanning mirror mechanisms to get full image hence such systems are bulky in size.
[0004] Another conventional technique discloses a dual FOV lens for wavelength spectrum of 8-12 µm. This lens provides 640X512 Focal Plane Array (FPA), 15 micron pixel size LWIR detector. Further, in surveillance cameras it is essential to see the large areas during surveillance and to see the details of the distant targets evidently. With single FOV any one of the two said requirements are fulfilled.
[0005] Another conventional technique discloses a double FOV infrared optical system for uncooled detectors. The uncooled detectors have more Noise Equivalent Temperature Difference (NETD) in the order of 40-50 mK as compared to the cooled detectors with Noise Equivalent Temperature Difference (NETD) values as low as 15mK. Thus, the system employing uncooled detectors cannot be used for long range viewing applications. The drawback of this disclosure is that it cannot be used with the cooled detectors.
[0006] Hence, there is still a need of a better or an alternative invention which addresses the above defined problems.
SUMMARY
[0007] This summary is provided to introduce concepts related to motorized Long Wave Infrared (LWIR) dual field of view lens module for cooled detectors. 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.
[0008] In an embodiment of the present invention, a Long Wave Infrared (LWIR) dual field of view lens module for cooled detectors is provided. In one embodiment herein, the dual FOV lens meets the requirements of the diffraction limited performance and the operation temperature. This lens is compatible with 640X512 elements, 15 micron pixel size LWIR detectors of F-Number 2.7. In this type of lens only a single lens moves along the optical axis to achieve optical compensated two fields of views (FOVs). The dual FOV lens is actively athermalized by moving the focus lens group along the optical axis. The lens is designed using aspheric and diffractive surfaces to keep the telephoto coefficient less than 0.5. Furthermore, this lens has near diffraction limited performance both in NFOV and WFOV. It provides a long back working distance which is the distance between the last Lens surface to protective window of the detector to incorporate the Non-Uniformity Correction (NUC) shutter which is essential for the LWIR cooled detectors.
[0009] In another embodiment, the dual FOV lens comprises two moving lens groups; one is for FOV change and another for focus compensation. Further, this lens provides a long back working distance for fitment of NUC shutter which is required for LWIR cooled thermal imaging systems.
[0010] In another embodiment, the motorized LWIR dual FOV lens captures infrared radiation from objects and focuses on to detector FPA in both NFOV and WFOV. The Dual FOV Lens comprises of fixed lens group comprising a single lens having a positive refractive power with aspheric surface on concave surface and said fixed lens is made of germanium material. Further, a FOV changer group with a single lens of negative refractive power made of germanium. The change in FOV is achieved by moving this FOV changer group axially along the optical axis. Further, a second fixed lens group of two spherical lenses made of germanium. This group has positive combined refractive power. Further, a focus lens group, which is selected to match the exit pupil of zoom lens with the cold shield of the detector while limiting the diameter of the front lens to the value calculated from the F-Number and the focal length in NFOV. The focus lens group is moved axially for focus adjustment and thermal focus compensation. This group consists of two lenses made of germanium. Further, a mechanical housing which consists of main housing, FOV barrel and focus barrel. The position of the FOV changer lens group is changed from the NFOV position to WFOV position by rotating the FOV barrel. The FOV barrel comprises of three cam slots to define FOV changer lens group position in WFOV and NFOV. The slot profile is chosen to nullify the degradation in image quality due to any external dynamic loads subjected on the dual FOV lens module. The required rotation of FOV barrel is attained by a mechanism consisting of gear train set that is driven by DC stepper motor. Further, a NUC shutter for non-uniform correction of the detector is provided. Further, a lens control module is provided to give required power to the FOV, focus and NUC shutter motors and to control the FOV and focus.
[0011] In another embodiment, the dual FOV lens comprises only six lenses made of germanium without employing Zinc Selenide (ZnSe) and ZnS materials.
[0012] In another embodiment, the dual FOV lens has telephoto ratio of only 0.49.
[0013] In another embodiment, the dual FOV lens has near diffraction limited MTF performance in both NFOV and WFOV.
[0014] In another embodiment, the dual FOV lens is compatible with 640X512 elements, 15 micron pixel size LWIR cooled detectors.
[0015] In another embodiment, this dual FOV lens comprises a complex opto-mechanical & control electronics module involving innovative optical design backed up by equally innovative mechanical and control PCB design. The electronics module (PCB) has been developed for control of Field Of View (FOV) motor, focus motors and the NUC shutter motor. The electronics also calibrates the differential encoders and performs BITE of Dual FOV on power on. The electronics module provides automatic focus at infinity at both FOVs and temperature compensation of focus. The electronics module interfaces with motor encoders and also with the signal processing electronics. The electronics module accepts commands from signal processing electronics for controlling the Dual FOV Lens and also provides accurate FOV, focus and NUC shutter position feedback.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
[0016] The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and modules.
[0017] Figure 1 is a dual FOV lens optical layout in WFOV & NFOV, according to an exemplary implementation of the present invention.
[0018] Figure 2 graphically illustrates the Modulation Transfer Function (MTF) curves at NFOV & WFOV, according to an exemplary implementation of the present invention.
[0019] Figure 3 graphically illustrates the distortion graphs at NFOV & WFOV, according to an exemplary implementation of the present invention.
[0020] Figure 4 is a block diagram showing the various sub-modules of the continuous zoom lens, according to an exemplary implementation of the present invention.
[0021] Figure 5 illustrates the FOV barrel diagram which shows cam slots of the FOV barrel, according to an exemplary implementation of the present invention.
[0022] 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 flowcharts, 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
[0023] In the following description, for the 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.
[0024] The various embodiments of the present invention provide a motorized long wave infrared dual field of view lens for cooled detectors.
[0025] 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.
[0026] References in the present invention to “one embodiment” or “an 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 one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
[0027] The present invention relates to a motorized long wave infrared dual field of view lens module for cooled detectors. This dual FOV lens module includes a first fixed lens group comprising a single lens that has a positive refractive power with aspheric surface on a concave surface. This fixed lens is made of germanium material. Further, the first fixed lens consists of a convex spherical surface facing towards an object and a concave aspherical surface facing the FOV changer group. It further includes a FOV changer group that has a single lens with a negative refractive power. a second fixed lens group comprising at least two spherical lenses, wherein said group provides a positive refractive power. This single lens of the FOV changer group is made of germanium material. Further, the FOV changer group is configured to move axially along the optical axis of the dual FOV lens for changing the FOV. Further, a focus lens group comprises at least two spherical lenses, wherein said focus lens group provides a positive refractive power. The focus lens is configured to move axially for focus adjustment and temperature compensation of focus. A mechanical housing unit comprises of a main housing, a FOV barrel and a focus barrel. The main housing comprises of three linear slots for the FOV changer group and the focus lens group. The FOV Barrel comprises of three cam slots to define FOV changer lens group position in WFOV and NFOV. The cam slot profile is chosen to nullify the degradation in image quality due to any external dynamic loads subjected on the dual FOV lens module. Further, a lens control module controls a FOV motor for change in FOV, a focus motor for automatic focus at infinity at a narrow FOV (NFOV) and a wide FOV (WFOV), temperature compensation of focus and a Non Uniform Correction (NUC) shutter motor controls a NUC shutter during NUC of detector.
[0028] In another implementation, the dual FOV lens comprises of at least six lenses. These at least six lenses are made of germanium material.
[0029] In another implementation, the dual FOV lens has a total throughput transmission of greater than 85%. Further, the dual FOV lens has a long back working distance of greater than 6 mm to incorporate the NUC shutter required for the LWIR cooled detectors. Further, the dual FOV provides a telephoto ratio of at least 0.49. Further, the dual FOV provides a distortion value of less than 2.7% in the NFOV and the WFOV.
[0030] In another implementation, the dual FOV lens provides a near diffraction limited Modulation Transfer Function (MTF) performance in the NFOV and the WFOV.
[0031] In another implementation, the dual FOV lens is compatible with 640X512 elements Focal Plane Array (FPA) and 15 micron pixel size LWIR cooled detector.
[0032] In another implementation, the FOV barrel and the focus barrel comprises of at least three linear slots.
[0033] In another implementation, the NUC shutter is arranged between the dual FOV lens and the cooled detector for NUC of the detector.
[0034] Figure 1 is a dual FOV lens optical layout in WFOV & NFOV, according to an exemplary implementation of the invention as disclosed in the present technical disclosure. The dual FOV lens (100) assembly is provided for Long Wave infrared (LWIR) spectrum for 640X512 elements, pixel size 15 micron LWIR cooled detector. The Dual FOV lens assembly provides change in NFOV of 2.29°(H) X1.83°(V) and WFOV of 9.14°(H)X7.32°(V) with F number of 2.7 in Both WFOV and NFOV. The optical layout of the dual FOV lens (100) at NFOV and WFOV are as shown in the figure 1. It further provides Dual FOV Optics, Near Diffraction Limited Image Quality, Low Telephoto ratio and Active Thermal Compensation.
[0035] Dual FOV Optics: For any thermal imager the area of surveillance should be as large as possible. At the same time, it should resolve more details of the target. For such requirements, optics assembly with variable focal length is provided herein. This requirement is achieved with dual FOV lens systems where the image is in focus for two FOVs that are WFOV and NFOV.
[0036] Near Diffraction Limited Image Quality: MTF (Modulation Transfer Function) is the important performance parameters for any optical systems used in surveillance cameras and these defines the quality of the image obtained using the respective optics module. The present disclosure provides 15 micron pixel pitch detector wherein 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 that are called the diffraction limited MTF performance.
[0037] Low Telephoto ratio: Telephoto ratio of an optical system is defined as the ratio between the optical track length to the focal length in NFOV. The dual FOV lens (100) has total track length of 118 mm with focal NFOV of 240 mm. Hence the telephoto ration is 118/240=0.49.
[0038] Active Thermal Compensation: The infrared radiation transmitting materials are highly sensitive to temperature variations due to large variation in its refractive index with temperature. This results in variation of the focus with temperature for same distant target. The focus lens group (104) is moved axially along the optical axis to compensate this focus variation due to temperature. The temperature sensor in control PCB senses the system temperature and sends commands to the focus lens group motor to move the focus group to the calibrated position for that temperature.
[0039] Figure 2 graphically illustrates the MTF curves at NFOV & WFOV, according to an exemplary implementation of the invention as disclosed in the present technical disclosure. The dual FOV lens provides near diffraction limited MTF Performance in both FOVs. The MTF curves for the WFOV and NFOV positions are provided herein.
[0040] Figure 3 graphically illustrates the distortion graphs at NFOV & WFOV, according to an exemplary implementation of the invention as disclosed in the present technical disclosure. Distortion is another performance metric of optical systems. The distortion graphs in both NFOV and WFOV are provided herein. In the Figure on X- Axis, the percentage of distortion is given and on the Y-axis, the image height is given. The figure shows that distortion values are less than 2.7 % in both FOVs.
[0041] Figure 4 is a block diagram showing the various sub-modules of the continuous zoom lens, according to an exemplary implementation of the invention as disclosed in the present technical disclosure. The various sub-modules of the continuous zoom lens which are shown in the figure 4 are: First Fixed Lens Group (108), FOV changer group (110), Second Fixed Lens Group (102), Focus Lens Group (104), Mechanical housing and NUC Shutter (412).
[0042] First Fixed Lens Group (108): The first fixed lens group (108) has single lens with positive power made of germanium material. This lens consists of convex spherical surface facing towards object and concave aspheric surface facing the FOV changer group (110).
[0043] FOV changer group (110): The FOV changer group (110) consists of single lens with negative refractive power made of germanium. The change in FOV is achieved by moving this FOV changer group (110) axially along the optical axis.
[0044] Second Fixed Lens Group (102): The second fixed lens group (102) of two spherical lenses are made of germanium. This group has positive combined refractive power.
[0045] Focus Lens Group (104): The focus lens group (104) is selected to match the exit pupil of zoom lens with the cold shield of the detector while limiting the diameter of the front lens to the value calculated from the F-Number and the focal length in NFOV. The focus lens group is moved axially for focus adjustment and thermal focus compensation. This group consists of two lenses which made of germanium.
[0046] Mechanical Housing: The mechanical housing comprises of main housing (402), the FOV barrel (404) and the focus barrel (406). The main housing (402) further comprises of two linear slots, FOV barrel (404) and focus barrel (406) consists of three cam slots. The position of the FOV changer lens group (110) is changed from NFOV position to WFOV position by rotating the FOV barrel (404). The required rotation of FOV barrel (404) is attained by a mechanism that consists of gear train set that is driven by DC stepper motor.
[0047] NUC Shutter (412): In LWIR cooled FPA detectors, the detector elements or pixels response varies with time, so it essential to correct the non-uniformities when the degradation in thermal imager performance is found using image processing. This NUC requires all the detector elements of FPA to check the uniform source/target. This is accomplished by incorporating an anodized aluminium shutter to the zoom lens as shown in the figure 4. This shutter can be placed in or out of the optical path by sending commands to the NUC Shutter motor (414) from lens control card.
[0048] Control PCB: Control PCB takes +12V DC power input from the power supply board assy. It supplies required voltages to the FOV motor (408), focus motor (410) and NUC shutter motor (414). It incorporates a RS422 serial communication interface. Depending on the user input for focus or Field of View (FOV) change or NUC, FPGA board sends respective commands to Control PCB via power supply board assy. Based on these commands; the Control PCB moves the motors to appropriate positions. Further, a provision has been given to store the auto focus values for different temperatures in the permanent memory in the Control PCB.
[0049] Figure 5 illustrates the FOV barrel diagram which shows cam slots of the FOV barrel, according to an exemplary implementation of the present invention. The FOV barrel comprises of three cam slots to define FOV changer lens group position in WFOV and NFOV. The slot profile is chosen to nullify the degradation in image quality due to any external dynamic loads subjected on the dual FOV lens module.
[0050] 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.
,CLAIMS:

1. A motorized dual Field of View (FOV) lens module (100) for cooled detectors comprising:
a first fixed lens group (108) comprising a single lens having a positive refractive power with aspheric surface on a concave surface;
a FOV changer group (110) comprising a single lens with a negative refractive power;
a second fixed lens group (102) comprising at least two spherical lenses, wherein said group provides a positive refractive power;
a focus lens group (104) comprising at least two spherical lenses, wherein said focus lens group (104) provides a positive refractive power;
a mechanical housing unit comprises of a main housing (402), a FOV barrel (404) and a focus barrel (406);
a lens control module is configured to control a FOV motor (408) for change in FOV, a focus motor (410) for automatic focus at infinity at a narrow FOV (NFOV) and a wide FOV (WFOV), temperature compensation of focus and a Non Uniform Correction (NUC) shutter motor (414) to control a NUC shutter (412) during NUC of detector.

2. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the fixed lens is made of germanium material.

3. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the FOV changer group (110) comprising the single lens is made of germanium material.

4. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the cooled detector is a Long Wave Infrared (LWIR) cooled detector.

5. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the dual FOV lens comprises of at least six lenses.

6. The motorized dual FOV lens module (100) as claimed in claim 5, wherein the at least six lenses are made of germanium material.

7. The motorized dual FOV lens module (100) as claimed in claim 1, wherein said lens having a total throughput transmission of greater than 85%.

8. The motorized dual FOV lens module (100) as claimed in claim 1, wherein said lens having a long back working distance of greater than 6 mm to incorporate the NUC shutter (412) required for the plurality of cooled detectors.

9. The motorized dual FOV lens module (100) as claimed in claim 1, wherein said lens provides a distortion value of less than 2.7% in the NFOV and the WFOV.

10. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the dual FOV lens provides a telephoto ratio of at least 0.49.

11. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the dual FOV lens provides a near diffraction limited Modulation Transfer Function (MTF) performance in the NFOV and the WFOV.

12. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the dual FOV lens is compatible with 640X512 elements Focal Plane Array (FPA) and 15 micron pixel size LWIR cooled detector.

13. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the first fixed lens (108) consists of a convex spherical surface facing towards an object and a concave aspherical surface facing the FOV changer group (110).

14. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the FOV changer group (110) is configured to move axially along the optical axis of the dual FOV lens for changing the FOV.

15. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the focus lens (104) is configured to move axially for focus adjustment and temperature compensation of focus.

16. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the main housing (402) comprises of at least three linear slots for the FOV changer group (110) and the focus lens group (104).

17. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the FOV barrel (404) and the focus barrel (406) comprises of at least three linear slots.

18. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the FOV barrel (404) includes two or more cam slots, wherein said cam slots are configured to define the FOV changer lens group position in WFOV and NFOV.

19. The motorized dual FOV lens module (100) as claimed in claim 1, wherein the NUC shutter (412) is arranged between the dual FOV lens and the cooled detector for NUC of the detector.