# Patent

## Title
Dual element passively athemalized infrared imaging systems

## Abstract

A passively athermalized infrared imaging system includes an object side meniscus lens that forms at least one aspheric surface, and an image side meniscus lens that forms two aspheric surfaces. Each of the meniscus lenses are formed of a material selected from the group consisting of a chalcogenide glass, germanium, silicon, gallium arsenide, zinc selenide and glass. An optical power of the image side meniscus lens is at least 1.6 times an optical power of the object side meniscus lens such that an effective focus position of the imaging system is athermalized over a range of 0 to +40 degrees Celsius.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to U.S. Provisional Patent Application No. 61/434,657, filed 20 Jan. 2011, which is incorporated by reference in its entirety.


U.S. GOVERNMENT SUPPORT
The present invention was made under Phase I SBIR Contract No. IIP-1047405 from the National Science Foundation. The Government has certain rights in this invention.


BACKGROUND
Conventional longwave infrared (“LWIR”) lenses are expensive to manufacture and often incorporate either moving parts or nested barrels for athermalization. Athermalization is a larger concern for infrared (“IR”) imaging systems than for systems that image visible radiation, due to the relative increased sensitivity to temperature of common LWIR materials in comparison to common visible spectrum materials. FIG. 1 illustrates one prior art manufacturing process 100 for producing a camera assembly. In process 100, aspheric lens elements are diamond turned individually in step 102, and a housing is fabricated in step 104. The lenses are placed into a lens mount with multiple component athermalization mechanisms in step 106. The lens mount is assembled into a camera in step 108. The camera is actively aligned and focused using feedback from acquired images, and then potted into place, in step 110. The camera is calibrated and tested in step 112.
FIG. 2 illustrates, in an exploded view, one prior art lens stack 200 that may be produced by steps 102 through 108 of process 100 of FIG. 1. Lenses 202 and 206 are diamond turned in step 102 of process 100. Housing 216 is fabricated in step 104. In step 106, lenses 202 and 206 mount with a spacer 204 therebetween and with a first barrel 210 and a second barrel 212 that has a high coefficient of thermal expansion. These components then mount with a threaded barrel 214 that has a low coefficient of thermal expansion. In step 108, threaded barrel 214 screws into housing 216. Lens stack 200 may integrate with a sensor 218 to form a camera.
As shown in FIG. 2, lens stack 200 includes a significant number of associated mounting hardware elements and interfaces between these elements and lenses. Those skilled in the art will appreciate that these elements generate tolerance stack-up issues (e.g., related to lens alignment, element thickness variations, tilt of the elements and mounting materials) that can make the performance of lens stack 200 nonideal. Therefore, lens stack 200 is usually actively aligned and focused in situ with a camera body in step 110 to mitigate the effects of these issues, but the active alignment process adds further cost to process 100.
Another prior art method of making cameras includes molding lenses onto one or both sides of a transparent glass substrate. Multiple wafers of lenses are then stacked on top of each other with spacer wafers between the lens wafers to achieve a required spacing between the lenses. Good imaging performance of the final lens assemblies requires precise positioning of all of the wafers in six degrees of freedom with respect to each other: typical Cartesian x and y coordinates for centering the lens elements; z spacing between the lens elements, and rotations known in the art as tip, tilt and theta. The required alignments are generally performed utilizing a mask aligner adapted from semiconductor processing equipment. Such equipment may be costly and time consuming to operate, and presents special challenges for assembly of IR optics. IR imaging instrumentation would generally be a nonstandard and costly addition to a mask aligner, and the longer wavelengths of IR as opposed to visible light may make alignment thereby less precise.
SUMMARY OF THE INVENTION
In an embodiment, a passively athermalized infrared imaging system includes an object side meniscus lens that forms at least one aspheric surface, and an image side meniscus lens that forms two aspheric surfaces. Each of the meniscus lenses are formed of a material selected from the group consisting of a chalcogenide glass, germanium, silicon, gallium arsenide, zinc selenide and glass. An optical power of the image side meniscus lens is at least 1.6 times an optical power of the object side meniscus lens such that an effective focus position of the imaging system is athermalized over a range of 0 to +40 degrees Celsius.
In an embodiment, a passively athermalized infrared imaging system includes an object side meniscus lens, formed of a first material that transmits infrared radiation, that forms at least one aspheric surface; and an image side meniscus lens, formed of a second material that transmits infrared radiation, that forms two aspheric surfaces. The object side and image side meniscus lenses cooperate to form a thermal image and are concave towards the image. The first and second materials have thermal glass constants Tg1 and Tg2 respectively, wherein Tg2 is at least 1.67 times Tg1, such that an effective focus position of the system is athermalized over a temperature range of 0 to +40 degrees Celsius.
In an embodiment, a method of aligning elements in a manufacturing process includes placing a middle element onto a base element, the base element forming first alignment features, the middle element forming apertures therethrough corresponding to the first alignment features. The method also includes placing second alignment features of an upper element onto the first alignment features such that the first and second alignment features cooperate, through the apertures, to align the upper element with the base element.
In an embodiment, a method of aligning elements in a manufacturing process includes placing a middle element onto a base element, the base element forming first alignment features, the middle element forming apertures therethrough corresponding to the first alignment features. The method also includes placing intermediate alignment elements upon the first alignment features, and placing second alignment features of an upper element onto the intermediate alignment features such that the first, intermediate, and second alignment features cooperate to align the upper element with the base element.
In an embodiment, a plurality of infrared lens systems includes at least one first lens wafer comprising a first base material that is opaque to infrared radiation, with infrared transmissive material inset into apertures therein to form first lenses, bonded with at least one second lens wafer having a plurality of second lenses, such that pluralities of the first and second lenses align to form the lens systems.
In an embodiment, a lens wafer for use in optical manufacturing includes a substrate forming apertures therein, the substrate being formed of a base material, and a plurality of lenses, each of the lenses comprising an optical material and disposed within a respective one of the apertures.
In an embodiment, an infrared lens assembly includes a lens formed of an infrared transmitting material that is disposed within a carrier of a base material, the lens being molded within the carrier with at least one feature that secures the lens to the carrier.
In an embodiment, a mold set includes an upper mold and a lower mold, at least one of the upper and lower molds having one or more features that are configured to hold an infrared lens in an aligned position, the upper and lower molds configured to provide a cavity for molding a moldable material into one of a lens carrier and a lens wafer about the infrared lens.
In an embodiment, an infrared lens assembly includes a lens formed of an infrared transmitting material that is disposed within a carrier of a base material, the carrier being molded around the lens with at least one feature that secures the lens to the carrier.
In an embodiment, an infrared lens stack includes a first lens formed of an infrared transmitting material and disposed within a first carrier, formed of a base material, that forms at least one first alignment feature; and a second lens formed of an infrared transmitting material and disposed within a second carrier, formed of a base material, that forms at least one second alignment feature configured to cooperate with the first alignment feature. The stack is formed by placing the first assembly atop the second assembly so that the alignment features cooperate.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates one prior art manufacturing process for producing a camera assembly.
FIG. 2 illustrates, in an exploded view, a prior art lens stack produced by steps of the process of FIG. 1.
FIG. 3 shows a parallel LWIR manufacturing process, in an embodiment.
FIG. 4 is a perspective view showing one exemplary lens stack produced by the process of FIG. 3, in an embodiment.
FIG. 5 illustrates a wafer that forms an array of apertures where lenses will be fabricated, in an embodiment.
FIG. 6 shows a portion of a wafer having a single aperture that is within recesses, in an embodiment.
FIG. 7 shows molds and moved into position for molding about the aperture so as to mold the slug into a lens, in an embodiment.
FIG. 8 shows the lens of FIG. 7 with the molds removed.
FIG. 9 is a cross-sectional view of a lens, in an embodiment.
FIG. 10 is a bottom plan view looking upward through the lens of FIG. 9.
FIG. 11 shows a mold set including upper and lower molds separated from the lens of FIG. 9, in an embodiment.
FIG. 12 shows the molds of FIG. 11 in contact with the lens of FIG. 9.
FIG. 13 is a cross-sectional view of a lens carrier produced by molding an appropriate base material around the lens of FIG. 9, using the molds of FIG. 11, in an embodiment.
FIG. 14 is a top plan view looking downward at the lens carrier of FIG. 13.
FIG. 15 schematically illustrates a lens carrier produced by mounting the lens of FIG. 9 to a lens carrier blank, in an embodiment.
FIG. 16 schematically illustrates a wafer that forms an array of apertures for integration of lens carriers therein, in an embodiment.
FIG. 17 is an exploded view that schematically illustrates integration of two lens wafers with a spacer wafer, in an embodiment.
FIG. 18 is an exploded view that schematically illustrates integration of two lens wafers, in an embodiment.
FIG. 19 is a flowchart for a method of aligning and bonding two elements, such as two lens wafers in an IR camera, with each other and with a spacer wafer, in an embodiment.
FIG. 20 shows a cross section of an exemplary LWIR lens stack (configured with an object side lens element fabricated on a first wafer, a stop and stray light control element, an image side lens element fabricated on a second wafer, and a spacer, in an embodiment.
FIG. 21A illustrates exemplary performance of the lens stack of FIG. 20 as a function of temperature.
FIG. 21B illustrates performance of the lens stack of FIG. 20 over a focus shift range as a function of temperature.
FIGS. 22, 24, 26, 28, 30, 32, 34, 36, 38, 40 and 42 illustrate athermalized lens configurations, in embodiments.
FIGS. 23A, 23B, 25A, 25B, 27A, 27B, 29A, 29B, 31A, 31B, 33A, 33B, 35A, 35B, 37A, 37B, 39A, 39B, 41A, and 41B illustrate performance of the lens configurations of FIGS. 22, 24, 26, 28, 30, 32, 34, 36, 38, and 40 respectively
FIG. 42 shows two lens carriers that may be integrated into a lens system, in an embodiment.
FIG. 43 shows an alternative arrangement that forms a lens system including a spacer that can provide optical spacing and an aperture stop between lens carriers, in an embodiment.
FIG. 44 illustrates performance of lens configurations having various combinations of object side and image side thermal glass constants.



DETAILED DESCRIPTION OF THE DRAWINGS
Introduction
Certain materials that are typically used for LWIR imaging have large changes in optical and mechanical properties with temperature. Therefore, creating an athermalized imaging design is a larger concern for IR imaging than visible light imaging, due to the relative increased sensitivity to temperature of common LWIR materials in comparison to common visible spectrum materials. Another challenge is that the LWIR imaging industry cannot rely on components, processes, or testing equipment that have been developed for the extremely high volumes of the visible imaging and display businesses. Although some of the general techniques that have been developed for visible wafer level solutions can be transferred, the technology is not readily adapted to the LWIR spectrum without significant changes to the process and the lens architecture to compensate for the unique challenges of imaging in the LWIR spectrum.
Lens Carrier Based Manufacturing and Passive Alignment
FIG. 3 illustrates a parallel longwave infrared (“LWIR”) manufacturing process 300. FIG. 4 is a schematic perspective view showing one exemplary lens stack 400 produced by process 300 of FIG. 3.
In process 300, FIG. 3, arrays of lens elements are simultaneously fabricated on carriers or “wafers,” stacked with an optional, athermalizing spacer “wafer,” passively aligned, bonded (optionally with the spacer), diced into complete IR optical modules and mounted onto IR sensors. (The carriers and spacer elements utilized in the manufacturing techniques herein are sometimes called “wafers” due to the similarity of such manufacturing to semiconductor processing that is performed on wafers cut from crystal boules.) “Sensors” herein are sensor chips such as microbolometer arrays that include a plurality, usually a large number, of pixels that are sensitive to incoming radiation and can form an image therefrom. The resulting lens stacks 400 thus enable a passively athermalized IR imaging system that does not rely on complicated arrangements such as nested barrels and/or moving parts for athermalized performance.
In step 310 of process 300, an array of lenses is fabricated in each of two or more wafers. Exemplary lens materials and prescriptions, and further details about how the lenses may be fabricated, are provided below. In step 320, a spacer wafer is fabricated. Step 320 is optional, as one or more of the lens wafers may be formed of sufficient thickness that a spacer wafer is not needed to provide appropriate spacing and/or appropriate coefficient of thermal expansion for an athermalized design. In step 330, the lens wafers are aligned with each other and optionally about the spacer wafer, and bonded thereto to form a wafer level lens assembly. Further details on alignment strategies that may be employed within step 330 are also provided below. In step 340, the wafer level lens assembly is diced into single lens assemblies. Step 350 is an optional lens test step to eliminate defective lens assemblies before they are mated to other camera components. In step 360, the lens assemblies are assembled to other components to form cameras. In step 370, the cameras are calibrated and tested.
Certain steps of process 300 may be performed in a different order than is shown in FIG. 3. For example, steps 310 and 320 may be performed in any order. Also, when step 350 is performed as part of process 300, it may be performed before or after dicing step 340 (that is, the wafer level lens assemblies formed in step 330 may be tested individually in wafer form before step 340, or as individual units after step 340).
It should also be clear that the steps of process 300 may be adapted to add further lens and/or spacer layers to a lens stack, as compared to the two lens stack produced by the minimum steps of process 300. For example, a common variation of process 300 will be to bond two lens wafers and two spacer wafers, to produce a lens stack that has spacing between two lenses and between an image side one of the lenses and a sensor. Another common variation of process 300 will be to bond a spacer atop the lens stack to form an input aperture stop. Addition of other features by repeating steps of process 300 or by adding other features that are compatible with the wafer level manufacturing scheme of process 300 are contemplated and fall within the scope of the present disclosure.
By forming arrays of lenses, with precise spacing on or within a monolithic wafer, the need for individual, active alignment of each lens stack is eliminated. A single alignment step between one or more lens array wafers and one or more spacer wafers accurately positions all lenses with respect to one another, thus improving off-axis image performance relative to conventional lens assembly by reducing decenter and tilt errors. This effectively eliminates the need for active alignment of each lens, a step that is currently a time-consuming, labor intensive adjustment required for each lens (e.g., step 110 of process 100 discussed above). By simplifying and grouping (e.g., across a wafer) the number of assembly steps involved, lens carrier and/or lens wafer based fabrication also enables production of IR cameras that are more compact and less costly than in the prior art. One key to cost reduction is to utilize more costly optical materials only where needed and to utilize appropriate, less costly base materials for structural support of the optical materials. Methods of doing so, and the cameras formed thereby, are now discussed.
FIG. 4 schematically shows the major elements of a lens stack 400 and a camera 420 that can be produced utilizing process 300, FIG. 3. A portion 402 of a first lens carrier includes a first lens 404. A portion 406 of a spacer wafer is disposed between, and bonded to, overlying portion 402 and an underlying portion 408 of a second lens carrier. Portion 408 includes a second lens 410. Lens stack 400 includes portions 402, 406 and 408, and can integrate with a sensor 412 to form camera 420.
FIGS. 5 through 8 schematically illustrate one way in which step 310 of process 300, FIG. 3 may be performed, in an embodiment. FIG. 5 illustrates a wafer 510 that forms an array of apertures 520 where lenses will be fabricated. Each aperture 520 may be within an optional recess 530. Slugs 540 of a moldable material correspond to each aperture 520 and are molded to form lenses therein, as explained below. Not all of apertures 520, recesses 530 and slugs 540 are labeled in FIG. 5, for clarity of illustration.
FIGS. 6 through 8 schematically illustrate how one slug 540 may be molded into one of the apertures 520 of wafer 510 to form a lens, in an embodiment. FIG. 6 shows a portion of wafer 510 having a single aperture 520 that is within recesses 530. A slug 540 is positioned within aperture 520. Molds 600(1) and 600(2) that include mold surfaces 610(1) and 610(2) are shown above and below slug 540, respectively. Mold surfaces 610(1) and 610(2) are complementary to desired surfaces of a lens to be fabricated within aperture 520. FIG. 7 shows molds 600(1) and 600(2) moved into position (e.g., in the directions shown by arrows, towards wafer 510) for molding about the aperture so as to mold the slug into a lens 700. FIG. 8 shows lens 700 with molds 600(1) and 600(2) removed. Lens 700 forms surfaces 810(1) and 810(2) that are complementary to mold surfaces 610(1) and 610(2) respectively.
Numerous variations on the process illustrated in FIGS. 5 through 8 are contemplated and/or would be considered equivalent to one skilled in the art. For example, in FIG. 8, lens 700 includes overmolded flanges 820(1) and 820(2) that may be advantageous for securing lens 700 within wafer 510. Although two overmolded flanges are shown in FIG. 8, it is contemplated that a molded lens could include only one, or no such flanges. Alternatively, wafer 510 may form recesses, flanges or protrusions that lens 700 can mold into or around, to secure the lens within the lens wafer.
In another example, instead of slugs 540, a moldable optical material may be injection molded to produce lens 700. Molds 600(1) and 600(2) may be individual molds for one surface of one lens each, or may form part of molds that simultaneously provide mold surfaces 610(1) and 610(2) for each of a plurality of lenses 700 that are concurrently fabricated in wafer 510. Surfaces 810(1) and 810(2) may be molded so as to provide their final optical surfaces, or one or more of surfaces 810(1) and 810(2) may be machined (e.g., diamond turned) to provide final optical surfaces.
Moreover, molding lenses into a wafer 510 is but one example of step 310, FIG. 3, for providing a lens wafer. Another way to provide a lens wafer includes first providing lenses, either alone or within individual lens carriers, then assembling the lenses (and carriers, if used) into the wafer. The assembly of the lenses and/or carriers into the wafer may utilize molding and/or adhesives.
FIGS. 9 through 13 schematically illustrate a process for molding a lens carrier or a lens wafer about one or more lenses, respectively. FIGS. 9 and 10 show a lens 900 that is integrated with a circumferential assembly flange 905. FIG. 10 is a bottom plan view looking upward through lens 900, and FIG. 9 is a cross-sectional view taken at line 9-9′ shown in FIG. 10. Lens 900 is for example injection molded to provide lens surfaces 910(1) and 910(2) and assembly flange 905 that includes an alignment feature 915. Alignment feature 915 is shown in FIGS. 9 and 10 as a groove, but it is contemplated that alignment feature 915 could be one or more of (a) one or more indentations, such as groove sections, or (b) protrusions, that can mate with complementary features of a lens carrier or lens wafer to align lens 900 to the respective carrier or wafer. Lens 900 and features thereof may be made by molding or machining bulk material, or by a combination of molding and machining before or after lens 900 is incorporated in a carrier or wafer. In particular, lens 900 may be molded to a shape wherein lens surfaces 910(1) and 910(2) are outside the final desired surface, such lens 900 may be incorporated into a carrier, then diamond turned to provide the final desired surface. Alignment feature 915 can also be added by machining before or after lens 900 is incorporated in a carrier or wafer, instead of being molded into lens 900.
FIGS. 11 and 12 show schematic cross-sections of a mold set including upper and lower molds 1010(1) and 1010(2) that are configured to mold base material to form a lens carrier about lens 900. FIG. 11 shows molds 1010(1) and 1010(2) separated from lens 900, while FIG. 12 shows molds 1010(1) and 1010(2) in contact with lens 900. FIG. 12 does not label individual features of molds 1010(1), 1010(2) and lens 900, for clarity of illustration. Upper mold 1010(1) includes a dam 1030(1) for contacting assembly flange 905 of lens 900 to keep molded base material away from lens surface 910(1). Lower mold 1010(2) includes a similar dam 1030(2) for contacting assembly flange 905 to keep molded base material away from lens surface 910(2), and a dam 1040 to form the outer edge of the lens carrier. A suitable base material would be injected into cavity 1200, as shown in FIG. 12 with upper and lower molds 1010(1) and 1010(2) contacting lens 900, to form the lens carrier. Lower mold 1010(2) also includes alignment features 1015(1) and 1015(2) that mate with alignment feature(s) 915 of lens 900, for precision alignment of final surfaces formed by lower mold 1010(2) relative to lens surfaces 910(1) and 910(2) of lens 900. Regions between alignment features 1015(1) and 1015(2) and dam 1030(2) are also considered portions of cavity 1200, as shown, because alignment features 1015(1) and 1015(2) may not extend fully about lens 900 (that is, alignment features 1015(1) and 1015(2) are shown for illustrative purposes in the cross-sectional plane shown in FIGS. 11 and 12, but may only exist at selected positions about a circumference of lens 900).
Lower mold 1010(2) also includes features 1035(1) and 1035(2) to provide alignment features in the lens carrier. Because alignment features 1015(1), 1015(2), 1035(1) and 1035(2) are all features of lower mold 1010(2), alignment of lens 900 to lower mold 1010(2) is sufficient to provide precision alignment of lens 900 (especially, lens surfaces 910(1) and 910(2) thereof) to known features in the finished lens carrier; that is, alignment of upper mold 1010(1) may not be as critical. However, it is contemplated that upper mold 1010(1) could also include physical alignment features for precision alignment to lens 900 (and lens 900 could include appropriate mating features therefor), or all of molds 1010(1), 1010(2) and lens 900 could include such features.
It should be clear upon reviewing and understanding FIGS. 11 and 12 that molds similar to upper and lower molds 1010(1) and 1010(2), but without dam 1040, and replicated to mold around an array of lenses, may be utilized to mold an entire lens wafer instead of a lens carrier.
FIGS. 13 and 14 show a schematic cross section of a lens carrier 1300 produced by molding an appropriate base material 1310 around lens 900, using upper and lower molds 1010(1) and 1010(2) as shown in FIGS. 11 and 12. FIG. 14 is a top plan view looking downward, and FIG. 13 is a cross-sectional view taken at line 13-13′ shown in FIG. 14. Base material 1310 is produced by molding a moldable base material within cavity 1200, FIG. 12, including the regions shown as alignment features 1015(1) and 1015(2) in FIG. 11, because these features may only exist in selected positions about a circumference of lens 900. As shown in FIG. 13, lens carrier 1300 includes alignment features 1315(1) and 1315(2) for subsequent precision alignment within a lens wafer. Base material 1310 may be a material that absorbs at least IR radiation, so that when lens carrier 1300 or a portion thereof becomes part of a camera assembly, stray IR radiation that impinges on base material 1310 is absorbed instead of continuing to propagate through the camera. Base material 1310 may also absorb visible and/or ultraviolet (“UV”) radiation. However, in some embodiments base material 1310 transmits visible and/or UV radiation, which can enhance manufacturability by facilitating cure of certain epoxies.
FIG. 15 schematically illustrates a lens carrier 1300(2) produced by mounting lens 900 to a lens carrier blank 1510 utilizing an adhesive 1517. Lens carrier blank 1510 may be made of a base material such as metal (e.g., aluminum) that is not necessarily moldable under conditions that lens 900 would survive. Lens carrier blank 1510 may be made for example by casting or by machining metal bar stock, and includes features for aligning lens 900 and for later aligning completed lens carrier 1300(2) to a lens wafer. Adhesive 1517 may be for example epoxy that is curable by applying visible or UV radiation; when such a curable epoxy is utilized, it may be advantageous to form lens carrier 1300(2) of a material that is transparent to the wavelength of radiation utilized to cure the epoxy. The amount and location of adhesive 1517 is exemplary; other embodiments of a lens carrier may utilize different amounts or placement of such adhesive. Alternatively, lens 900 may simply be press-fit into lens carrier blank 1510 to form completed lens carrier 1300(2) without any adhesive.
FIG. 16 schematically illustrates a wafer 1610 that forms an array of apertures 1620 for integration of lens carriers 1300 therein. Lens carriers 1300 (e.g., either of lens carriers 1300(1), 1300(2) or equivalents thereof) correspond to each aperture 1620 and are mounted therein utilizing any one or combination of mechanical fit, adhesives (such as epoxy), welding, soldering, and/or mating mechanical features associated with the lens carriers and the apertures. Not all of apertures 1620 and lens carriers 1300 are labeled in FIG. 16, for clarity of illustration.
FIG. 17 is an exploded view that schematically illustrates integration of two lens wafers 1700, 1700′ with a spacer wafer 1780, as in step 330 of process 300, FIG. 3. Lens wafers 1700, 1700′ are populated with lenses 1710, 1710′ respectively and may be fabricated as described in connection with FIGS. 5 through 8 or FIGS. 9 through 16, in embodiments. Alternatively, either of lens wafers 1700, 1700′ may be entirely molded and/or machined out of a single piece of a suitable optical material. Fabricating an entire lens wafer from optical material consumes more of the optical material itself, which may be costly, and can reduce the opportunity to utilize a base material that absorbs stray radiation, as noted above. Lens wafer 1700 has at least a substantially planar upper surface 1702 and lens wafer 1700′ has at least a substantially planar lower surface 1702′, except for respective alignment features 1715 and 1715′, discussed below.
Spacer wafer 1780 has substantially planar, counterfacing surfaces 1782 and 1784, separated by a thickness d. Apertures 1786 are shown in FIG. 17 and are usually formed in spacer wafer 1780 for mechanical clearance for lenses 1710, 1710′, and/or to allow IR (and/or visible radiation) to pass without refraction or absorption between lenses 1710 and 1710′. Apertures 1786 may be omitted if spacer wafer 1780 is formed of an IR (and/or visible radiation) transparent material, if mechanical clearance is not needed and if refraction by spacer wafer 1780 is included in the optical prescription of imaging optics thus formed. Apertures 1786 are shown as rounded rectangles in FIG. 17, but may be of other shapes such as circles that are matched to an optically active area of the imaging optics. Spacer wafer 1780 may be formed for example of metal (e.g., aluminum, steel, titanium, brass, copper or alloys thereof), plastic, ceramic or composite materials that may be selected on the basis of mechanical strength, coefficient of thermal expansion, and/or absorptivity with respect to IR or visible radiation (e.g., to absorb stray radiation, as discussed above).
Lens wafers 1700, 1700′ also include alignment features 1715, 1715′ that are utilized for passive alignment to one another, as discussed below. Also shown in FIG. 17 are optional intermediate alignment elements 1720. Spacer wafer 1780 forms apertures 1788 that correspond with alignment elements 1720, allowing alignment elements 1720 to contact both alignment features 1715 and 1715′. Apertures 1788 are large enough to provide clearance around alignment elements 1720 such that spacer wafer 1780 remains free to move somewhat in the planes defined by the upper and lower surfaces of the spacer wafer. (Not all of lenses 1710, 1710′, apertures 1786, 1788 or alignment features 1715, 1715′ are labeled in FIG. 17, for clarity of illustration.) The clearance of apertures 1788 relative to alignment elements 1720 avoids overconstraining alignment elements 1720, so that alignment elements 1720 align solely with alignment features 1715, 1715′, yet constrains alignment of spacer wafer 1780 so that the material of spacer wafer 1780 around each aperture 1786 can be utilized as an aperture stop in a resulting lens assembly or camera. For example, alignment features 1715, 1715′ may be conical or spherical indentations in lens wafers 1700, 1700′ respectively, and intermediate alignment elements 1720 may be spheres such as precision ball bearings. In another embodiment, alignment features 1715, 1715′ may be oval or oblong features that provide additional degrees of freedom for alignment elements 1720, as discussed below in connection with FIG. 18. In still other embodiments, intermediate alignment elements 1720 are not present, and alignment features 1715 and/or 1715′ protrude substantially from their respective surfaces through apertures 1788 and mate directly with each other. Alignment features 1715, 1715′ may be formed in lens wafers 1700, 1700′ by machining, molding, etching, or printing, for example.
When lens wafers 1700, 1700′ are assembled with spherical alignment elements 1720 and spacer wafer 1780, lens wafer 1700′ will naturally settle into precise alignment with lens wafer 1700 without the use of an active alignment step (e.g., using a mask aligner or other positioning device). When the opposing surfaces of lens wafers 1700, 1700′ are perfectly planar, intermediate alignment elements 1720 are perfectly spherical, and three each of alignment features 1715, 1715′ with diameters that match alignment elements 1720 are formed in an equilateral triangle in lens wafers 1700, 1700′, the lens wafers will be perfectly positioned and constrained in all six degrees of freedom. Adhesives or localized bonding techniques (e.g., localized welding or soldering, performed on the stacked wafers as a unit or repeated for each of the lens and spacer sets formed by the wafers) are utilized to bond lens wafers 1700, 1700′ with spacer wafer 1780 such that the alignment of lens wafers 1700, 1700′ remains precise across all of the lens and spacer sets thus formed.
It should also be noted that dimensions of alignment features 1715, 1715′ and/or a size of intermediate alignment elements 1720 set a spacing between lens wafers 1700, 1700′ that is at least thickness d of spacer wafer 1780, and which spacing may exceed thickness d. This allows for application of an adhesive (e.g., epoxy) between base lens wafer 1700 and spacer wafer 1780, and between spacer wafer 1780 and upper lens wafer 1700′. In this manner, each set of elements (a lens in each of lens wafers 1700, 1700′ and a portion of spacer wafer 1780) are bonded and can be diced with the spacing intact. Other attachment means (e.g., local welding, soldering) may also be utilized.
FIG. 18 is an exploded view that schematically illustrates integration of two lens wafers 1700(2), 1700(2)′, also as in step 330 of process 300, FIG. 3, but without showing a spacer wafer, for clarity of illustration. Lens wafers 1700(2), 1700(2)′ are populated with lenses 1710, 1710′ respectively and may be fabricated as described in connection with FIGS. 5 through 8 or FIGS. 9 through 16, or may be entirely molded and/or machined out of a single piece of a suitable optical material, in embodiments. Lens wafers 1700(2), 1700(2)′ are similar to lens wafers 1700, 1700′ except for alignment features 1715(2) and 1715(2)′ formed therein, respectively. Alignment features 1715(2) formed in lens wafer 1700(2) are shown as grooves that are radial with respect to the center of lens wafer 1700(2), and are matched in width to intermediate alignment elements 1720. Alignment features 1715(2)′ formed in lens wafer 1700(2)′ are shown as semispherical depressions like alignment features 1715′. Since alignment features 1715(2) have a width that matches intermediate alignment elements 1720 but with a length that allows for some motion, lens wafer 1700(2) can be brought into and remain in rotational alignment with lens wafer 1700(2)′ with some room for expansion and/or contraction of either of the lens wafers. This can be advantageous for testing individual pairs of lenses 1710, 1710′ in wafer form, which is often more economical than testing individual units. For example, as is known in semiconductor fabrication, testing in wafer form can often be done on a sample basis to statistically verify the performance of tested units without testing each one. Such testing can also be done across lens wafers 1700(2), 1700(2)′ at various temperatures to verify an exact temperature to which the lens wafers should be brought when they are joined to form a plurality of lens stacks that will later be singulated into individual lens stacks.
Although alignment features 1715(2) are shown as radial grooves that allow expansion and alignment features 1715(2)′ are shown as semispherical depressions that fix positions of intermediate alignment elements 1720, it is appreciated that these features may vary. In particular, an upper wafer may include grooves while a lower element may contain depressions, or alignment features 1715(2) and 1715(2)′ may be designed to cooperate with each other directly rather than utilizing intermediate alignment elements 1720. For example, alignment elements 1715(2) may be grooves as illustrated in FIG. 18, with alignment elements 1715(2)′ being semispherical or conical protrusions.
FIG. 19 is a flowchart for a method 1900 of aligning and bonding two elements, such as two lens wafers in an IR camera, with each other and with a spacer wafer, as shown in FIG. 17. Method 1900 may be utilized for example as step 330 of process 300, FIG. 3. An optional step 1910 can for example utilize lens wafer 1700 as a base element, and couple intermediate alignment elements 1720 with alignment features 1715 of lens wafer 1700. An optional step 1920 applies one or more adhesives to the base, middle, and/or upper elements. In an embodiment, an adhesive is applied to the base and/or middle element so that they adhere when they come into contact. In another embodiment, the adhesive is curable and is applied to one or more of the base, middle, and/or upper elements so that the elements may be brought into contact with one another but the adhesive does not immediately bond them into place, allowing for small alignment adjustments (such as facilitated by mating alignment elements 1715 of the base and upper elements). Step 1920 may be omitted when step 1950, below, adheres all of the base, middle and upper elements at once. Step 1930 places a middle element (e.g., spacer wafer 1780) onto the base element. Step 1940 places a top element (e.g., lens wafer 1700′) over the middle element such that alignment features of the top element engage alignment features of the base element or the intermediate alignment elements, if placed in step 1910. Step 1950 bonds the top element to the base element and to the middle element, for example by curing a curable epoxy applied in step 1920. It may be advantageous to utilize lens wafers, spacer wafers and/or lens carriers that are transparent to a curing wavelength of such a curable epoxy, for example when IR lens elements are not transparent to the curing wavelength. Alternatively, in embodiments it is possible to utilize localized welding, soldering or mechanical attachments to bond the top element to the base element, keeping in mind that the attachments must remain secure while and after the lens stacks so formed are singulated.
In addition to lens wafer based manufacturing as discussed above, it is appreciated that lens carriers may be utilized in manufacturing individual lens systems. For example, FIG. 42 shows two lens carriers 1300(3), 1300(4) that may be integrated into a lens system 4200, similar to lens stack 400 (FIG. 4). Lens carriers 1300(3), 1300(4) may be made by the techniques discussed above, especially in connection with FIG. 5 through FIG. 12. Lens carriers 1300(3), 1300(4) do not include alignment features that mate with features of corresponding parts, but are formed with a precise outside diameter so that they can be stacked within a cylindrical alignment jig 4210 that holds them in alignment, as shown. Lens carriers 1300(3), 1300(4) may then be bonded together in alignment with one another to form lens system 4200. It is appreciated that the technique disclosed here can be utilized not only with circular carriers and a cylindrical alignment jig, but are easily adaptable to lens carriers and corresponding alignment jigs that are square or rectangular, or are circular with keyed features that force their positioning into a known rotational alignment. Alignment jigs 4210 may be removable or may form part of the finished lens systems. Alignment jigs 4210 may facilitate bonding of lens carriers 1300(3), 1300(4) to one another (e.g., may be transparent to a curing wavelength of an epoxy, may transfer heat to lens carriers 1300(3), 1300(4) for welding or soldering, or may form a mold for molding a further material about lens carriers 1300(3), 1300(4)).
FIG. 43 shows an alternative arrangement that forms a lens system 4300 including a spacer 4220 that can provide optical spacing and an aperture stop between lens carriers 1300(3), 1300(4); spacer 4220 can also be made of a material with a coefficient of thermal expansion that provides athermalization of lens system 4300. The lateral alignment of spacer 4220 may not be as critical as the alignment of lens carriers 1300(3), 1300(4) to one another, because optically it serves only to provide spacing and possibly as an aperture stop for lens system 4300. Therefore spacer 4220 can be manufactured to a slightly smaller diameter and looser diameter tolerance than lens carriers 1300(3), 1300(4). The addition of spacer 4220 is but one modification of lens carrier based manufacturing. Those skilled in the art will appreciate that the features discussed in the context of lens wafer based manufacturing elsewhere in this disclosure may also be adapted or modified for use in lens carrier based manufacturing.
LWIR Material Selection and Athermalization Techniques
The goal of athermalizing a system is to design is to provide imaging performance that remains good across a temperature range. This is a difficult characteristic to achieve in current LWIR systems, for two reasons. First, as discussed above, many materials commonly used in LWIR systems are sensitive to temperature. Second, the typical applications for LWIR imaging require operation over a large temperature range. Without athermalization, the result would be reduced image quality (e.g., a blurred image due to inability to hold focus) at temperatures away from some nominal temperature. This would be unacceptable for automotive, outdoor security and military applications where large temperature swings are commonplace.
There are two common methods for athermalizing an LWIR imaging system. The first uses an active mechanism similar to auto focus, but instead of being distance dependent, it is temperature dependent. The second is a multi-component, “nested” lens barrel that leverages a material's coefficient of thermal expansion (CTE) to shift the image plane towards the lens as the power of the elements are increased, and away when the power is decreased, due to temperature changes. However, wafer based manufacturing combined with selection of materials used therein based both on their imaging and thermal expansion properties can yield a passively athermalized IR imaging system that does not require either active focus correction or nested barrels, as now discussed.
A useful definition of an athermalized lens is one for which a polychromatic, through-focus modulation transfer function (“MTF”) is at least 0.1 across a specified temperature range, at 1⁄2 of an optical cutoff frequency of the lens (1/((f/#)*λ)); wherein “f/#” refers to the f-number of the lens and λ refers to wavelength. Performance of athermalized lenses described herein meet this criterion.
One class of materials that may be used in embodiments is that of chalcogenide glasses, several of which are known to be moldable and to have high transparency to infrared radiation. Chalcogenide glasses that may be utilized in embodiments may be obtained from several sources. One is Amorphous Materials, Inc. of Garland, Tex., selling such glasses under the trademarked name “AMTIR” (an acronym for Amorphous Material Transmitting Infrared Radiation). These include AMTIR-1 (Ge33As12Se55), AMTIR-2 (AsSe), AMTIR-3 (GeSbSe), AMTIR-4 (AsSe), AMTIR-5 (AsSe) and AMTIR-6 (As2S3). Another supplier is SCHOTT North America Inc., which sells chalcogenide glasses under the trade names IG2 (Ge33As12Se55), IG3 (Ge30As13Se32Te25), IG4 (Ge10As40Se50), IG5 (Ge28Sb12Se60) and IG6 (As40Se60). Another supplier is Umicore Electro-Optic Materials, (Belgium) which sells a chalcogenide glass called GASIR®1 Infrared Transmitting glass.
Other materials that are IR transmissive and may be utilized include plastic, glass, crystalline materials (e.g., germanium, gallium arsenide, zinc selenide), and salts.
It is also important to utilize aspheric optics to control aberrations, field curvature and astigmatism. In embodiments herein, an object side meniscus lens forms at least one aspheric surface and an image side meniscus lens forms two aspheric surfaces. In a two element configuration it is important to have aspheric degrees of freedom on three of the four surfaces. The aspheric surface on the front element, near an aperture stop, is used to control spherical aberration. The two aspheric surfaces on the second lens are used to control higher order field curvature and astigmatism. These three aspheric surfaces are required to enable the passively athermalized two meniscus configuration that provides high performance. A fourth asphere can optionally be used to slightly increase performance or manufacturability.
FIG. 20 shows a cross section of an exemplary LWIR lens stack 2000 (e.g., lens stack 400 of FIG. 4) configured with an object side optical element 2002 fabricated on a first wafer, a spacer 2004 that may also serve as an aperture stop and stray light control element, an image side optical element 2006 fabricated on a second wafer, and a spacer 2008. Lens stack 2000 forms an image at an image plane 2010. Lens stack 2000 includes embedded athermalization materials that form a combination of the lens element materials and spacer materials for passive athermalization. Lens stack 2000 is also shown coated with an IR blocking and protective external covering 2012, for example to block stray IR radiation and/or to improve structural integrity of the lens stack. FIG. 21A illustrates exemplary performance of lens stack 2000 as a function of temperature, where each line 2102, 2104, and 2106 shows a modulation transfer function (“Modulation”) of lens stack 2000 at a different temperature. FIG. 21B illustrates exemplary performance of lens stack 2000 at 50 cycles/mm (which is half the optical cutoff for an F/1 lens) over a focus shift range of −0.1 to +0.1 mm from best focus, where each line 2108, 2110 and 2112 shows modulation at a different temperature. The performance of lens stack 2000 meets the athermalized lens criterion noted above, since at the zero focus position, none of the different temperature curves drops below a modulation of 0.1.
In the example of FIGS. 20, 21A and 21B, an object side AMTIR-4 optical element 2002, a germanium image side optical element 2006, and A1 6061 spacers 2004, 2008 are used in combination with the thermally dependent optical power and thicknesses designed into optical elements 2002 and 2006 to balance thermally induced focus errors through temperature. Optical elements 2002 and 2006 are both meniscus lenses with aspheric surfaces. In this configuration, optical element 2002 is considered an object side meniscus lens (because it is closest to the object(s) being imaged) and optical element 2006 is considered an image side meniscus lens. By embedding the athermalization in this way, lens stack 2000 does not require an athermalizing barrel or an expensive focus device, and can be built as lens stacks formed from a lens wafer and spacer wafers, as discussed above.
Spacer elements can be utilized advantageously for stray light control in embodiments. In prior art lens stacks (e.g., lens stack 200, FIG. 2), stray light is typically controlled by the combined characteristics of the barrel and the lenses. In low cost prior art imagers, where stray light is not specifically controlled and may therefore arrive at the image plane, the imager may have reduced image contrast. In embodiments herein, spacers not only form at least a part of the athermalization architecture, but may also act as stray light baffles. For example, in FIG. 20, spacers 2004 and 2008 of lens stack 2000 allow light rays 2020 (solid lines) to pass through optically active portions of optical elements 2002 and 2006 and on to image plane 2010, but out-of-field light rays 2030 (dashed lines) that enter the system are blocked by the spacers.
In the example of FIG. 20, lens stack 2000 has a volume (2.87 cm3) that is 40% smaller than its closest commercially available counterpart (which is not entirely passively athermalized, and requires additional components beyond those used in this analysis). For this example, the first order parameters of the two solutions have been scaled to be equivalent for a fair comparison. The use of a barrel in the prior art commercial solution adds significant size, whereas the use of protective external covering 2012 on the sides of lens stack 2000 substantially reduces the size of lens stack 2000, as compared to commercially available counterparts that use a lens barrel for the same purpose. External protective covering 2012 may be made from a variety of materials, including IR blocking paints and coatings, or a low cost metal sleeve that is press fit over the entire lens stack 2000.
Another advantage provided by the manufacturing techniques herein is that hermetically sealed units can be produced thereby. A typical uncooled IR sensor is a microbolometer that requires a hermetically sealed environment for long term reliability. Nested barrel type lens systems typically cannot be hermetically sealed; prior art IR cameras typically utilize a window made of IR transparent material such as germanium or gallium arsenide to seal a microbolometer into a cavity before mounting the sealed sensor to the nested barrel lens system. The window adds cost and can generate a loss in signal due to Fresnel reflections from the high refractive index of the IR transparent material.
Several advantageous strategies may be utilized to design an IR imaging system that is passively athermalized. For example, it may be advantageous to constrain thermal glass constants of the object and image side lenses. A thermal glass constant is defined as




  
    
      T
      gi
    
    =
    
      
        
          
            d
            
              n
              i
            
          
          
            d
            T
          
        
        
          (
          
            
              n
              i
            
            -
            1
          
          )
        
      
      -
      
        α
        i
      
    
  
  ,




where

    
    
        i is an index variable, that is, i=1 or 2 for the object and the image side meniscus lenses respectively,
        ni is a refractive index of lens i,
        ai is a thermal expansion coefficient of lens i, and
        T is temperature, and dni/dT referss to the differential equation notation of the term ni/T.
    
    


When considering an objective that consists of two elements, the change in focus due to temperature is approximated by:




  
    dEFL
    =
    
      
        -
        
          
            EFL
            *
            Δ
            ⁢
            
              
            
            ⁢
            T
          
          4
        
      
      ⁢
      
        (
        
          
            3
            ⁢
            CTE
          
          +
          
            2
            ⁢
            
              T
              
                g
                ⁢
                
                  
                
                ⁢
                1
              
            
          
          +
          
            T
            
              g
              ⁢
              
                
              
              ⁢
              2
            
          
        
        )
      
    
  
  ,




where

    
    
        EFL=system focal length and dEFL=a change in system focal length,
        ΔT=a change in temperature
        CTE=CTE of a spacer material between the image side and object side meniscus lenses
        Tg1, Tg2 are the thermal glass constants of the object and the image side meniscus lenses, respectively.
    
    


Given these definitions, the approximate condition for an athermalized objective is:




  
    T
    
      g
      ⁢
      
        
      
      ⁢
      1
    
  
  =
  
    
      -
      
        1
        2
      
    
    ⁢
    
      
        (
        
          
            3
            ⁢
            CTE
          
          +
          
            T
            
              g
              ⁢
              
                
              
              ⁢
              2
            
          
        
        )
      
      .
    
  




In exemplary embodiments herein, Tg1 is at least 26×10−6 less than Tg2, and more particularly, Tg1 may be at least 60×10−6 less than Tg2
Lens stack 2000, FIG. 20, is an example of a system where Tg2 is at least 1.67 times Tg1. Parameters for the object side meniscus lens (i.e., surfaces 1 and 2) and the image side meniscus lens (i.e., surfaces 4 and 5) are given in Table 1. The material between the object side and image side lenses in this and in all other examples below, is aluminum.







TABLE 1





Example 1 lens stack parameters, with object side meniscus lens


formed of AMTIR-4 and image side lens formed of Germanium



















Surf
Radius
Thickness
Glass
Diameter





1
14.02745
2.002162
AMTIR-4
11.79292


2
19.7889
1.017775

10.48084


STO
Infinity
8.611625

9.6


4
12.51413
2.998523
GERMANIUM
14.28097


5
16.27892
4.112299

11.7186


Tg2-Tg1
171.4















Surf
Coeff on r 4:
Coeff on r 6:
Coeff on r 8:







1
2.16E−05
−3.94E−07 
3.74E−10



2
2.83E−05
 2.63E−07
−5.20E−09



STO



4
0.000272422
−290E−06
3.58E−08



5
0.000778757
−126E−05
1.49E−07










Another example of a system where Tg2 is at least 1.67 times Tg1 is shown in FIG. 22. Only the object side meniscus lens 2210, image side meniscus lens 2220, and exemplary rays passing through the lens system to an image plane 2230 are shown in FIG. 22. Performance of the system shown in FIG. 22 is illustrated in FIG. 23A and FIG. 23B; each of the lines of each graph illustrates performance at one of temperatures −40 C, 10 C and 60 C. The performance of the system meets the athermalized lens criterion noted above meets the athermalized lens criterion noted above, since at the zero focus position, none of the different temperature curves drops below a modulation of 0.1. Parameters for object side meniscus lens 2210 and image side meniscus lens 2220 are given in Table 2.







TABLE 2





Example 2 lens stack parameters, with object side meniscus lens


formed of AMTIR-2 and image side lens formed of Gallium Arsenide





















Surf
Radius
Thickness
Glass
Diameter







1
13.24067
2.001792
AMTIR-2
13.68846



2
18.44871
2.115521

12.56471



STO
Infinity
6.967737

9.6



4
10.80241
3.0006
GAAS
12.95704



5
14.5185
3.616475

10.57044



Tg2-Tg1
77.7
















Surf
Coeff on r 4:
Coeff on r 6:
Coeff on r 8:







1
5.07E−06
1.07E−06
−4.66E−08



2
4.27E−05
8.23E−07
−6.79E−08



STO



4
0.000295523
−249E−06 
3.57E−08



5
0.000978647
−1.14E−05 
9.16E−08










Another example of a system where Tg2 is at least 1.67 times Tg1 is shown in FIG. 24. Only the object side meniscus lens 2410, image side meniscus lens 2420, and exemplary rays passing through the lens system to an image plane 2430 are shown in FIG. 24. Performance of the system shown in FIG. 24 is illustrated in FIG. 25A and FIG. 25B; each of the lines of each graph illustrates performance at one of temperatures −40 C, 10 C and 60 C. The performance of the system meets the athermalized lens criterion noted above meets the athermalized lens criterion noted above, since at the zero focus position, none of the different temperature curves drops below a modulation of 0.1. Parameters for object side meniscus lens 2410 and image side meniscus lens 2420 are given in Table 3.







TABLE 3





Example 3 lens stack parameters, with object side meniscus lens


formed of AMTIR-2 and image side lens formed of Germanium



















Surf
Radius
Thickness
Glass
Diameter





1
14.201
1.994121
AMTIR-2
11.72873


2
19.40331
0.9784842

10.40591


STO
Infinity
8.664542

9.6


4
12.51486
2.996764
GERMANIUM
14.34517


5
16.29145
4.079677

11.78007


Tg2-Tg1
145.6















Surf
Coeff on r 4:
Coeff on r 6:
Coeff on r 8:







1
2.17E−05
−2.89E−07
−6.34E−10



2
2.76E−05
1.56E−07
1.33E−09



STO



4
0.000272429
−2.90E−06
3.57E−08



5
0.000777671
−1.26E−05
1.47E−07










Another example of a system where Tg2 is at least 1.67 times Tg1 is shown in FIG. 26. Only the object side meniscus lens 2610, image side meniscus lens 2620, and exemplary rays passing through the lens system to an image plane 2630 are shown in FIG. 26. Performance of the system shown in FIG. 26 is illustrated in FIG. 27A and FIG. 27B; each of the lines of the graph illustrates performance at one of temperatures −40 C, 10 C and 60 C. The performance of the system meets the athermalized lens criterion noted above. Parameters for object side meniscus lens 2610 and image side meniscus lens 2620 are given in Table 4.







TABLE 4





Example 4 lens stack parameters, with object side meniscus lens


formed of AMTIR-2 and image side lens formed of Silicon





















Surf
Radius
Thickness
Glass
Diameter







1
13.88453
2.001792
AMTIR-2
11.78243



2
19.06958
0.9562742

10.56639



STO
Infinity
8.037013

9.6



4
12.12694
3.000315
SILICON
13.46339



5
17.44629
4.186328

11.20679



Tg2-Tg1
82.3
















Surf
Coeff on r 4:
Coeff on r 6:
Coeff on r 8:







1
−1.59E−06
−5.13E−07
−2.78E−08



2
2.42E−05
−1.29E−06
−3.37E−08



STO



4
0.000275359
−2.81E−06
3.38E−08



5
0.000800679
−1.31E−05
1.57E−07










Another example of a system where Tg2 is at least 1.67 times Tg1 is shown in FIG. 28. Only the object side meniscus lens 2810, image side meniscus lens 2820, and exemplary rays passing through the lens system to an image plane 2830 are shown in FIG. 28. Performance of the system shown in FIG. 28 is illustrated in FIG. 29A and FIG. 29B; each of the lines of each graph illustrates performance at one of temperatures −40 C, 10 C and 60 C. The performance of the system meets the athermalized lens criterion noted above. Parameters for object side meniscus lens 2810 and image side meniscus lens 2820 are given in Table 5.







TABLE 5





Example 5 lens stack parameters, with object side meniscus lens


formed of Zinc Selenide and image side lens formed of Germanium



















Surf
Radius
Thickness
Glass
Diameter





1
12.7184
2.000568
ZNSE
12.48927


2
18.97205
1.497532

11.22439


STO
Infinity
7.944457

9.6


4
12.0265
3.000467
GERMANIUM
13.70417


5
14.90827
4.013358

11.11599


Tg2-Tg1
90.5















Surf
Coeff on r 4:
Coeff on r 6:
Coeff on r 8:







1
2.55E−05
9.27E−07
−3.27E−08



2
7.92E−05
8.24E−07
−4.87E−08



STO



4
0.000281568
−2.77E−06
3.43E−08



5
0.000810695
−1.14E−05
1.30E−07










Another example of a system where Tg2 is at least 1.67 times Tg1 is shown in FIG. 30. Only the object side meniscus lens 3010, image side meniscus lens 3020, and exemplary rays passing through the lens system to an image plane 3030 are shown in FIG. 30. Performance of the system shown in FIG. 30 is illustrated in FIGS. 31A and 31B; each of the lines of each graph illustrates performance at one of temperatures −30 C, 10 C and 50 C. The performance of the system meets the athermalized lens criterion noted above, although the temperature range of this particular lens system is reduced as compared to other lenses disclosed herein. Parameters for object side meniscus lens 3010 and image side meniscus lens 3020 are given in Table 6.







TABLE 6





Example 6 lens stack parameters, with object side meniscus lens formed


of Gallium Arsenide and image side lens formed of Germanium



















Surf
Radius
Thickness
Glass
Diameter





1
14.66519
2.0004
GAAS
12.46912


2
18.43001
1.432689

11.16288


STO
Infinity
8.030219

9.6


4
12.52965
2.999147
GERMANIUM
13.83558


5
16.19574
3.996068

11.34395


Tg2-Tg1
67.9















Surf
Coeff on r 4:
Coeff on r 6:
Coeff on r 8:







1
1.07E−05
−2.05E−07
−1.41E−08



2
3.41E−05
−6.22E−07
−1.84E−08



STO



4
0.000273252
−2.91E−06
3.50E−08



5
0.000768881
−1.27E−05
1.54E−07










Another example of a system where Tg2 is at least 1.67 times Tg1 is shown in FIG. 32. Only the object side meniscus lens 3210, image side meniscus lens 3220, and exemplary rays passing through the lens system to an image plane 3230 are shown in FIG. 32. Performance of the system shown in FIG. 32 is illustrated in FIG. 33A and FIG. 33B; each of the lines of the graph illustrates performance at one of temperatures −40 C, 10 C and 60 C. The performance of the system meets the athermalized lens criterion noted above. Parameters for object side meniscus lens 3210 and image side meniscus lens 3220 are given in Table 7.







TABLE 7





Example 7 lens stack parameters, with object side meniscus lens


formed of AMTIR-4 and image side lens formed of Gallium Arsenide





















Surf
Radius
Thickness
Glass
Diameter







1
13.7553
2.000217
AMTIR-4
11.30208



2
19.48751
0.6224783

10.177.08



STO
Infinity
8.107027

9.6



4
12.1874
2.999143
GAAS
13.35351



5
18.36456
4.334889

11.19408



Tg2-Tg1
103.5
















Surf
Coeff on r 4:
Coeff on r 6:
Coeff on r 8:







1
−3.95E−06
−8.23E−07
−3.22E−08



2
1.79E−05
−1.41E−06
−4.64E−08



STO



4
0.000273914
−2.88E−06
3.39E−08



5
0.000784306
−1.28E−05
1.55E−07










Another example of a system where Tg2 is at least 1.67 times Tg1 is shown in FIG. 34. Only the object side meniscus lens 3410, image side meniscus lens 3420, and exemplary rays passing through the lens system to an image plane 3430 are shown in FIG. 34. Performance of the system shown in FIG. 34 is illustrated in FIG. 35A and FIG. 35B; each of the lines of each graph illustrates performance at one of temperatures −40 C, 10 C and 60 C. The performance of the system meets the athermalized lens criterion noted above. Parameters for object side meniscus lens 3410 and image side meniscus lens 3420 are given in Table 8.







TABLE 8





Example 8 lens stack parameters, with object side meniscus lens


formed of AMTIR-4 and image side lens formed of AMTIR-2





















Surf
Radius
Thickness
Glass
Diameter







1
13.41408
2.002164
AMTIR-4
11.89383



2
19.0776
0.9713252

10.82319



STO
Infinity
7.267797

9.6



4
11.69409
2.99037
AMTIR-2
12.81551



5
21.42909
4.344554

11.16564



Tg2-Tg1
25.8
















Surf
Coeff on r 4:
Coeff on r 6:
Coeff on r 8:







1
−3.74E−05
−1.06E−06
−4.55E−08



2
−1.78E−05
−2.78E−06
−3.60E−08



STO



4
0.000279049
−2.82E−06
−6.75E−11



5
0.000881254
−1.61E−05
9.87E−08










Another example of a system where Tg2 is at least 1.67 times Tg1 is shown in FIG. 36. Only the object side meniscus lens 3610, image side meniscus lens 3620, and exemplary rays passing through the lens system to an image plane 3630 are shown in FIG. 36. Performance of the system shown in FIG. 36 is illustrated in FIG. 37A and FIG. 37B; each of the lines of each graph illustrates performance at one of temperatures −40 C, 10 C and 60 C. The performance of the system meets the athermalized lens criterion noted above. Parameters for object side meniscus lens 3610 and image side meniscus lens 3620 are given in Table 9.







TABLE 9





Example 9 lens stack parameters, with object side meniscus lens


formed of AMTIR-4 and image side lens formed of Silicon





















Surf
Radius
Thickness
Glass
Diameter







1
13.79301
2.00216
AMTIR-4
11.28861



2
19.53293
0.6495318

10.10299



STO
Infinity
8.186761

9.6



4
12.25933
2.999695
SILICON
16.99682



5
17.67549
4.313474

14.34624



Tg2-Tg1
108.1
















Surf
Coeff on r 4:
Coeff on r 6:
Coeff on r 8:







1
3.82E−07
−8.27E−07
−3.94E−08



2
2.13E−05
−1.33E−06
−5.36E−08



STO



4
0.00027401
−2.88E−06
3.40E−08



5
0.000779558
−1.28E−05
1.60E−07










In another strategy to design an IR imaging system that is passively athermalized, it is advantageous to constrain two aspheric meniscus lenses such that an optical power of the image side meniscus lens is at least 1.6 times an optical power of the object side meniscus lens. This is useful for both aberration correction and for athermalization. Also, when thermal glass constants of the two lenses are not constrained, certain resulting lens stacks may utilize two lenses of the same material, which may help in reducing cost.
An example of a system where an optical power of the image side meniscus lens is at least 1.6 times an optical power of the object side meniscus lens is shown in FIG. 38. Only the object side meniscus lens 3810, image side meniscus lens 3820, and exemplary rays passing through the lens system to an image plane 3830 are shown in FIG. 38. Performance of the system shown in FIG. 38 is illustrated in FIG. 39A and FIG. 39B; each of the lines of each graph illustrates performance at one of temperatures −40 C, 10 C and 60 C. The performance of the system meets the athermalized lens criterion noted above. Parameters for object side meniscus lens 3810 and image side meniscus lens 3820 are given in Table 10.







TABLE 10





Example 10 lens stack parameters, with object side meniscus


lens and image side lens both formed of Silicon





















Surf
Radius
Thickness
Glass
Diameter







1
14.60529
1.973347
SILICON
12.77556



2
18.07738
1.594933

11.59552



STO
Infinity
7.414748

9.6



4
12.12362
3.000314
SILICON
13.21068



5
17.39618
4.028685

11.17493



Tg2-Tg1
0
















Surf
Coeff on r 4:
Coeff on r 6:
Coeff on r 8:







1
5.97E−06
−7.02E−07
−2.20E−08



2
3.09E−05
−1.91E−06
−2.04E−08



STO



4
0.00026331
−2.94E−06
1.75E−08



5
0.00079123
−1.46E−05
1.07E−07










A further example of a system where Tg2 is at least 1.67 times Tg1 is shown in FIG. 40. (In this example, both of the lenses are formed of AMTIR-2, which has a negative Tg, so Tg2 is not as negative as 1.67 times Tg1.) Only the object side meniscus lens 4010, image side meniscus lens 4020, and exemplary rays passing through the lens system to an image plane 4030 are shown in FIG. 40. Performance of the system shown in FIG. 40 is illustrated in FIG. 41A and FIG. 41B; each of the lines of each graph illustrates performance at one of temperatures −40 C, 10 C and 60 C. The performance of the system meets the athermalized lens criterion noted above. Parameters for object side meniscus lens 4010 and image side meniscus lens 4020 are given in Table 11.







TABLE 11





Example 11 lens stack parameters, with object side meniscus


lens and image side lens both formed of AMTIR-2





















Surf
Radius
Thickness
Glass
Diameter







1
10.78755
1.362582
AMTIR-2
12.18862



2
13.89166
1.816183

11.22592



STO
Infinity
7.200517

9.6



4
9.866962
3.00269
AMTIR-2
12.34325



5
14.30628
3.749132

10.04798



Tg2-Tg1
0
















Surf
Coeff on r 4:
Coeff on r 6:
Coeff on r 8:







1
3.99E−05
6.17E−06
−4.67E−08



2
8.95E−05
1.03E−05
−1.29E−07



STO



4
0.000362993
−5.56E−06
1.09E−07



5
0.001165077
−1.73E−05
3.17E−07










In exemplary embodiments herein, Tg2 is greater than (1.67*Tg1) in the solutions that meet the athermalization criteria noted above. More particularly, Tg2 may be greater than (1.25*Tg1+50×10−6), as can be shown in FIG. 44. FIG. 44 has units of 10−6 and shows the top performing material combinations, indicated by diamonds 4100, and the worst performing material combinations, indicated by squares 4102. For the sake of clarity, not all material combinations are numbered.
Combinations of Features
Features described above as well as those claimed below may be combined in various ways without departing from the scope hereof. The following examples illustrate possible, non-limiting combinations the present invention has been described above, it should be clear that many changes and modifications may be made to the process and product without departing from the spirit and scope of this invention:
(a) A passively athermalized infrared imaging system includes an object side meniscus lens that forms at least one aspheric surface, and an image side meniscus lens that forms two aspheric surfaces. Each of the meniscus lenses is formed of a material selected from the group consisting of a chalcogenide glass, germanium, silicon, gallium arsenide, zinc selenide and glass. An optical power of the image side meniscus lens is at least 1.6 times an optical power of the object side meniscus lens, such that an effective focus position of the imaging system is athermalized over a range of 0 to +40 degrees Celsius.
(b) In system denoted as (a), athermalized may be defined as a polychromatic, through-focus modulation transfer function (“MTF”) being at least 0.1 over the range of 0 to +40 degrees Celsius, at 1⁄2 of an optical cutoff frequency of the lens (1/((f/#)*λ)); wherein “f/#” refers to the f-number of the lens and λ refers to wavelength.
(c) In the system/s denoted as (a) or (b), a thermal glass constant of the object side meniscus lens may be equal to a thermal glass constant of the image side meniscus lens.
(d) In the system denoted as (c), the thermal glass constants may be defined as




  
    
      T
      gi
    
    =
    
      
        
          
            d
            
              n
              i
            
          
          
            d
            T
          
        
        
          (
          
            
              n
              i
            
            -
            1
          
          )
        
      
      -
      
        α
        i
      
    
  
  ,




where
i is 1 or 2 for the object and image side meniscus lenses, respectively,
ni is a refractive index of lens i,
ai is a thermal expansion coefficient of lens i, and
T is temperature, and
dni/dT refers to the differential equation notation of the term ni/T.
(e) In the system/s denoted as (a)-(d), each of the object side meniscus lens and the image side meniscus lens may include chalcogenide glass.
(f) In the system/s denoted as (a)-(d), each of the object side meniscus lens and the image side meniscus lens may include silicon.
(g) In the system/s denoted as (a)-(f), the system may be free of powered optics except for the object side and image side meniscus lenses.
(h) In the system/s denoted as (a)-(d) and (h), the object side meniscus lens may be molded of a chalcogenide glass.
(i) In the system/s denoted as (a)-(h), a thermal image may be formed at an image plane. A distance between the image side meniscus lens and the image plane is set by a first fixed spacer element therebetween, and a distance between the object side and image side meniscus lenses is set by a second fixed spacer element therebetween.
(j) In the system/s denoted as (a)-(i), the effective focus position of the imaging system may be athermalized over a range of −40 to +60 degrees Celsius.
(k) A passively athermalized infrared imaging system includes an object side meniscus lens, formed of a first material that transmits infrared radiation, that forms at least one aspheric surface; and an image side meniscus lens, formed of a second material that transmits infrared radiation, that forms two aspheric surfaces. The object side and image side meniscus lenses cooperate to form a thermal image and being concave towards the image. The first and second materials have thermal glass constants Tg1 and Tg2 respectively, wherein Tg2 is at least 1.67 times Tg1, such that an effective focus position of the system is athermalized over a temperature range of 0 to +40 degrees Celsius.
(I) In the system denoted as (k), athermalized may be defined as a polychromatic, through-focus modulation transfer function (“MTF”) being at least 0.1 over the range of 0 to +40 degrees Celsius, at 1⁄2 of an optical cutoff frequency of the lens (1/((f/#)*λ)); wherein “f/#” refers to the f-number of the lens and λ refers to wavelength.
(m) In the system/s denoted as (k) or (l), the system may be free of powered optics except for the first and second lens.
(n) In system/s denoted as (k)-(m), the thermal image forms at an image plane, and a distance between the second lens and the image plane may be set by a first fixed spacer element therebetween. A distance between the first lens and the second lens may be set by a second fixed spacer element therebetween.
(o) In system/s denoted as (k)-(n), the first and second materials may be selected from the group consisting of a chalcogenide glass, germanium, silicon, gallium arsenide, zinc selenide and glass.
(p) In system/s denoted as (k)-(o), each of the first and second materials may be chalcogenide glass.
(q) In system/s denoted as (o), Tg2 may be at least (1.25 times Tg1 plus 50×10−6), and each of the first and second materials may be a different one of said group.
(r) In system/s denoted as (k)-(o) or (q), the first material may be a chalcogenide glass, and the second material may be germanium.
(s) In system/s denoted as (k)-(o) or (q), the first material may be a chalcogenide glass and the second material may be gallium arsenide.
(t) In system/s denoted as (k)-(o) or (q), the first material may be a chalcogenide glass and the second material may be silicon.
(u) In system/s denoted as (k)-(o) or (q), the first material may be zinc selenide and the second material may be germanium.
(v) In system/s denoted as (k)-(o) or (q), the first material may be gallium arsenide and the second material may be germanium.
(w) In system/s denoted as (k)-(v), the effective focus position of the imaging system may be athermalized over a range of −40 to +60 degrees Celsius.
(x) A method of aligning elements in a manufacturing process includes placing a middle element onto a base element, the base element forming first alignment features, the middle element forming apertures therethrough corresponding to the first alignment features. Second alignment features of an upper element are placed onto the first alignment features such that the first and second alignment features cooperate, through the apertures, to align the upper element with the base element.
(y) In the method denoted as (x), the steps of placing the middle element and the second alignment features aligns the middle element to the base element and the upper element with a tolerance corresponding to a clearance of the apertures around the first and second alignment features.
(z) In the method/s denoted as (x) or (y), the base, middle and upper elements may be bonded together while the alignment features cooperate, to form a composite element; and the composite element may be diced to generate a plurality of subunits that do not include any portion of the first or second alignment features, but in which respective portions of the base and upper elements remain in alignment.
(a1) In the method/s denoted as (z) bonding includes utilizing an adhesive.
(b1) A method of aligning elements in a manufacturing process includes placing a middle element onto a base element. The base element forms first alignment features, and the middle element forms apertures therethrough corresponding to the first alignment features. Intermediate alignment elements are placed upon the first alignment features; and second alignment features of an upper element are placed onto the intermediate alignment features such that the first, intermediate, and second alignment features cooperate to align the upper element with the base element.
(c1) In the method denoted as (b1), placing the intermediate alignment elements includes placing substantially spherical elements into recesses in the base element that form the first alignment elements.
(d1) In the method/s denoted as (b1) or (c1), the steps of placing the middle element, the intermediate alignment elements and the second alignment features may align the middle element to the base element and the upper element with a tolerance corresponding to a clearance of the apertures around the intermediate alignment elements.
(e1) A plurality of infrared lens systems includes at least one first lens wafer having a first base material that is opaque to infrared radiation, with infrared transmissive material inset into apertures therein to form first lenses. The first lens wafer is bonded with at least one second lens wafer having a plurality of second lenses, such that pluralities of the first and second lenses align to form the lens systems.
(f1) In the plurality of systems denoted as (e1), the infrared transmissive material may be inset into carriers that are inset into the first base material.
(g1) In the plurality of systems denoted as (e1) or (f1), the first base material may transmit at least one of visible and ultraviolet radiation.
(h1) In the plurality of systems denoted as (e1)-(g1), the second lens wafer may include a second base material that is opaque to infrared radiation, with infrared transmissive material inset into apertures therein to form the second lenses.
(i1) In the plurality of systems denoted as (h1), the first base material may transmit at least one of visible and ultraviolet radiation.
(j1) In the plurality of systems denoted as (e1)-(i1), the first base material may be one of metal, plastic, ceramic and composite.
(k1) In the plurality of systems denoted as (e1)-(j1), the first and second wafers may be joined to one another.
(l1) In the plurality of systems denoted as (e1)-(k1), the first and second lens wafers may be mechanically joined.
(m1) In the plurality of systems denoted as (e1)-(l1), the first and second lens wafers may be joined with an adhesive.
(n1) In the plurality of systems denoted as (e1)-(m1), the first and second lens wafers may be diced to separate each of the lens systems from one another.
(o1) In the plurality of systems denoted as (n1), after dicing, the lens systems may not include alignment features utilized to align the first and second lens wafers.
(p1) The plurality of systems denoted as (e1)-(o1) may further include a sensor aligned to at least one of the lens systems.
(q1) In the plurality of systems denoted as (p1), the sensor may be hermetically sealed to the at least one of the lens systems.
(r1) The plurality of systems denoted as (e1)-(o1) may further include a plurality of spacers, each of the spacers being disposed between and bonded with one of the first lenses and one of the second lenses, each of the spacers forming an aperture along an optical axis formed by the ones of the first and second lenses.
(s1) The plurality of systems denoted as (e1)-(q1) may further include a spacer layer forming a plurality of apertures between each of the first and second lenses, the spacer layer being disposed between and bonded with the first and second lens wafers.
(t1) In the plurality of systems denoted as (s1), the spacer layer may be opaque to infrared energy.
(u1) In the plurality of systems denoted as (s1) or (t1), the spacer layer may be one of metal, plastic, ceramic and composite.
(v1) The plurality of systems denoted as (e1)-(u1) may further include a sensor array layer having a plurality of sensors, each sensor aligning to a corresponding one of the lens systems.
(w1) In the plurality of systems denoted as (v1), each of the sensors may be hermetically sealed with the corresponding one of the lens systems.
(x1) In the plurality of systems denoted as (v1) or (w1), each of the sensors is hermetically sealed with the corresponding one of the lens systems without a cover plate over the sensor array.
(y1) A lens wafer for use in optical manufacturing includes a substrate forming apertures therein, the substrate being formed of a base material; and a plurality of lenses, each of the lenses including an optical material and disposed within a respective one of the apertures.
(z1) In the wafer denoted as (y1), the base material may be or include one of metal, plastic, ceramic and composite.
(a2) In the wafer denoted as (y1) or (z1), each of the plurality of lenses may be at least 200 microns from each other of the plurality of lenses such that the substrate can be diced to singulate portions of the substrate having a single lens therein without damaging any of the plurality of lenses.
(b2) In the wafer denoted as (y1)-(a2), the optical material may be transparent to IR radiation.
(c2) In the wafer denoted as (y1)-(b2), the optical material may be or include one or more of a chalcogenide glass, a crystalline material, a salt, plastic or glass.
(d2) The wafer denoted as (y1)-(c2) may further include mechanical alignment features therein for alignment of the lens wafer to a second lens wafer.
(e2) In the wafer denoted as (y1)-(d2), each of the lenses may be disposed within its respective aperture in a nonfinal form, and may be subsequently shaped to a final form while in the aperture.
(f2) In the wafer denoted as (e2), each of the lenses may be shaped to the final form by one of polishing, grinding, diamond turning, magnetorheological finishing, compression molding or injection molding.
(g2) In the wafer denoted as (y1)-(f2), each of the plurality of lenses may form an outwardly facing flange, and the substrate may be or include plastic that is overmolded about each of the flanges.
(h2) In the wafer denoted as (g2), each of the outwardly facing flanges may form an alignment feature that cooperates with a mold, to align each respective lens within the lens wafer as the plastic is overmolded about each of the flanges using the mold.
(i2) In the wafer denoted as (g2), the substrate may include alignment features for subsequent alignment of the substrate, and the alignment features may be formed as the plastic is overmolded about each of the flanges using the mold.
(j2) In the wafer/s denoted as (y1)-(g2), each of the plurality of lenses may be held by a respective lens carrier.
(k2) In the wafer denoted as (j2), the lens carriers may be held within respective apertures of the substrate with an adhesive.
(l2) In the wafer/s denoted as (j2) or (k2), each of the lens carriers may be or include one or more of metal, plastic and ceramic.
(m2) In the wafer/s denoted as (j2)-(l2), each of the lens carriers may include alignment features for alignment of the lens carriers with the lens wafer.
(n2) An infrared lens assembly includes a lens formed of an infrared transmitting material that is disposed within a carrier of a base material, the lens being molded within the carrier with at least one feature that secures the lens to the carrier.
(o2) In the assembly denoted as (n2), the lens may include one or more of a chalcogenide glass, plastic, glass, a crystalline material, and a salt.
(p2) In the assembly/ies denoted as (n2) and (o2), the carrier may form one of a recess and a protrusion that engages the at least one feature of the lens.
(q2) In the assembly/ies denoted as (n2)-(p2), the carrier may form at least one alignment feature for aligning the carrier to another carrier.
(r2) In the assembly/ies denoted as (n2)-(q2), the base material may include plastic.
(s2) In the assembly/ies denoted as (n2)-(q2), the base material may transmit visible radiation and be opaque to infrared radiation.
(t2) A mold set includes an upper mold and a lower mold, at least one of the upper and lower molds having one or more features that are configured to hold an infrared lens in an aligned position, the upper and lower molds configured to provide a cavity for molding a moldable material into one of a lens carrier and a lens wafer about the infrared lens.
(u2) In the mold set denoted as (t2), at least one of the upper and lower molds may include a dam for preventing the moldable material from obscuring an optical surface of the infrared lens.
(v2) An infrared lens assembly includes a lens formed of an infrared transmitting material that is disposed within a carrier of a base material, the carrier being molded around the lens with at least one feature that secures the lens to the carrier.
(w2) An infrared lens stack includes a first lens formed of an infrared transmitting material and disposed within a first carrier, formed of a base material, that forms at least one first alignment feature; and a second lens formed of an infrared transmitting material and disposed within a second carrier, formed of a base material, that forms at least one second alignment feature configured to cooperate with the first alignment feature.
(x2) In the infrared lens stack/s denoted as (v2) or (w2), the carriers may be bonded together.
(y2) In the infrared lens stack/s denoted as (v2)-(x2), any of the base materials may include plastic.
(z2) In the infrared lens stack/s denoted as (v2)-(y2), plastic may transmit at least one of visible and ultraviolet radiation but be opaque to infrared radiation.
Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.

## Claims

1. A passively athermalized infrared imaging system, comprising:
a single, positive object side meniscuslens that forms at least one aspheric surface, and a single, positive image side meniscus singlet lens that forms two aspheric surfaces, each of the meniscus lenses formed of a material selected from the group consisting of a chalcogenide glass, germanium, silicon, gallium arsenide, zinc selenide and glass;
an optical power of the image side meniscus lens being at least 1.6 times an optical power of the object side meniscus lens;
such that an effective focus position of the imaging system is athermalized over a range of 0 to +40 degrees Celsius;
wherein the object side meniscus singlet lens and the image side meniscus singlet lens are the only powered lenses in the passively athermalized infrared imaging system.

2. The system of claim 1, wherein athermalized is defined as a polychromatic, through-focus modulation transfer function (“MTF”) being at least 0.1 over the range of 0 to +40 degrees Celsius, at 1⁄2 of an optical cutoff frequency of the lens (1/((f/#)*λ)); wherein “f/#” refers to the f-number of the lens and λ refers to wavelength.

3. The system of claim 1, a thermal glass constant of the object side meniscus lens being equal to a thermal glass constant of the image side meniscus lens.

4. The system of claim 3, the thermal glass constants being defined as




  
    
      T
      gi
    
    =
    
      
        
          
            d
            
              n
              i
            
          
          
            d
            T
          
        
        
          (
          
            
              n
              i
            
            -
            1
          
          )
        
      
      -
      
        α
        i
      
    
  
  ,



where
i is 1 or 2 for the object and the image side meniscus lenses respectively,
ni is a refractive index of lens i,
ai is a thermal expansion coefficient of lens i, and
T is temperature, and dni/dT is the first order derivative of ni/T.

5. The system of claim 3, each of the object side meniscus lens and the image side meniscus lens comprising chalcogenide glass.

6. The system of claim 3, each of the object side meniscus lens and the image side meniscus lens comprising silicon.

7. The system of claim 1, wherein the system is free of powered optics except for the object side and image side meniscus lenses.

8. The system of claim 1, wherein the object side meniscus lens is molded of a chalcogenide glass.

9. The system of claim 1, wherein the system forms a thermal image at an image plane, and wherein
a distance between the image side meniscus lens and the image plane is set by a first fixed spacer element therebetween, and
a distance between the object side and image side meniscus lenses is set by a second fixed spacer element therebetween.

10. The system of claim 1, wherein the effective focus position of the imaging system is athermalized over a range of −40 to +60 degrees Celsius.

11. A passively athermalized infrared imaging system, comprising:
a single object side meniscuslens, formed of a first material that transmits infrared radiation, that forms at least one aspheric surface; and
a single image side meniscus singlet lens, formed of a second material that transmits infrared radiation, that forms two aspheric surfaces;
the object side and image side meniscus lenses cooperating to form a thermal image and being concave towards the image;
the first and second materials having thermal glass constants Tg1 and Tg2 respectively, wherein Tg2 is at least 1.67 times Tg1,
such that an effective focus position of the system is athermalized over a temperature range of 0 to +40 degrees Celsius;
wherein the object side meniscus singlet lens and the image side meniscus singlet lens are the only powered lenses in the passively athermalized infrared imaging system.

12. The system of claim 11, wherein athermalized is defined as a polychromatic, through-focus modulation transfer function (“MTF”) being at least 0.1 over the range of 0 to +40 degrees Celsius, at 1⁄2 of an optical cutoff frequency of the lens (1/((f/#)*λ)); wherein “f/#” refers to the f-number of the lens and λ refers to wavelength.

13. The system of claim 11, wherein the system is free of powered optics except for the first and second lens.

14. The system of claim 11, the thermal image forming at an image plane, and wherein
a distance between the second lens and the image plane is set by a first fixed spacer element therebetween, and
a distance between the first lens and the second lens is set by a second fixed spacer element therebetween.

15. The system of claim 11, the first and second materials being selected from the group consisting of a chalcogenide glass, germanium, silicon, gallium arsenide, zinc selenide and glass.

16. The system of claim 15, each of the first and second materials comprising chalcogenide glass.

17. The system of claim 15, wherein Tg2 is at least (1.25 times Tg1 plus 50×10−6), and wherein each of the first and second materials is a different one of said group.

18. The system of claim 17, the first material comprising a chalcogenide glass, the second material comprising germanium.

19. The system of claim 17, the first material comprising a chalcogenide glass, the second material comprising gallium arsenide.

20. The system of claim 17, the first material comprising a chalcogenide glass, the second material comprising silicon.

21. The system of claim 17, the first material comprising zinc selenide, the second material comprising germanium.

22. The system of claim 17, the first material comprising gallium arsenide, the second material comprising germanium.

23. The system of claim 11, wherein the effective focus position of the imaging system is athermalized over a range of −40 to +60 degrees Celsius.