Dual field of view optics with non-mechanical switching

An all-refractive optical system that images a scene at two different fields of view or FOVs, with switching between FOVs enabled by switchable lens elements is disclosed. The two fields of view vary in focal length by a factor of three. The wide FOV images broad-band Short Wave InfraRed SWIR radiation at an f/number of 1.7, while the narrow FOV images narrow-band illuminated SWIR at f/4.9. A voltage change across the switchable lens elements generates an optical power change between finite focus and infinite focus. Situated among static optical elements, the switching elements enable FOV changes with no mechanical movement. The given f/numbers at each FOV are a result of a fixed aperture in the system. The smaller throughput in the narrow FOV mode is augmented by narrow-band illumination of the scene to maintain equivalent sensor response between the two FOVs.

FIELD OF THE INVENTION

This invention relates in general to optics, and more particularly, to dual field of view optics.

BACKGROUND OF THE INVENTION

Smaller size and weight factor greatly into user selection for many handheld, head-mounted, or airborne imaging systems, even when the added weight or size can provide otherwise useful capabilities. The conventional means for field of view (FOV) switching generally requires more elements and/or longer optical paths over equivalent fixed FOV systems. As reconfigurable optic technologies are becoming more mature, switchable optic elements integrated with static optics can provide FOV switching capability in a smaller size and weight than a conventional zoom system. A variety of reconfigurable lenses are discussed in literature. Ye, et al. (2004) and Li, et al. (2010) give examples of two different types of Liquid Crystal (LC) lenses that vary the radial index profile of the LC material by changing the voltage across different electrode configurations. See, Ye et al., “Liquid-crystal lens with a focal length that is variable in a wide range.” Applied Optics, Vol. 43, No. 35, pp. 6407-6412 (2004). See, also, Li et al., “Liquid crystal lenses: Liquid crystals promise compact lenses with variable focus.” Laser Focus World, December 2010, accessible online. As long as the effects of diffraction are small, color dispersion is expected to be similar to that experienced by a conventional lens of the same optical power. Liquid Crystal Diffractive Lenses (LCDLs) are discussed in Valley, et al. (2010) and U.S. Pat. No. 5,751,471 to Chen, et al (1998). See, Valley et al., “Nonmechanical bifocal zoom telescope,” Optics Letters, Vol. 35, No. 15, pp. 2582-2584 (2010). See, also, U.S. Pat. No. 5,751,471, entitled, “Switchable lens and method of making,” issued May 12, 1998 to Chen et al. The color dispersion in LCDLs is much more pronounced.

Bagwell, et al. (2005 and 2006) describe several LC and non-LC reconfigurable elements assembled into non-mechanical zoom configurations, including one employing Liquid Crystal Diffractive Lenses (LCDLs). See, Bagwell et al., “Adaptive Optical Zoom Sensor,” Sandia Report SAND2005-7208, Sandia National Laboratories, November 2005. See, also, Bagwell et al., “Liquid crystal based active optics,” Proc. SPIE 6289, Novel Optical Systems Design and Optimization IX, 628908-1 (2006). From the above cited literature, two main challenges exist when integrating LCDLs into a zoom system, namely large color dispersion and sensor illumination changes with FOV.

SUMMARY OF THE INVENTION

The present disclosure has resolved these challenges and enables a dual FOV lens to be made using LCDLs. Specifically, an exemplary compact optical imager that can switch field of view (FOV) without mechanical motion is disclosed. Alternatively, an exemplary compact two field of view imager which uses switchable lenses to generate two different effective focal lengths for the system. Yet, in another aspect, an exemplary compact two field of view refractive imager is disclosed which uses the finite focus (“lens”) and infinite focus (“clear”) states of switchable lenses to alter the effective focal length of the imager such that two different f/numbers and FOVs are achieved. While the various exemplary solutions may have been developed for the particular case of LCDLs, it should be noted that the described exemplary solutions also encompass other reconfigurable elements that may be configured in the disclosed exemplary FOV switching systems.

An exemplary five-element (two reconfigurable and three static) imaging system is disclosed where the field of view is changed by switching the two reconfigurable elements between a finite focus (“lens”) state and an infinite focus (“clear”) state. The wide FOV images broad-band Short Wave InfraRed (SWIR) radiation at an f/number of 1.7, while the narrow FOV images narrow-band illuminated SWIR at f/4.9. The f/number in both cases is defined by a fixed aperture. The ratio of focal length change between the two FOVs is three. This results in an FOV change of the same ratio.

Two aspects of the present invention help to resolve the challenges encountered when implementing LCDLs in a FOV switching imager. First, the disclosed exemplary dual field of view system is configured such that the switchable diffractive elements are switchable between a “lens” state and a “clear” state. In the “clear” state, the LCDLs have no optical power and color dispersion is effectively zero. This state is designed into the wide FOV, such that suitable optical performance is achieved across the natural broad-band SWIR illumination in a scene. The five elements are configured to be well corrected for color dispersion in broad-band SWIR at the wide FOV setting. A second aspect of this invention that resolves much of the concern over high dispersion and low sensor illumination in the narrow FOV is that the system is configured to use a narrow-band illuminator on the scene at this FOV, in a manner similar to US Patent Application Publication US 2013/0044221 (Vizgaitis, 2011), incorporated herein by reference. See, U.S. Patent Application Publication No. US 2013/0044221 A1 by Vizgaitis, J, entitled, “Dual Field of View Multi-Band Optics,” published Feb. 21, 2013. In the narrow FOV configuration, the LCDLs are in the “lens” state. Narrow-band illumination ensures that 1) the narrow-band signal overwhelms any broad-band signal that would otherwise be highly dispersed by the LCDLs and 2) the sensor has sufficient illumination to compensate for the smaller throughput at this larger f/number. This helps the narrow FOV illumination to be comparable to that of the larger throughput wide FOV.

DETAILED DESCRIPTION

Before entering into the detailed description of one embodiment of the present invention according to the accompanying Figure, the theory of the present invention will be explained hereinafter.

An exemplary five-lens imaging system is disclosed where the field of view is changed by electrically switching1and5between an infinite focus state and finite focus states. Electrically switchable lenses are separately described below with respect toFIGS. 3a-3c.

In one aspect,Figure 1ashows an exemplary optical raytrace of the five-element imager in the wide FOV with the rays traced for broad-band SWIR radiation (e.g., of 1.0-1.7 μm). As tabulated inFIG. 2, both the front and back aperture diameters of an electrically switchable lens1can be 38.1 mm. The electrically switchable lens1is depicted inFIG. 1aas receiving wide FOV broadband rays (e.g., of 1.0-1.7 μm) on its front aperture. Lens2is configured as an exemplary meniscus lens with its front aperture facing the back aperture of the electrically switchable lens1. The front aperture of lens2can have an aperture diameter of 33 mm and Radius of Curvature (ROC) of 37.33 mm, its back aperture having an aperture diameter of 29 mm and ROC of 112.85 mm. Further, the material composition of lens2can be based on ZnS. Lens3is configured as an exemplary negative lens with its front aperture facing the back aperture of lens2. The front aperture of lens3can have an aperture diameter of 17 mm and ROC of −56.55 mm, its back aperture having an aperture diameter of 20 mm and ROC of 33.23 mm. Further, the material composition of lens3can be based on AMTIR-1. Lens4is configured as an exemplary positive lens with its front aperture facing the back aperture of lens3. The front aperture of lens4can have an aperture diameter of 30 mm and ROC of 67.46 mm, its back aperture having an aperture diameter of 30 mm and ROC of −58.07 mm. Further, the material composition of lens4can be based on ZnSe. Electrically switchable lens5configured to face the back aperture of lens4can have both its front and back aperture diameters being 25.4 mm. Its front aperture is configured to face the back aperture of lens4. The wide FOV of such an exemplary five-element imager as depicted inFIG. 1acan be achieved by electrically setting both electrically switchable lenses1and5in an infinite focus state. The infinite focus (“clear”) state allows the lens elements to pass broad-band SWIR illumination, which when coupled with the achromatic configuration of lens2through lens4allows unfiltered light to be collected into a highly resolved image on an appropriate SWIR focal plane array6.

In another aspect,FIG. 1bshows an exemplary optical raytrace of the five-element imager in the narrow FOV with the rays traced for a narrow spectral band centered around 1.55 microns (e.g., 1.54-1.56 μm). As tabulated inFIG. 2, both the front and back aperture diameters of an electrically switchable lens1can be 38.1 mm. The electrically switchable lens1is depicted inFIG. 1bas receiving narrow FOV rays centered around 1.55 microns (e.g., 1.54-1.56 μm) on its front aperture. Lens2is configured as an exemplary meniscus lens with its front aperture facing the back aperture of the electrically switchable lens1. The front aperture of lens2can have an aperture diameter of 33 mm and Radius of Curvature (ROC) of 37.33 mm, its back aperture having an aperture diameter of 29 mm and ROC of 112.85 mm. Further, the material composition of lens2can be based on ZnS. Lens3is configured as an exemplary negative lens with its front aperture facing the back aperture of lens2. The front aperture of lens3can have an aperture diameter of 17 mm and ROC of −56.55 mm, its back aperture having an aperture diameter of 20 mm and ROC of 33.23 mm. Further, the material composition of lens3can be based on AMTIR-1. Lens4is configured as an exemplary positive lens with its front aperture facing the back aperture of lens3. The front aperture of lens4can have an aperture diameter of 30 mm and ROC of 67.46 mm, its back aperture having an aperture diameter of 30 mm and ROC of −58.07 mm. Further, the material composition of lens4can be based on ZnSe. Electrically switchable lens5configured to face the back aperture of lens4can have both its front and back aperture diameters being 25.4 mm. The FOV change to the depicted narrow FOV ofFIG. 1boccurs by electrically switching lens1to a state of positive-focus, and electrically switching lens5to a state of negative-focus towards an appropriate SWIR focal plane array6. Active illumination of a narrow FOV scene helps to boost the total light energy collected at the focal plane array6.

In the infinite focus (“clear”) state (FIG. 1a), the elements pass broad-band SWIR illumination, which when coupled with the achromatic design of elements2through4allows unfiltered light to be collected into a highly resolved image on an appropriate SWIR focal plane array6. The light throughput in this FOV is high, such that the imager operates in passive image collection mode. The FOV is switched to narrow mode (FIG. 1b) by changing the states of elements1and5to positive focus and negative focus, respectively. The focus states of these elements are highly dispersive with wavelength, and the optical performance of the imager depends on active illumination of the scene with a narrow-band illuminator, having a 20 nanometer bandwidth and center wavelength of 1.55 microns. The clear aperture of element2serves as a fixed system aperture, rendering a roughly 3× increase in f/number based on the 3× focal length increase when switching from wide FOV to narrow FOV. The active illumination of the narrow FOV scene helps the total light energy collected at6to match that collected in the wide FOV. Light throughput is also dependant on the transmission of the optical elements, which are designed to pass SWIR illumination from 1.0 to 1.7 microns. Zinc Sulfide, AMTIR 1, and Zinc Selenide are used for the static elements and suitable anti-reflection coatings are needed for high SWIR transmittance. The switching elements are also designed to transmit SWIR light.

FIG. 2depicts exemplary raytrace parameters for the imager shown inFIG. 1. Lenses2and3are entirely made with spherical surfaces, the first surface of 2 being also used for the aperture stop. Lens4has a 10thorder even asphere on the front surface and a spherical surface on the back. The aspheric terms contribute to the lens sag (linear departure from the vertex plane) z:

z=r2R+R2-(1+k)⁢r2+A1⁢r4+A2⁢r6+A3⁢r8+A4⁢r10,
where r is the radial distance from the vertex, R is the radius of curvature, k is the conic constant, and Anindicates aspheric coefficients. The asphere helps to correct spherical aberration and coma that would otherwise render the image at6highly aberrated. The full aperture of elements4and5is only used in the wide FOV, as the narrow FOV requires a much longer system focal length and thus the rays have a much smaller footprint on these last two elements. The smaller footprint on5for the narrow FOV also means that any patterning used to generate the lens state for that element need only extend as far as the smaller footprint requires. The remaining aperture can be clear, as this is the state element5will be using in the wide FOV (full aperture).

FIGS. 3a-3cvariously illustrate three exemplary states of focus for switchable lenses:FIG. 3aillustrates an exemplary infinite focus state;FIG. 3billustrates an exemplary positive focus state; andFIG. 3cillustrates an exemplary negative focus state. The positive and negative focus states impose a radially varying phase change on the light passing through the lens, applying curvature to the initially flat optical wavefront. The infinite focus state imparts a radially uniform phase, such that the passing wavefront maintains its original form. The states of a switchable lens (e.g.,FIGS. 3a-3c) enable the FOV change in the exemplary imager shown inFIGS. 1aand1b. A wavefront of light, shown in this case as a plane wave7, enters the switchable lens8and either passes through unchanged9or changes10curvature, based on whether the lens is infinite focus (FIG. 3a), positive focus (FIG. 3b), or negative focus (FIG. 3c).

A common procedure for generating a switchable lens is to write a holographic image of the desired lens power into the photosensitive layer of an LC cell.FIG. 4illustrates a side profile view of an exemplary electronically switchable lens generated by a holographic technique. An LC material12is suspended in a glass cell13that is highly transparent to light. The inside walls of the cell11are coated with a transparent electrode, such that an electrical signal applied to wires14generates an electric field across the LC material12. The holographic pattern that constitutes a lens phase distribution resides in the LC material layer12. The lens pattern thus recorded is a periodic phase profile, whose amplitude is controlled by applying a voltage signal across the LC cell. Light passing through the LCDL will undergo deformation of its wavefront according to this phase profile. A special subset of LCDLs using concentric polarization gratings (Oh, 2009) is constructed to switch between two possible states: clear (FIG. 3a) and a superposition of both lens states (FIGS. 3b-c). Oh, C., “Broadband Polarization Gratings for Efficient Liquid Crystal Display, Beam Steering, Spectropolarimetry, and Fresnel Zone Plate.” PhD Dissertation, North Carolina State University, 2009 is hereby incorporated by reference. In the lens state, a polarizer is used to select between positive focus and negative focus with high efficiency.

FIGS. 5aand5bshow microscope images of an exemplary electronically switchable lens generated with the above referenced technique.FIG. 5awas captured while the element was in the lens state with no voltage applied to the element. In the lens state, the concentric rings characteristic of a lens holographic diffraction pattern can be seen in the image.FIG. 5bshows the element switched to the clear state as a result of applying a 3.0 volt peak to peak square wave oscillating at 2 kilohertz to the LC cell wires14. In the clear state, the diffraction pattern clears and the illumination from the microscope backlighting is seen in the image. Both microscope images are taken through parallel polarizers with a narrow-band SWIR filter centered on 1.55 μm.

FIGS. 6aand6billustrate the Optical Path Difference (OPD) imposed by element1in the positive focus state. The OPD is measured in optical waves and indicates departure from a plane (flat) wavefront. Specifically, the total OPD over the full exemplary aperture is shown inFIG. 6a, whereasFIG. 6billustrates an exemplary close-up view of the OPD near the edge of the aperture showing the difference between the total OPD (focus and 4th order aspheric term) and the contribution by the focus term alone.

The positive-focus state of element1in the above exemplary embodiment imposes a radial profile to the wavefront described inFIGS. 6aand6bas the Optical Path Difference (OPD) from a plane wave. The curvature of the OPD increases with distance from the vertex of the element. The effective focal length of this element is 233.03 millimeters at a design wavelength of 1.55 microns. This particular element was designed not only to have positive optical power in the lens state, but it also imparts small amounts of additional OPD (as seen inFIG. 6b) as a 4thorder correction to what would otherwise be spherical aberration at the image. The amount of 4thorder correction is 4.548 waves at 1.55 microns. Element5is designed to have a focal length of 18.46 millimeters at 1.55 microns, and there is no 4thorder term. Negative focus is selected for element5when in the lens state.

An alternate exemplary embodiment for this invention is shown inFIGS. 7aand7b. Specifically, the figures illustrate the optical raytrace of an alternate embodiment of the invention using alternate materials. Electrically switchable lenses have been separately described above with respect toFIGS. 3a-3c,4, and5a-5b.

In one aspect of the alternate exemplary embodiment,FIG. 7ashows an exemplary optical raytrace of the alternate five-element imager in the wide FOV with the rays traced for broad-band SWIR radiation (e.g., of 1.0-1.7 μm). As tabulated inFIG. 8, both the front and back aperture diameters of an electrically switchable lens15can be 38.1 mm. The electrically switchable lens15is depicted inFIG. 7aas receiving wide FOV broadband rays (e.g., of 1.0-1.7 μm) on its front aperture. Lens16is configured as an exemplary meniscus lens with its front aperture facing the back aperture of the electrically switchable lens15. The front aperture of lens16can have an aperture diameter of 33 mm and Radius of Curvature (ROC) of 35.82 mm, its back aperture having an aperture diameter of 29 mm and ROC of 141.22 mm. Lens17is configured with its front aperture facing the back aperture of lens16. The front aperture of lens17can have an aperture diameter of 22 mm and ROC of 78.65 mm, its back aperture having an aperture diameter of 19 mm and ROC of 20.84 mm. Lens18is configured with its front aperture facing the back aperture of lens17. The front aperture of lens18can have an aperture diameter of 29 mm and ROC of 97.31 mm, its back aperture having an aperture diameter of 29 mm and ROC of −74.69 mm. Electrically switchable lens19can have both its front and back aperture diameters of 25.4 mm. Its front aperture is configured to face the back aperture of lens18. The wide FOV of such an exemplary five-element imager as depicted inFIG. 7acan be achieved by electrically setting both electrically switchable lenses15and19in an infinite focus state. The infinite focus (“clear”) state allows the lens elements to pass broad-band SWIR illumination, which when coupled with the achromatic configuration of lens16through lens18allows unfiltered light to be collected into a highly resolved image on an appropriate SWIR focal plane array20.

In another aspect of the alternate exemplary embodiment,FIG. 7bshows an exemplary optical raytrace of the alternate five-element imager in the narrow FOV with the rays traced for a narrow spectral band centered around 1.55 microns (e.g., 1.54-1.56 μm). As tabulated inFIG. 8, both the front and back aperture diameters of an electrically switchable lens15can be 38.1 mm. The electrically switchable lens15is depicted inFIG. 7bas receiving a narrow spectral band centered around 1.55 microns (e.g., 1.54-1.56 μm) on its front aperture. Lens16is configured as an exemplary meniscus lens with its front aperture facing the back aperture of the electrically switchable lens15. The front aperture of lens16can have an aperture diameter of 33 mm and Radius of Curvature (ROC) of 35.82 mm, its back aperture having an aperture diameter of 29 mm and ROC of 141.22 mm. Lens17is configured with its front aperture facing the back aperture of lens16. The front aperture of lens17can have an aperture diameter of 22 mm and ROC of 78.65 mm, its back aperture having an aperture diameter of 19 mm and ROC of 20.84 mm. Lens18is configured with its front aperture facing the back aperture of lens17. The front aperture of lens18can have an aperture diameter of 29 mm and ROC of 97.31 mm, its back aperture having an aperture diameter of 29 mm and ROC of −74.69 mm. Electrically switchable lens19can have both its front and back aperture diameters of 25.4 mm. Its front aperture is configured to face the back aperture of lens18. The FOV change to the depicted narrow FOV ofFIG. 7boccurs by electrically switching lens15to a state of positive-focus, and electrically switching lens19to a state of negative-focus towards an appropriate SWIR focal plane array20. An active illumination of a narrow FOV scene helps to boost the total light energy collected at the focal plane array20.

Like the exemplary embodiment described inFIGS. 1aand1b, the alternate exemplary imager (e.g., ofFIGS. 7aand7b) is comprised of five elements, with elements15and19being switchable lenses and elements16through18being static optics.FIG. 8is a table that shows the exemplary raytrace parameters and switchable lens specifications used in this alternate exemplary embodiment. Switchable element19is identical to5, and element15differs only slightly from1. The effective focal length of element15is 243.86 millimeters at a design wavelength of 1.55 microns. The amount of its 4thorder correction is 2.757 waves at 1.55 microns.

The static optics for the alternate exemplary embodiment are based on glass and ceramic materials developed by Naval Research Laboratories (Bayya, et al., 2013). See, Bayya et al., “New Multiband IR Imaging Optics,” Proc. SPIE 8704, Infrared Technology and Applications)XXXIX, 870428 (2013), incorporated herein by reference. These materials are more amenable to molding than are crystalline materials, which may be an attractive economic consideration. Lens16has a conic term and a 10thorder even asphere on the front surface and a spherical surface on the back. The front surface of16is also used for the aperture stop. Lens16is made of Miltran ceramic. Lens17is made of NRL4glass and has only spherical surfaces. Lens18has a conic and a 10thorder even asphere on the front surface and a spherical surface on the back. NRL7glass is used for this element. By comparison to the exemplary embodiment inFIGS. 1aand1b, the alternate exemplary embodiment illustrated inFIGS. 7aand7buses two aspheric surfaces (instead of one), but the NRL materials do not need to be as thick, saving on overall weight. Aside from these noted differences, the functional description of this alternate exemplary embodiment is the same as described for that inFIG. 1.