Patent Application
20250147289
The invention relates to an infrared objective lens.
The low-cost infrared thermal imaging market is dominated by “uncooled” sensor assemblies which include optical objective lenses that focus an image onto a bolometer focal plane detector array, which then in turn converts the optical image into electronic signals. The standard spectral waveband for such sensors is 8-12 microns for the wavelength of light. A typical infrared objective lens assembly for a wide-field of view of ±20.0° and fast F # on the order of 1.0-1.2 requires at least two lens singlets (Latimer & Fantozzi), of which at least one is typically made out of Germanium lens material. See, e.g., David G. Latimer and Louis R. Fantozzi, “Fast 8-12 m Objectives Utilizing Multiple Aspheric Surfaces”, SPIE Conference on Infrared Technology and Applications XXV, 882 Orlando, Florida, April 1999 SPIE Vol. 3698, 0277-786×1991, which shows standard two-lens Ge design forms for most uncooled applications.
It has been noted that field curvature is a significant optical aberration for these wide field, fast-F # objective lens design forms (Schuster & Franks). See, Norbert Schuster, John Franks, “Two-lens designs for modern uncooled and cooled IR imaging devices,” Proc. SPIE 8896, Electro-Optical and Infrared Systems: Technology and Applications X, 889604, 25 Oct. 2013; doi: 10.1117/12.2028716, wherein field curvature is primary aberration of concern for WFOV lens. Desroches, et. al., provided a list of infrared materials and their relative cost compared to Germanium as the common baseline. See, Gerard Desroches, Kristy Dalzell, Blaise Robitaille, “Technical considerations for designing low-cost, long-wave infrared objectives,” Proc. SPIE 9070, Infrared Technology and Applications XL, 907026 (24 Jun. 2014); doi: 10.1117/12.2050570. Their results indicate that Zinc Sulphide (ZnS) is perhaps the most affordable material overall. However, attempts to create designs which do not use high-index material such as Ge in lieu of cheaper materials such as ZnS typically result in more than two lenses being required (Zang, et. al.), thereby negating the potential for cost reduction. See, Evan Zhang, Vivian W. Song, James S. Zhang, Cunwu Yang, “Non-Ge optics and low-cost electronics designs for LIR imagers,” Proc. SPIE 4820, Infrared Technology and Applications XXVIII, (23 Jan. 2003); doi: 10.1117/12.469690, which taught non-Ge solution required three fat lenses, best solution was Ge & AMTIR-1.
Recently, the ability to fabricate infrared bolometer focal arrays upon curved instead of traditionally flat substrates has been demonstrated (Fendler, et. al.), with the implications that if the focal plane curvature matches the normal amount of optical field curvature, then the burden upon the lens design of correcting field curvature is reduced or possibly eliminated. See, M. Fendler, D. Dumas, F. Chemla, M. Cohen, P. Laporte, K. Tekaya, E. Le Coarer, J. Primot, H. Ribot, “Hemispherical infrared focal plane arrays: a new design parameter for the instruments,” Proc. SPIE 8453, High Energy, Optical, and Infrared Detectors for Astronomy V, 84531P (25 Sep. 2012); doi: 10.1117/12.925379.
It is therefore the purpose of the present invention to demonstrate that indeed the mitigation of field curvature considerations permits the use of alternative lower cost materials without adding extra lens components as compared to the current art.
The subject invention is a novel low-cost objective lens design, using only ZnS optical material, suitable for use with an uncooled infrared bolometer having a curved focal plane. The objective lens field of view is at least ±20.0° over a 1280×720 pixel array with 0.0012 mm pitch. Lens chromatic aberrations are corrected over at least the 8-11 micron infrared wavelength band. The objective lens operates at a relatively fast F #/1.0 which is common in the art for bolometer applications.
An exemplary optical system uses a single material ZnS for two lenses, each lens having aspheric curvatures on both faces, resulting in a low positive power (≈5 diopter) from the first lens and stronger positive power (≈35 diopter) from the second lens. Each lens has a negative-power diffractive surface on its 2nd surface to provide correction of chromatic aberrations for a focal plane having a concave radius of curvature.
Thus, subject invention demonstrates that indeed the mitigation of field curvature considerations permits the use of alternative lower cost materials without adding extra lens components as compared to the current art.
Additional advantages and features will become apparent as the subject invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:
The subject invention is a novel low-cost objective lens design, using only ZnS optical material, suitable for use with an uncooled infrared bolometer having a curved focal plane. The objective lens field of view is at least ±20.0° over a 1280×720 pixel array with 0.0012 mm pitch. Lens chromatic aberrations are corrected over at least the 8-11 micron infrared wavelength band. The objective lens operates at a relatively fast F #/1.0 which is common in the art for bolometer applications. The description of the invention is best explained via reference to
Light rays 1 enter the optical system from four very distant object points. The first optical element 2 provides a weak positive power of 5.15 diopters, and is made using Zinc Sulphide (ZnS) infrared transmitting material with a diameter of at least 28 mm. The first surface of element 2 has an aspherical curvature to enable correction of higher order optical aberrations. The second surface of element 2 has a hybridized surface profile containing both an aspheric base curvature with an overlaid diffractive pattern. The light rays progress over an intervening air gap and are incident upon the second lens element 3 which provides a strong optical power of 35.26 diopters and is also made from ZnS optical material with a diameter of at least 30.5 mm. The first surface of element 3 has an aspherical curvature to enable correction of higher order optical aberrations. The second surface of element 3 has a hybridized surface profile containing both an aspheric base curvature with an overlaid diffractive pattern. In each case of an aspheric surface, the profile is defined by the standard optical equation as follows where z is the surface sag, c is the curvature, r is the radial height above the optical axis, k is the conic constant, and then α1, α2, etc. are the aspheric coefficients:
In the instances where diffractive surfaces are present, the diffractive profile adds phase (i.e., effective sag) to the raytrace per the following relationship where ϕ is the added phase, M is diffraction order, n is the whole number count of terms in the equation, the An are coefficients, and ρ is the normalized radial aperture coordinate:
ϕ=MΣnAnρ2n.
In the preferred embodiment, the maximum order polynomial for ϕ is n=2, with the n=1 terms being negative, n=2 terms having positive values of lesser magnitude. The diffractive phase profiles thus have a net negative power for each of element 2 and 3, thereby providing a large degree of correction for color aberrations across the infrared spectrum from 8-11 micron wavelengths. After being acted upon by the two lens elements 2 and 3, the light rays 1 pass through a flat window 4 and then come to a focus onto a curved focal plane 5. The curved focal plane 5 has a concave radius of curvature of roughly 52.5 mm, and thus relieves the lens design from having to correct field flatness aberrations.
Accordingly, an exemplary optical system uses a single material ZnS for both lenses, each lens 2 & 3 having aspheric curvatures on both faces, resulting in a low positive power (≈5 diopter) from the first lens and stronger positive power (≈35 diopter) from the second lens. Each lens 2 & 3 have a negative-power diffractive surface on the 2nd surface to provide correction of chromatic aberrations, forming a focal plane with a concave radius of curvature.
The total level of aberration correction and focused image quality of the preferred embodiment is captured in
It is obvious that many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as described.
The invention described herein may be manufactured, used, sold, imported, and/or licensed by or for the Government of the United States of America.
