With the advancement in sensors, hyperspectral imaging in short wave infrared (SWIR 0.9 μm to 1.7 μm) now has wide applications, including night vision, haze-penetrating imaging, etc. Most conventional optical glasses can be material candidates for designing in the SWIR as they transmit up to 2.2 μm. However, since SWIR is in the middle of the glasses’ major absorption wavebands in UV and IR, the flint glasses in SWIR are less dispersive than in the visible spectrum. As a result, the glass map in the SWIR is highly compressed, with crowns and flints all clustering together. Thus correcting for chromatic aberration is more challenging in the SWIR, since the Abbé number ratio of the same glass combination is reduced. Conventionally, fluorides, such as CaF<sub>2</sub> and BaF<sub>2</sub>, are widely used in designing SWIR system due to their unique dispersion properties, even though they are notorious for poor manufacturability or even high toxicity. For lens elements in a zoom system, the ray bundle samples different sections of the each lens aperture as the lens zooms. This creates extra uncertainty in correcting chromatic aberrations. This paper focuses on using only commercially available optical glasses to color-correct a 3X dual-band zoom lens system in the VIS-SWIR. The design tools and techniques are detailed in terms of material selections to minimize the chromatic aberrations in such a large spectrum band and all zoom positions. Examples are discussed for designs with different aperture stop locations, which considerably affect the material choices.
A design study is compiled for a VIS-SWIR dual band 3X zoom lens. The initial first order design study investigated zoom motion, power in each lens group, and aperture stop location. All designs were constrained to have both the first and last lens groups fixed, with two middle moving groups. The first order solutions were filtered based on zoom motion, performance, and size constraints, and were then modified to thick lens solutions for the SWIR spectrum. Successful solutions in the SWIR were next extended to the VIS-SWIR. The resulting nine solutions are all nearly diffraction limited using either PNNP or PNPZ (“Z” indicating the fourth group has a near-zero power) design forms with two moving groups. Solutions were found with the aperture stop in each of the four lens groups. Fixed f-number solutions exist when the aperture stop is located at the first and last lens groups, while varying f-number solutions occur when it is placed at either of the middle moving groups. Design exploration included trade-offs between parameters such as diameter, overall length, back focal length, number of elements, materials, and performance.
Broadband high-resolution micro-objectives for optical biopsy imaging applications benefit from radial gradient-index lenses, improving performance and potentially decreasing manufacturing sensitivity. Achromatization is achieved with fewer elements as gradient-index lenses can replace homogeneous doublets or triplets.
An all-plastic high-performance eyepiece design utilizing a polymer spherical gradient-index optical element is presented. The use of a gradient-index lens in the eyepiece offers better off-axis and chromatic aberration correction, as well as overall performance improvement compared to a similar eyepiece with all homogeneous lenses.
High-performance eyepiece designs have been carried out using both spherical and radial gradient-index (GRIN)
elements. Eyepiece designs of both geometries are shown to offer superior imaging performance with fewer elements
when compared to purely homogeneous systems. These GRIN lenses are formed from monomer diffusion between
polymethyl methacrylate (PMMA) and polystyrene (PSTY) during the polymerization process, resulting in a copolymer
of the two homogeneous materials.
A process for fabricating spherical GRIN elements is discussed where copolymer axial GRIN blanks are thermally
compressed using spherical surface molds. This process curves the nominally-straight isoindicial surfaces of the axial
GRIN rod to be consistent with the shape found during optimization of the design. Once compressed, the spherical
blanks are diamond-turned for final surface figure and finish. Measurement of the GRIN profile is carried out using the
Schmidt immersion technique in a Mach-Zehnder interferometer. Tolerances specific to GRIN elements are identified
and determined to be readily achievable using the aforementioned manufacturing process.
A 40-deg full field-of-view high-performance eyepiece design utilizing a polymer spherical gradient-index (GRIN) optical element is presented. In the design process, the GRIN lens material is constrained to current manufacturing capabilities. Several spherical GRIN lens blanks are fabricated from a thermoformable axial GRIN polymethyl methacrylate polystyrene copolymer material. One is diamond turned into a lens for the eyepiece, and the additional blanks are used to characterize the fabrication process. The spherical GRIN profile is evaluated in the original design, and a tolerance analysis is provided.
Radial and spherical polymer gradient-index (GRIN) eyepiece designs are presented. The chromatic behavior of GRIN profiles is constrained to real material properties of a polymethyl methacrylate polystyrene copolymer gradient-index system. Single-element, two-element, and multielement eyepiece design configurations each demonstrate significant spot diameter and modulation transfer function performance improvements with the use of a GRIN element. A high-performance spherical GRIN eyepiece design, with 48-deg full field-of-view and 3% distortion, is compared to a similar homogeneous glass solution.
Results that characterize a fast-diffusing titania silicate gradient-index (GRIN) glass in a sodium (Na + ) for lithium (Li + ) ion exchange are presented. Manufacturing challenges associated with diffusion in titania silicate glass are addressed in order to extend its manufacturability to large-diameter radial diffusions. Two 20-mm diameter radial GRIN lenses have been fabricated in less than 4-weeks diffusion time, demonstrating the potential for fabricating large-diameter radial GRIN glass elements.
Over 100 doublets were designed using a polychromatic gradient-index (GRIN) design model to analyze the benefits of radial GRIN profiles in broadband visible to short-wave infrared (vis-SWIR) imaging system applications. The polychromatic GRIN design model can be applied to any GRIN material, but for this work, titania silicate glass was investigated. A multielement design study with Petzval lenses was performed to show improved color correction when using GRIN elements. Results from the doublet and Petzval designs illustrate that in broadband vis-SWIR imaging applications, GRIN can either improve system performance or reduce a cemented homogeneous doublet to a GRIN singlet.