A new type of lens design features broadband achromatic performance as well as telecentricity, using a minimum number of spherical elements. With appropriate modifications, the lens design form can be tailored to cover the range of response of the focal-plane array, from Si (400–1,000 nm) to InGaAs (400–1,700 or 2,100 nm) or InSb/HgCdTe reaching to 2,500 nm. For reference, lenses typically are achromatized over the visible wavelength range of 480–650 nm.
In remote sensing applications, there is a need for broadband achromatic telescopes, normally satisfied with mirror-based systems. However, mirror systems are not always feasible due to size or geometry restrictions. They also require expensive aspheric surfaces. Non-obscured mirror systems can be difficult to align and have a limited (essentially one-dimensional) field of view. Centrally obscured types have a two-dimensional but very limited field in addition to the obscuration. Telecentricity is a highly desirable property for matching typical spectrometer types, as well as for reducing the variation of the angle of incidence and cross-talk on the detector for simple camera types.
This rotationally symmetric telescope with no obscuration and using spherical surfaces and selected glass types fills a need in the range of short focal lengths. It can be used as a compact front unit for a matched spectrometer, as an ultra-broadband camera objective lens, or as the optics of an integrated camera/spectrometer in which the wavelength information is obtained by the use of strip or linear variable filters on the focal plane array. This kind of camera and spectrometer system can find applications in remote sensing, as well as in-situ, applications for geological mapping and characterization of minerals, ecological studies, and target detection and identification through spectral signatures. Commercially, the lens can be used in quality-control applications via spectral analysis.
The lens design is based on the rear landscape lens with the aperture stop in front of all elements. This allows sufficient room for telecentricity in addition to making the stop easily accessible. The crucial design features are the use of a doublet with an ultra-low dispersion glass (fluorite or S-FPL53), and the use of a strong negative element, which enables flat field and telecentricity in conjunction with the last (field lens) element. The field lens also can be designed to be in contact with the array, a feature that is desirable in some applications.
The lens has a 20° field of view, for a 50-mm focal length, and is corrected over the range of wavelengths of 450–2,300 nm. Transverse color, which is the most pernicious aberration for spectroscopic work, is controlled at the level of 1 μm or below at 0.7 μm field and 5 μm at full field. The maximum chief ray angle is less than 1.7°, providing good telecentricity. An additional feature of this lens is that it is made exclusively with glasses that provide good transmission up to 2,300 nm and even some transmission to 2,500 nm; thus, the lens can be used in applications that cover the entire solar-reflected spectrum. Alternative realizations are possible that provide enhanced resolution and even less transverse color over a narrower wavelength range.
This work was done by Pantazis Mouroulis of Caltech for NASA's Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
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Refer to NPO-44059, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Broadband Achromatic Telecentric Lens
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Overview
The document outlines NASA's development of a Broadband Achromatic Telecentric Lens, detailed in Technical Support Package NPO-44059. This innovation addresses the limitations of traditional mirror-based telescopes used in remote sensing applications, which often face challenges related to size, geometry, and alignment. Mirror systems can be expensive due to the need for aspheric surfaces and may have restricted fields of view, making them less suitable for certain applications.
The new lens design features all spherical surfaces and utilizes preferred glass types, allowing for a compact and unobscured optical system. It is particularly beneficial for short focal lengths, making it ideal for laboratory, in-situ, or low-altitude remote sensing applications. The lens is designed to cover a wide range of wavelengths, from 450 nm to 2300 nm, with some transmission capabilities extending to 2500 nm. This broad spectral response is crucial for applications that require capturing the entire solar reflected spectrum.
Key design elements include a doublet made from ultra-low dispersion glass (such as fluorite or S-FPL53) and a strong negative element that ensures flat field performance and telecentricity. The lens achieves a 20-degree field of view at a 50 mm focal length, with transverse color aberration controlled to levels as low as 1 micrometer at 0.7 field and 5 micrometers at full field. The maximum chief ray angle is less than 1.7 degrees, which contributes to the lens's excellent telecentricity.
The document emphasizes the novelty of this lens design, highlighting its broadband achromatic performance and telecentricity achieved with a minimal number of optical elements. This advancement represents a significant improvement over prior art, providing a more efficient and effective solution for remote sensing and spectroscopic applications.
For further inquiries or detailed information, the document provides contact details for NASA's Jet Propulsion Laboratory, encouraging collaboration and exploration of this innovative technology. Overall, the Broadband Achromatic Telecentric Lens stands as a promising development in optical technology, with potential applications across various scientific and commercial fields.
