Valentina Doushkina, University of California, Irvine, Extension and Doushkina Optics
Development of infrared detector technology has brought to market high-performance infrared cameras for demanding thermal imaging applications in the short-wave, mid-wave and long-wave spectral bands. There is a vast demand for high-performance imaging optics for infrared cameras in a variety of markets and applications.
Thermal stability, impact resistance and low manufacturing cost make hybrid glass-polymer optics well suited for IR applications. Courtesy of Diverse Optics Inc. (formerly Qioptiq Polymer Inc.)
Low manufacturing cost, thermal stability and impact resistance are some unbeatable characteristics of infrared-application hybrid glass-polymer optics. Hybrids combine the thermal stability of glass with the low manufacturing cost of polymers, reducing component cost, weight, design complicity and alignment, while enhancing the appeal of the products. The narrow range of choices in polymer materials, for IR applications in particular, is counterbalanced by using sophisticated optical surfaces such as aspheric, asphero-diffractive, toroidal and free-form off-axis optics.
The above advantages are not achievable when polymers or glass optics are used on their own. The integration of polymer and glass for IR applications offers high resolution and diffraction-limited image quality.
In this article, we look at the design and analysis process of combining glass and polymer optics in the IR spectral range, challenging applications including fast optics with a low f number, wide field-of-view lenses or systems, and free-form optics.
Thermally stable imaging
One limitation of polymer optics is the high thermal coefficient of expansion of optical-grade polymers, which shifts the image plane in the optical system, resulting in the bluer image or the spot size.1 But thermally stable imaging using polymers is possible and can be applied to the glass-polymer solutions known as hybrid glass-optics.2 The high material and manufacturing process cost for IR transmitting optics has led to a call for alternative, cost-efficient advances and products.
Figure 1. Transmission of a 130-μm polyethylene film at wavelengths from 2.5 to 14.9?μm.
The hybrid solution discussed here for the proposed fast (low f number) zoom lens and the imaging lens designs incorporates thin sheets of polyolefin plastics or (high-density polyethylene) applied on silicon, resulting in the diffraction-limited optics transmitting in the MWIR (mid-wave infrared). The significant advantage of the designs is in the relatively low cost of the lenses, a result of the reduction in the number of components and materials costs; the considerably less expensive manufacturing process makes the technology very attractive for a variety of applications.
In the following design examples, the wavelength is 10 µm, based on the typical IR emission spectrum of an object at 25 °C. Using polyethylene in the design provides a material with good transmission characteristics in the MWIR and excellent suitability for injection molding. The index of refraction of polyethylene at 10 µm is n ≈ 1.51. P.T. Tsilingiris, in “Comparative evaluation of the infrared transmission of polymer films,”3 has shown the transmission of a 130-µm polyethylene film at wavelengths from 2.5 to 14.9 µm.
Design examples
(1) MWIR, f/3, diffraction-limited, continuous 3x zoom lens, refractive
Typical applications for this kind of lens could be search and rescue, surveillance, tracking, target recognition, low-power laser scanning systems and the like. The MWIR spectral range for detection purposes allows users to see through smoke and fine particulate, making this lens suitable for fire and police applications.
The simplicity of the design and the assembly shown in Figure 2 are possible because of the thin layer of polymer applied to the first outer surface of the lens, which does not affect the thermal stability and does not shift the image plane of the system, while correcting the aberration resulting from the aspheric prescription.
Figure 2. MWIR f/3 continuous 3x zoom diffraction-limited.
The thermal stability of the optical system within a broad thermal range is necessarily a factor for optical device stability and functionality. When polymer and glass optics are integrated in a hybrid glass-polymer solution, this stability can be achieved via athermalization of the design. Thermal stability, while still retaining a relatively low manufacturing cost, is one of the great advantages of hybrid systems, which offer the thermal stability of glass and the low manufacturing cost of polymer optics.1 When the glass component(s) carries the optical power, and a thin layer of the aspherized polymer component is used to correct for aberrations, the thermal behavior of the hybrid is as stable as that of glass optics.
Figure 3. MWIR f/2 diffraction-limited imaging lens.
(2) MWIR, f/2, fast diffraction-limited imaging lens
The low weight and compact packaging of glass-polymer hybrids are very desirable features, as demonstrated in the example of the fast diffraction-limited lens in Figure 3. The fast f/2 lens, with a wide field of view, becomes possible because of the thin layer of polymer applied to the second outer surface of the lens, which also does not affect the thermal stability and does not shift the image plane of the system, while correcting the aberration resulting from the aspheric prescription. For a wide spectral range, the solution would result in asphero-diffractive surface prescription.
Figure 4. MWIR f/2 diffraction-limited imaging lens; modulation transfer function.
For many years, infrared optics have been used for the applications mentioned earlier. Today they are finding wide use in the biomedical field for sensing temperature and 3-D imaging of various features in a broad array of applications.
Meet the author
Valentina Doushkina, lead instructor at the University of California, Irvine, Extension, teaches the certificate program in optical engineering and instrument design. She is the founder and president of Doushkina Optics, which provides optical design consulting to companies. She also is president of the Optical Society of Southern California; e-mail: [email protected].
References
1. V. Doushkina and E. Fleming (2009). Optical and mechanical design advantages using polymer optics. Proc SPIE, Vol. 7424, p. 74240Q.
2. V. Doushkina (January 2011). Polymer optics for thermally stable imaging, BioPhotonics, pp. 32-35.
3. P.T. Tsilingiris (November 2003). Comparative evaluation of the infrared transmission of polymer films, Energ Conv and Mgmt, Vol. 44, Issue 18, pp. 2839-2856.