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Polymer Optics for Thermally Stable Imaging

Jan 2011
Valentina Doushkina, Qioptiq Polymer Inc.

There is currently a soaring demand for high-performance optics and optical systems applications in the fields of optometry, biomedicine, microscopy, and a number of related scientific and industrial fields.

Due to advances in the microelectronics and semiconductor industries, the pixel size of optical sensors is decreasing to less than 2 x 2 µm, placing high demands on optical performance. At the same time, miniature cameras for surgical, diagnostic and other biomedical applications, coupled with fiber optics, place even higher demands on image resolution; the result is complex, bulky multi-element glass optical systems. However, improvements in polymer and integrated hybrid glass-polymer optical systems and components offer many efficient and compact solutions to meet these demands, while at the same time significantly reducing cost, weight and complexity of such optical systems.

Polymer and integrated hybrid glass-polymer optics can provide practical and economic applications for these technologies and markets. For example, new and smaller endoscopic systems are emerging for clinical use, creating a demand for low-cost, high-performance and high-volume production optics delivered in a compact package to support them. The versatility and design freedom of polymer and hybrid glass-polymer optics, compared to multilens glass assemblies, provide many advantages. They significantly lower manufacturing costs while offering high optical performance, a wide field of view, a low f number and, in the case of hybrid optics, thermal stability. Both types of optics provide solutions not possible in the past and help bring the products and new technology to the broad consumer market.1

Optical solutions that are based on polymer and hybrid glass-polymer components and systems can be desirable for multiple markets and/or practical applications. These systems can be used in specific applications, such as imaging fast lenses with a low f number for megapixel cameras used as C-mount lenses in applications requiring high image quality and high resolution; for robotics and machine vision for high-resolution line-scan applications, industrial inspection and document scanning; and for telecentric lenses for noncontact optical metrology solutions requiring a high degree of accuracy and resolution.

Micro-optics applications, markets, solutions

“Recent advances in the fields of optometry, medicine, biomedicine and neurology for surgical and diagnostic applications have put optics manufacturers into situations where they have reached their manufacturing limits or have gone beyond the range of their current capabilities,” Doushkina said. Micro-optics with geometry measured in tenths and low hundredths of microns have been successfully manufactured on supporting substrates. However, producing an optical system containing a number of micro-optical stand-alone components and having them aligned, assembled and sealed present a significant manufacturing challenge – one requiring a different approach and design philosophy to successfully meet the system and product requirements.

Figure 1.
Polymer optics can be used in miniature endoscope cameras, and integration of the optics and mechanical design can establish levels of performance not achievable with prior technologies. Images courtesy of Qioptiq Polymer Inc.

A miniature endoscope camera, shown in Figure 1, is one of multiple examples where polymer optics has enabled the integration of optical and mechanical design, breaking the limits of prior technology. The result is high resolution and stable performance over the broadband spectral range with the high light-gathering power of the fast lens system. Diffraction-limited performance is shown in Figure 2, where the Airy disk encircles the geometric aberrations over the entire wide field of view.

One matrix defining the resolution of an optical system can be specified in terms of the finest detail that the system can image. This property is defined by the modulation transfer function (MTF). The following is a common definition of MTF:

Figure 2. This diagram shows diffraction-limited performance, where the Airy disk encircles the geometrical aberrations over the entire wide field of view and the broad spectral range.

If an object with sinusoidally varying brightness I and a frequency of lp (line pairs) per mm is imaged through the optical system, the modulation of the object (Mobj) is defined as follows: Mobj = (Imax − Imin)/(Imax + Imin), where Imax is the maximum intensity and Imin is the minimum intensity of the sine modulation.

The image is characterized in a similar manner: The modulation of the image (Mimg) is defined as Mimg = (Imax − Imin)/(Imax + Imin). Therefore, the MTF is the ratio of the modulation of the image to that of the object: MTF = Mimg / Mobj. The limiting resolution of the system is defined by the frequency lp/mm at which the MTF falls below a minimum detectable level (typically 5 percent of MTF).

Figure 3. A modulation transfer function that almost reaches the diffraction limit, even at the high frequencies of 200 line pairs per millimeter, is suitable for use with modern megapixel sensors of 2 x 2 μm, and with a smaller single-pixel size.

Shown in Figure 3 is a highly resolved system with an MTF function that nearly reaches the diffraction limits, even at high frequencies of 200 lp/mm; it is suitable for use with both modern megapixel sensors and the smaller single-pixel size. The pixel size can be derived from the Nyquist frequency, also called the Nyquist limit:

F (Nyqyust) = ----------------
                        2* PixelSize

Thus, with an average of 40 percent MTF at 200 line pairs per millimeter, the designed lens is highly compatible with a sensor of 2 x 2-µm pixel size, and the resolution of the image is “pixel-size limited,” which meets the demands of the latest state-of-the-art megapixel sensors.

The significant advantage of the optical system presented, when it is integrated with the mechanical housing, is that the tolerances of the optomechanical housing are integrated into the optical design, resulting in precise mounting and assembly of the only two components. Compared to a conventional approach, where the tolerance budget is shared between the optical and the mechanical components, the optical tolerances are more relaxed.2 This design philosophy will not be possible with glass optics. Monte Carlo statistical tolerance analysis has confirmed the feasibility of the design and of a single drop-in assembly step with no alignment needed.

The sample design presented in Figure 3 can be used in many other optical imaging applications (where the temperature range is not broad), offering the features of (1) high image resolution, (2) compact packaging, (3) low weight, (4) high repeatability of the process and (5) low manufacturing cost.

Hybrid glass-polymer systems

There are multiple markets and applications in which the thermal stability of the optical system within a broad thermal range is necessarily a factor for optical device/system functionality. A disadvantage of polymer stand-alone optics is the lack of thermal stability due to the high coefficient of thermal expansion.

Figure 4.
Thermal stability and low manufacturing cost are great advantages of hybrids, which have the thermal stability of glass and the low manufacturing cost of polymer optics.

However, when polymer and glass optics are integrated into a hybrid glass-polymer solution, this limitation can be resolved. One of the great advantages of hybrid systems is that they maintain the thermal stability of glass while offering the relatively low manufacturing cost of polymer optics.1 When the glass component 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 – or even better, since the dn/dt, the temperature coefficient of refractive index (for a given wavelength), is negative for polymers and positive for the optical glass.

Figure 5.
Glass-polymer hybrid optics offer multiple advantages in terms of efficiency and compactness, and also can reduce the cost, weight and complexity of optical systems.

Additionally, the low weight and compact packaging of a hybrid system (resulting in a minimal number of optical components) are very desirable. An example of high-performance hybrid optics is shown in Figure 4. The system can be successfully used in the automotive industry, for example, to track lane position or to recognize objects as part of a warning system.

Considering the wide field of view offered by the system, its high resolution and image stability over a broad temperature range and wide spectrum, it offers excellent adaptability for driverless driving technology. This technology is finding its way into consumer vehicles, and it answers a product demand for low-cost, high-performance and high-production-volume optics in a compact package. Variations of this technology can also be used for robotics and machine sensing for high-resolution line-scan applications, industrial inspection and document scanning.

Figure 6.
Qioptiq Polymer Inc. produces polymer prisms (top left), polymer total internal reflection optics (top right), polymer diffractive optics (bottom left), and polymer collimating lenses and housings (bottom right).

Meet the author

Valentina Doushkina is director of engineering at Qioptiq Polymer Inc. in La Verne, Calif., and the lead instructor at the University of California Irvine, Extension. She also is president of the Optical Society of Southern California; e-mail:


1. V. Doushkina (2010, Vol. 7788). Innovative hybrid optics: Combining the thermal stability of glass with low manufacturing cost of polymers. Proc SPIE, p. 778809.

2. V. Doushkina, E. Fleming (2009, Vol. 7424). Optical and mechanical design advantages using polymer optics. Proc SPIE, p. 74240Q.

A standard lens interface initially made for 16mm movie cameras and now used primarily on closed-circuit television cameras. It is a 1-in.-diameter, 32-thread-per-inch interface with a flange-to-image plane distance of 0.69 in.
The science of measurement, particularly of lengths and angles.
Contraction of "picture element." A small element of a scene, often the smallest resolvable area, in which an average brightness value is determined and used to represent that portion of the scene. Pixels are arranged in a rectangular array to form a complete image.
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