Dual field-of-view midwave infrared optical design and athermalization analysis Chih-Wei Kuo,1,* Chih-Lung Lin,2 and Chien-Yuan Han3 1Electro-Optics Section, Materials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology, Lung-Tan 325, Taiwan, China 2Department of Electronic Engineering, Hwa Hsia Institute of Technology, Taipei 235, Taiwan, China 3Department of Electro-Optical Engineering, National United University, Miao-Li 360, Taiwan, China *Corresponding author: [email protected] Received 2 April 2010; revised 31 May 2010; accepted 2 June 2010; posted 2 June 2010 (Doc. ID 126354); published 23 June 2010 A step-zoom and reimaging structure were utilized to construct a dual field-of-view optical design for high-magnification switching in the 3–5 μm spectral band. The design has a flexible optomechanical lay- out, which means it can be utilized for multipurpose applications. The effects of the surrounding envir- onmental temperature and axial gradient temperature are analyzed using the concept of thermal resistance, and the thermal compensation is discussed. A description of the zooming mechanism and optomechanical control is offered. © 2010 Optical Society of America OCIS codes: 110.3080, 220.3620. 1. Introduction xial analysis of mechanically compensated zoom The applications of infrared dual field-of-view lenses can be expressed in terms of Gaussian brackets (DFOV) optical systems have been widely discussed [9,10]. The focus of the zoom lens can be solved by [1–5] for both the midwave infrared (MWIR) spectrum describing the relationships among focal length, lens (3–5 μm) and longwave infrared spectrum (8–12 μm). position, ray height, and direction in relation to the These optical systems utilize the wide-angle field of matrix. The zoom process can be expressed by a view (WFOV) to search the scenery and the narrow unified varifocal differential equation with a stable angle (NFOV) mode to identify the target of interest image plane being the constraint condition [11]. Ther- close up. The algorithm for assessing a surveillance mal imaging systems for target-tracking purposes, image is derived from the Johnson criteria [6] and such as rocket and missile launching applications, then used to deduce the target being detected, recog- require WFOV systems for monitoring and NFOV nized, or identified [7]. Variations in the characteris- systems for identification. DFOV infrared optical sys- tems should have FOV switching without target loss. tic size of a particular object, and the distance from They can be classified as a subset of continuous-zoom the sensor to the target plane, both result in the dif- structures that utilize only axial steps to move lenses ferent magnification ratio requirements obtained by between two extreme magnifications. A nonaxial the computation of the Johnson criteria. Previously, moving DFOV variant, called the rotating-in scheme, the Delano diagram has been used to aid in the design was developed by combining reverse-telephoto and of zoom lenses [8], composed using the height of the telephoto structures with/without two separate lens paraxial marginal and chief rays as the longitudinal groups controlled by a rotation mechanism. This is and transverse coordinates, respectively. The para- distinct from the step-zoom scheme [12]. Most infrared semiconductor detectors are cryo- 0003-6935/10/193691-10$15.00/0 genically cooled and assembled in a thermally insu- © 2010 Optical Society of America lated Dewar flask. This is necessary to achieve the 1 July 2010 / Vol. 49, No. 19 / APPLIED OPTICS 3691 maximum signal-to-noise ratio and avoid anomalous ble system. The methodology and process for stabiliz- images. A special baffle, called a cold stop, located in- ing the optical qualities by temperature compensa- side the Dewar flask, causes the light cone to strike tion is called athermalization. Active athermalization the focal plane array (FPA), and the FPA records the is appropriate for quick responses to overcome rapid objective space energy exclusively. It has been deter- error inputs while operating in a thermally dynamic mined that 100% cold-stop efficiency is required [13]. environment [17]. To facilitate athermalization, a mo- This can be accomplished by the aperture stop and torized lens adjusts the position and thermal sensors cold stop coinciding. The optical system is still opera- measure the temperature difference. The lens and tional if the effective focal length (EFL) is small. the sensor are controlled by a programmed micro However, to identify or classify a distant but small processor. target, a longer EFL is essential. The layout of the An advantageous feature of the DFOV infrared op- aperture stop inside the Dewar flask will make the tical system is the incorporation of motorized me- diameter of the front end of the lens larger than chanical parts that allow it to travel quickly to the entrance pupil, because of the off-axis light cone compensate for the variation of the optical group over accommodation. The only way to shrink the aperture an exact distance. In other words, the optomechani- size of the front end of the lens while maintaining cal design can ensure the functioning of the optical 100% cold-stop efficiency and offering a longer EFL design. The mechanical layout of the zoom system is is through reimaging. The extra relay lens creates an adapted to meet overall system requirements. Inno- image of the aperture stop located on the front end vations and inventions have led to the development lens that is equal in diameter to the entrance pupil. of numerous patents. For example, two coaxial lens The reimage element is the primary objective, fol- sleeves have been assembled together. With this type lowed by the intermediate focal plane, relay lens, of device, the translation of optical elements is en- cold/aperture stop, and FPA. abled by the relative rotation of the sleeves. The Stray light caused by double reflection in the visi- cam grooves provide a predetermined locus [18]. The ble lens can create ghost images, especially in a scene barrel is made more compact and portable when not with a signal difference source against a dark back- in operation by the movement of a thrust member ground. A high transmission rate of antireflection back and forth in the optical-axis direction [19]. coating in the IR spectrum is harder than in the visi- The diameter of the whole barrel can be reduced ble one; hence, an IR system is more vulnerable to by replacing the coaxial sleeve layout with an off-axis double-reflection stray light. Besides, the cryogeni- cam attached to the lens rim [20]. The traditional cally cooled detector acts as a strong light in a dark cam groove located on the cylindrical surface is re- scene because of its low temperature compared with placed by the plane surface [21]. The cam provides the warmer surroundings of the lens barrel or the ob- a reliable lens walkway but the mechanical manufac- served target. Therefore, an infrared FPA may reflect turing process is expansive and complex. Thus, the a single reflection of its own image. This is called the lens movement path can be controlled by a pro- narcissus effect [14,15], and this retroreflected non- grammed motor utilizing a spur gear to drive the uniformity thermal image reduces the contrast of a feed bar [22]. The optomechanical alignment must dim object. In a scanning system, the narcissus effect ensure that the line of sight (LOS) remains unaf- is noticed as a retroreflected ghost image that moves fected during the change in the FOV, especially for on the focal plane as the scanning mirror rotates. The automatic target-tracking applications. The methods starring array system can negate the narcissus effect for LOS stabilization commonly used to counteract if the optical system meets the requirements of non- jitter introduced from the optical system platform uniformity calibration, a constant surrounding tem- and surroundings can be categorized in three types: software, platform, and steering [23]. perature, and no lens movement. Otherwise, the narcissus effect is monitored by an optical design process. 2. Lens System Scheme Temperature change in the operating environment A SELEX HgCdTe FPA sensor (F=4, 384 × 488 pixels can result in variation of the physical properties of an and 20 μm square pixel size) was used in this study. optical assembly (i.e., lens thickness, air space, refrac- The optical design offers a dual EFL of 20=250 mm. tive index, structural dimensions) [16]. These effects This specification can improve the broad scene while can cause the system to lose focus or elements to be- still giving high enough resolution for most infrared come misaligned. Temperature gradients may cause optical system requirements. This is a reimaging type tilting in the axial or radial directions, reversing the system that allows for reduction of the lens aperture effects of homogenization on the materials and dis- and for compact instrument space. To ensure image torting the optical surface. The quality of the final im- quality, we must strictly consider a better minimum age can become degraded as the adverse temperature resolvable temperature difference (MRTD). The poly- effects increase. Most materials for infrared refrac- chromatic modulation transfer function (MTF) within tive lenses have high rates of index variation with the 3–5 μm spectrum approaches the diffraction limit temperature, which can result in rapid focus shifts. beneath the Nyquist spatial frequency. Degradation from thermal variations in an infrared In a Gaussian design, a primary objective is imple- system is an order of magnitude higher than in a visi- mented to generate the system optical power and to 3692 APPLIED OPTICS / Vol. 49, No. 19 / 1 July 2010 focus on the intermediate focal plane under infinite conjugate conditions. This is reimaged by a relay lens to the final focal plane (i.e., the cryogenic detector ar- ray).