Optical System Design of Dual-band/dual-field Panoramic Aerial Camera

Aerial cameras are precision optical instruments installed on aircraft to take pictures of the ground. They are widely used in flood control, urban planning, land surveying, mapping, and other fields. There are three main types of aerial cameras: frame type, push-broom type, and panoramic type. Compared with the former two, the panoramic aerial camera uses the objective lens of the ordinary field of view to sweep through the front reflector, that is, it can achieve an ultra-wide field of view photography and obtain a larger amount of information.

Since the target shows different optical characteristics in different wavebands, in order to increase the amount of target information obtained and improve the accuracy of processing and interpretation, multi-band aerial cameras are more and more widely used. The development of multi-band optical systems has been carried out very early in foreign countries. At present, visible light/infrared aerial cameras with all-weather application capability are widely used. The domestic research on visible light/infrared dual-band imaging technology started late, and the research mostly focused on frame-type aerial cameras. The system structure mostly adopts the Cassegrain primary optical common aperture system and uses the beam splitter to realize dual-band beam splitting. There are few reports on the research on dual-band panoramic aerial cameras. In addition, in order to improve the flexibility and accuracy of aerial reconnaissance, the demand for dual field-of-view aerial cameras is also increasing. It can meet the needs of different flight altitudes for ground coverage and achieve large-field target searching and accurate small-field target observation. 

In this paper, a visible light/infrared dual-band dual-field imaging optical system for panoramic aerial cameras is designed. The optical system has the advantages of compact structure, small volume, lightweight, and low cost.

The zooming observes the principle of object image exchange, which changes the focal length of the entire system by the movement of a group of lenses in the optical system, ensuring good image quality in the large and small fields of view and the unchanged image plane position. As shown in Figure 1, for any lens or lens group in the optical system, when it moves from position A to position B, it can ensure that the conjugate distance remains unchanged and the magnification changes. The zooming of the dual-field system observes the principle of object image exchange.

Figure 1:

2 Camera Optical System Design

2.1 Optical Design Index

The main optical design indicators are shown in Table 1.

There are three types of optical system structures: refractive, catadioptric, and reflective. Regarding the design of visible light/infrared dual-field imaging optical systems, Cassegrain catadioptric optical system and the total reflection optical system are mostly used in China. However, these two types of systems can only bear a small field of view angle. The infrared and visible light large field of view systems in this paper has a large view angle, so they are not considered. The refractive optical system has the advantages of a large field of view and high imaging quality. Therefore, the structure of the refractive optical system is adopted in this paper, and the plane mirror is appropriately used to fold the optical path and facilitate miniaturization.

The zooming methods of the optical system include axial movement zooming and cut-in and cut-out zooming. Since the cut-in and cut-out zooming method requires a large structural space, the axial movement zooming method is adopted. The cold diaphragm of the cooled infrared detector is set to eliminate the interference of stray light outside the field of view. The matching of the exit pupil and the cold diaphragm must be considered in the optical design to ensure 100% cold diaphragm efficiency, which is realized by directly using the cold diaphragm as the aperture diaphragm or placing the exit pupil of the optical system on the cold diaphragm while keeping its size consistent with the cold diaphragm. In addition, to avoid a too large aperture of optical parts at a long focus, the aperture of optical parts is compressed by means of secondary imaging, and a field diaphragm is set at the primary image plane to suppress stray light.

The Gaussian optics theory is used to reasonably distribute the optical power. After calculating the initial structure, the optical design software Zemax is used to set the boundary conditions and optimize the initial structure. To make a compact optical system, both the infrared and the visible-light systems adopt the “positive-negative-positive” optical focal structure. The front fixed group and the rear fixed group are positive lens groups, and the zoom group is a negative lens group. In the optical path of the system, to reduce the volume, the infrared system uses three mirrors to fold the optical path, and the visible light system uses two mirrors to fold the optical path.

In order to facilitate the correction of off-axis aberrations and keep the size of the aperture diaphragm unchanged on large and small fields of view switching while considering the front and rear dimensions of the optical system, the aperture diaphragms of the infrared system and visible light system are placed near the middle of the optical path and before the rear fixing group. On-axis aberrations are corrected by using a lens near the position of the curved aperture stop, while the lens surface with a large light incidence angle is bent toward the stop to reduce advanced aberrations. In order to reduce the number of lenses in the infrared system and improve the transmittance and obtain satisfactory image quality, the infrared system introduces three high-order aspheric surfaces on the rear surface of lens 2, the front surface of lens 3, and the rear surface of lens 4 to balance on-axis spherical aberration and field curvature.

To ensure the material supply channel, the lens materials of the infrared system are commonly used single-crystal germanium, silicon, and zinc selenide, and the lens materials of the visible light system are selected from the materials with high frequency and excellent performance produced by Chengdu Guangming.

In this paper, a visible light/infrared dual-field imaging optical system for panoramic aerial reconnaissance cameras is designed, the detailed optical design indicators are given, and the structure of the designed optical system is analyzed. The visible light and infrared dual-field optical systems can be integrated by using the ground object scanning mirror and the spectroscope. The design results show that the imaging quality of the optical system is close to the diffraction limit, which can meet the practical needs of engineering.

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