Spectral imaging detects two-dimensional (2D) spatial information about multiple wavelengths with two spatial coordinates and one spectral dimension.^[1–7] Polarization imaging acquires the information about target’s features, shapes, texture, and material composition.^[8–10] Multispectral and polarization imaging^[11,12] can obtain the polarization information of an object in different wavelengths and thus becomes an effective detection technique^[13–15] to improve target detection, biomedical imaging, remote sensing, and disease diagnosis. Image multiplexing methods produce multiple images by taking advantages of lens arrays,^[16] beamsplitters,^[17] image slicers,^[18] Wollaston prisms,^[19] light pipes^[20,21] or active optical elements.^[22] The simultaneous acquisition of multispectral and polarization images is a critical factor of the performance of multispectral and polarimetric imaging system. However, the majority of current multispectral imaging devices still feature the major limitation of measuring polarization and spectral information simultaneously. The simultaneous acquisition of polarization and spectral information leads to the dramatical increase of either system complexity^[23] or time consumption.^[24] For example, a multispectral single-shot polarimeter^[25] can obtain only the polarization information of a linear pixel array. Therefore, scanning has to be included to enable the simultaneous acquisition of the multispectral and polarization information of a two-dimensional (2D) scene.

In this work, we design a relatively simple system for achieving the simultaneous acquisition of multispectral and polarization optical images. The system utilizes a catadioptric optical system to acquire polarization information in two diverse wavebands simultaneously. An off-axis reflection optical system is employed as a common-aperture telescope^[26,27] to maintain a wide field of view (FOV), compared with some of the literature like Ref. [27] whose FOV is 1°. The optical system enhances the identification ability of the whole system by functioning in the full waveband. The off-axis three-mirror system gets rid of central obscuration by decenter and/or tilt^[28] and stabilizes overall system resolution and energy. A beamsplitter separates light into the visible (400 nm–750 nm) and long-wave infrared (LWIR, ) waveband. Visible light and LWIR light converge on two detectors once they pass through two sets of transmissive lenses. The simulation results indicate that the performance of the proposed system is quite satisfactory to obtain clear images in the visible spectrum and LWIR.

As schematically illustrated in Fig. 1, the multispectral and polarization camera system can simultaneously acquire the spatial and polarization characteristics of an object in two different wavebands. The whole system functions in the visible light (400 nm–750 nm) and LWIR ( ) wavebands and employs a catadioptric optical system. Two wavebands share a non-confocal optical system as a telescope. The optical system is mounted on the front of the camera system and overcomes the disadvantages encountered in the broad spectrum by the anti-reflection (AR) film coatings of refractive optical system. The non-confocal optical system can maintain a wide FOV (4.5°) and enhances the identification ability of the overall system by utilizing an off-axis three-mirror optical system as a common-aperture telescope. A beamsplitter is used to separate the emergent light path into the visible light and LWIR wavebands. These two different wavebands are employed in a refractive optical system for secondary imaging. The refractive optical system use a secondary telescope, a polarizer, and an objective to focus the visible light and LWIR wavebands on the detector. The common-aperture multispectral and polarization optical imaging system is thus realized.

Fig. 1.

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	Fig. 1. Schematic diagram of common-aperture multispectral and polarization optical imaging system.

The target is imaged multiple times to limit the size of the beamsplitter and polarizer aperture.

2.1. Common-aperture off-axis reflection optical system

Options for materials with the desired thermal properties, resilience, and color correction ability for a multispectral system are limited. Simultaneous optical imaging in the visible light and LWIR is necessary to increase the level of target discrimination and identification and reduce the rate of false alarm. We present a non-confocal common-aperture optical system in which an off-axis reflection system is used to achieve these objectives. Reflection systems have been attracting considerable attention from researchers because they do not exhibit chromatic aberration while possessing compact and light-weight structures. The off-axis three-mirror optical system has sufficient tunable parameters to reduce aberrations while getting rid of central obscuration and influences on resolution and energy. In addition to these advantages, there are other two reasons for employing this common-aperture off-axis reflection optical system: our front-end off-axis reflection optical system can not only achieve common-aperture in visible spectrum and LWIR, but also compress the beam to the ratio of 3.

The designed off-axis three-mirror common-aperture optical system is shown in Fig. 1. This system can maintain a wide FOV (4.5°) and meanwhile enhance the identification ability of the overall system. The focus of the combined first and second mirror is the same as that of the third mirror. Thus, a primary imaging plane is present between plane mirror and third mirror. The optical length L of the non-confocal optical system is given by the following formula:

where F_1,2 is the focal length of the combination of the first and second mirror, and F₃ is the focal length of the third mirror. A plane mirror that is inclined 45° is used to reduce the volume of the off-axis reflection system. The diameter of the exit beam is assumed to be limited to the diameter of the beamsplitter ( ) and polarizer ( ). The magnification of the non-confocal optical system must be reduced to increase angular magnification within a finite range and to limit the diameter of the exit beam. We select the magnification of the non-confocal optical system MR as 1:3 in consideration of the FOV; that is,where D_EXP is the diameter of the exit pupil, and D_ENP is the diameter of the entrance pupil.

2.2. LWIR optical system

2.2.1. Secondary telescope

We should further limit the diameter of the exit beam, given the small size of the polarizer ( ). As shown in Fig. 2, we add a secondary telescope, such as a Kepler telescope, to limit the aperture diameter of the exit beam. We then limit the axial and lateral sizes of the objectives sequentially. An intermediate image exists between the first and second lens group.

Fig. 2.

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	Fig. 2. Schematic diagram of Kepler telescope.

The optical length L_IR of the secondary telescope is given by

where f_IR1 and f_IR2 are the focal length of the first lens group and that of the second lens group, respectively. The magnification of secondary telescope M_IR is selected as 2:3, that is,The FOV of the incident beam of secondary telescope changes from 4.5° to 13.5° after the incident beam has passed through the off-axis reflection optical system with a magnification of 1:3.

2.2.2. Objective

The objective converges beams on the detector and corrects residual aberration. These functions are illustrated schematically in Fig. 3.

Fig. 3.

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	Fig. 3. Schematic diagram of objective.

The diameter of objective d₀ is given as

2.3. Visible-spectrum optical system

The principle of the visible-spectrum optical system is similar to that of the LW system, which includes a secondary telescope and objective. These two systems can be distinguished as follows: A Kepler telescope is employed in the LW system, whereas a Galileo telescope is used in the visible-spectrum optical system. These differences are presented in Fig. 4. The optical length L_LW of the secondary telescope is given by

where f_V1 and f_V2 are the focal length of the first lens group and that of the second lens group, respectively. Here the magnification of the secondary telescope M_V is chosen as 2:3, that is,Like the LW system, the visible-spectrum optical system has an FOV of 4.5°.

Fig. 4.

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	Fig. 4. Schematic diagram of Galileo telescope.

We allocate the power of system elements and calculate the initial structural parameters in accordance with primary aberration theory and the design theory presented in Section 2. At first, the paraxial lens is used to meet first order requirements and then replaced by a real lens. After the elimination of aberration and a series of optimization, the common-aperture multispectral and polarization optical imaging system with an FOV of 4.5° is achieved. The system enables the aberration correction and the balance between primary aberration and advanced aberration. The diagram in Fig. 5 illustrates the design results. The whole system employs a catadioptric optical system. Two wavebands share a non-confocal optical system that is used as the front-end telescope. A beamsplitter is used to split the emergent light into visible light and LWIR. The refractive optical system employs a secondary telescope, a polarizer, and an objective to focus visible light and LWIR on the detector. The common-aperture multispectral and polarization optical imaging system is thus realized.

Fig. 5.

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	Fig. 5. Optical layout of the common-aperture multispectral and polarization optical imaging system. (a) Optical path of whole system. (b) Optical path of LWIR optical system. (c) Optical path of visible-spectrum optical system.

To reduce the system aberration, the conic surfaces, rather than aspheric surfaces, are employed in three mirrors, which can facilitate processing and alignment and reduce the manufacturing cost. The values of the parameters of the conic surfaces are shown in Table 1. We use spheric surfaces to describe all other surfaces of the visible and LWIR transmission systems.

Table 1.

Table 1.

Table 1. Values of parameters of off-axis reflection optical system. . Surfaces Y radius Conic constant First mirror −1,242.34 −0.96 Second mirror −581.62 −4.98 Third mirror 1,022.17 −1.30

Table 1. Values of parameters of off-axis reflection optical system. .

Figure 6 shows the spot diagrams and modulation transfer function (MTF) plots of the whole common-aperture multispectral and polarization optical imaging system at a full FOV of 4.5°. Figures 6(a) and 6(b) show the spot diagrams and MTF plots of the visible-spectrum optical system with the common-aperture off-axis reflection optical system. The maximum RMS radius of the spot diagram is 4.86 , which is less than the minimum pixel size ( ) of common visible-light detector. Figures 6(c) and 6(d) show the spot diagrams and MTF plots of the LWIR optical system with the common-aperture off-axis reflection optical system. The maximum RMS radius of the spot diagram is , which is less than the minimum pixel size ( or ) of common LWIR detector. The MTF plots of both wavebands are close to the diffraction limits as illustrated in Figs. 6(b) and 6(d). These characteristics satisfy the technical requirements.

Fig. 6.

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	Fig. 6. Spot diagram and MTF plots of the common-aperture multispectral and polarization optical imaging system. (a) Spot diagram of visible-spectrum optical system and (b) its corresponding MTF plot. (c) Spot diagram of the LWIR optical system and (d) its MTF plot.

We have just finished the optical design of the system and set about mechanical design and optical elements fabrication recently. This experiment has a long project cycle, high cost and considerable difficulty. The experimental results will be shown in detail when the fabrication and alignment of the system are completed in the near future.

In this work, we introduce a common-aperture multispectral and polarization optical imaging system with a full FOV of 4.5°. Instead of incorporating two discrete transmission systems, we employ an off-axis three-mirror optical system as a front-end common-aperture telescope to improve the target discrimination and identification rates and reduce the false alarm rate. This system can maintain a wide FOV and enhance the identification ability of the system. At the same time, the off-axis three-mirror system gets rid of central obscuration but maintains the resolution and energy of the whole system. The whole system is composed of the reflective and refractive component. The reflective component employs an off-axis three-mirror optical system. The refractive component is divided into visible light and LWIR wavebands by a beamsplitter and they are converged on two detectors through two groups of lenses. This system enables the simultaneous acquisition of the spatial, spectral, and polarization information of a target. Given these characteristics, this system indicates remarkable potential applications in target detection and biomedical imaging.