2.1.

Working Concept

PI is achieved using specialized imaging devices to obtain a hemispheric FOV of more than 180 deg in the vertical and horizontal direction of 360 deg FOV.⁷ In order to acquire a full range of IR image data, the proposed design incorporates three IR objective lenses to divide the horizontal FOV for ensuring the integrity of data for two adjacent field parts, considering that fields of view of two adjacent lens need certain overlap. For that reason, the infrared lens field of each imaging channel is set to 122 deg, and then a complete PI system is obtained by splicing the images for all directions.

Three IR objective lenses in the horizontal direction receive the detected target information with different FOV. After transferring this information to the imaging system, detected light waves of various channels reach the focal plane array. Finally, after subsequent image splicing and matching, the panoramic image is displayed on a suitable display device. This process is shown in Fig. 1.

Fig. 1

The imaging principle of IR panoramic system.

2.2.

Structural Design of the Panoramic System

The structure and deployment of IR in a panoramic system is critical for producing high quality panoramic images.⁵ Panoramic systems consist of an IR objective system, a transferring image optical system, an IR receiver lens and detectors. A cobeam prism cemented together with the four right-angle prisms lies in the center position of the sphere. The four flat mirrors are denoted by M1, M2, M3, and M4. Optical path 1 enters the system from the objective lens A and arrives at the beam splitter prism after one reflection on the cobeam prism; optical path 2 enters the system from the objective lens B, arrives at the cobeam prism after two reflections from mirrors M1 and M2, and then reaches the beam splitter prism after another reflection; optical path 3 enters the system from the objective lens C, arrives at the partially reflecting mirror after reflections from mirrors M3 and M4, and then passes through the beam splitter prism after the cobeam prism; optical path 4 enters the system from objective lens D, through the partially reflecting mirror and the cobeam prism, and then arrives at the beam splitter prism. MWIR and LWIR are separated using the beam splitter, making images on two high-resolution IR detection focal planes IFPD1 (infrared focal plane array) and IFPD2, respectively. Thereby, a complete panoramic image is created by splicing subsequent images. The four IR lenses are designed according to the common path structure so that the system can respond to MWIR and LWIR bands simultaneously. On the one hand, this system design avoids the need to switch lenses since the optical paths of the lenses are common, and also makes the system more compact. On the other hand, the system can fully exploit the benefits of MWIR and LWIR to meet the practical application needs (Fig. 2).

Fig. 2

Structural representation of the panoramic system.

3.1.

Specifications of Optical System

The pixel size and resolution of MWIR and LWIR detectors is $28\mu \mathrm{m}$ and $640\times 512\text{pixels}$, respectively. The specifications of the optical system, determined by detector models and practical considerations, are shown in Table 1.

Table 1

Optical design specifications.

3.2.

Structure Design of the Optical System

The system adopts a hybrid refractive-diffractive structure, which can attain a larger FOV and meet our requirements. On the one hand, the introduction of the diffractive surfaces can weaken the vertical axis aberration and the chromatic aberration by taking advantages of unique dispersion characteristics of diffractive element; on the other hand, it can reduce the diameter of the system, making the system lightweight and portable.

It is necessary to consider the cold stop characteristics of the system itself with regard to a cooled IR system.⁸ Cold stop determines the location and size of the exit pupil of the optical system. Attaining a 100% cold stop efficiency indicates that the exit pupil of the optical system is able to match the cold stop of the detector. Otherwise, there is a risk of beam cutting which can reduce the system sensitivity or cause additional incident hybrid heat radiation entering the detector, which in turn can reduce the SNR of system.

Wide-field imaging systems typically have short effective focal lengths. In order to ensure a sufficient working distance, our system uses a double imaging configuration design known as the quadratic imaging configuration. A single imaging system requires a large aperture of the cooled IR system in order to achieve 100% efficiency of cold stop. Therefore, adding a secondary imaging configuration behind the first imaging position can reduce the radial dimension of the system. Moreover, for the double imaging configuration system, the first configuration group burdens the major diopter of the system, and latter configuration mainly plays a part in secondary imaging with the completion of residual aberration balance. The principal ray diagram of double imaging system is shown in Fig. 3.

Fig. 3

Principal ray diagram of a double-band optical system.

In Fig. 3, $L_4$, $h_4$, $h_5$, $a_1$, $a_2$ have been identified. According to equation for a paraxial optical system

In the above equation, $M$ denotes the total lateral magnification of double imaging system. We can obtain the following relationships from the geometrical arrangement shown in Fig. 3:

Combining with the equations from Eqs. (1)–(9), $L_3$ can be derived as follows:

From these results, we can deduce that: (1) the distance from exit pupil to the final image surface shortens with the included angle between the principal ray and the optical axis gradually decreasing, which affects the aberration unfavorably. In order to achieve 100% cold stop efficiency, it needs to coincide with aperture stop of the system. This is required for eliminating stray light and ensuring that the detector detects only the image in its FOV. (2) With the increase of axial magnification of the double imaging system, spacing from the exit pupil to the final image surface gradually decreases. Although this can make the system more compact, the $F$ number of the system can increase, should be avoided.

4.1.

System Design

Figure 4 shows the optical arrangement for the proposed MWIR/LWIR dual-band panoramic system, with an FOV of 122 deg and the $F$ number equivalent to 2. A hybrid refractive-diffractive structure was employed in the design. Optical surfaces marked with numbers 1, 6, and 11 are aspheric, surfaces 2, 4, and 13 are diffractive, while the remaining surfaces are spherical. IR materials such as ZnSe, ZnSe, and ZnS were chosen to match, with the total length of system set to 220 mm.

Fig. 4

Initial structure of signal IR objective optical system.

Surfaces 1, 6, and 11 are even, while 2, 4, and 13 are binary aspheric. One of the imaging channels of the IR dual-band panoramic system is shown in Fig. 5. Each imaging channel contains two configurations of MWIR and LWIR, a coaperture, a cobeam a prism, a beam splitter prism, a receiving objective lens of IR detector, and a focal plane of MWIR and LWIR. Dual-band coaperture form is applied in the former configuration of IR objective lens to achieve the synchronized imaging of MWIR and LWIR without the need for a switching mechanism.

Fig. 5

Structure of the dual-band co-aperture optical system. (a) Light path 1, (b) light path 2, (c) light path 3, and (d) light path 4.

Five lenses were used in the coaperture design of the system. The cobeam prism in the middle was composed of four identical isosceles right angle prisms that were mutually glued together at the oblique side coated with a semi-reflective film. Antireflection film was coated on the surfaces of rectangular prism, to minimize any loss of the incident radiation. According to the transmittance of half transparent half reflecting mirror and cobeam prism is 50%, and the beam splitter will not attenuate energy, which splitting wavelength rather than splitting energy, it can be estimated that the transmittance of light path 1 and 2 is about 50%, and the light path 3 and 4 is about 25%. Four lenses were used in the IR detector objective lens, for allocating the optical diopter of system and balancing the overall system aberrations. Ge, ZnSe, and ZnS satisfy the required physical and chemical properties. They were selected according to their refractive indexes and Abbe numbers for achromatism and athermalization.

4.2.

Image Quality Evaluation

4.2.1.

Modulation-transfer-function

In this optical system, image quality of light path 1 is similar to that of light path 4, with light paths 2 and 3 also having similar image quality.

In light paths 1 and 4, from the modulation-transfer-function (MTF) analysis of MWIR and LWIR shown in Figs. 6(a) and 6(b), it can be observed that the MTF values are all close to the diffraction limit at the Nyquist frequency of 18 lp/mm. The MTF of $\mathrm{MWIR}>0.72$ and the MTF of $\mathrm{LWIR}>0.45$, satisfying the requirements of imaging quality.⁹

Fig. 6

The MTF of light paths 1 and 4. (a) MWIR and (b) LWIR.

The MTF analysis of MWIR and LWIR for light paths 2 and 3 is shown in Figs. 7(a) and 7(b). It can be seen that the MTF is higher than 0.70 in MWIR and 0.35 in LWIR, at the Nyquist frequency of 18 lp/mm.

Fig. 7

The MTF of 2 and 3 light path. (a) MWIR and (b) LWIR.

4.2.2.

Energy distribution and calculation

The energy distribution of the light spot along with radial variations of light paths 1 and 4 can be obtained from Zemax software, as shown in Fig. 8. It can be seen that the energy concentration rate within a single pixel reached $\sim 85\%$ and 75% under the full FOV for MWIR and LWIR, respectively.

Fig. 8

Energy distribution of optical paths 1 and 4. (a) MWIR and (b) LWIR.

The energy concentration rate of light focused on Fig. 9 shows the light spot energy distribution of MWIR and LWIR along with radial variations for light paths 2 and 3. It can be seen that the energy concentration rate within one pixel size reached $\sim 80\%$ and 63% under the full FOV for MWIR and LWIR systems, respectively.

Fig. 9

Energy distribution of the optical system. (a) MWIR and (b) LWIR.

Although the energy concentration rate of light focused on the detector focal plane can reach above 75% ideally, the optical system exhibits energy losses mainly due to three effects: (a) reflection loss of MWIR and (b) absorption loss of LWIR.

4.2.3.

Detection blind area range calculation

Figure 10 shows the detection blind area range schematic of the PI system. The system uses the A, B, and C IR lenses to divide the horizontal FOV, achieving 360 deg PI in the horizontal direction. Each IR lense has an FOV of 122 deg, so that the detection FOV of two adjacent lens has overlaps in the horizontal direction in order to ensure a complete panoramic image. A blind area denoted by AOCH exists where all IR lenses are incapable of detecting. $O$ denotes the center point of the system, and $A$, $B$, and $C$ denote the entrance pupil centers of the IR lenses. From the design parameters, it follows that $\mathrm{AO}=136.241\mathrm{mm}$.

Fig. 10

Detection blind area range of the panoramic system.

The geometric relations are given as

Eq. (10)

$$\frac{\sin \angle AOH}{AH}=\frac{\sin \angle AHO}{AO}.$$

From this relationship AH is found to be 6.760 m. Evidently, if the horizontal direction 360 deg FOV is divided between the three IR lenses, the target can evade detection when it enters the range within about 6.76 m. This structure with three IR lenses in the horizontal direction and one IR lens in the vertical axis direction constitutes the four-channel PI system. This approach remarkably reduces the volume and weight of the system, while meeting the design requirements.

5.1.

Optical Passive Athermalization

Athermalization has to be considered in the system in order to account for ambient temperature changes that affect the refractive index, thickness, interval, and the radius of curvature for IR materials. The system is analyzed in the $-40°\mathrm{C}$ to 60°C temperature range in order to find the dependence of defocus on temperature of the two bands, with the goal of eliminating defocus by compensating through athermalization. For achieving passive optical athermalization, the thermal expansion coefficient and the dispersion factor need to meet the following criteria:¹⁰

Figures 11 and 12 denote the MTF of MWIR and LWIR systems at the temperature of $-40°\mathrm{C}$, 20°C, and 60°C, respectively. It can be seen that for all temperatures, the MTF values for the MWIR and LWIR systems at the Nyquist frequency of 18 lp/mm are greater than 0.6 and 0.4, respectively. This indicates that the image quality meets the design requirements after athermalization.

Fig. 11

Exit pupil of optical system.

Fig. 12

The MTF of MWIR after athermalization. (a) $-40°\mathrm{C}$, (b) 20°C, and (c) 60°C.

5.2.

Narcissus Analysis

Narcissus is the self-image of “cold light” emitted from the focal plane of the cooled IR detector. It is formed when the IR radiation passes the surface reflection of system lenses and then refocuses on the focal plane. Besides the detected target information, the detector also receives its own image, thereby seriously affecting the sensitivity of the system. Hence, during the design process of a cooled IR optical system, narcissus phenomenon has to be considered.¹¹ Typically, YNI value is used to characterize the amount of narcissus, where $Y$ is the incident height of paraxial rays on one optical surface, $N$ is the refractive index of one optical material, and $I$ is the incident angle of marginal rays at one optical surface. If the YNI value is greater than 1, it indicates that the impact of narcissus to the optical system can be ignored. For values of YNI below 1, further analysis is required.

Due to the limitation of space, the analysis of the narcissus of the 1 light path is given. The YNI values are shown in Table 2.

Table 2

YNI contribution value of 1 light path.

It can be seen from Table 2, the YNI value of surfaces 5, 7, 19 is less than 1, so it needs to undergo reverse ray tracing analysis, as shown from Figs. 13(a)–13(c).

Fig. 13

The MTF of LWIR after athermalization. (a) $-40°\mathrm{C}$, (b) 20°C, and (c) 60°C.

It can be seen from Figs. 13(a)–13(c) that the reflected light from the 5th, the 7th, and the 19th surface is diverging and cannot reach the IR focal plane, hence it can be ignored. Comprehensive analysis shows narcissus does not affect the image quality of the optical system (Fig. 14).

Fig. 14

Narcissus analysis of optical path 1. (a) Reverse ray tracing schematic of fifth surface. (b) Reverse ray tracing schematic of seventh surface. (c) Reverse ray tracing schematic of 19th surface.