Objective With the advancement and development of science and technology, the demand for miniaturized optical systems is becoming increasingly significant. The total length of the optical system can be reduced by folding the optical path in the annular aperture folding imaging system. However, the annular aperture folding imaging system only uses a piece of base material, so the annular aperture folding imaging system cannot achieve high-quality imaging in a wide temperature range. In order to reduce the impact of temperature on the imaging quality and simplify the optical system structure, the wavefront encoding method is introduced to design a light-digital combined annular aperture folding imaging system. Methods The design principle of the annular aperture folding imaging system based on light-digital is studied. The relationship between the obscuration ratio and the phase mask parameters is studied to achieve defocus consistency (Fig.5). In the image decoding part, the image restoration effect is analyzed. The synthetic PSF model is studied through simulated annealing algorithm (Fig.2). High-quality imaging in a wide temperature range is achieved through a light-digital combination method. Results and Discussions A long-wave infrared annular aperture folding imaging system is designed (Fig.3). The focal length is 70 mm, the system aperture is 98 mm, the full field of view is 8°, and the total length is 25 mm. The synthesized PSF is constructed by simulated annealing algorithm. When over a wide temperature range, the high-quality image restoration is achieved through a single filter. Although the PSNR of the restored image dropped by 3.572 3 dB at the design temperature, the PSNR of the restored image at −40 ℃ also increased from 19.417 3 dB to 24.461 5 dB, which increased by 5.044 2 dB. The PSNR of the restored image at 60 ℃ also increased from 19.751 9 dB to 24.460 9 dB, which increased by 4.709 0 dB. This method outperforms traditional PSF image restoration at the design temperature. Image artifacts and blur are significantly reduced by this method. Conclusions Athermalization of infrared annular aperture folding imaging system is achieved. The light-digital combination method is introduced into the annular aperture folding imaging system. The annular cubic phase mask is introduced into the annular aperture folding imaging system, and the restored image is achieved through image restoration. The relationship between central obscuration and phase mask parameters is studied. The conclusion that increasing central obscuration will reduce PSF consistency is studied. The synthesized PSF is constructed by simulated annealing algorithm. When over a wide temperature range, high-quality image restoration is achieved through a single filter. In order to verify the effectiveness of this theoretical model, an annular aperture folding imaging system based on light-digital combination is designed. The total length is 25 mm, the focal length is 70 mm, the system aperture is 98 mm, and the full field of view is 8°. High-quality image restoration is achieved by synthesizing PSF when the temperature is between −40 ℃ and 60 ℃. Although the PSNR of the restored image dropped by 3.572 3 dB at the design temperature, the PSNR of the restored image at −40 ℃ also increased from 19.417 3 dB to 24.461 5 dB, which increased by 5.044 2 dB. The PSNR of the restored image at 60 ℃ also increased from 19.751 9 dB to 24.460 9 dB, which increased by 4.709 0 dB. Compared with PSF image restoration at design temperature, this method significantly reduces image artifacts. The study not only simplifies the infrared imaging optical system, but also uses a light-digital combination method to overcome the temperature limitations of the annular aperture folding imaging system. A new idea is provided for the miniaturization of infrared systems across a wide temperature range. © 2024 Chinese Society of Astronautics. All rights reserved.
