Objective  For the current space environment single infrared band detection target false alarm rate is high, low sensitivity and other challenges, a double-color infrared optical system design method based on cold optical technology is proposed. The front optical path of the optical system adopts a common aperture structure, and the spectral band splitting is performed by a splitting plate, and the cold apparatus matching is realized by the secondary image of the relay mirror in order to ensure the light miniaturization of the system. In order to enhance the detection sensitivity of the long-wavelength system, a cold optical design is carried out to reduce the impact of the system's own radiation of detection performance. The working wavelengths are 3.7-4.8 μm and 7.9-9.3 μm, the F-number is 1.2, the total dimension of the optical structure is 260 mm×150 mm×80 mm, the aberration of the medium-wavelength system is less than 2.8%, about 82% of the energy is concentrated in a pixel of the detector, and the aberration of the long-wavelength system is less than 0.33%, about 70% of the energy is concentrated in one image element of the detector. The system can detect dim space targets at a long distance, and has the advantages of low false alarm rate, high sensitivity and compact structure.

  Methods   The common optical systems included refractive, reflective and reflexive, and the different optical structures have their unique advantages and disadvantages. Refractive systems have no center obscuration and high efficiency, but the variety of optical materials is small and not easy to correct chromatic aberration. Secondary image system is easy to match the cold screen, the optical components are small in size and light in weight, but the number of pieces is more. After comprehensive consideration, the refractive secondary image system is selected.

  Results and Discussions  According to the design index of infrared optical system and the design principle of light miniaturization and high energy transmission rate, it is decided to choose refractive secondary imaging system as the initial structure. The front optical path of the system adopts a common aperture type structure, and then the spectroscopic plate is used for spectroscopy, and the relay mirror set adopts the secondary imaging method to realize the cold apparatus matching and ensure the compactness of the system. In the process of system design optimization, aspheric surface is introduced to correct the aberration. The aberration of the medium-wavelength system is less than 2.8%, and about 82% of the energy is concentrated in one image element of the detector, while the aberration of the long-wavelength system is less than 0.33%, and about 70% of the energy is concentrated in one image element of the detector. Each mirror of the medium-wavelength system and long-wavelength system meets the requirements of the system cold reflection. After setting a reasonable tolerance value, the system image quality still meets the use requirements. After the completion of processing and assembly, the experimental verification, the system detection distance meets the expected target, to meet the design requirements.

  Conclusions  As the demand for space target detection grows, multi-band detection will become one of the future directions of infrared detection technology. A cold dual-color infrared detection system is designed in the paper. Through experimental verification, the detection capability of the system meets the expected target.