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The Design Principles of Infrared Optical System

The infrared optical system refers to the system that works in the infrared band of the optical wave, which is the optical system that receives or sends infrared light waves. In general, the infrared optical system is a category of optical system, which is not different from other optical systems in light energy reception, transmission, imaging and other optical concepts. However, due to the infrared optical system working wavelength infrared region and the photoelectric detector as the receiving element, so it has its own characteristics, different from the general optical system.

The wavelength range of infrared optical system tends to be wide, and there are few kinds of infrared transmission materials available at present. Aberration correction, especially chromatic aberration, is very difficult. Therefore, the infrared optical system adopts the non-spherical reflection system or refraction - reflection system in the structure. With the continuous expansion of infrared application scope and the continuous development of infrared thermal imaging technology, the projective objective cannot meet the requirements of large field and large aperture. Therefore, in recent years, high refractive index and low dispersion crystal materials have been widely used for various refractive lens. Infrared optical system is an infrared detector. In order to improve the detection sensitivity, increase the signal-to-noise ratio, the system USES the package live immersion lens, field lens and light cones of secondary condensing system (also known as detector optical system), and a light engine scanning with all kinds of optical scanner. From the principle of design, most infrared optical systems are designed using geometrical optics.

The characteristics of infrared optical system

Because of the unique properties of infrared radiation, compared with the general optical system, especially the visual and photographic system, the infrared optical system have different characteristics.

1) the radiation band of infrared radiation source is located in the invisible zone above 1 um. Ordinary optical glass is not transparent to 2.5 um. In all materials that are possible to penetrate infrared wavelengths, only a few materials have the necessary mechanical properties and can be given a certain size. This greatly restricts the application of lens system in the design of infrared optical system, which makes the reflection type and the reflex optical system occupy a more important position.

2) nearly all infrared systems are optoelectronic systems. Its receiver is not a person's sleep or a photographic plate, but a variety of optoelectronic devices. Therefore, the performance and quality of the corresponding optical system should be based on the sensitivity and signal-to-noise ratio of the optical system, rather than the resolution of the optical system. This is because the resolution tends to be limited by the size of the photoelectric device, thus reducing the requirements for the optical system accordingly.

3) small field and large aperture. In the case of the application unit detector, because the infrared detector has smaller reception area, the optical field of the general infrared optical system is not very large, and the outer aberration of the axis can be considered less.
Because the reflection system has no color difference, in most cases, it is possible for such a reflection system to eliminate the ball and satisfy the sine condition. At the same time, the requirements of such system objects are not too high, and they require high sensitivity. Therefore, most of the optical systems with large relative aperture, namely small F, are adopted. In general, due to the limitation of processing, the number of F is 2-3.

4) the application of various scanners has become more and more, in order to achieve the goal of scanning the space target by tracking the heating of the imaging and thermal imaging technologies. The scanner can be placed before the imaging system, which has a large size and high power consumption, but has minimal impact on the image quality of the optical system. In addition, the optical system with this scanning system requires the rear focal length of the principal and some special requirements for the it.

5) the wavelength of the infrared band is about 5-20 times that of visible light. In this way, the temperature of the thermal imaging system is lower because of the diffraction limit, which means that the thermal imaging system with high resolution must have a large aperture. This makes the system heavy and costly.

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