where D is the entrance pupil diameter; F# is the F-number; λ is the wavelength; M is the number of pixels required to confirm the target Δx; C is the scene contrast; d is the size of the detector element; η is the detector collection efficiency; ΦB2π represents Background radiant flux for a 2π field of view; τint is the integration time.
It can be seen from formula (1) that keeping the volume of the infrared optical system unchanged means fixing the entrance pupil diameter D, then the larger the F#, the larger the working distance, but the sensitivity NETD decreases with the increase of F, which indicates that in the second-generation optical system a single F# in the system cannot meet the requirements of range and sensitivity at the same time. Therefore, the third-generation infrared optical system is firstly designed with a variable F-number, so as to optimize simultaneously the working distance and sensitivity of the infrared/thermal imaging system.
2) Under the condition of minimizing the number of lenses and maximizing the transmittance of the optical system, it can achieve clear imaging in the mid-wave infrared and long-wave infrared bands and dual-band pixel-level fusion imaging at the same time.
The feature of the dual-band infrared focal plane detector using the stacked pixel structure is that the pixel size and pixel scale of the dual-band are the same, and the independent readout circuits read out the dual-band data respectively. The issues that need to be considered when designing the infrared optical system of the dual-band infrared focal plane detector are as follows:
Dual-band infrared radiation of the same scene should be focused on the same focal plane detector without refocusing, and sufficiently high image quality should be obtained;
The double band reaches the diffraction limit at each field of view
The focal length of the dual bands should be the same;
Distortion should be the same for both bands;
Chromatic aberration caused by the difference in the dispersion of infrared optical materials in the mid-wave and long-wave infrared bands should be corrected.
In response to these problems, the dual-band third-generation infrared optical system greatly simplifies the field of view registration of the two bands by adopting a common aperture design. At the same time, a catadioptric optical system with a wide spectrum, small chromatic aberration, small axial size, and flexible design is adopted. In addition, a dual-band "picture-in-picture" infrared optical system has been developed.
3) Reducing the volume, weight, power consumption, and cost of infrared/thermal imaging systems is an eternal requirement: "there is no best, only better".
In response to this requirement, the third generation of infrared optical systems has developed micro-optical systems, free-form optical systems, etc.
4) Infrared optical systems for computational imaging, the potential of optical systems should be fully exploited through intelligent computing.
In response to this requirement, optical multiplex imaging has been developed. Brief descriptions are given below.
3.1 Development of variable F-number cooling infrared imaging optical system
The variable F-number infrared imaging optical system can give full play to the advantages of the third-generation infrared detectors with high sensitivity and high spatial resolution (large area array), and optimize the spatial resolution and sensitivity of the system while maintaining the volume of the original thermal imaging system, improve the signal-to-noise ratio when searching for a target with a wide field of view, and maintain the ability to point to and track the target at a long distance (i.e. when the field of view is narrow). For the limited entrance pupil diameter in practical applications, choose a large F-number in a narrow field of view, and focus on the action distance; choose a small F-number in a wide field of view, and focus on the field of view and sensitivity. For multi-band detectors, choose a large F-number for the mid-wave infrared band, and a small F-number for the long-wave infrared band.
3.2 Development of catadioptric infrared imaging optical system
According to the characteristics of the catadioptric infrared optical system with a wide spectrum, small chromatic aberration, small axial size, and flexible design, the infrared imaging with common aperture, double F-number, dual/multi-band, multi-field/large zoom ratio continuous zoom has been developed. The optical system meets the requirements of integrated and automated target search, confirmation, and tracking of weapon platforms with limited installation space.
3.3 Development of dual-band "picture-in-picture" infrared imaging optical system
A dual-band "picture-in-picture" infrared imaging optical system is developed to simultaneously search and identify targets through spatial information and spectral information of different optical magnifications on one screen.
For common ground object scenes, the medium-wave and long-wave infrared images output by the medium-wave/long-wave infrared dual-band focal plane detector have a high degree of correlation (that is, there is no obvious difference between the images), and the operator needs to zoom repeatedly between the wide field of view and narrow field of view to search and identify objects of interest. In order to make full use of the ability of dual-band focal plane detectors to synchronously obtain information in separate bands, and to take advantage of the fact that the peak radiation wavelength of ground objects is located in the long-wave infrared band and the contrast of medium-wave infrared images is high, the dual-band “picture-in-picture” infrared imaging optical system has been developed in recent years. It uses wide-field long-wave infrared images to perceive the situation of ground objects and scenes, and narrow-field medium-wave infrared images to obtain high-contrast target images.
3.4 Development of a catadioptric peripheral infrared imaging optical system
According to the advantage of the large scale of the third-generation infrared focal plane detection pixels, a catadioptric circular infrared imaging optical system based on quadratic surface mirrors was developed, and a single infrared focal plane detector was used to record a wide field of view and even a 360°circumference view Field information to meet the needs of infrared warning and alarm.
3.5 Development of free-form surface infrared imaging optical system
With the support of advanced optical manufacturing and measurement technology, free-form surface infrared imaging optical systems are developed to overcome the difficulties such as widening the field of view, correcting aberrations, simplifying the structure of optical systems, controlling volume, and reducing weight.
Freeform surfaces provide complex geometries that are not rotationally symmetric, enabling unconventional image acquisition and aberration correction. In 2014, Kyle Fuerschbach of the University of Rochester, Jannick P. Rolland, and Kevin P. Thompson pointed out that the free-form surface can be completely described using the existing aberration theory. This study found that optical designers can break the limitation of rotational symmetry, and design free surfaces of any shape according to the current mathematical models, to obtain a completely unobstructed infrared optical system, completely composed of reflective optical elements. The three researchers also designed and verified a fully reflective infrared optical system using only three free surfaces, with diffraction limit as low as 5um, F-number 1.9, high compactness, high thermal stability, and lightweight diagonal field of view 10, which can be installed in a complex three-dimensional space without a folding mirror.
3.6 Development of miniature uncooled infrared imaging optical system
Ultra-thin, miniature uncooled infrared imaging optical systems were developed to meet high-performance infrared imaging requirements under the constraints of volume, weight, and power consumption (SWaP).
With the substantial reduction in the volume and weight of uncooled infrared focal plane detector components, the demand for miniature infrared imaging optical systems with optical path lengths less than 1/2 of the focal length is increasing. Low-cost, simple processed spherical reflectors are widely used for folding optical paths, refractive optical elements are also used to reduce the occlusion of the reflection surface to the optical path.
The infrared imaging optical system is extending from the objective lens to the infrared focal plane detector chip and its signal processing circuit by reducing its size. When the size of the optical element unit is reduced to the same size as the detector, operations such as spectrum, polarization, and phase encoding at the pixel level can be realized. For example, the "infrared retina" proposed by Sanjay Krishna in 2009 regards each pixel of the infrared detector as a cone cell in the retina, and couples the interactions between these cells through later information processing technology, thereby imitating the human eye’s function of perceiving scene information and recognizing the situation of the scene; when the optical element unit is further reduced to the micrometer or nanometer scale, due to the surface effect, volume effect and quantum size effect, its optical performance will show characteristics significantly different from the macroscopic optical element unit, such as superior absorption, antireflection or convergence properties. It can be said that the development trend of infrared optical systems is to integrate with infrared detectors, and nanophotonics has become the driving force for the development of the fourth generation of infrared focal plane detectors.
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