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Design of shortwave infrared imaging optical system

In recent years, with the development and improvement of infrared detector technology and its processing technology, infrared optical systems have been widely used in the field of machine vision. Visible light and short-wave infrared are currently more commonly used optoelectronic imaging bands. Short-wave infrared mainly comes from the infrared radiation in the reflection environment of ground objects and objects. It has strong anti-interference ability, good concealment, and is not limited by lighting conditions. However, the infrared imaging system has the disadvantages of low spatial resolution, weak contrast, poor stereoscopic effect and low signal-to-noise ratio. Therefore, in the case of poor lighting conditions, use infrared light imaging to realize the observation of the target, and make up for the defects of visible light in poor lighting conditions; Infrared light acquires more detailed information about the target.


In this paper, combined with the needs of machine vision engineering applications, a wide-spectrum visible-short-wave infrared imaging optical system is designed using the optical design software ZEMAX.


Optical system selection


Optical system structure selection


The choice of optical system structure is closely related to the application scenario of the system. In the field of machine vision, imaging systems in the short-wave infrared band often have the characteristics of large field of view, small distortion and stable imaging quality. Reasonable selection of the optical system structure can reduce the complexity of the design. Commonly used optical system structures are divided into three types: refractive system, reflective system, and hybrid system. Different optical system structures have their own advantages and disadvantages.


(1) Refractive optical system structure. The refraction system observes through the transmitted light after refraction, which is a widely used form in the selection of optical structures. According to conventional processing and assembly methods, the precision requirements can be met, and it has the characteristics of stable image quality, small stray light, and high component transmittance. However, the refractive system is prone to aberrations, and the types of transmission materials that can be used in a wide spectrum are relatively limited. A typical refractive optical structure is shown in Figure 1.


Figure 1 Refractive optical system structure


(2) Reflective optical system structure. The reflective system is widely used in the field of infrared thermal imaging to observe with reflected light through reflective accessories. It has the characteristics of no chromatic aberration, wide working band, and easy realization of athermalization. However, the reflective structure increases the volume of the optical system, and the central occlusion of the coaxial reflective system causes the loss of luminous flux, which reduces the modulation transfer function and signal-to-noise ratio of the system; the off-axis reflective system solves the occlusion problem, but causes difficulties for system installation and adjustment . Typical structures of two reflective optical systems are shown in Figure 2


Figure 2 Schematic diagram of reflective optical system

(3) Catadioptric optical system structure. The catadioptric optical system is designed by combining the characteristics of refraction and reflection to meet the needs of actual engineering. Combining the advantages of different optical system structures can reduce the complexity of the optical system structure and the difficulty of aberration correction. However, the processing of the aspheric mirror is difficult, the cost is high, and the stability is poor, and the fine structure of the aspheric surface and the diffraction element increases the difficulty of camera processing and adjustment. The structure of a typical catadioptric optical system is shown in Figure 3.


Figure 3 Structure of catadioptric optical system


To sum up, for the wide-spectrum visible-short-wave infrared imaging optical system in this paper, as an industrial lens for machine vision inspection, it should meet the design requirements of light weight and large field of view as much as possible, while maintaining a high luminous flux. It is difficult to correct the off-axis aberration of the reflective structure, and it is difficult to expand the field of view; the secondary mirror of the catadioptric structure forms a central obscuration, and with the increase of the field of view and relative aperture, the obscuration ratio increases rapidly, making the imaging contrast and resolution and reduced detection capabilities. Regardless of imaging performance or cost performance, the refractive system is the ideal choice for this case with a large field of view, low distortion, and compact structure.


Calculation of Aspherical Surface Parameters


Aspheric optical element refers to the optical element whose surface shape is determined by multiple high-order equations, and the radii of each point on the surface shape are different. Although the processing and manufacturing of aspheric surfaces is difficult, it has the obvious effect of eliminating monochromatic aberration, and it is also more and more widely used in industrial lenses for machine vision inspection. Compared with the spherical surface, the aspheric surface has great advantages: the aspheric surface can increase the relative aperture ratio of the system, expand the field of view, improve the beam quality, and the number of lenses is less than that of the spherical surface, and the shape of the lens is miniaturized, which can reduce the system pressure. quality etc.


System Optical Design


Determination of overall lens design index and initial structure


The design of an optical system is generally divided into the following steps: (1) Determine the structural parameters of the optical system; (2) Use the PW method to solve the initial structural parameters or select the corresponding initial structural parameters; (3) Aberration correction; ( 4) Image quality evaluation; (5) Determine the tolerance of each optical component; (6) Draw the optical system diagram and the optical component parts diagram.


According to the actual application situation, the main parameters considered in the structural design of the wide-spectrum visible-short-wave infrared imaging optical system are: lens material, working band, focal length, F-number, field of view, and total system length. Through the analysis of the parameters of the wide-spectrum infrared imaging system, the final optical system chooses a refractive structure, using a CMOS area array detector with a resolution of 2448 pixel×2048 pixel, a pixel size of 3.45 μm, and a target size of 2/3 in (1 in=2.54 cm), the lens has stable optical performance and good imaging quality in the operating temperature range of 0~50 ℃. The performance parameters of the optical system are shown in Table 1.


A good initial structure can make aberrations converge rapidly during the correction process. The first key step in optical design is how to choose an appropriate initial structure. If the structural parameters of the optical system are known, the optical system can Tracking, calculation of aberrations and evaluation of imaging quality. When the optical system is relatively simple, according to the relevant initial system design indicators, the aberration balance equation and PW method are used to solve the initial structure by using the Seidel aberration theory. With the development of computer-aided optimization technology, a large number of optical system models are built into modern optical design software. When the optical system is more complex, the initial structure similar to the initial system design index can be compared according to parameters such as focal length and relative aperture. In this design, relevant patents were inquired, and the second method was used to determine the initial structure of the system.


Aberration correction and optimization evaluation


After the initial structure of the system is determined, the final system with good imaging performance needs to go through the subsequent repeated aberration correction process, and also needs to further optimize its structure. The specific optimization design process is as follows.


According to the optical design indicators given in Table 1, by changing the surface parameters of the lens, changing the thickness of the lens and the distance between the lenses, and changing the lens material, the aberration of the lens can be gradually reduced. In the optical design software ZEMAX, the parameters such as the radius of curvature and thickness of the lens, the focal length and length of the system, and the lens spacing are set as variables. The default optimization evaluation function of ZEMAX is used, and only the first-order optical parameter limit is added. Variables from less to more, order The number is gradually optimized from low to high. After each step of optimization is completed, observe the changes in various aberrations of the system after each optimization, increase the weight coefficient of the operand of the aberration item that contributes more to the imaging performance of the system, optimize it in a targeted manner, and judge whether it is necessary to continue Increase the optimization variables, and finally design an optical system that meets the various indicators of the system.


The structure of the wide-spectrum visible-short-wave infrared imaging optical system after optimization is shown in Figure 4. The system can image visible light and short-wave infrared bands. It uses 10 lenses in 7 groups. The diaphragm is located on the back surface of the fourth lens, and the front surface of the tenth lens is aspherical. The total system length is 79.6 mm, the entrance pupil diameter is 9.9 mm, and the F number is 2.8.


Figure 4 Optical system structure diagram


Figure 5 shows the spot diagram of the wide-spectrum visible-shortwave infrared imaging system. It can be seen from the figure that the maximum RMS radius is 5.623 μm, and the maximum GEO radius is 26.431 μm. The smaller the radius of the diffuse spots in the spot diagram, the better the imaging quality of the optical system. The spot diagrams of the five fields of view are all close to the Airy disk, close to the diffraction limit, and meet the imaging requirements.


Figure 5 Optical system spot diagram


Figure 6 shows the modulation function curve of the wide-spectrum visible-short-wave infrared imaging system. The optical transfer function is currently recognized as the evaluation index that can fully reflect the actual imaging quality of the system. When the Nyquist frequency is 100 lp/mm, the MTF values of each field of view are higher than 0.4, close to the diffraction limit, and the imaging quality is good.


Figure 6 Modulation transfer function curve


Figure 7 shows the astigmatism, field curvature and distortion curves of the wide-spectrum visible-shortwave infrared imaging system. It can be seen from the figure that the maximum astigmatism and field curvature are 0.1 mm, and the maximum distortion is 1.4%, which are very small values and meet the requirements of system design for field curvature and distortion.


Figure 7 Astigmatism, field curvature and distortion curves


Feasibility Analysis

Tolerance Analysis


After the system design is completed, it is necessary to conduct tolerance analysis on it, taking into account the relevant errors generated during actual processing and assembly, and giving a certain tolerance component. In the process of tolerance analysis of the system, it is necessary to allocate reasonable tolerances to all optical components, so that the detection performance of the optical detection system can meet the requirements, and the cost of optical components, assembly and calibration can be minimized, so that the performance of the entire system reach the optimum.


According to the experience and the actual process level, a relatively loose predetermined tolerance value of each parameter is given first, and the tolerance analysis is carried out on the design results to find out the particularly sensitive tolerance and redistribute it.


Systematic tolerance analysis is carried out with the average of diffraction MTF as the evaluation standard. Figure 8 is the MTF curve diagram of 100 Monte Carlo analysis, and Table 2 is the result of Monte Carlo analysis after tolerance optimization. The analysis shows that: at 100 lp/mm, the nominal value of MTF is 0.559, the best is 0.554, the worst is 0.333, the average is 0.481, and the standard deviation is 0.052; 90% of the lens MTF≥0.410, 50% of the lens MTF≥0.427, 10% The lens MTF≥0.540. It can be seen from this that the MTF meets the technical specification requirements under the given tolerance.


Figure 8 Monte Carlo analysis MTF curve

Table 2 Monte Carlo analysis results



Experimental proof


In the actual detection process, the whole system is composed of light source, lens, camera, image processing unit, image processing software and output unit. In order to further verify the imaging performance of the designed optical system, the imaging observation comparison in the laboratory is shown in Figure 9. The first comparison: Figure 9 (b1) is the effect of agricultural products taken by a camera in the visible light band, and Figure 9 (a1) is the effect of a short-wave infrared camera. The bruises inside the agricultural products can be detected by short-wave infrared penetrating the epidermis. And this is invisible to the human eye. This is due to the fact that when the fruit is bruised, the cell wall will rupture, and the water content in this area is higher, and the water in the bruise is almost black, because it has strong adsorption at 1450~1900 nm, and this adsorption This makes bruises clearly visible in the imaged object image by SWIR imaging. Second comparison: Figure 9 (b2) is a plastic that is opaque at visible wavelengths, and Figure 9 (a2) is a plastic that becomes translucent in the SWIR range, the ability of SWIR to penetrate plastics for this light to detect sealed plastic containers A new method is provided for the product volume within.



in conclusion

With the increasing demand for composite image information in machine vision, modern optical imaging technology will inevitably expand beyond the visible and near-infrared bands. Short-wave infrared will be more widely used in the future because of its resolution comparable to visible light and unique optical properties. In this paper, a wide-spectrum visible-short-wave infrared imaging system that can work in the 0.4~1.7 μm band is designed. The system is composed of 7 groups of 10 lenses. The modulation transfer function value is at the Nyquist frequency of 100 lp/mm Greater than 0.4, the system F number is 2.8, the distortion is less than 1.4%, all kinds of aberrations have been well corrected and balanced, and it has good imaging performance. The discussed method has certain reference value for the design of similar optical systems in the future.




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