In terms of uncooled thermal imaging cameras, compared with the modularized uncooled detectors and imaging circuits, the optical system plays an important role in reducing product quality, size, and cost. Size, Weight, Power and Cost) characteristics of the main factors.
The design of light, small, low-cost, high-performance uncooled infrared optical system needs to consider the following aspects: small number of lenses, short overall length of the optical system, large objective lens with small diameter, high optical modulation transfer function (MTF) and environmental adaptability good.
According to Memes Consulting, recently, the research team of Kunming Institute of Physics published an article on the theme of "Compact and Low-cost Uncooled Long-Wave Infrared Continuous Zoom Optical Design" in the journal "Infrared and Laser Engineering". The first author of this article is Tang Han, a senior engineer, who is mainly engaged in the research of infrared light-mechanical system technology.
This paper introduces three groups of linkage zoom technology to balance aberration and compress the total length of the system, adopts variable F# design technology to constrain the system's large objective lens diameter, and uses active compensation athermalization technology to realize clear imaging of the system under high and low temperature conditions, and constructs a four-lens structure. The uncooled long-wave infrared continuous zoom optical system has the characteristics of short overall length, low cost, good environmental adaptability, and high performance. It can be widely used in handheld reconnaissance equipment or unmanned system platforms to meet the growing market demand.
Three-group linkage continuous zoom model
The three-group linkage continuous zoom system is a mechanical compensation zoom system that continuously moves the three lens groups in the axial direction to change the combined focal length of the optical system while keeping the image plane position fixed and has good imaging quality during continuous zooming. The common form of the three-group linkage continuous zoom optical system is composed of five groups of lenses: the front fixed group, the zoom group, the compensation group, the second compensation group and the rear fixed group. By establishing a mathematical model, the zoom process can be quickly analyzed, the Gaussian optical parameters of the zoom system can be determined, and the initial paraxial optical structure can be obtained. The three-group linkage continuous zoom system motion model is shown in Figure 1.
Figure 1 Schematic diagram of the three-group linkage continuous zoom system
Optical System Evaluation and Analysis
Design Flow
The design process of the uncooled long-wave infrared continuous zoom optical system is shown in Figure 2. Firstly, the calculation program is compiled according to the three groups of linkage continuous zoom models, and the initial optical structure is optimized according to the design indicators from the aspects of system total length, focal power distribution, and part spacing, etc., and an ideal optical model is established; secondly, the optical power of the components is reasonably selected. , set the evaluation function to enter optimization and global optimization; thirdly, evaluate the imaging quality of room temperature and high and low temperature environments according to the convergence results of the evaluation function; The quality evaluation and tolerance analysis links are iterated repeatedly until the design technical index requirements are met; finally, the system zoom curve renormalization operation is carried out to complete the system design.
Figure 2 Flow chart of continuous zoom optical system design
Design specifications
Based on the current mainstream 640×512@12 μm uncooled vanadium oxide focal plane detector, a compact, low-cost, high transmittance, and full-temperature uncooled long-wave infrared continuous zoom optical system was designed. The main technical indicators of the system are shown in Table 1.
Table 1 Optical system technical indicators
designing process
According to the design process, firstly, according to the three-group linkage continuous zoom model, the initial parameter calculation program of the three-group linkage zoom system is compiled. According to the design index of the optical system, the Gaussian optical parameters of the continuous zoom system (ie, the optical power of the elements and the distribution of intervals) are solved to establish the paraxial optical system. The three-group linkage continuous zoom system contains five components. To realize the four-element lens structure, one component needs to be subtracted. From the analysis of the difficulty of correcting aberrations, the second compensation group also has the ability of the rear fixed group to balance aberrations. It is reasonable and feasible to fix the group after subtraction.
Consider setting the aperture stop position. The position of the aperture stop has a significant effect on the diameter of the large objective lens and the aberration balance of the system. After analysis, setting the aperture diaphragm on the compensation group can effectively reduce the diameter of the large objective lens and reduce the difficulty of aberration correction. The fixed aperture diaphragm is adopted, and the F# of the system changes with the field of view through the variable F# design technology, combined with the automatic gain algorithm of the imaging circuit to reduce the impact of variable F#.
Input the parameters of each focal length section calculated by the program into the optical aided design software system, set multiple structures, reasonably select the lens shape and lens material according to the focal power of each component, and set the optimization evaluation function, set the binary diffraction surface and high-order aspheric surface, In order to provide more optimization variables and design freedom, improve the imaging quality of the optical system. The initial architecture of the system at three focal length positions (short focal length 20.7 mm, medium focal length 80 mm, and long focal length 126 mm) is shown in Figure 3.
Figure 3 Continuous zoom optical multiple system diagram
design result
The final design result of the compact and low-cost uncooled long-wave infrared continuous zoom optical system is shown in Figure 4. The whole system uses a total of four lenses, the maximum lens processing diameter is 116 mm, the total length of the optical system is 180 mm, the total mass of optical parts is 418 g, and the telephoto ratio is 1.44. The front fixed group is a germanium lens with positive refractive power; the zoom group is a germanium lens with negative refractive power; the compensation group is a germanium lens with positive refractive power; the second compensation group is a chalcogenide glass lens with positive refractive power and low temperature refractive index . The system uses a binary diffraction surface and three aspherical surfaces, and the second compensation group is used as an adjustment link for active cooling of the system and focusing of the viewing distance. The aperture diaphragm is set on the front surface of the compensation group. During the continuous zooming process from the large field of view to the small field of view, the linear change range of F# of the system is 1.05~1.2, the focal length change range is 20.7~126 mm, and the corresponding field of view change range is 21°× 16.8°~3.5°×2.8°, the zoom process is continuous, the image quality is good, and it meets the design requirements.
Figure 4 Layout of continuous zoom optical system
Optical System Evaluation and Analysis
System room temperature image quality evaluation
Optical modulation transfer function: The MTF corresponding to an ideal optical system is the diffraction limit of the system transfer function. The MTF of the optical system is shown in Figure 5. The MTF of the system in the three focal length states is close to the diffraction limit, and the imaging quality is good.
Figure 5 MTF curve of continuous zoom optical system
Spot diagram: the optical system takes the intersection point of the chief ray as the reference point, calculates the distance corresponding to the point farthest from the point as the geometric radius of the diffuse spot (SPT), and uses the least squares algorithm to calculate the average distance between each point and the reference point , called the root mean square (RMS) radius of the diffuse spot. The spot diagram of the optical system is shown in Figure 6. The RMS radius of the largest diffuse spot of the system under the three fields of view is 6.8 μm, which shows that the system has a clear image and meets the requirements for use.
Figure 6 Spot diagram of continuous zoom optical system
Distortion: It is the difference between the ideal image height and the actual chief ray height. The distortion of the optical system is shown in Figure 7. In the small field of view, the maximum distortion is 0.92%, and in the large field of view, the maximum distortion is 3.08%. The distortion of the system has no obvious influence on the imaging during the continuous zooming process.
Figure 7 Distortion of small field of view (a) and large field of view (b) of continuous zoom optical system
System high and low temperature image quality evaluation
The imaging quality of the uncooled long-wave infrared continuous zoom optical system in high and low temperature environments is affected by factors such as large relative aperture and high temperature refractive index coefficient of commonly used lens materials. The system adopts active compensation technology, that is, by moving the second compensation group, the imaging quality of the optical system in the temperature range of −40~+60 ℃ can meet the requirements of use.
Figure 8 shows the optical modulation transfer function of the system after compensation at high and low temperatures when the telephoto is 126 mm and the short focal length is 20.7 mm. Figure 9 is a system spot diagram after compensation at high and low temperatures when the telephoto is 126 mm and the short focal is 20.7 mm. It can be seen from the high and low temperature transfer diagrams and spot diagrams that the continuous zoom system meets the requirements of use in the range of −40~+60 °C through active compensation.
Fig.8 MTF curve of continuous zoom optical system in high and low temperature environment
Fig.9 Spot diagram of continuous zoom optical system in high and low temperature environment
Optical System Tolerance Analysis
Tolerance analysis can fully evaluate the impact of various tolerances on the imaging quality of the optical system, and evaluate the processing technology of optical parts and the difficulty of optical-mechanical assembly. The manufacturing tolerances and assembly tolerances that have a great impact on the imaging quality of the system need to be adjusted appropriately and require multiple iterative optimizations.
Optical design software uses statistical algorithms to estimate tolerances. For medium and high-precision optical systems, modify the default tolerance table according to Table 2, and use sensitivity analysis to obtain a statistical error evaluation table in the small field of view and normal temperature state of the system. Among them, the items with the most serious tolerances are shown in Figure 10.
Table 2 Commonly used Zemax tolerance table
Figure 10 The most serious items
It can be seen from Figure 10 that the local tolerances of the tilt of the first lens and the second lens and the aperture of the front surface of the second lens are the "most serious items", but they have little impact on the imaging quality of the system, and the tolerance is controllable.
Figure 11 and Figure 12 show the RMS and MTF tolerance analysis results of the system speckle respectively. It can be seen from Fig. 11 that the design value of the RMS radius of the diffuse spot of the system is 5.3 μm, the statistical average value of the change is 0.78 μm, and the estimated value of the RMS radius of the diffuse spot after processing and assembly is 6.09 μm, which satisfies the actual use well. It can be seen from Figure 12 that the design value of MTF at 40 lp/mm of the system is 0.358, the change is 0.0227, and the estimated value of MTF after processing and assembly is 0.336, which meets the actual use requirements.
Figure 11 Estimation of the RMS radius of the diffuse spot
Figure 12 MTF estimates
Systematic Binary Diffraction Surface Processing Analysis
The system introduces a binary diffraction surface on the front surface of the compensation group to balance the chromatic aberration of magnification. The parameters of the binary diffraction surface introduced on the germanium substrate are Norm Radius=22 mm, H1=−10.53, H2=−1.48. The calculation shows that the number of rings on the binary diffraction surface is 1, and the ring depth is 3.18 μm. The relationship between the phase and period of the binary diffraction surface and the diameter of the element is shown in Figure 13. The binary diffraction surface of the germanium substrate can be processed by single-point diamond turning. The binary diffraction element has few diffraction rings, low hardness of the base material, simple processing, and the cost is not much different from that of the aspherical lens.
Figure 13 The relationship between the phase and period of the binary plane and the diameter of the element
In the working band range of 8.0-12.0 μm, the center wavelength is 9.6 μm, and the average diffraction efficiency of the band is obtained by using the diffraction efficiency calculation formula, which is shown in Figure 14. Considering the shading effect caused by the processing of optical parts and the beam scattering caused by surface roughness, the average diffraction efficiency of the used band is about 92.0%. Therefore, the transmittance of the optical system is: τ=0.96×0.975×0.92×0.975=0.84. Meet the system optical transmittance requirements.
Figure 14 Diffraction efficiency of binary diffraction surface
System Cam Curve Renormalization
The final stage of system design also needs to calculate the displacement of the zoom group, compensation group and second compensation group with the change of focal length. In the optimization design process, only the five-fold structure of the zoom system is given, including the system zoom range (long focus, short focus) and the three focal length positions in the middle. The actual cam structure design needs to cover the entire zoom range, and a complete zoom curve is required, that is, The zoom interval is an independent variable, the equation of the curve as a function of the focal length of the system, or a sufficiently dense data table. This operation is called "renormalization" of the zoom curve.
Different from the one-to-one relationship between the zoom group and the compensation group in the positive group compensation or negative group compensation system of two moving components, there are multiple compensation solutions for each zoom group position in the three-group linkage zoom system, so the three groups The cam curve smoothness and monotonicity need to be taken into account in the cam curve reformation of the linkage zoom system. Set the radius of curvature and aspheric parameters of the system as fixed values, set the zoom interval as a variable, insert multiple structures from short focus to long focus and assign values at certain intervals. Generally, when the zoom ratio is less than 10, 200 focal length position data pairs The design of the cam structure is sufficient, and the ZPL macro program is compiled to automatically set the operands, change the focal length value to optimize successively and control the relative movement of the adjacent focal length zoom group and compensation group, and obtain the zoom interval and evaluation function convergence of the entire zoom range. The system cam curve is shown in Figure 15. It can be seen from the figure that the maximum stroke of the zoom group is 56 mm, the maximum stroke of the compensation group is 17.5 mm, and the maximum stroke of the second compensation group is 4.5 mm; the curves of the zoom group and the compensation group are smooth and have no inflection points. Figure 16 shows the relationship between the pressure rise angle of the multiplying group and the compensating group. The second compensation group is driven by a single motor, which is conducive to system line-of-sight focusing and active heat dissipation. Considering the high and low temperature heat dissipation compensation stroke, the maximum movement of this group is 6.6 mm.
Figure 15 Cam curve of continuous zoom optical system
Figure 16 Relationship between cam rotation angle and pressure rise angle
in conclusion
With the rapid iteration of thermal imaging cameras towards SWaP-C, the zoom optical system that affects the size, volume, quality, price, etc. High, good environmental adaptability and other aspects of development to meet the application needs of civil, military and other aspects. Based on an uncooled focal plane detector with a pixel pitch of 640×512 and a pixel pitch of 12 μm, a compact and low-cost four-element lens structure is realized by adopting the variable F# design method, introducing three groups of linkage zoom design technology, and actively compensating for adiabatic difference. Continuous zoom optical system design. The range of focal length of the system is 20.7~126 mm, the corresponding F# changes between 1.05~1.2, the corresponding field of view changes range is 21°×16.8°~3.5°×2.8°, the zoom ratio is 6.0×, and the maximum aperture of the objective lens is 116 mm , the total optical length is 180 mm, the telephoto ratio is 1.44, the total mass of optical parts is 418 g, the processing technology of parts is mature, the tolerance of processing and adjustment is good, the curve of zoom cam is smooth, the cam track is easy to process, the servo control of motion components is simple, and the system The image is clear in the environment of −40~+60 ℃, which meets the requirements of high and low temperature use. The compact and low-cost uncooled long-wave infrared continuous zoom optical system will have broad market prospects in unmanned system platforms such as navigation, reconnaissance, warning, search and tracking, or handheld thermal imager products, and will promote uncooled infrared thermal imagers to further develop It is developing in the direction of reducing SWaP-C.
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