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红外成像系统的环境温度变化引起红外光学系统的光学元件几何尺寸、折射率以及光学元件之间的间隔变化,最终导致红外光学系统发生热离焦。波前编码红外成像技术通过在光阑附近加入光学相位板,使得光学系统对环境温度变化不敏感,其基本原理归纳如下。
由环境温度变化引起红外成像系统的热离焦量为[16]:
$$ {\Delta _f} = - \gamma f{\Delta _T} $$ (1) 式中:
$ {\Delta _f} $ 由温度变化引起的热离焦量;$ \gamma $ 为热差系数;$ f $ 为红外光学系统的焦距;$ {\Delta _T} $ 表示温度变化量。最大波像差
$ {W_{020}} $ 与实际离焦量$ {\Delta _f} $ 的关系满足[16]:$$ {W_{020}} = \frac{1}{k}\frac{{\pi {\Delta _f}}}{\lambda }{\left( {\frac{1}{{2F}}} \right)^2} = \frac{{{\Delta _f}}}{{8{F^2}}} $$ (2) 式中:
$ k $ 为波数;$ \lambda $ 为波长;$ F $ 为光学系统的$ F $ 数。将公式(1)代入公式(2)可得最大波像差
$ {W_{020}} $ 与温度变化量$ {\Delta _T} $ 的关系为:$$ {W_{020}} = - \frac{{\gamma f}}{{8{F^2}}}{\Delta _T} $$ (3) 在发生热离焦情况下,红外光学系统的广义光瞳函数表示为:
$$ P(x,y,{W_{020}}) = p(x,y)\exp \left[jk{W_{020}}\left({x^2} + {y^2}\right)\right] $$ (4) 式中:
$ p(x,y) $ 表示不考虑热离焦波像差情况下的广义光瞳函数,$ x $ 和$ y $ 表示归一化的光瞳坐标。由信息光学理论可知,光学系统的OTF(Optical Transfer Function)为广义光瞳函数的自相关,可表示为:
$$ \begin{split} H(u,v,{W_{020}})& = \iint {P\left( {x - \frac{u}{2},y - \frac{v}{2},{W_{020}}} \right)} \hfill \times\\ &{P^*}\left( {x{\text{ + }}\frac{u}{2},y{\text{ + }}\frac{v}{2},{W_{020}}} \right){\rm{d}}x{\rm{d}}y \hfill \end{split} $$ (5) 式中:
$ u $ 和$ v $ 表示归一化的空间频率。若在红外光学系统的光阑处添加三次光学相位板:
$$ {z}(x,y) = \alpha ({x^3} + {y^3}) $$ (6) 式中:
${z}$ 表示矢高;$ \alpha $ 表示光学相位板的面形参数;$ x $ 和$ y $ 为二维空间坐标。相应地,波前编码光学系统的广义光瞳函数变为:
$$ \begin{split} {P_{WFC}}&\left(x,y,{W_{020}}\right) = p\left(x,y\right) \hfill \times\\ &\exp \left[jk{W_{020}}\left({x^2} + {y^2}\right) + j\alpha \left({x^3} + {y^3}\right)\right] \hfill \end{split} $$ (7) 由广义光瞳函数自相关运算,得到波前编码红外光学系统的光学传递函数(OTF)的表达式为[17]:
$$ \begin{split} H(u,v,{W_{020}})& \approx {\left( {\frac{\pi }{{12\left| {\alpha uv} \right|}}} \right)^{1/2}} \hfill \times\\ &\exp \left[ {j\frac{{\alpha ({u^3} + {v^3})}}{4}} \right]\exp \left[ { - j\frac{{{k^2}W_{020}^2(u + v)}}{{3\alpha }}} \right] \hfill \end{split} $$ (8) 式中:相位因子由两项组成。第一项相位因子exp
$\left[{{j\alpha ({u^3} + {v^3})} \mathord{\left/ {\vphantom {{j\alpha ({u^3} + {v^3})} 4}} \right. } 4}\right]$ ,与离焦参数无关的;第二项相位因子$\exp \left( - j{k^2}W_{20}^2(u + v)/(3\alpha) \right)$ 与离焦参数有关。只要光学相位板的面形参数$ \alpha $ 足够大,第二项的值趋向于1,对传递函数的影响可以忽略不计,则公式(8)可简化为:$$ H(u,v,{W_{020}}) \approx {\left( {\frac{\pi }{{12\left| {\alpha uv} \right|}}} \right)^{1/2}}\exp \left[ {j\frac{{\alpha ({u^3} + {v^3})}}{4}} \right] $$ (9) 由公式(9)可知,此时OTF值和最大波像差
$ {W_{020}} $ 几乎没有关系,结合公式(3)进一步可知,OTF值与环境温度变化量$ {\Delta _T} $ 也几乎无关。因此在温度变化量为$ {\Delta _T} $ 范围内,近似保持了OTF不变。在波前编码红外成像系统的数字解码处理环节,可以利用波前编码红外光学系统自身的点扩散函数作为滤波器,对模糊的中间编码图像进行数字解码处理,得到比较清晰的数字解码图像。上述讨论,从原理上表明了波前编码技术扩展红外光学系统无热化工作温度范围的可行性。 -
前述基本原理部分从数学上阐述了光学编码如何实现波前编码光学传递函数对环境温度变化不敏感。数字解码器的模型是理想的,受光学相位板制备工艺水平的限制,实际加工出来的光学编码器(光学相位板)不是理想的,结果导致光学编码与数字解码在信息空间中存在“不匹配”问题。该问题最终引起解码图像质量下降,具体表现为解码图像伪像严重、图像分辨率降低。由于光学相位板是波前编码红外成像系统完成光学编码的物理载体,光学相位板的面形设计、加工制备、装配、热变形等环节直接影响光学编码效果,因此光学编码是波前编码红外成像系统的一项关键技术。接下来,文中重点阐述光学相位板的面形参数设计与优化、光学相位板的制备材料及工艺、热特性分析。
由于三次相位板具有大范围的无热化温度范围,国际上波前编码红外成像系统的光学相位板普遍采用三次相位板[18],其形式如公式(6)所示。
在此基础上进行改进的面形,采用广义三次面形为[19]:
$$ {z}(x,y) = \alpha ({x^3} + {y^3}) + \beta ({x^2}y + x{y^2}) $$ (10) 在此基础上增加了2次和5次项的多项式面形[20]:
$$ \begin{split} {{z}}(x,y)= &{\alpha ({x^2} + {y^2}) + \beta ({x^3} + {y^3})} + \gamma ({x^5} + {y^5}) \end{split} $$ (11) 参考文献[21]提出将三次相位面与球面表面集成在同一表面的方案:
$$ \begin{split} {{z}}(x,y) = &{\dfrac{{c({x^2} + {y^2})}}{{1 + \sqrt {1 - (1 + k){c^2}({x^2} + {y^2})} }}} + \alpha ({x^3} + {y^3}) \end{split} $$ (12) 在光学相位板面形参数优化设计方面,目前优化算法[22- 23]主要是以提高温变的MFT (或PSF)与常温MFT (或PSF)的相似性为优化目标函数,同时以常温MTF值高于给定的经验值为约束条件。
在光学相位板材料选择方面,可用于光学相位板制备的红外材料有锗、硒化锌、硫化锌、硅、硫系玻璃等。现有波前编码红外成像系统中光学相位板采用锗[19, 23-26]和硒化锌[27]、硫化锌[22]。冯斌等[27]对比分析了两种材料用于制作三次相位板区别,结果表明:在光程差给定条件下,由于硒化锌材料的折射率低,硒化锌材料光学相位板具有的允许制造误差是锗材料光学相位板的2.14倍。
在光学相位板制备工艺方面,由于波前编码红外成像系统中的光学相位板通常属于自由曲面且用于制作光学相位板的红外材料通常为脆性材料,在加工过程中容易发生“脱落”现象,为了保证光学相位板具有高的光洁度,需要在具有快刀伺服系统的单点金刚石车床上制备完成[25]。粒子注入辅助纳米加工(NiIM)方法[28- 29],通过粒子注入辅助方式改变被加工材料表层的可加工性能,实现硬脆材料平面及复杂面形的高效切削加工,解决了硬脆材料无法采用切削技术制造光学自由曲面的难题。
在制备过程中,为了保证光学相位板被吸附,将光学相位板元件中与三次相位面对应的表面设计为平面[25]。在两透镜的波前编码红外成像系统研制过程中,为了减少波前编码红外成像系统的透镜数量,与三次相位面对应的表面设计为非球面[23]。
在光学相位板的热效应研究方面。2012年,哈尔滨工业大学的陈守谦等[30]分析了基于波前编码技术的无热化红外光学系统中光学相位板的自身热效应,得到了不同温度下在光瞳处光学相位板的相位函数变化形态。2016年,冯斌等[31]考虑到波前编码无热化红外成像系统在温变过程中光学相位板也会发生温变,揭示了波前编码红外成像系统中光学相位板的温变对解码图像的影响机理,构建了温变对点扩散函数(PSF)一致性的影响模型;在该模型中,推导出将温变对三次相位板的影响等效为在常温下增加一片虚拟的三次相位板。
在光学编码对PSF一致性或数字解码图像质量的影响方面。2018年,Cai等[32]利用SolidWorks和ZEMAX软件的配合简化光、机、热的集成分析流程, 基于有限元分析法对温度梯度下波前编码系统的热效应及成像质量进行定量分析,比较了温变条件下波前编码成像系统的PSF(MTF)一致性。2020年,冯斌等[15]从理论上分析了波前编码红外成像系统中光学点扩散函数(PSF)数字化偏离对解码图像质量的影响机理,并以结构相似性(MSSIM)指标[33]进行了定量评价,同时给出实测波前编码PSF 图像的实验装置和方法。
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波前编码红外成像链路中,光学编码的点扩散函数与用于数字解码的点扩散函数在信息空间的“不匹配”,将引起解码图像存在伪像,最终影响解码图像的清晰程度。波前编码红外成像链路中,由于探测器输出的中间编码图像包含着探测器噪声,数字解码处理造成噪声放大的缺陷[25],如何抑制解码图像的伪像和噪声是个有待解决的问题。因此,数字解码是波前编码红外成像系统的另一项关键技术。目前波前编码红外成像系统的数字解码研究,主要围绕寻找最优数字解码算法、解码算法的实时硬件设计、中间编码图像的噪声抑制,解码图像质量影响等开展。波前编码红外成像系统中数字解码处理的算法主要有:逆滤波器, Wiener滤波图像复原方法[34]、Richardson-Lucy图像复原方法[19]、约束最小二乘(CLS)解码算法[35]等。在数字解码过程中,数字解码算法容易将中间编码图像的噪声放大,张程硕等[25]提出将收缩映射函数(Shrinkage function)用于波前编码红外成像系统的数字解码,并取得了良好的解码图像效果。在数字解码算法的硬件实时性方面,英国Qioptiq公司的Hasler等[35],针对Wiener滤波图像复原在频域内不适合用FPGA硬件电路来实现并行计算,提出改进数字解码算法并开发了具有实时性的数字解码硬件电路。
Review on athermalized infrared imaging technology based on wavefront coding (Invited)
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摘要: 波前编码红外成像技术是一种结合光学编码和数字解码两步成像的计算光学成像技术。波前编码无热化红外成像系统通过在红外光学系统的光阑附近增加特殊面形的光学相位板,对场景红外辐射进行编码调制,使得在宽的环境温度范围内红外焦平面探测器输出的中间编码图像具有高度一致性,再对中间编码图像进行数字解码得到清晰红外图像。近年来,国内外学者开展了大量波前编码无热化红外成像技术的理论分析和原理验证,表明其无热化特性的有效性。文中结合作者近年来的研究工作,主要介绍波前编码无热化红外成像技术的研究背景、基本原理、关键技术、国内外典型的设计方案和原理样机、并展望了波前编码红外成像技术的应用价值和发展趋势。Abstract: As a novel computational optical imaging technology, wavefront coding infrared imaging technology includes two stages of optical coding and digital decoding. The wavefront coding athermalized infrared imaging system is realized by mounting a special-form phase mask near the stop of an infrared optical system to modulate the incident radiation. Its infrared detector produces intermediate coded images. Those intermediate coded images have high similarity over a wide range of ambient temperature. An intermediate coded image is decoded to produce a decoded image with sharpness. In recent years, researchers have made much contribution to its theory and experiment to prove its validity for athermalization. In this paper, combining their previous related works, the authors have respectively introduced research background, basic principle, key technique, typical design scheme and experimental prototype, and prospected its application and development trend in future.
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Key words:
- wavefront coding /
- athermalization design /
- infrared imaging /
- optical coding /
- digital decoding
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