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普通光学系统中,目标景物经光学系统成像于光学系统像面处,根据傅里叶光学理论,不考虑噪声的情况下,成像的物理过程可描述为目标景物与光学系统点扩散函数的卷积,空间频域为目标景物与光学传递函数的乘积。
$$G(u,v) = H(u,v)F(u,v)$$ (1) 式中:
$G(u,v)$ 为已知观测图像的频域;$H(u,v)$ 为光学系统点扩散函数PSF的频域,即光学系统的OTF(当系统存在像差时,OTF为退化函数);$F(u,v)$ 为目标景物的频域;u,v为像素强度对应的空间频率。如图1所示,波前编码系统中,通过在普通光学系统的光瞳面上加入一个相位掩模板对波前进行编码,使光学系统PSF和OTF对系统存在的像差不敏感,在探测器上成一系列模糊的编码图像,再对编码图像以图像复原的方式进行解码,得到清晰图像。
其成像过程的物理模型为:
$$\begin{split} {G_{\rm{encode}}}(u,v) =\;& {H_{\rm{encode}}}(u,v)F(u,v)\\ {G_{\rm{decode}}}(u,v) =\;& {H_{\rm{decode}}}(u,v){G_{\rm{encode}}}(u,v)= \\ & {H_{\rm{decode}}}(u,v){H_{\rm{encode}}}(u,v)F(u,v) \end{split} $$ (2) 式中:
${G_{\rm{encode}}}(u,v)$ 为光学系统像面上获得的编码图像的频域;${H_{\rm{encode}}}(u,v)$ 为光学系统编码过程的OTF;${G_{\rm{decode}}}(u,v)$ 为解码图像的频域,即为最终图像;${H_{\rm{decode}}}$ 为光学系统解码过程的OTF。一般情况下,解码过程就是在已知系统编码过程OTF情况下完成图像复原过程。由于OTF为复数,计算时一般采用其模量MTF。波前编码系统为两步成像,系统的MTF由编码和解码过程的MTF组成。图2为波前编码系统MTF与普通系统的差别。普通系统MTF随离焦变化明显,幅度迅速下降,离焦值较大时,空间频率出现截止现象,说明其成像质量对离焦像差较为敏感。波前编码系统中,编码MTF随离焦几乎无变化,说明编码图像质量对离焦不敏感。尽管编码MTF值相对衍射受限系统有所下降,但空间频率无截止,由于编码MTF各离焦位置较为稳定,可通过同一解码OTF实现不同离焦位置图像的解码,解码后全系统的MTF接近于衍射限。
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所谓像差钝化设计是指通过设计特定的相位掩模板使得波前编码系统成像特性在所需的景深或动态像差范围内不变。Fisher信息理论优化方法是相位掩膜板参数优化的常用方法[4-5]。使用OTF构造的Fisher评价函数可表示为:
$${\left[ {\frac{1}{{MN}}\sum\limits_{i = 1}^M {\sum\limits_{j = 1}^N {{{\left| {\frac{{{H^*}\left( {{u_i},{v_j},{W_{20}} = 0} \right){H_0}\left( {{u_i},{v_j},{W_{20}} = 0} \right)}}{{{{\left| {H\left( {{u_i},{v_j},{W_{20}} = 0} \right)} \right|}^2} + K}}} \right|}^2}} } } \right]^{\frac{1}{2}}} \leqslant \sigma $$ (3) 式中:H为规格化的OTF;K为噪声与场景功率谱之比;σ为最小可接受的噪声放大。
考虑掩模板制造和装调,掩模板相位采用三次多项式表征:
$$f(x,y) = a{x^3} + a{y^3}$$ (4) 式中:a为多项式参数。
光学系统F/#为1.4,工作波长为6.5~8.5 μm(IR1)和9~10.5 μm(IR2),则其最小焦深为Δ≈±2λ(F/#)2=±25 μm。焦深若扩大10×,即为±250 μm,将使得弹载光学系统完全满足力热环境和几乎不考虑双色对准精度。在10倍焦深位置分别提取系统各视场PSF,建立相位掩模板参数优化函数实现相位掩膜板参数优化,得到a=5.12×10−6,将其代入光学设计软件,得到如图3所示的光路。图4给出了在±250 μm离焦范围内波前编码光路在两个波段内的离焦特性,可以看出:波前编码系统在离焦范围内MTF调制度变化不大,说明其具备较高的离焦像差抑制能力。
Double bands missile-borne infrared detection system of extended focus depth based on wavefront coding
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摘要: 波前编码技术通过在光学系统光瞳位置加入特殊的相位掩模板,对目标信号光波进行编码调制,并在图像处理端对该编码信号进行解码而恢复原图像,由于被编码调制后的波前对离焦等像差的不敏感度扩大了十数倍,能显著扩大光学系统焦深。因此,波前编码技术能在编码与解码之间解决恶劣力热条件或多色制导体制对弹载红外探测系统带来的离焦和对准误差。文中基于波前编码扩大焦深基本原理,对一长长双色红外光学系统进行了10倍焦深扩大的波前编码像差钝化设计。集成样机后,进行了波前编码成像实验。以小孔点靶编码像作为PSF解码10倍离焦位置处的十字靶和四条靶图像,十字靶和四条靶解码图像清晰可辨,证明波前编码技术对于系统像差或离焦像差的抑制是有效的。最后,对波前编码成像效果进行了分析:解码图像的水波纹是由于空间采样PSF不足导致的,可提取不同视场位置PSF,使用空间变化解码算法实现条纹消除;由于解码图像会在放大信号的同时放大噪声,因此,解码算法需要进一步研究噪声抑制算法,以期满足弹载高能量、高信噪比应用的要求。Abstract: In the technology of wavefront coding, a special phase mask is put at the position of optical system stop. Then the goal signal wavefront will be coded. At the image processing part, the coded signal will be decoded to the original image. Because the insensitivity of the coded wavefront to the defocus and other aberrations is ten times more, the focus depth will be extended ten times more. So the wavefront coding can solve the defocus and alignment error caused by terrible mechanics and heat environment using coding and decoding. Based on the extended focus depth principle of wavefront coding, an aberration inactivation long/long double bands infrared optical system was designed. The focus depth was extended to be 10 times more. Imaging test was carried out with the wavefront coding prototype. The coding image of the spot-bar was used as the PSF to decode the image at the 10 times defocus position of cross-bar and four-bar. After decoding, the image of cross-bar and four-bar was with high resolution. At the last, the imaging results of wavefront coding were analyzed. The ripple of decoding picture was caused by not enough spatial sampling. After sampling PSF of different field, the ripple could be eliminated by using decoding algorithm of spatial transformation. The signal and noise would be extended at same time while the image was decoding. So the noise suppression algorithm used in decoding should be further researched to satisfy the missile-borne demand of more energy and higher signal to noise ratio.
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Key words:
- wavefront coding /
- focus depth /
- infrared detection /
- aberration inactivation
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