Volume 49 Issue 10
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Ren Hong, Bu Yuanzhuang, Wang Xi, Li Yu, Du Yanli, Gong Qiaoxia, Li Jinhai, Ma Fengying, Su Jianpo. Incoherent digital holographic color imaging with high accuracy image registration[J]. Infrared and Laser Engineering, 2020, 49(10): 20200022. doi: 10.3788/IRLA20200022
Citation: Ren Hong, Bu Yuanzhuang, Wang Xi, Li Yu, Du Yanli, Gong Qiaoxia, Li Jinhai, Ma Fengying, Su Jianpo. Incoherent digital holographic color imaging with high accuracy image registration[J]. Infrared and Laser Engineering, 2020, 49(10): 20200022. doi: 10.3788/IRLA20200022

Incoherent digital holographic color imaging with high accuracy image registration

doi: 10.3788/IRLA20200022
  • Received Date: 2020-03-15
  • Rev Recd Date: 2020-04-18
  • Available Online: 2020-05-26
  • Publish Date: 2020-11-03
  • Fresnel incoherent correlation holography is a new technology which can record incoherent object holograms and has important applications in biomedical imaging and 3D remote sensing. The problem of image fusion registration in hyperspectral imaging, three groups of double lens phase masks with constant focal lengths at 492 nm, 562 nm and 672 nm were designed and fabricated. The spatial light modulator called the masks of three wavelengths in turn and recorded the holograms of the objects under the corresponding wavelengths, which owed to the spatial light modulator were programmable. Because of the three-color recorded light was modulated by the corresponding wavelength mask, the spot position size of the CCD surface was the same. The reconstructed images had constant lateral magnification, which could improve the image registration accuracy and avoid complicated spatial registration algorithm of spectral images. The system truly achieved high-precision registration and real-time fusion of holographic color imaging. The color 3D image obtained from the dice holograms had high color reconstruction after numerical reconstruction and color fusion.
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Incoherent digital holographic color imaging with high accuracy image registration

doi: 10.3788/IRLA20200022
  • School of Physics, Zhengzhou University, Zhengzhou 450001, China

Abstract: Fresnel incoherent correlation holography is a new technology which can record incoherent object holograms and has important applications in biomedical imaging and 3D remote sensing. The problem of image fusion registration in hyperspectral imaging, three groups of double lens phase masks with constant focal lengths at 492 nm, 562 nm and 672 nm were designed and fabricated. The spatial light modulator called the masks of three wavelengths in turn and recorded the holograms of the objects under the corresponding wavelengths, which owed to the spatial light modulator were programmable. Because of the three-color recorded light was modulated by the corresponding wavelength mask, the spot position size of the CCD surface was the same. The reconstructed images had constant lateral magnification, which could improve the image registration accuracy and avoid complicated spatial registration algorithm of spectral images. The system truly achieved high-precision registration and real-time fusion of holographic color imaging. The color 3D image obtained from the dice holograms had high color reconstruction after numerical reconstruction and color fusion.

    • 近年来,人们对三维成像的需求不断提高,三维成像技术在信息科学、生命科学、材料科学和无损检测等领域的应用受到越来越多的关注[1]。数字全息技术因能方便、高效地获取并重建物光波的振幅及相位信息,在三维捕获和显示方面具有广泛的应用前景[2-3],并成功应用于生物医学成像[4-5]、工业无损检测[6]、微观粒子测量与跟踪[7]等领域。但由于数字全息采用激光光源,存在不可避免的激光散斑噪声和由光学元件引入的寄生干涉条纹,这些都严重影响了全息图的成像质量[8]。非相干数字全息是在非相干光源照明下,利用某种分光技术[9-14]将来自任一物点的光分成两束自相干光实现全息图的记录。其中,由Joseph Rosen和Gary Brooker[15]首次提出的菲涅尔非相干相关数字全息(Fresnel incoherent correlation holography,FINCH)记录系统由于具有光路简单、系统稳定、无扫描、成像速度快、分辨率高且易于与现有成熟光学系统匹配等优点而成为研究热点[16]。系统的核心为由空间光调制器(Spatial Light Modulator,SLM)和电荷耦合器(Charge-Coupled Device,CCD)组成的同轴非相干干涉仪,SLM为可编程器件,兼具分光和移相的功能,CCD为成像探测器[17]。非相干数字全息解决了数字全息对相干光源的依赖[18],将数字全息术的应用范围拓展至自适应光学[19]、彩色全息[20-21]、光学扫描全息[22]、编码孔径相关全息[23-24]、超分辨率成像[25]、光谱全息[26]等多个领域。

      由于全息图的记录不再依赖相干光源,为非相干全息技术在荧光成像、自然光全息、光谱成像及彩色成像方面的应用提供了可能。2007年,J. Rosen和Gary Brooker通过分析单透镜FINCH系统的成像原理,提出SLM上加载的衍射透镜在三个成像波长处菲涅尔数与波长乘积不变的方法,来实现系统的横向放大率和轴向放大率恒定,并对红、绿荧光骰子成像。但由于SLM孔径利用率低,从而导致分辨率低,图像质量差。2013年,Kim采用迈克尔逊干涉光路结合望远系统搭建了自然光全息相机,采用压电驱动器提供相移,系统存在不可避免的机械误差[27]。2014年,Y. H. Wan采用双透镜搭建离轴傅立叶三角干涉光路记录三色全息图,通过对全息图进行零点补充、横向位置优化和反射率估计的方法实现对色差的补偿,获得了真彩色全息成像[28]。2018年,M. N. Cuong采用一个曲面镜和一个平面镜搭建共路离轴干涉光路记录三色全息图,同样采用零填充技术补偿放大率色差。采用离轴光路记录全息图,虽然解决了单次曝光问题,但无法避免由此引入的相位畸变问题;另外,无论是通过旋转平面镜实现离轴记录还是通过CCD轴向运动记录不同颜色的全息图,光路均存在机械运动,引入了机械误差;而采用零填充技术补偿放大率色差,在频域拉伸过程中不可避免地会出现空间位置失配,导致融合后的图像出现颜色失真,并未真正解决图像的配准问题[29]

      文中提出将液晶可调谐滤波器(Liquid Crystal Tunable Filter,LCTF)引入FINCH系统实现彩色全息图的记录。LCTF可连续输出峰值波长400~720 nm、光谱半高宽20 nm的单色光,且相比于声光可调滤波器(AOTF)具有更高的光通量。设计制作焦距不随波长改变的双透镜相位掩模,利用SLM的可编程特性,对于LCTF输出的每一波长,SLM调用相应波长的相位掩模,CCD记录该波长下物体的全息图,且三色全息图记录过中保持记录距离不变(记录光路无运动部件)。采用该方法得到的全息图及重建图像具有恒定的放大率,能够实现像素级图像配准,抑制图像配准误差,提高重建精度。采用该方法对非荧光彩色骰子进行全息拍摄,获得了具有高颜色重现性的彩色三维重建像。

    • 图1为FINCH记录光路。物体可看作无穷多个非相干点源的集合,任一物点$P({x_s},{y_s},{z_s})$反射或发射的球面波经透镜L准直后入射至SLM表面,经SLM上的双透镜掩模分束后在CCD表面干涉,得到点源全息图。

      SLM中像素被随机分成两部分,分别加载焦距为${f_{{\rm{d}}1}}$${f_{{\rm{d}}2}}$的衍射透镜相位,其振幅反射系数为:

      式中:$B$$B'$为常数;${\theta _j}$为相移常数;${\lambda _0}$为掩模波长。

      Figure 1.  Optical path diagram of FINCH record. LCTF, liquid crystal tunable filter; SLM, spatial light modulator; CCD, charge-coupled device

      由于存在衍射色差,透镜的有效焦距与光波长成反比[30]

      式中:r为衍射透镜的半径;N为菲涅尔数。

      当波长为$\lambda $时,SLM上加载的两透镜的有效焦距分别为$\dfrac{{{\lambda _0}{f_{d1}}}}{\lambda }$$\dfrac{{{\lambda _0}{f_{d2}}}}{\lambda }$,此时,SLM的振幅反射系数变为:

      为实现分束后两干涉波前的最大交叠从而获得干涉条纹的最佳对比度,由图1几何关系可得CCD与SLM之间的距离${z_h}$,需满足:

      即记录距离需随波长$\lambda $的变化不断调整。CCD表面的点源全息图为:

      式中:CC1为复常数;${M_T}(\lambda ) = - \dfrac{{{z_h}(\lambda ){f_0}}}{{{z_s}{f_0} + d({f_0} - {z_s})}}$为系统的横向放大率,${z_s}$为物距,${f_0}$为准直透镜焦距,d为透镜与SLM之间的距离;${z_r}(\lambda ) = \dfrac{{[{f_1} + {z_h}(\lambda )][{f_2} + {z_h}(\lambda )]}}{{{f_2} - {f_1}}}$为重建距离,${f_{1,2}} = \dfrac{{{f_{d1,2}}[{z_s}{f_0} + d({f_0} - {z_s})]}}{{({f_{d1,2}} - d)({f_0} - {z_s}) - {z_s}{f_0}}}$

      由系统横向放大率和重建距离公式可以看出:两者均与记录距离有关。若采用FINCH系统实现彩色全息,有以下两种途径:(1)彩色全息图记录过程中保持记录距离${z_h}$随波长不断变化;(2)彩色全息图记录过程中保持记录距离${z_h}$不变。对于第一种情况,最终获得的全息图和重建像横向放大率随波长改变,图像融合时需补偿放大率色差,会带来如参考文献[27-28] 中的图像匹配精度低、颜色失真等问题,而且也会引入机械运动,使FINCH系统无机械扫描的优点大打折扣;对于第二种情况,彩色全息图记录过程中保持记录距离${z_h}$不变有两种途径:(1) 记录距离不变,SLM上加载的相位掩模也保持不变,则此时由于不满足最佳干涉条件(公式(4)),记录的全息图条纹对比度较低,重建像像质较差;(2) 记录距离不变,掩模波长始终保持与记录波长一致,此时SLM上加载的两透镜有效焦距在光谱全息图记录过程中不变,记录距离也恒定且满足各波长下的最佳记录条件,系统横向放大率与波长无关。因此,记录过程中无需通过调节CCD的位置满足最佳记录条件,从而实现无扫描、无放大率色差补偿、无位置补偿、无相位畸变补偿的非相干数字彩色全息记录。

      彩色全息记录时,LCTF每输出一个波长,SLM依次调用该波长下3张具有不同相位常数的相位掩模,CCD依次记录3张全息图,将3张全息图进行数值处理消除孪生像和零级像后重建,获得重建像S(x′,y′,zr,λ):

      采用上述成像方法获得的重建图像横向放大率恒定,可实现图像的像素级配准,免去了繁杂的补偿算法,且图像颜色重现精度高。

    • 图2为基于LCTF的非相干数字彩色全息记录系统。光源为氙灯(CELTCX250,250W),液晶可调滤波器为PerkinElmer公司生产的VariSpecVisLCTF (波长范围400~720 nm,带宽20 nm)。透镜L1,L2和L3的焦距选择分别为${f_{L1}}={f_{L2}}$=60 mm和${f_{L3}}$=250 mm。CCD采用Q-IMAGING数码相机RETIGA 6000 (像素尺寸${\sigma _c} = 4.54\;{\text{μ}}{\rm{ m}}$,分辨率 $2\;750 \times 2\;200$),为便于后续处理,仅使用$2\;048 \times 2\;048$ 像素。调整偏振片$P$的偏振方向与SLM (纯相位空间光调制器Holoeye Pluto,$1\;920 \times 1\;080$像素,8 μm像素间距)的偏振方向保持一致。其他实验参数为:fd1=245 mm,fd2=255 mm,zh=250 mm,d=150 mm。

      Figure 2.  Experimental installation. BS, beam splitters; P, polaroid; L1, L2, L3, converging lens

      图3(a)~(d)(e)~(f)分别为采用零填充技术补偿放大率色差和文中方法获得的分辨率板的三色重建像及融合图像。可以看出:相比于零填充技术,文中方法省却了繁杂的图像配准算法,且获得的三色融合图像具有更好的颜色重现性和更高的信噪比。

      Figure 3.  (a)−(d) Three color reconstruction images with different magnification and three color fusion image obtained by using zero-filling technique to compensate the magnification color difference; (e)−(h) Three color reconstruction image and three color fusion image obtained by this method

      同时,采用该方法对三维非荧光物体骰子记录彩色全息图。图4(a)~(c)为0°相移下波长分别为492.8、562.8、672.8 nm的双透镜相位掩模,焦距分别为245、255 mm。可以看出:随波长的增加,相位掩模的同心环越来越稀疏。采用图2光路记录绿底红点骰子的全息图,三色重建结果如图5所示。可以看出:三色重建像的横向放大率相同。图6(a)为三色重建像融合后的图像。为方便对比,用手机拍摄了骰子的实物图像,如图6(b)所示。可以看出:彩色重建图像与手机拍摄的图像之间存在较大颜色差异,这主要是未对单色图像进行反射率校正。由于系统内部各光学元器件自身的光学特性,导致系统的光谱响应不是理想的矩形分布,重建像存在反射率误差,从而造成彩色融合图像颜色失真。

      Figure 4.  Double lens with 245 mm and 255 mm focal lengths,respectively; (a) Mask wavelength 492.8 nm; (b) Mask wavelength 562.8 nm; (c) Mask wavelength 672.8 nm

      Figure 5.  Holograms of the dice at ((a) 672.8 nm, (b) 562.8 nm, (c) 492.8 nm) and corresponding reconstructed images (d)−(f)

      假设物体表面反射率为Ri,j(λ),氙灯光谱功率分布为I(λ),LCTF的光谱透过率为T(λ),SLM为纯相位空间光调制器,光谱反射率为1,CCD的光谱灵敏度函数为$S(\lambda )$,重建图像的光谱功率分布为Ii,j(λ)。光源的光谱功率分布、LCTF和CCD的光谱响应函数如图7所示。则物体反射率校正函数为:

      Figure 6.  (a) Dice color image after fusion; (b) An image taken by a mobile phone

      Figure 7.  Spectral response function of main optical components

      图8(a)为单色重建图像经反射率校正后的彩色融合图像,与图6(a)相比虽然具有更好的颜色还原效果和亮度,但由于校正过程会同时放大噪声,导致图8(a)噪声较大。为解决上述问题,笔者采用物体全息图与背景全息图相减的方法去除背景噪声,最终以光强比R:G:B=1.6:1.4:0.6合成彩色重建像,如图8(b)所示。可以看出,融合图像具有更好的颜色还原度,且信噪比较高。

      采用该系统对前后相距约2 cm的两个彩色骰子成像。彩色重建图像如图9所示。其中图9(a)9(b)分别为红色骰子和蓝色骰子在最佳焦平面上的彩色重建图像。图9(c)9(d)9(e)9(f)图9(a)9(b)中矩形方框内图像的放大,不难看出彩色三维成像效果。

      Figure 8.  (a) Color image after reflectivity correction; (b) Color image after removing background noise and color difference compensation

      Figure 9.  Color three dimensional image of the dice. (a) Reconstructed image focused on the front face of the blue die; (b) Reconstructed image focused on the front face of the red dice; (c)-(f) are magnification of the images in the green rectangular box shown in (a), (b)

    • 文中将LCTF引入非相干数字全息系统实现高精度配准彩色全息成像。通过设计相位掩模,使系统记录的不同波长全息图具有恒定的横向放大率和重建距离。采用该系统记录非荧光骰子的彩色全息图。结果表明,该方法不仅可以同时获取物体的三维空间信息,而且具有较好的颜色重现性,更重要的是该方法无需空间和时间扫描。与激光彩色全息和其他非相干彩色全息系统相比,本系统结构简单、操作便捷、配准精度高、颜色重现性好,具有较高的实用价值。

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