Large aperture phase-coded diffractive lens for achromatic and 16° field-of-view imaging with high efficiency

Diffractive lenses (DLs) can realize high-resolution imaging with light weight and compact size. Conventional DLs suffer large chromatic and off-axis aberrations, which significantly limits their practical applications. Although many achromatic methods have been proposed, most of them are used for designing small aperture DLs, which have low diffraction efficiencies. In the designing of diffractive achromatic lenses, increasing the aperture and improving the diffraction efficiency have become two of the most important design issues. Here, a novel phase-coded diffractive lens (PCDL) for achromatic imaging with a large aperture and high efficiency is proposed and demonstrated experimentally, and it also possesses wide field-of-view (FOV) imaging at the same time. The phase distribution of the conventional phase-type diffractive lens (DL) is coded with a cubic function to expand both the working bandwidth and the FOV of conventional DL. The proposed phase-type DL is fabricated by using the laser direct writing of grey-scale patterns for a PCDL of a diameter of 10 mm, a focal length of 100 mm, and a cubic phase coding parameter of 30π. Experimental results show that the working bandwidth and the FOV of the PCDL respectively reach 50 nm and 16° with over 8% focusing efficiency, which are in significant contrast to the counterparts of conventional DL and in good agreement with the theoretical predictions. This work provides a novel way for implementing the achromatic, wide FOV, and high-efficiency imaging with large aperture DL.     

Development of a pinhole gamma camera for environmental monitoring

The collimator design for a nuclear monitoring system should be considered differently from the collimator design for medical environments because it has to be used in high-energy radiation environments. The purpose of this study was to determine the optimum pinhole design and to evaluate its performance for acquiring good-quality image in a high-energy radiation field. Simulations using the Geant4 Application for Tomographic Emission (GATE) were performed to model the pinhole gamma camera system. The gamma camera consists of a pyramid-shaped lead collimator with a tungsten pinhole insert, and a CsI(Tl) scintillation crystal with thickness of 6.0 mm and area of 50.0 mm × 50.0 mm. The acceptance angle of the pinhole collimator and the distance from pinhole to scintillator crystal were set to 45° and 60 mm, respectively. The intrinsic spatial resolution and sensitivity were simulated by changing the pinhole diameter and channel height. The point source was located 60 mm above the center of the pinhole, and the transmitted image was estimated for pinhole diameter values from 2.0 mm to 4.0 mm, while the channel heights were fixed between 2.0 mm and 6.0 mm. The optimal ranges of channel height and pinhole diameter were determined by evaluating the intrinsic resolution and sensitivity tradeoff curves. The pinhole parameters were selected based on these analyses, and we verified the simulation results through experimental tests of three types of collimators (general purpose, high sensitivity, and high resolution). The simulated and experimental results agreed, with discrepancies of 4.5% and 6.4% in the sensitivity and spatial resolution, respectively. The results demonstrate that the pinhole collimator designed in this study could be utilized to perform radiation monitoring.     

紧凑型自由曲面离轴三反系统设计

针对小型化反射式光学系统的设计要求,采用新型的离轴三反光路形式和自由曲面,实现了F#为1.8,对角线视场角7.5°的紧凑型自由曲面离轴三反系统,光线在系统内多次重叠,系统波像差全视场平均值为0.017λ(λ=10 μm)。相比非球面离轴三反系统,该系统具有体积小、透过率高、大视场、无中心遮拦、多波段成像等优点。     

高分辨率制冷型中波广角红外成像系统的光学设计

设计了一个F数为2,工作波段为3.7～4.8μm,全视场2ω=111.2°的高分辨率制冷型中波广角红外成像光学系统。该系统采用二次成像构型,通过Si、Ge、ZnSe三种材料六片式对称布局,利用折/衍混合器件及非球面,实现了光学被动消热差设计,使系统在-55℃至+80℃的宽温范围内,在空间频率为33lp/mm处的光学传递函数(MTF)均大于0.4,系统在15μm的像素尺寸内,能量集中度大于70%;采用f-θ设计,使成像系统对不同视场具有相同的角分辨率;通过引入光阑像差和控制像方视场角,使像面具有较好的均匀性,边缘视场最低相对照度为中心视场的90.9%,且具有近100%的冷光阑效率,同时,系统具有较好的冷反射抑制效果,该光学系统适用于像素为15μm,分辨率为640pixel×512pixel的中波制冷探测器。     

Wide field-of-view foveated imaging system

We have designed and demonstrated a simple, wide field-of-view (FOV = 60°) foveated imaging system utilizing a deformable mirror. The deformable mirror is used to dynamically correct the off-axis aberrations that limit the useful FOV of the system. The system mimics the operation of human eye through creating an image with variable spatial resolution and can be made significantly smaller and more compact than a conventional wide FOV system. Experiments show that this system can be used in many areas where size, weight, and data transmission bandwidth are critical.     

星载均匀像面低畸变广角气溶胶探测仪的研制

为满足空间遥感的迫切需求,设计并研制了一个星载均匀像面低畸变广角气溶胶探测仪样机.该仪器通过利用光阑像差产生的有效像差渐晕提高像面照度的均匀性,解决了广角系统中像面照度不均匀的问题.合理选择结构型式校正了畸变,并且采用全球面光学系统,易于加工和检测.广角气溶胶探测仪的中心波长为670 nm,带宽20 nm,全视场72°,相对孔径1/3.6,焦距20 mm.实验结果表明:研制的星载广角气溶胶探测仪镜头其入瞳大小5.6 mm,边缘视场的相对照度达到95.6%,在36 lp/mm处,轴上视场的调制传递函数值大于0.61,轴外视场的调制传递函数值高于0.58,最大畸变量为-1.95%,完全满足设计指标要求,体积小,适合空间遥感应用.     

太赫兹成像准光学系统设计

为了设计出适用于太赫兹波段成像的光学系统,综合分析了各种成像系统的特点,其中离轴三反射式光学系统具有独特的优势。通过对离轴三反射式光学系统的设计原理、设计方法和步骤的详细介绍,利用ZEMAX光学设计软件,通过编写简单的ZPL宏指令以及光阑和视场离轴,设计了工作在太赫兹波段、入瞳大小250 mm、瞬时视场1 mrad、焦距800 mm、F数3.2的系统,实现了可用于太赫兹波段无中心遮拦的离轴三反射式准光学成像系统设计。     

制冷型中/长红外双波段双视场全景光学系统设计

为了同时探测中波红外和长波红外两个波段信息,实现两个不同视场快速切换,采用空间多镜头图像拼接全景成像法,设计了四通道制冷型中/长红外双波段双视场全景成像光学系统。该全景系统由周视方向3个互成120的红外物镜和顶视方向一个红外物镜构成,每一个成像通道光学系统采用二次成像结构。F数为2,工作波段为中波3.5 m～4.8 m、长波7.8 m～9.8 m,双视场两档焦距之比为5,通过轴向移动变倍组可以完成122/44.49双视场转换。利用折/衍混合器件及非球面设计技术,采用光学被动式消热差法对光学系统进行了温度补偿。设计结果表明,该双视场光学系统具有100%冷光阑效率和良好的冷反射抑制能力。在-40℃～+60℃范围内,在奈奎斯特频率18 lp/mm位置处,中波红外系统MTF值均大于0.5,长波红外系统MTF值均大于0.3。     

“聚龙一号”装置不同光子能段X射线积分成像系统研制

成功研制了用于"聚龙一号"装置上Z箍缩实验中等离子体图像诊断的三通道不同光子能段的X射线积分成像系统。该系统主要由针孔阵列调节组件、X射线真空飞行管道、X射线分光器件及成像板组成,各通道的光谱响应分别为277eV,小于700eV和大于800eV,其对应的成像针孔直径为100,100和50μm,成像放大倍率2倍,成像空间分辨率均优于0.2mm。在"聚龙一号"装置上完成了该成像系统的性能考核,获取了不同光子能段的等离子体时间积分图像。结果表明,该成像系统满足"聚龙一号"装置上Z箍缩实验的诊断需求,在提升了单台相机光谱响应范围的同时提高了低能段图像的空间分辨率。     

远距离成像汽车平视显示光路结构设计

针对汽车AR-HUD显示虚像视距更远、虚像视场角更大的需求,采用自由曲面离轴反射光路结构,设计了一种焦距为-309 mm,虚像视距为7.5 m,虚像视场角为9.8°×5.5°的虚像显示光路。为保证驾驶员观察到的虚像的清晰度,结合人眼典型分辨角为1′,源图像显示模块采用分辨率为854×480 pixel的背投式DLP微投影系统。将人眼与虚像显示光路作为整体进行设计,使用Eyebox(孔径光阑偏移范围)模拟驾驶员眼睛活动范围,对于不同的孔径光阑位置,该光学系统的虚像面在奈奎斯特空间频率0.33 lp/mm处,中心视场MTF值大于0.6,全视场MTF值大于0.3。此外,使用蒙特卡罗方法对该系统取不同入瞳位置时进行了初步公差分析,系统90%的MTF值大于0.49,表明该系统容差能力较强。     

像方扫描技术研究

基于显微摄影的成像原理,研究了像方扫描以扩大视场的途径,并建立了一个二次成像的设计模型,包括一个大视场的固定前置物镜组和一个运动轨迹为球面的中继透镜组。物镜组所成的一次像面优化了场曲,中继透镜组则根据该场曲进行运动,对一次像面不同区域成像,并采用光学被动消热差以保证不同温度的像质。该模型的相对孔径1∶3,波长3.7 m~4.8 m,焦距90 mm,瞬时凝视视场为4,扫描视场达24,采用7片透镜3个非球面,在全视场范围内具有接近衍射限的像质。     

超短脉冲激光瞄准装置光学系统设计

为了提高超短脉冲激光的瞄准精度,基于自准直原理提出瞄准装置光学系统。以670 nm光纤耦合激光器为光源,设计指示光准直、扩束光学系统,准直光的不平行度达到3.2,设计焦距为350 mm,相对孔径1/5,离轴量50 mm的主激光离轴抛物面镜,其成像质量达到衍射极限,基于准直束光学系统和离轴抛物面镜,设计可适应670 nm和800 nm两种波长的20和100的瞄准和监测成像光学系统。提出一种小孔准直的安装调试方法,以指示光进行实验验证,结果表明:设计的光学系统成像光斑均匀,其物方分辨率达到4.1 m。     

Characteristics of geometric distortion correction with increasing field-of-view in open-configuration MRI

Open-configuration magnetic resonance imaging (MRI) systems are becoming increasingly desirable for musculoskeletal imaging and image-guided radiotherapy because of their non-claustrophobic configuration. However, geometric image distortion in large fields-of-view (FOV) due to field inhomogeneity and gradient nonlinearity hinders the practical applications of open-type MRI. We demonstrated the use of geometric distortion correction for increasing FOV in open MRI. Geometric distortion was modeled and corrected as a global polynomial function. The appropriate polynomial order was identified as the minimum difference between the coordinates of control points in the distorted MR image space and those predicted by polynomial modeling. The sixth order polynomial function was found to give the optimal value for geometric distortion correction. The area of maximum distortion was < 1 pixel with an FOV of 285 mm. The correction performance error was increased at most 1.2% and 2.9% for FOVs of 340 mm and ~ 400 mm compared with the FOV of 285 mm. In particular, unresolved distortion was generated by local deformation near the gradient coil center.     

基于CdZnTe像素阵列探测技术的伽玛源成像

根据高能射线针孔成像理论,采用CdZnTe像素阵列探测器建立了直接成像探测模式的伽玛源针孔探测系统。测试分析了CdZnTe像素阵列探测器的能量分辨力及峰值效率,讨论研究了针孔成像探测系统的调制传递函数和附加噪声特性,测试获得直径5mm137Cs源的探测图像,采用Lucy-Richardson迭代算法得到了137Cs源的复原图像。实验结果表明:CdZnTe探测器对662keV137Cs源的能量分辨力为6.25%～7.50%,峰值效率65.0%～72.5%;成像系统探测图像存在一定扩散现象,所采用的Lucy-Richardson迭代复原算法能较好地修正图像扩散,提高探测图像中心区域细节分辨力;估算所得137Cs源尺寸误差约0.5mm,所建立的CdZnTe针孔成像探测系统能有效得到小尺寸伽玛源的辐照强度分布及尺寸信息。     

A study of the pointed observation methods and sensitivity of HXMT

The Hard X-ray Modulation Telescope (HXMT) is an X-ray astronomy satellite consisting of three slat-collimated instruments, the High Energy X-ray Instrument (HE), the Medium Energy X-ray Instrument (ME), and the Low Energy X-ray Instrument (LE). HXMT will carry out an all sky survey and make pointed observations in the 1-250 keV energy band. In order to get the source and background fluxes simultaneously in the pointed observations, two methods, i.e., the combined field of view (FOV) method and the off-axis pointing method are proposed in this paper. Comprehensive analyses of the sensitivities of the three instruments by using these two methods are presented, respectively. It is found that the off-axis pointing method has a higher sensitivity for HE and ME but a lower sensitivity for LE. Since the axes of the three instruments are aligned along the same direction, the off-axis pointing method is recommended as the main method in the pointed observation for HXMT; the combined FOV method can be used when LE is the most relevant instrument in order to satisfy the scientific objective of the observation.     

基于ZEMAX的手机摄像镜头设计

在光学工程软件ZEMAX的辅助下,配套采用像元大小为1.75μm的CMOS图像传感器,设计了一款500万像素的手机镜头,镜头视场角60°,F/#2.8,半像高2.87 mm,镜头总长为6 mm,镜头为3P1G结构,第1、3、4片镜片采用非球面塑料,第2片镜片采用球面透镜。各个视场的横向像差均小于20μm,均方根半径(RMS Radius)都在艾利斑之内,在1/2奈奎斯特频率处绝大部分视场MTF值都大于0.6,可以获得优质的成像效果。     

Versatility of a hard X‐ray Kirkpatrick–Baez focus characterized by ptychography

In the past decade Kirkpatrick–Baez (KB) mirrors have been established as powerful focusing systems in hard X‐ray microscopy applications. Here a ptychographic characterization of the KB focus in the dedicated nano‐imaging setup GINIX (Göttingen Instrument for Nano‐Imaging with X‐rays) at the P10 coherence beamline of the PETRA III synchrotron at HASLYLAB/DESY, Germany, is reported. More specifically, it is shown how aberrations in the KB beam, caused by imperfections in the height profile of the focusing mirrors, can be eliminated using a pinhole as a spatial filter near the focal plane. A combination of different pinhole sizes and illumination conditions of the KB setup makes the prepared optical setup well suited not only for high‐resolution ptychographic coherent X‐ray diffractive imaging but also for moderate‐resolution/large‐field‐of‐view propagation imaging in the divergent KB beam.     

大视场航空相机光学系统设计

针对航空相机复杂的使用环境以及需在高速运动中进行高分辨率成像的特点, 设计了一种大视场航空照相机光学系统。该系统光学结构采用双高斯准对称结构形式,通过双成像模块光学拼接扩大视场角,调整最后一片透镜实现内置调焦,且通过控制地物反射镜的3种工作模式,分别实现航空相机垂直照相、自动调焦及前向像移补偿功能,避免了航拍过程中温度、气压、航高等环境条件变化时引起的图像质量大幅下降,确保整个视场内成像质量不受影响。该光学系统设计实现了全视场无渐晕, 全视场最大畸变＜0.5‰,在91 lp/mm处MTF接近衍射极限,物镜在全视场范围内成像质量一致。通过实验室及航拍试验验证,该光学系统具有成像清晰、视场大、可靠性高、体积小、质量轻等优点,满足了航空相机在比较复杂环境下清晰成像的要求。     

一种有效提高太赫兹时域光谱装置成像空间分辨率的方法

太赫兹波成像技术一个最显著的制约因素是其有限的空间分辨率。提出通过在样品前加小孔的方法来提高传统太赫兹时域光谱装置成像的空间分辨率。采用在样品前约2 mm处加直径为0.5 mm小孔的方法使成像的空间分辨率从1.276 mm提高到0.774 mm,提高0.502 mm,约39％。通过这个简单的方法在传统的太赫兹时域光谱成像装置上实现了空间分辨率从毫米量级到亚毫米量级的提高。聚乙烯板上直径为1 mm的小孔被作为成像的研究对象,分别采用传统的太赫兹时域光谱装置对样品进行直接成像和在样品前约2 mm处加直径为0.5 mm的小孔后对样品成像两种方式,并采用损失成像中信噪比较好的能量损失成像,对比两种方式得到的样品的太赫兹像,结果显示聚乙烯板上小孔的边界加小孔后成像比不加小孔直接成像明显清晰。证实了在样品前加小孔可以有效的提高太赫兹成像系统的空间分辨率。从理论上对通过在样品前加小孔提高系统空间分辨率的方法进行了分析,指出小孔尺寸越小,系统的空间相干长度越大,空间分辨率越高,但同时太赫兹信号的强度会相应减小。该方法可以简单有效的提高太赫兹时域光谱装置成像的空间分辨率,从而进一步拓展太赫兹谱成像技术的应用领域。     

Adaptive field-of-view imaging for efficient receive beamforming in medical ultrasound imaging systems

Quadrature demodulation-based phase rotation beamforming (QD-PRBF) is commonly used to support dynamic receive focusing in medical ultrasound systems. However, it is computationally demanding since it requires two demodulation filters for each receive channel. To reduce the computational requirements of QD-PRBF, we have previously developed two-stage demodulation (TSD), which reduces the number of lowpass filters by performing demodulation filtering on summation signals. However, it suffers from image quality degradation due to aliasing at lower beamforming frequencies. To improve the performance of TSD-PRBF with reduced number of beamforming points, we propose a new adaptive field-of-view (AFOV) imaging method. In AFOV imaging, the beamforming frequency is adjusted depending on displayed FOV size and the center frequency of received signals. To study its impact on image quality, simulation was conducted using Field II, phantom data were acquired from a commercial ultrasound machine, and the image quality was quantified using spatial (i.e., axial and lateral) and contrast resolution. The developed beamformer (i.e., TSD-AFOV-PRBF) with 1024 beamforming points provided comparable image resolution to QD-PRBF for typical FOV sizes (e.g., 4.6% and 1.3% degradation in contrast resolution for 160 mm and 112 mm, respectively for a 3.5 MHz transducer). Furthermore, it reduced the number of operations by 86.8% compared to QD-PRBF. These results indicate that the developed TSD-AFOV-PRBF can lower the computational requirement for receive beamforming without significant image quality degradation.