可变焦液晶微透镜阵列成像研究

Imaging of Variable Focusing Liquid Crystal Microlens Array

提出一种利用四电压驱动的液晶微透镜阵列对物体直接进行成像并减小成像色散的方法。液晶微透镜阵列内部的单个微透镜方格有多根上下垂直的电极,这些电极可以准确控制微透镜的波前分布。利用液晶微透镜阵列电控调焦的优点,在不移动光学器件的情况下对不同波段光透射的分辨率板清晰成像。采用图像合成算法处理成像结果,减少了互补型金属氧化物半导体(CMOS)红绿蓝滤光片非中心波长光的干扰,最后对处理结果进行图像拼接算法重建,得到具有大视场角、高清晰度的完整图像。

Objective Traditional microscopes have limitations such as large size and restricted field of view,necessitating comprehensive scanning for complete imaging of large-scale samples.In contrast,microlens array imaging systems feature a larger imaging range and simpler setup,thus becoming a research hotspot.Each sub-lens within a microlens array possesses a unique optical axis,and their optical performances are similar under identical parameters.Integration of these unit structures forms a unified optical axis within the microlens array.Compared to traditional single lenses,microlens arrays exhibit exceptionally high parallelism,and thus each sub-lens can independently transmit optical signals without interference,essentially forming numerous two-dimensional parallel optical paths.This characteristic enables each sub-lens to perform functions such as transmitting,transforming,and conducting imaging on optical information,thereby facilitating large-area imaging.The variable focusing feature of liquid crystal microlens arrays further promotes the miniaturization of imaging systems and can be utilized to address chromatic aberrations during imaging.Unlike traditional microscopes that require lens movement to adjust the focal plane,variable focusing liquid crystal microlens arrays can alter the focal plane without moving the lens to enhance the flexibility and portability of imaging systems.Methods We develop a high-performance liquid crystal microlens array.Each microlens unit within the proposed array consists of multiple vertical electrodes,allowing precise wavefront distribution control.Leveraging the advantages of electrically controlled focusing in the liquid crystal microlens arrays,we achieve clear imaging of different spectral bands without physically moving optical components.Meanwhile,the imaging results are processed by adopting an image synthesis algorithm to mitigate interference from non-central wavelength light filtered by the CMOS red-green-blue filters.Subsequently,a reconstruction algorithm is applied to the processed results for image stitching.During image restoration with the stitching algorithm,we first calibrate the imaging positions of each microlens,invert the imaging results at the calibration points,and then translate these images to form a complete image.Additionally,weighting is applied to different regions of the stitched image to reduce the impact of overlap on lens imaging after translation.The final output is a comprehensive image characterized by a large field of view and high definition.Results and Discussions The performance testing results of the proposed array indicate that the lens focal length varies linearly with the voltage difference when the center voltage and apex voltage range from 1.6Vrms to 2.5Vrms,which suggests that the lens operates within the linear voltage region of the liquid crystal material.Imaging results show that during focusing with white light,only the green light band is in focus,while the red and blue light bands are out of focus.By comparing the contrast of the in-focus and out-of-focus segments of the red light band under the red channel and blue light band under the blue channel,the blue calibration point in Fig.10 shows that the contrast of stripes in Figs.10(a)and(b)is 0.125 and 0.101 respectively,while the contrast of stripes in Figs.10(i)and(h)is 0.129 and 0.104 respectively.This indicates an increase in contrast of 23.8%and 24%for Figs.10(a)and(b)respectively after improving the defocusing phenomenon caused by dispersion.Image reconstruction is presented in Fig.12,from which the modulation transfer function(MTF)is obtained as shown in Fig.13.Figure 13 reveals that compared to white light imaging,direct image synthesis focusing on the red,green,and blue bands improves overall image quality by approximately 1.01%.After synthesizing the images,the overall image quality is enhanced by approximately 16.9%compared to white light imaging,with a more significant improvement in the mid-frequency range.For the low-frequency range where stripe intervals are larger,the impact of defocusing at stripe edges on contrast is minimal.In contrast,for the high-frequency range with smaller stripe intervals,the defocusing phenomenon of other bands in the high-frequency range is not significant due to the dispersion of single-band light itself,resulting in a smaller stripe contrast change.Finally,the imaging system achieves a spatial frequency resolution limit of approximately 100 lp/mm,corresponding to a resolvable line width of 5μm.Conclusions We propose a liquid crystal microlens array with higher electrode density and provide a detailed derivation of the driving method for this array.By utilizing theoretical results to drive the liquid crystal microlens array for imaging of a resolution target,we adjust the driving voltage to focus different spectral bands of light transmitted through the resolution target,aiming to reduce imaging dispersion.Employing an image synthesis algorithm,we remove some of the non-central wavelength light contamination from the CMOS filters and then restore the image of the resolution target using an image reconstruction algorithm.The results indicate that compared to direct imaging with white light,the processed image exhibits an overall contrast enhancement of 16.9%,and the minimum resolvable width reaches 5μm.