含单层衍射元件的可见宽波段计算成像系统设计

Design of Visible Broadband Computational Imaging System with Single-Layer Diffractive Element

为消除单层衍射元件在可见光宽波段中低的衍射效率对成像质量的影响,根据探测器的量子效率,提出了设计波长的选择方法,构建了可见光宽波段折衍混合系统受衍射效率影响的点扩散函数(PSF)模型。使用构建的PSF模型进行图像复原,提高了折衍射混合系统的图像质量。为了验证所提方法的有效性,将单层衍射光学元件引入目前已有的专利物镜系统中进行优化设计,优化后的系统中不仅光学元件的数量得到了减少,还将波段范围从486.1~656.3 nm扩展至400.0~800.0 nm。利用所提方法对波段范围扩展后的低衍射效率图像进行复原,复原后的图像质量不论在主观上还是客观上都有明显提升,这说明所提方法可用于含单层衍射元件的可见光宽波段系统设计。

Objective As the low diffraction efficiency of the single-layer diffractive element in the visible broadband results in blurred images and poor contrast,an optical-digital joint design method is proposed to solve this problem.Diffractive optical elements have special dispersion characteristics and temperature characteristics.When they are applied to the traditional refraction imaging system,the structure of the system can be simplified,and the weight of the system can be reduced.The performance index that is difficult to achieve in the traditional system can be achieved.The single-layer diffractive optical element features a simple structure,easy processing,and low costs.However,when the incident wavelength is far away from the central wavelength,the diffraction efficiency of the single-layer diffraction optical element will be significantly reduced,and the imaging quality will be severely affected by the low diffraction efficiency.Therefore,the single-layer diffractive optical element can only be applied to an optical system with a narrow wavelength range.As a result,multi-layer diffractive optical technology has emerged to improve the diffraction efficiency in the broadband.Despite the high diffraction efficiency of the multi-layer diffractive element in the broadband,its structure is complex,and the diffraction efficiency is easily affected by factors such as processing errors and ambient temperature compared with the single-layer diffractive element.Therefore,we hope to propose a design method combined with computational imaging to improve the visible broadband imaging quality of the single-layer diffractive element and expand the applicable band of the single-layer diffractive element.Methods The imaging process of the optical system is the process of image degradation.After a clear image passes through the optical system,convolves with the point spread function(PSF)of the optical system,and is added with noise,a new image is obtained.Therefore,the PSF can be used as a restoration function to perform a deconvolution operation on the obtained image to produce a clear image.Since the PSF obtained in the optical design software does not consider diffraction efficiency,the diffraction efficiency of each order is assumed to be 100%,and hence,it is necessary to reconstruct a PSF affected by diffraction efficiency.In this paper,the PSF model of the visible broadband affected by diffraction efficiency is constructed in three steps.Firstly,the energy distribution of a certain characteristic wavelength in a certain analysis level needs to be constructed.Secondly,the energy distribution of the wavelength in all analysis levels needs to be calculated and superimposed.Thirdly,the energy distribution of all characteristic wavelengths is obtained according to the method of the first two steps.The obtained energy distribution is superimposed with the quantum efficiency of the detector by weight and then normalized,and thus,the PSFs of the R,G,and B channels are obtained.After the PSF model is deconvolved with the grayscale images of the three channels of the blurred image,the three grayscale images obtained are recombined to obtain a clear color image free from the influence of diffraction efficiency.Results and Discussions Firstly,the existing patent optical system(Table 2)is optimized and adjusted,and a single-layer diffractive element is introduced.Without considering the diffraction efficiency,the image quality remains unchanged(Fig.5 and Fig.7),while the band range is expanded from 486.1-656.3 nm to 400.0-800.0 nm,and the number of lenses is reduced from six to four(Fig.6).Then,the R,G,and B three-channel PSF model of the optimized system affected by diffraction efficiency is constructed according to the previous method(Figs.8-10).The Richardson-Lucy algorithm and the constructed PSF model are used to deconvolute the three-channel grayscale images of the simulated image.After that,the restored grayscale images(Fig.12)are combined to obtain a restored color image(Fig.11).The grayscale mean gradient(GMG)function and the blind image quality index(BIQI)function are employed to evaluate the images before and after restoration(Table 3).The BIQI value of the R-channel grayscale image decreases by 5.00%,and the GMG value increases by 51.61%after restoration.The BIQI value of the G-channel grayscale image decreases by 5.44%,and the GMG value increases by 24.28%after restoration.The BIQI value of the B-channel grayscale image decreases by 1.23%,and the GMG value increases by 48.79%after restoration.The effectiveness of this method is proven by the evaluation results.Conclusions The design method in this paper can effectively improve the visible broadband imaging quality of the optical system with a single-layer diffractive optical element.The image obtained by restoration with the PSF model and the unrestored image are evaluated,and the evaluation results are as follows.Subjectively,the image quality after restoration is significantly improved as the image is clearer and has higher contrast.Objectively,the GMG function and the BIQI function are used for evaluation.The GMG evaluation value of the restored image increases by 40.33%,and the BIQI evaluation value decreases by 4.30%,which all indicate that the image quality after restoration is better.The simulations show that this method can be used in the design of a system with a single-layer diffractive element in the visible broadband.