Computational Imaging-based Method For Expanding The Applied Waveband Range Of Single-layer Diffraction Elements

Diffractive optical elements(DOEs)has unique imaging characteristics,so it can simplify the structure of conventional optical systems and reduce the weight of the system,and further,the performance targets that are difficult to obtain in conventional systems can be achieved.Single-layer diffractive optical elements(SLDOEs)are characterized by simple structure,easy processing,and low cost.However,the diffraction efficiency of the SLDOEs will be significantly reduced when the incident wavelength is far from the central wavelength,so the image quality will be affected seriously by the low diffraction efficiency.Therefore,the SLDOEs can only be applied to optical systems with a narrow wavelength range.In order to improve the diffraction efficiency over a wide wavelength range,multilayer diffractive optics techniques have emerged.Although multilayer diffractive optics can achieve high diffraction efficiency in a wide wavelength range,its structure is complicated compared with that of the SLDOES,and the diffraction efficiency is easily affected by factors such as processing errors and ambient temperature.Therefore,we propose a design method to improve the imaging quality of diffractive-refractive optical system in the wide visible wavelength range by combining computational imaging techniques,and the application wavelength of the SLDOE can be extended in this paper.Firstly,the theoretical basis of diffractive optics based on the scalar diffraction theory is investigated,the diffraction efficiency characteristics of the SLDOES are analyzed,the diffraction efficiency formula for the SLDOES is discussed and the effect of diffraction efficiency on the imaging of SLDOES is investigated in this paper.Then,we propose a method to extend the application range of the SLDOES in the wide visible band based on the image degradation model,and a point spread function(PSF)model for the wide visible band is constructed by combining the quantum efficiency of detectors which is influenced by diffraction efficiency;Finally,a wide visible band diffractive-refractive optical system with a SLODE has been designed using the method described in this paper.The system has a focal length of 50 mm,a F-number of 5.6 and a field of view of 34°.The blurred images resulting from the low diffraction efficiency of the system are recovered.The recovered images were evaluated before and after the restoration.The recovered images are clearer,the contrast was significantly enhanced compared with that before the restoration,and the image quality was significantly improved.The evaluation value of the recovered images is4.30% lower than that of the original images when the BIQI function was used to evaluate the color images before and after the restoration;the evaluation value of the recovered images is 40.33% higher than that of the original images when the GMG evaluation function was used to evaluate the color images before and after the restoration.The evaluation results show that the image quality is significantly improved.The result also proves that the method described in this paper can eliminate the influence of low diffraction efficiency on the imaging quality,and can expand the application range of the SLDOES in the visible wavelength effectively.