Research On Biometric Imaging System Based On Liquid Crystal Lens

The optical system is an important part of image acquisition,and the quality of the acquired image directly affects the subsequent calculation of image information in the image processing process.At present,in biometrics such as iris recognition,face recognition,etc.,the imaging system used mostly uses a fixed focus system due to the limitation of the device volume.During the recognition process,human cooperation is required and it is not convenient.The use of an electronically controlled liquid crystal lens that does not require moving parts is one of the methods to solve this problem.The liquid crystal lens is prepared according to the birefringence effect of liquid crystal molecules,and the deflection angle of the liquid crystal molecules is changed by an external electric field to achieve the focal length of the liquid crystal lens.Electronic control adjustment.At present,the research of liquid crystal lenses mainly focuses on the performance of a single liquid crystal lens,but due to the limited range of the focal length of the liquid crystal lens itself,it is difficult to apply a single liquid crystal lens independently.This article mainly combines the liquid crystal lens and the optical lens to achieve optical focusing without mechanical moving parts.The specific research contents are as follows:1.The calculation methods of focal length and optical path difference of liquid crystal lens are given.The resistance-capacitance equivalent model of the liquid crystal lens is used to calculate the refractive index distribution of the liquid crystal lens.According to the Newton optical imaging formula,the theoretical basis for the optical focusing of the liquid crystal lens and traditional glass lens combination system is obtained.2.The Zemax optical simulation software was used to model the four-separated optical lens.According to the requirement of iris image pixels larger than 80 pixels in iris recognition,the longest object distance is 620 mm,which is taken as the longest object distance of the imaging system.The refractive index distribution of the liquid crystal lens is modeled with the gradient 2 in Zemax,and combined with a four-separation optical lens to form a focusable optical system,and the imaging performance of the system is simulated.3.A mode-controlled liquid crystal lens was prepared.The effect of the number of spin coatings on the sheet resistance of the high-resistance film was studied when the sol-gel method was used to prepare the high-resistance film.The sheet resistance of the high-resistance film directly determines the frequency of the applied AC drive voltage.By controlling the size of the high-resistance layer sheet resistance,the power consumption of the liquid crystal lens can be controlled,which facilitates the preparation of low-power liquid crystal devices.After the preparation of the liquid crystal lens,based on the birefringence effect of the liquid crystal material,the optical interference characteristics of the liquid crystal lens were measured.After using Labview to reduce the noise of the measured interference ring,the liquid crystal lens was calculated using the optical path difference calculation formula of the liquid crystal lens Refractive index distribution.4.In order to verify the actual focusing effect of the mode-controlled liquid crystal lens,a liquid crystal lens focus drive control system was built.Based on Labview Vision,an auto-focus program that uses the image boundary gray gradient as the focus evaluation function is written,and the system is used to test the focus performance.The results show that after the liquid crystal lens is added,when the object distance is greater than 440 mm,the focusing range of the imaging system increases by 4.5 times,but it still has a certain gap with the ideal focusing effect.To solve this problem,a method of segmenting the sheet resistance of the high-resistance layer of the liquid crystal lens is proposed to correct the refractive index distribution of the liquid crystal lens so that the refractive index is close to the ideal parabolic distribution.Finally,when the refractive index distribution of the liquid crystal lens is parabolic,the imaging effect of the optical system is calculated,and a feasible solution for preparing a high-performance liquid crystal lens is proposed.