The Study On Fingerprint Imaging Of Mobile Phone Based On The Self-focusing Lens Array

In recent years,biometric identification has become more and more widely used in mobile phones.Fingerprint recognition,iris recognition and face recognition have emerged successively.Fingerprint recognition has become a relatively common way in mobile phones because of its simplicity,convenience and high practicability.At present,the fingerprint unlocking function of mobile phone mainly through the mobile phone pre-identification or post-identification module,inductive fingerprint RF signal or voltage difference to form fingerprint image.In view of the development trend of the full screen of mobile phone,the existing physical fingerprint unlocking mode can not meet the need of unlocking the fingerprint under the screen,and the traditional optical system is not suitable to be built into the mobile phone.It is more urgent to develop optical micro-lens array as a new fingerprint imaging carrier.Micro-lens arrays are compact,light weight,and widely used in optical interconnection,medical devices,etc.As imaging devices,high-resolution images can also be obtained by algorithm reconstruction.In this paper,the single lens can be used to realize conjugate imaging,and the array can be constructed to realize the optical synthetic imaging directly.The scheme of fingerprint imaging of self-focusing lens array under the screen of mobile phone is mainly studied in this paper.Based on the scalar diffraction theory,the invariance of the imaging space of the self-focusing lens and its array system is analyzed in this paper.The optical field distribution of the ideal image plane is obtained by using Fresnel diffraction integral and the point diffusion function of both.The ray theory is used to discuss the array imaging,and the condition of array integrated imaging is obtained.The parameters of the paraxial secondary refractive index distribution of the lens are fitted by the method of multiple measurement of the posterior intercept.The accuracy of the fitting parameters is further verified by the theoretical analysis and experiment of the optical field near the focal plane of the lens.Finally,the resolution,numerical aperture and other performance parameters of the lens are measured by the imaging method.The lens chromatic aberration curves are obtained by using the lens parameters at different wavelengths.The accurate refractive index distribution of lens is obtained by interferometry and the coefficient of higher order term is obtained by fitting.The relationship between lens aberration and refractive index distribution is analyzed.The effect of the change of aperture diameter in different field of view on the point diffusion function is explained qualitatively by Zemax.It is shown that the large off-axis point of view will seriously reduce the resolution of the edge imaging of the lens as well as the overall resolution of the array.The effects of wavelength,space frequency,array length and focal depth on the transfer function of the array are analyzed,and the conclusion that the array length determines the image quality is obtained.After the lens parameters are determined,the array imaging simulation model is established by using the imaging theory,and the array integrated imaging is realized by MATLAB.In fabrication,we eliminate edge aberration by chemical etching and obtain three sets of lens arrays.After constructing the imaging optical path and combining with the OLED mobile phone screen,it is preliminarily verified that the self-focusing lens array can realize fingerprint imaging.