Research On Freeform Varifocal Optical Imaging Systems

Compared to the traditional spherical or aspherical optical element,the freeform optical element has higher degrees of freedom.We can adjust the local surface shape flexibly according to the requirements of the optical design and control the propagation of lights purposely.In some off-axis or non-rotational symmetry optical systems,freeform surface with a high degree of freedom can effectively simplify the system structure and reduce the whole optical system's volume and weight,improve the optical performance.With the degree of freedom of freeform optical elements being increased,the fuction for charactering a surface will contain more variables and the the complexity of optimization will also be increased.It is much more difficult to design freeform optical elements than traditional optical system.Especially for some optical systems with spatial surface-plane mapping between object and image,the difficulty of design is more bigger.A method to control the local surface of freeform surface is proposed,which is the inhomogeneous field-of-view meshing optimization method.By configuring the layout of multi field of views,curvature of local surface can be controlled.The method is applied to two kinds of typical optical system with freeform surface,and the validity and practicability of this method to the design the freeform surface optical system has been verified.In this paper,two freeform varifocal optical systems have been designed,they are progressive multifocal lens and panoramic annular varifocal system,corresponding to the single varifocal lens and multi varifocal lenses respectively.The progressive multifocal lens and eyeball optical system can be combined into a imaging optical system.We use the inhomogeneous field-of-view meshing optimization method to control and optimize the optical power distribution.Considering the human eye's habit of progressive stadia,we successfully designed progressive multifocal lens suitable for presbyopia,which can meet the mapping between progressive stadia surface and the retina surface.The panoramic annular varifocal system contains several freeform optical elements and its focal length varies around the azimuth direction with no astigmatism.The panoramic annular varifocal lens rotates around the optical axis,and its field of view obeys the cosine curve from the 70°~110° trim angle at 0° azimuth angle to 80°~110° trim angle at ±180° azimuth angle,specifying the central field of view as a constant during the rotation.Its object-image mapping relationship is 3D cylindrical surface-2D plane mapping.We use the local field-ofview dense meshing optimization method to control the curvature distribution of the freeform surface,significantly reducing the meshing distortion aberration.