Study On Flat-plate Imager With Extended Depth Of Field

Flat-plate lens is an ultra-thin folded optic lens with annular aperture.The lens uses multiple coaxial aspheric reflectors to fold the internal light path,significantly reducing thickness and weight while maintaining high resolution and good light collection.This ultra-thin camera will become an important branch of the future camera market,and will have a broad application prospect in the fields of airborne photoelectric imagers and portable infrared telescopes.In view of the narrow depth of field and the poor imaging quality of existing flat-plate lens,this paper deeply studies the principle and characteristics of the flat-plate lens.A four-folded flat-plate lens and an improved four-folded flat-plate lens combined with wavefront coding technique are designed.Main work of this paper:1.A four-folded flat-plate lens is designed,the function of folding and imaging is realized.The outer diameter is 40 mm,the obscuration ratio is 0.8,the effective focal length is 78.4mm,the total thickness is 19.5mm,and the full field of view is 4°.The performance of the flat-plate lens is simulated and analyzed,the results show that the lens has high imaging quality in the depth of field of 222.67m-577.40 m.The imager is processed and assembled,the resolution and imaging tests were performed.2.In view of the shortcomings of the first flat-plate lens,an improved four-folded flat-plate lens is designed.The outer diameter is 20 mm,the obscuration ratio is 0.8,the effective focal length is 19.8mm,the total thickness is 6.7mm,and the full field of view is 12°.The performance of the improved flat-plate lens is simulated and analyzed,the results show that the lens has high imaging quality in the depth of field of 46.11m-185.40 m.In addition,the gap adjustment of 8.34μm enables the focal plane position of the lens to be extended from 10 m to infinity.3.In order to extend the depth of field of the improved flat-plate lens,the encoding theory based on phase mask and the decoding theory based on digital image processing are studied.A fifth-order phase mask with XY polynomial term is designed,the imaging characteristics of the improved flat-plate lens under the phase mask coding are simulated.The simulation results show that the depth of field is extended to 20m-∞m.In order to restore the encoded intermediate image,a combined filtering method based on wiener filtering and constrained least squares filtering is proposed,the simulation results show that the filtering method has a certain improvement.