Design Of Compact Non-blind Area PAL System And Its Zoom Elements

Panoramic annular lens(PAL)is a special kind of optical system consists of a panoramic front block unit and a relay lens system,which can image a very large field of view by refraction and reflection.Surrounding beams transmit into PAL block from its first refractive surface,their propagation directions are changed by twice reflection within this block,and then transmit out with a small angle between propagation direction and its optical axis.After passing through the relay lens system,these beams are finally focused onto the sensing plane of detector.PAL imaging system has the feature of transforming a cylindrical field of view into a ring-shaped plane,which makes it very useful in computer vision and endoscopy.Conventional PAL system mainly has three drawbacks.1)Central blind area.Since the second reflective surface of PAL block is orthogonal to optical axis,the beams from its front field of view cannot pass through,which makes the central pixels on image plane cannot capture any optical signal and form a round blind area.It will reduce the efficiency of detector greatly and arise user's uncomfortable feeling.2)Large block size.The beams are reflected within PAL block twice,so the block size will be much larger than the relay lens system.Thus,it is hard to control the entire system scale and causes an unbalanced weight.3)Because the PAL system transforms a very large field of view onto an image plane with limited size,its resolution is always low compared with other imaging system.With the purpose of solving the problem of central blind area,we propose an effective way to realize the feature of non-blind area panoramic imaging by using optical components with the selectivity of transmittance and reflection.Based on wavelength bands,we coat a dichroic filter on the second reflective surface,which can make the beam in some band reflect,propagate along the PAL imaging path,and focus on the image plane at last.At the same time,this filter makes the beam in some other band transmit,and finally focus on the central part of image plane.Based on polarization,we cement a polarizing plate beam splitter on the second reflective surface,which can make the beam with S polarization state reflect and image on the outer ring,while make the beam with P polarization state transmit and focus on the central part.With the purpose of compress the size of PAL block,we also establish the mathematical model of PAL block by ray tracing.According to this model,we find an effective optimization method to control the lateral PAL block scale.And wo present our optical design of endoscopy system by this method.With the purpose of improving the resolution of PAL system,we introduce the zoom elements with Alvarez freeform surface into PAL system.By adding Alvarez-surface based lens groups after an telecentric PAL system,we can change the local system focal length with lateral movement.Meanwhile,we present a laser beam expander to verify the result of zooming function of Alvarez freeform surface.