Study On The Cornel Image Acquisition System Based On Placido Disk

Corneal topographer is a kind of important medical testing instrument, which is used in the modern ophthalmic area. It is used for quantitative detection of the morphology of human corneal. The information of the corneal surface morphology could be given in the form of image or data. And it provides convenience for the ophthalmologists to test the human corneal. It can be used in the inspection of the corneal disease, such as the examination of the corneal before Lasik surgery and the cornea measurement of the Rigid gas permeable contact lenses (RGP) and Orthokeratology lenses fitting.This work is mainly about the corneal image acquisition system study as a part of the corneal topographer system development which is based on the placido disk in the corneal image acquisition. The corneal image acquisition system study includes the design of the corneal image acquisition-driven, projection system design and Optical lens design in the corneal image acquisition system. These three parts will be described detailed in the article.For corneal image acquisition requirements, Researcher chooses the Sony ICX424AL area array CCD as the image sensor of the image acquisition system. After analysis drive timing requirements of the ICX424AL, the application of the MAX2Series EPM240T100C5N and high-performance analog front-end chip AD9949A together for the CCD to provide a driving pulse signals. With the verilog hardware description language realize the logic design of the Drive timing. And use the ALTERA company software Quartus Ⅱ to drive the functional simulation of the timing generator. And use EPM240T100C5N chip hardware adaptation, and ultimately realize the ICX424AL drive design with CPLD and AD9949A.The projection system design includes placido disk and backlight design. To realize the acquisition of corneal surface topography, system need to project clear, uniform, soft, concentric rings on the human cornea. So the placido disk structure and production process conducted in-depth discussed and the design of backlight and power drive.According to design requirements selecting an initial lens structure, researcher with the adaptation method to finish the optical structure optimization design. Using the CODE V optical design software to achieve lens optimized which’s focal length is20mm, field of view is20°and entrance pupil diameter is2mm.With the image acquisition system developed, corneal placido disk concentric rings projected on the cornea meet the accuracy requirements of the host computer to calculate the corneal image for the corneal image acquisition. The images collected with a resolution of640(H)×480(V) and the image acquisition rate of60fps. By comparing with the Italian CSO corneal topographer, the image acquisition system meets the extraction of the corneal surface sampling point requirements using image processing software.