The Study Of The Image Processing Software Of Corneal Topography Apparatus Based On Placido Disc

Measurement of the cornea is an important component in modern ophthalmic medical testing, where the corneal topography apparatus is one of the essential measurement instruments, which can measure the surface morphology of the human cornea quantitatively and provide much important information through analyzing the data and processed images. Corneal topography apparatus has a very wide range of applications in corneal disease detection, corneal topography examination before Lasik surgery, rigid gas permeable contact lenses (RGP) and orthokeratology (OK lens) fitting.This thesis mainly studies the software design and image processing in the developing of Placido disc corneal topography apparatus. The main contents include four parts mentioned below. The first part is the communication and control between software systems and lower machine. The second part is CCD image acquisition and data extraction. The third part is mathematical modeling and calculation of the cornea surface height values and eyes’diopters values. And the last part is drawing the map of the 3-D surface.The study realized the control between software system and lower machine through the analyzing and comparison the relation between underlying circuit and CCD camera used in Sony Point Grey Digital Camera. Our circuit design can discriminate the difference between right and left eyes and control the camera status. In addition, the CCD-frame video output formation was converted effectively in order to achieve the consistency of the entire software system on image processing.In order to solve the focus problem of the limited depth of focus of the optical system on the convex object in the process of collecting images, a method of image sharpness evaluation function using the gradient of image gray value and the distance weighted was proposed theoretically. With this method, the problem of image gathering center was solved effectively, and excellent imaging results of Placido disk image on the cornea were achieved.Because the high accuracy of image valid information extracted from the image, which was obtained from the corneal topography apparatus based on the Placido disk, we use the method of twice extract in the point of center image to improve the positioning accuracy of the image center point. As a result, the data of each ring in Placido disk image can be extracted accurately under the process of gradient information obtain after the smoothing the gray value of image center point and sub-pixel edge detection using cubic curve fitting.Through the mathematical modeling of images of the topography based on the Placido disk, many accurate results of corneal were obtained including the surface topography, surface height difference maps, diopters map and three-dimensional surface topography.