Improved Biometry of the Anterior Eye Segment

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Abstract
In this paper, a method is presented to evaluate the true values of several optical parameters of the anterior eye segment. These are: the thicknesses of the cornea and lens, the depth of anterior chamber and the anterior and posterior curvatures of the cornea and lens. First, a photo is taken of each patient''s eye with a Topcon SL-45 Scheimpflug camera. There are two effects distorting the photo. As the film plane and the object plane are perpendicular to each other, the imaging scale is not constant over the photo, a phenomenon which is called the camera distortion. As the light passes through different refractive media on its way from the inner structures of the eye to the camera, the rays of light are refracted, which is called the refractive distortion of the eye. Following these distortions, the proportions on the photo do not correspond to those in the human eye. The intention of this paper is to calculate the true optical parameters of the anterior eye segment. The seeming optical parameters are taken from the photo and the path of light from a point in the film plane through the camera and the refracting surfaces into the eye is constructed by ray tracing. A set of representative points calculated according to ray tracing provides a basis to evaluate the true optical parameters. This work is done using a Basic program, that accepts the seeming parameters from the photo as input values and provides the true values as output. The adjectives seeming and true added to the optical parameters indicate a value correlating with a distorted photo and a value that was corrected by the theory. The two terms were chosen by mnemonic reasons, although another pair of adjectives such as distored and corrected might have been of higher precision. For an average adult human eye the optical parameters from the photo were compared with the calculated ones. A phantom eye with well-known optical parameters was constructed and photos were taken from this eye to verify the calculations.

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