Improved Biometry of the Anterior Eye Segment

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 the 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 distorted 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.

Refractive correction method for digital charge-coupled device-recorded Scheimpflug photographs by means of ray tracing

“…However, both techniques, A scan ultrasonography and slitlamp Scheimpflug photography, have their respective drawbacks: ultrasonography requires the velocity of sound in the lens, which may be age-dependent, whereas Scheimpflug photography requires knowing the refractive index of the cornea, the anterior chamber, and the eye lens to enable correction of the photographs for the distortion due to the refraction of the cornea and lens ͑after Dubbelman et al 8 ͒. One of the major advantages of the algorithm described in this paper is the fact that it is based on digital-CCD-recorded Scheimpflug photographs instead of digitally scanned Scheimpflug photographs ͑see Richards et al 4 and Kampfer et al 5 ͒. Thus, additional errors are avoided in the analysis of the original Scheimpflug photographs, namely inaccuracies that occur by processing and scanning of the original Scheimpflug photograph.…”

“…However, for a quantitative evaluation, for which especially digital Scheimpflug photographs are suitable, optical/refractive distortions must be taken into account. Such distortions arise not only from the anterior corneal surface ͑ACS͒ alone; images of deeper structures of the eye such as the posterior corneal surface, the pupil, and the anterior lens surface are also distorted [2][3][4][5] due to the differences in refractive indices and the various radii of curvature of the refractive surfaces.…”

“…In earlier work, e.g., by Richards et al 4 and Kampfer et al, 5 optical corrections have been applied to scanned Scheimpflug photographs. In their work, the authors had to overcome problems such as photo/camera distortion and distortions resulting from projection of Scheimpflug negatives for digitization in addition to the natural refractive distortion of the eye.…”

Correction on the distortion of Scheimpflug imaging for dynamic central corneal thickness

“…13,19 In our previous study with the Corvis ST (OCULUS Optikgeräte GmbH), the directly measured CCT at highest concavity was found to be reduced by ∼8% in 42 healthy subjects 20 without considering the Scheimpflug distortion. 21 The overall objective of our study is to predict the influence of Scheimpflug distortion on the CCT measurement, to propose a method to correct the distortion for dynamic CCTs, and to apply the method to study the CCT variation in clinical cases.…”

“…An early study reported that the directly observed CCT was underestimated by ∼43% in comparison with the true value. 21,23 The authors corrected this error by using the ray tracing technique with a Basic program. An introduction to the theory of ray tracing and its computation implementation can be found in Ref.…”