Aufbau und Validierung einer digitalen Messstrecke für die dynamische Bestimmung der Krümmungsradien und der Lage der menschlichen Augenlinse unter natürlichen Sehbedingungen

Kirschkamp, Thomas; Reim, Martin (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2002)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2002


Purpose: In this thesis an experimental setup was to be developed in order to quantify the radii of curvature and the position of the human crystalline lens. There had to be determined the anterior and posterior radii of curvature of the crystalline lens as well as the decentralisation and rotation of the crystalline lens with regard to the corneal axis. The experimental setup had to permit static measurements under constant accommodation on the one hand and, on the other, dynamic measurements with continuous changes of accommodation under natural conditions of vision. Methods: The eye was illuminated by two infrared light sources. In this way reflecting images were produced at the anterior corneal surface and the anterior and posterior lens surface, the so-called Purkinje images I, III, and IV. They were then registered and measured by a camera taking pictures of infrared light. The size of these Purkinje images is linked with the radius of curvature of each of the reflecting surfaces. The position of the Purkinje images in relation to the front part of the eye is dependent on the rotation and decentralisation of each of the surfaces. This thesis is divided into three parts: 1. Because of the completely new demands there had to be developed, in several steps, collimated light sources (because of the infrared light the subjects were not dazzled), a fixation object with variable stimuli of accommodation, a telecentric objective without perspective errors due to different axial positions of the Purkinje images (otherwise refocusing had been necessary, so that the accommodation had to be constant during this time and no dynamic measurements under accommodation changes had been possible) and biconvex plastic lenses for a model eye (for the simulation of the human crystalline lens during the following validation measurements). Afterwards all components were tested carefully. 2. Then, the human eye was simulated by the model eye to verify the accuracy of the experimental setup. There were determined the following measuring errors: Radius of curvature of the anterior lens surface r3 ±0,38 mm, radius of curvature of the posterior lens surface r4 ±0,51 mm, position of the corneal axis Kappa±0,56°, crystalline lens rotation Alpha±0,26° and crystalline lens decentralisation d ±0,02mm. 3. Finally, the described parameters were determined for 9 emmetropic subjects in a pilot study. In this study, the horizontal components were measured, the vertical components can be determined in the same way. The measurements were done under static conditions (accommodation to 25 cm and cycloplegia) as well as under dynamic conditions (periodic and continuous changes between accommodated and relaxed eye). Results: The static as well as the dynamic measurements with a change of accommodation of nearly 4 D showed that the contribution to the accommodation of the anterior lens surface was 2/3 and of the posterior surface 1/3. For the orientation of the cornea and the crystalline lens the following values were found: Position of the corneal axis Kappa= 5,98° ± 2,26° in temporal direction, rotation of the crystalline lens Alpha= -0,14° ± 1,10° to nasal, lens decentralisation d = -0,11 mm ± 0,07 mm to nasal. So we can say that the influence of accommodation on these values can be neglected within the fixation and measuring tolerances. These results can be interpreted that normally the angle Kappa is positive, that the crystalline lens is nearly not inclined and that the lens shows - as well as the centre of the pupil - a small decentralisation to nasal.