Bernhard Rassow

Correlation of optotypes with the Landolt ring : a fresh look at the comparability of optotypes

The legibility of three selected sets of optotypes: (1) letters; (2) Snellen E; and (3) KOLT test have been compared to the Landolt ring in a study based on the German Standard DIN 58220, Part 2 and the International Standard ISO 8597. The letters were printed in the standard typeface used on German street signs (sans serif Linear-Antiqua, Typeface B). The main results are: 1. A set of letters can be selected in such a way that the differences in legibility displayed between different letters in the set are smaller than the ISO/DIN allowance of 0.05 logarithmic units of the visual angle, although the letters are not constructed according to the traditional 5 by 5 or 5 by 4 construction principles. 2. The eight letters C, D, E, K, N, P, U, Z are proposed for standardized visual acuity tests because of their almost equal legibility. 3. The height of the typeface used for this selected set of letters should be 5% less than the diameter of the Landolt ring in order to achieve the same legibility as the latter. 4. Both shape optotypes (Snellen E and KOLT test) must be approximately 15% smaller than the diameter of the Landolt ring in order to obtain comparable visual acuity scores.

Light hazards to the patient's retina from ophthalmic instruments

The spectral radiance and irradiance of several common ophthalmic instruments were measured. The respective spectral retinal irradiance a patient might receive was calculated. To evaluate the potential light hazard to the patients retina, we calculated the temperature rise resulting from light absorption at the fundus, compared retinal irradiance with threshold values for photochemical retinal damage, and correlated them with safety guidelines. A thermal hazard to the patients retina can result from operation microscopes if light output is maximal and exposure prolonged. Comparison of light exposures from different ophthalmic instruments with thresholds for photochemical-induced lesions showed that one has to be more concerned about the actinic effect of light exposure. At maximum lamp voltage, the light exposure from operation microscopes reaches thresholds for photochemical damage even after a few minutes. Other ophthalmic instruments present no particular hazard if they are used at common exposure durations. Application of laser safety guidelines confirm these results. In some cases, light from operation microscopes exceeds the safety margin in <1 min.

Light-intensity distribution in eccentric photorefraction crescents

We try to improve the accuracy of eccentric photorefraction by taking more information into account than just the size and tilt of the crescent. Based on Gaussian optics and the assumption of an isotropic scattering retina, a theoretical analysis of the light-intensity distribution in the pupils of astigmatic eyes is presented. The method is applied to different photorefractor setups (point light source, long linear light source, knife-edge aperture, and circular aperture). In the case of a knife-edge aperture the crescent structure can be formulated analytically. In the case of a circular aperture an analytic description is possible only for spherical refractive errors, but astigmatic refractive errors can be determined from crescent parameters with neural networks.

Neuere Tendenzen der objektiven Refraktionsbestimmung – Kinderrefraktion

Zusammenfassung Die objektiven Methoden der Refraktionsbestimmung werden bei der Untersuchung von Patienten eingesetzt, deren Mitarbeitsfahigkeit oder -bereitschaft fur eine subjektive Refraktionsbestimmung nicht ausreicht. Die klassische Methode der objektiven Refraktionsbestimmung ist die Skiaskopie, die jedoch viel Ubung und Erfahrung des Untersuchers voraussetzt. Sie wird immer mehr von den opto-elektronischen Augenrefraktometern abgelost. Unter diesen nehmen die noch in der Entwicklung befindlichen Photorefraktometer einen besonderen Platz ein. Sie erlauben auch die Untersuchung von Kleinkindern. Bei diesen ist im Fall einer Fehlsichtigkeit eine fruhzeitige Versorgung mit der richtigen Sehhilfe fur die spater erreichbare Sehleistung von entscheidender Bedeutung. Die Praxis der Photorefraktion wird an Beispielen vorgestellt und die Theorie wird, insbesondere im Hinblick auf noch offene Fragen, erlautert.

Interferometric measurement of the refractive error of the eye

A Michelson interferometer is used to determine the refractive error of the eye. A coherent infrared point source radiates into the eye; at the fundus the light is scattered. The light wave emerging from the eye depends on its refractive error. The wave is analyzed by means of the interferogram which results from superposing a reference light wave. The refractive error is determined in two steps: (1) Speckle size is enlarged by moving the point source towards the plane conjugate with the eyes fundus. (2) Interference fringes which appear when speckle size exceeds fringe intervals are evaluated by Fourier analysis after suitable transformation of coordinates.