Development of software-based methods to evaluate the eye safety of laser based optical systems

Abstract

Laser systems have long since become an integral part of countless fields, such as industry, manufacturing, medicine, information technology, science and research. Their importance is continuously growing and their field of application is becoming even broader. However, with lasers also comes a potential hazard, as the radiation can cause damages to the eye and skin. The eye is the most sensitive part as radiation in the visible and near-infrared wavelength region can be highly focused on the retina. Retinal injuries are irreversible and are accompanied by a loss of vision in the damaged regions. Therefore, the topic of eye safety will always be an essential part of the development and application of laser systems. This thesis focuses on this topic with a software-based approach addressing the laser safety standard IEC 60825-1:2014. The laser safety standard provides a set of emission limits that are connected to the different types of damage mechanisms, namely the photochemical, thermal and thermomechanical damage. In the first study of this thesis, the retinal image analysis is developed. By means of this analysis, it is possible to perform eye safety calculations for radiation imaged on the retina. The aim of this study is to introduce an unambiguous and general evaluation method that leads to restrictive results. In a second study, this procedure is applied to Gaussian beams. Their special characteristic for eye safety evaluations is that the evaluation can be done analytically with the beam width. This approach is commonly used by laser safety laboratories or by laser safety officers. In this study, the eye safety of Gaussian beams is evaluated by placing an eye model in front of the beams and simulating the retinal image using wave optics. It was found that due to the diffraction at the eye pupil, the analytical approach yields incorrect results and underestimates the hazard potential. An extended analytical description is developed which is verified by the simulation and provides safe results. The last study of this thesis addresses the validity of the evaluation scheme from the laser safety standard. The eye safety of radiation that is imaged on a non-circular symmetric area on the retina is evaluated by averaging over two orthogonal widths. This procedure is not supported by any experimental data as these only includes circular symmetric profiles. The question arises if this evaluation procedure is valid and sufficiently safe. For this reason, a three-dimensional computer model which simulates retinal thermal injuries was developed. It consists of an eye model to evaluate the retinal image and a finite-element-method simulation of the retinal tissue to calculate the heat distribution. In the study, three different profiles, namely an elliptical Gaussian, an elliptical top-hat and a rectangular top-hat, are investigated with varying parameters like the image widths and the laser pulse durations. For these profiles, the retinal injuries are simulated with the computer model and the maximum allowed energies according to the standard are evaluated. Both results are compared to each other by calculating the reduction factor which is the ratio between both values. In summary, the evaluation procedure from the standard was validated for the investigated profiles. The developed computer model is an important tool to simulatively extend the dataset of retinal thermal injuries which can be used for research purposes and to further improve the laser safety standard.