Capabilities and limitations of a new thermal finite volume model for the evaluation of laser-induced thermo-mechanical retinal damage.

Many experimental studies focus on the physical damage mechanisms of short-term exposure to laser radiation. In the nanosecond (ns) pulse range, damage in the Retinal Pigment Epithelium (RPE) will most likely occur at threshold levels due to bubble formation at the surface of the absorbing melanosome. The energy uptake of the melanosomes is one key aspect in modeling the bubble formation and damage thresholds. This work presents a thermal finite volume model for the investigation of rising temperatures and the temperature distribution of irradiated melanosomes. The model takes the different geometries and thermal properties of melanosomes into account, such as the heat capacity and thermal conductivity of the heterogeneous absorbing melanosomes and the surrounding tissue. This is the first time the size and shape variations on the melanosomes' thermal behavior are considered. The calculations illustrate the effect of the geometry on the maximum surface temperature of the irradiated melanosome and the impact on the bubble formation threshold. A comparison between the calculated bubble formation thresholds and the RPE cell damage thresholds within a pulse range of 3 to 5000 ns leads to a mean deviation of µ=22mJ/cm2 with a standard deviation of σ=21mJ/cm2. The best results are achieved between the simulation and RPE cell damage thresholds for pulse durations close to the thermal confinement time of individual melanosomes.

A prediction model for ocular damage - Experimental validation.

With the increasing number of laser applications in medicine and technology, accidental as well as intentional exposure of the human eye to laser sources has become a major concern. Therefore, a prediction model for ocular damage (PMOD) is presented within this work and validated for long-term exposure. This model is a combination of a raytracing model with a thermodynamical model of the human and an application which determines the thermal damage by the implementation of the Arrhenius integral. The model is based on our earlier work and is here validated against temperature measurements taken with porcine eye samples. For this validation, three different powers were used: 50mW, 100mW and 200mW with a spot size of 1.9mm. Also, the measurements were taken with two different sensing systems, an infrared camera and a fibre optic probe placed within the tissue. The temperatures were measured up to 60s and then compared against simulations. The measured temperatures were found to be in good agreement with the values predicted by the PMOD-model. To our best knowledge, this is the first model which is validated for both short-term and long-term irradiations in terms of temperature and thus demonstrates that temperatures can be accurately predicted within the thermal damage regime.

Prediction of temperature and damage in an irradiated human eye-Utilization of a detailed computer model which includes a vectorial blood stream in the choroid.

The work presented here describes the development and use of a three-dimensional thermo-dynamic model of the human eye for the prediction of temperatures and damage thresholds under irradiation. This model takes into account the blood flow by the implementation of a vectorial blood stream in the choroid and also uses the actual physiological extensions and tissue parameters of the eye. Furthermore it considers evaporation, radiation and convection at the cornea as well as the eye lid. The predicted temperatures were successfully validated against existing eye models in terms of corneal and global thermal behaviour. The model׳s predictions were additionally checked for consistency with in-vivo temperature measurements of the cornea, the irradiated retina and its damage thresholds. These thresholds were calculated from the retinal temperatures using the Arrhenius integral. Hence the model can be used to predict the temperature increase and irradiation hazard within the human eye as long as the absorption values and the Arrhenius coefficients are known and the damage mechanism is in the thermal regime.

Large aperture at low cost three-dimensional time-of-flight range sensor using scanning micromirrors and synchronous detector switching.

In this article the problem of achieving fast scanning of a time-of-flight range sensor with a large optical receiver aperture at low system cost is targeted. The presented approach to solve this problem consists of a micromirror-based transmitter unit and a receiver unit consisting of a large aperture lens system with a small field of view and a detector array. A concept, which is called synchronous detector switching, is applied to the detector array. Thereby electronic steering of the small receiver field of view is possible. The overall approach is compared to alternative approaches, and the underlying concept of synchronous detector switching is demonstrated experimentally in an implementation of a three-dimensional time-of-flight range sensor. It is theoretically shown that the presented concept is potentially cheaper than the alternative approaches for applications with a field of view of less than 60×60°. After a discussion of the strengths and limitations of the approach, its effect on broader scientific issues is outlined.

Eye safety for scanning laser projection systems.

In the growing field of pico-projectors, laser-based scanning systems may be advantageous over DLP- or LCoS-based imagers due to their potential for miniaturization, enhanced optical efficiency and cost reduction. The high energy density of a combined laser beam can, however, be hazardous to the human eye. Laser projection systems must therefore be identified with the laser class, depending on their maximum optical output power. This power limits the brightness of the displayed image, which is of particular interest for mobile applications. Various approaches to classifying laser devices by their wavelength and output power are described within the standards for laser safety. It is found that actual safety regulations cannot be directly applied to scanning systems. A detailed analysis of the optical conditions in terms of a two-dimensional extended light source is appropriate for the consideration of laser scanner devices. In this article, alternative ways of applying laser standards for scanning systems are discussed. The dependencies of maximum luminous flux from scanning system parameters are reviewed. It is shown that the evaluation of retinal light exposure in terms of existing laser regulations leads to an overestimation of the hazardous potential. Advanced investigations are proposed to support the definition of suitable criteria for the classification of laser scanning projectors.