Simulation of laser-induced retinal thermal injuries for non-uniform irradiance profiles and the applicability of the laser safety standard

Abstract

Significance: A validated biophysical computer model simulating retinal thermal damage thresholds is used to investigate elongated retinal images. The International Commission on Non-Ionizing Radiation Protection Guideline and the laser safety standard IEC 60825-1:2014 include a method for averaging non-uniform extended sources, however, there are no studies that have examined the applicability in detail. Our study represents a method that can also support future research in the field of eye safety. Aim: As there is currently no experimental data available for non-uniform irradiance profiles, the calculation procedure given in the laser safety standard is derived from symmetric retinal images. We aim to verify this calculation procedure for such profiles on the retina in the thermal hazard regime. Approach: A three-dimensional computer model, which solves the heat transfer equation and the Arrhenius equation describing the denaturation of the proteins in the retina, is used to simulate the threshold values for the retinal thermal injury. Three different non-uniform irradiance profiles, elliptical Gaussian, elliptical top-hat, and rectangular top-hat distributions, are investigated for a wavelength of 530 nm. The profiles are varied in their sizes and simulated for different single-pulse durations. By applying the laser safety standard, the maximum allowed energies are calculated and divided by the corresponding threshold values to obtain the reduction factor (RF) which is a crucial parameter. Results: Due to the thermal behavior in the retinal tissues, the Gaussian irradiance profiles yield larger threshold values than both top-hat profiles. Furthermore, the ratio between the threshold values and the maximum allowed energies are found to be the lowest for the Gaussian profiles. Conclusion: The simulated retinal thermal injury thresholds for the three investigated non-uniform irradiance profiles show larger RFs than the minimum RF found for symmetric profiles. This supports the applicability of the evaluation scheme of the laser safety standard for non-uniform retinal images.