Thermal versus photochemical damage in the retina--thermal calculations for exposure limits.

Different kinds of damage can be produced by intense light irradiation of the retina. These types of damage can be separated into changes which are induced by photochemical or thermal effects in the tissue. Photochemical damage occurs mainly with intense irradiation of a few seconds or longer whereas thermal effects appear to prevail at irradiation times in the 1 ms to 1000 ms-range. Distinction between photochemical and thermal damage is very difficult because both types of injury appear as disorders in the retinal pigment epithelium and the outer segments of the photoreceptors. A thermal model has therefore been developed which takes into account the light absorption in the different layers of RPE and choroid and allows the calculation of the temporal development of temperature at each site and depth of the fundus. Model calculations are used to correlate exposure parameters (wavelength, irradiance, exposure time etc.) with the resulting temperature profiles. This correlation leads to a better distinction between exposure limits for thermal and photochemical damage of the retina.