The resulting photoproducts damage the cells, impairing their ability to respond to light, and in extreme cases can destroy them. Depending on the severity of the damage, an affected observer experiences either a temporary or permanent loss of visual function. This photochemical retinal injury mainly occurs when the retina is exposed to blue and green light. Longer wavelengths of visible light and near-IR radiation are absorbed by the dark pigment epithelium below the retina, and converted into heat which can literally cook the exposed tissue. This thermal damage also occurs during extended exposure to blue and green light.
Photocoagulation destroys the rods and cones, leaving a permanently blind area in the retina.
Each wavelength of optical radiation has an associated threshold retinal exposure level that must be exceeded in order for retinal damage to be observed - shorter wavelengths are more effective in that less energy is needed. Because the threshold exposure levels for photic damage to the retina at each wavelength of the optical spectrum are well known, it is fairly simple to calculate the maximum permissible filter transmittance that will provide adequate retinal protection from sunlight. The ratio between the threshold retinal irradiance for light damage to the solar spectral irradiance at each wavelength provides a starting point for this. Back to Index Measuring Filter Transmittance Filter materials that were tested are shown in Table 1 . Solar filter materials were randomly selected from the manufacturers' stock.