5.0 Viewing the SPS from the Earth Through Optical Instruments. Up to this point we have analyzed hazards to the naked eye from the SPS and have not specifically considered the potential hazards to the eye resulting from viewing any of the sources of reflected sunlight through binoculars or a telescope. Viewing any hazardous light source through magnifying optics normally increases the hazard to the retina; the exact degree of increase is dependent upon whether the source presents a thermal or a blue-light hazard. If the hazardous light source already extends greater than 10 mradian and is only a blue-light photochemical retinal hazard, then the increased angular source size does not increase the hazard, since there is no image-size dependence for this injury mechanism. It is important here to note that a magnifying optical instrument increases only the image size; it cannot increase the retinal irradiance (W • cm ) because of the Law of Conservation of Radiance, or Brightness (16). The blue-light hazardous viewing time (tmax) cannot be reduced below that of viewing the sun directly — 0.84 sec. as shown in Equation (7). On the other hand, if the source is potentially a retinal thermal injury hazard, the hazard actually increases proportionately with increasing image size. That is, because of the poorer conditions for retinal heat conduction for a larger image, the retinal threshold of injury decreases inversely as the increase in image diameter. The studies of Ham and others, as reported by Sliney and Wolbarsht (16) show that a retinal thermal injury is not possible for the unaided 159.Um image size of the sun on the retina. However, if the sun were viewed by an optical instrument of sufficient power to increase the retinal image to 1 mm or greater (i.e., 7X or greater) a retinal thermal injury could result, on the tenable assumption of an effective 3 mm or larger pupil.
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